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HF antenna placement question
Howdy, y'all
As most of you know, we're in an extensive refit. We're finally to the point of addressing the stern of the boat. Most of you know we added an arch, on which we've put wind and solar, satellite, gps and vhf antennas, davits, and other goodie-hangers. Our prior HF antenna location was about midway on the starboard stern side, attached by a standoff to the stern rail, 24" up. Tuner is very close to that location, being mounted on the side of the propane locker. As it turns out, my solar module frame is 1" tube and is in such a position as to allow the standoff from my whip to attach to it (much more support, of course, being 8' in the air, rather than 2' up, like the rail was before). However, to make that work, I'd have to put the whip centerline, or very close to it. The base would be right next to the chain plate for the aft stay. The way I've got the aft entry, now, it's not a problem to do that in convenience and use terms. The question is, given that it's marginally closer (don't know that I'd change the end of the cable) to the tuner, is that a good placement? There's no parallel metal anywhere nearby; the closest angle is the back stay, going forward, and, 4'+ away, the arch itself. Everything else is right angles. So, is that a good location for the whip? I'd expected to have to put it very close to the side of the boat in order to keep it out of the way. That would put it close to the arch itself, and some impediment to access to some stuff on that side. Thanks for any informed opinions :{)) L8R Skip, refitting as fast as I can in the last two days aboard before surgery Morgan 461 #2 SV Flying Pig http://tinyurl.com/384p2 - The vessel as Tehamana, as we bought her "Twenty years from now you will be more disappointed by the things you didn't do than by the ones you did do. So throw off the bowlines. Sail away from the safe harbor. Catch the trade winds in your sails. Explore. Dream. Discover." - Mark Twain |
Our main VHF antenna is up at the top of the mast. I think it came
that way. We have a flagpole on the stern, which was a cell phone antenna. The SSB antenna is the backstay and the tuner is down on the deck on the stern. "Skip Gundlach" wrote: Howdy, y'all As most of you know, we're in an extensive refit. We're finally to the point of addressing the stern of the boat. Most of you know we added an arch, on which we've put wind and solar, satellite, gps and vhf antennas, davits, and other goodie-hangers. Our prior HF antenna location was about midway on the starboard stern side, attached by a standoff to the stern rail, 24" up. Tuner is very close to that location, being mounted on the side of the propane locker. As it turns out, my solar module frame is 1" tube and is in such a position as to allow the standoff from my whip to attach to it (much more support, of course, being 8' in the air, rather than 2' up, like the rail was before). However, to make that work, I'd have to put the whip centerline, or very close to it. The base would be right next to the chain plate for the aft stay. The way I've got the aft entry, now, it's not a problem to do that in convenience and use terms. The question is, given that it's marginally closer (don't know that I'd change the end of the cable) to the tuner, is that a good placement? There's no parallel metal anywhere nearby; the closest angle is the back stay, going forward, and, 4'+ away, the arch itself. Everything else is right angles. So, is that a good location for the whip? I'd expected to have to put it very close to the side of the boat in order to keep it out of the way. That would put it close to the arch itself, and some impediment to access to some stuff on that side. Thanks for any informed opinions :{)) L8R Skip, refitting as fast as I can in the last two days aboard before surgery Morgan 461 #2 SV Flying Pig http://tinyurl.com/384p2 - The vessel as Tehamana, as we bought her "Twenty years from now you will be more disappointed by the things you didn't do than by the ones you did do. So throw off the bowlines. Sail away from the safe harbor. Catch the trade winds in your sails. Explore. Dream. Discover." - Mark Twain grandma Rosalie |
In article . com,
"Skip Gundlach" wrote: Howdy, y'all As most of you know, we're in an extensive refit. We're finally to the point of addressing the stern of the boat. Most of you know we added an arch, on which we've put wind and solar, satellite, gps and vhf antennas, davits, and other goodie-hangers. Our prior HF antenna location was about midway on the starboard stern side, attached by a standoff to the stern rail, 24" up. Tuner is very close to that location, being mounted on the side of the propane locker. As it turns out, my solar module frame is 1" tube and is in such a position as to allow the standoff from my whip to attach to it (much more support, of course, being 8' in the air, rather than 2' up, like the rail was before). However, to make that work, I'd have to put the whip centerline, or very close to it. The base would be right next to the chain plate for the aft stay. The way I've got the aft entry, now, it's not a problem to do that in convenience and use terms. The question is, given that it's marginally closer (don't know that I'd change the end of the cable) to the tuner, is that a good placement? There's no parallel metal anywhere nearby; the closest angle is the back stay, going forward, and, 4'+ away, the arch itself. Everything else is right angles. So, is that a good location for the whip? I'd expected to have to put it very close to the side of the boat in order to keep it out of the way. That would put it close to the arch itself, and some impediment to access to some stuff on that side. Thanks for any informed opinions :{)) L8R MF/HF Marine Antennas should ALWAYS follow these simple rules: 1. Higher and longer, is better than lower and shorter..... 2. Autotuners were invented to replace Compitant Radiomen, with just about every other "Dufus" who thought that he could install a SSB Radio by himself, and save himself a pile of money..... 3. It doesn't matter how big, or long, your antenna is, if your RF Ground is totally crap........ 4. If you have a GOOD RF Ground, a "Wet Noodle" will radiate better then the "Best Antenna" with a poor RF Ground..... 5. It is the RF Ground, Sonny, the RF Ground...... Me one who has been around the docks for 40 years...... |
Me wrote: In article . com, "Skip Gundlach" wrote: Howdy, y'all As most of you know, we're in an extensive refit. We're finally to the point of addressing the stern of the boat. Most of you know we added an arch, on which we've put wind and solar, satellite, gps and vhf antennas, davits, and other goodie-hangers. Our prior HF antenna location was about midway on the starboard stern side, attached by a standoff to the stern rail, 24" up. Tuner is very close to that location, being mounted on the side of the propane locker. As it turns out, my solar module frame is 1" tube and is in such a position as to allow the standoff from my whip to attach to it (much more support, of course, being 8' in the air, rather than 2' up, like the rail was before). However, to make that work, I'd have to put the whip centerline, or very close to it. The base would be right next to the chain plate for the aft stay. The way I've got the aft entry, now, it's not a problem to do that in convenience and use terms. The question is, given that it's marginally closer (don't know that I'd change the end of the cable) to the tuner, is that a good placement? There's no parallel metal anywhere nearby; the closest angle is the back stay, going forward, and, 4'+ away, the arch itself. Everything else is right angles. So, is that a good location for the whip? I'd expected to have to put it very close to the side of the boat in order to keep it out of the way. That would put it close to the arch itself, and some impediment to access to some stuff on that side. Thanks for any informed opinions :{)) L8R MF/HF Marine Antennas should ALWAYS follow these simple rules: 1. Higher and longer, is better than lower and shorter..... 2. Autotuners were invented to replace Compitant Radiomen, with just about every other "Dufus" who thought that he could install a SSB Radio by himself, and save himself a pile of money..... 3. It doesn't matter how big, or long, your antenna is, if your RF Ground is totally crap........ 4. If you have a GOOD RF Ground, a "Wet Noodle" will radiate better then the "Best Antenna" with a poor RF Ground..... 5. It is the RF Ground, Sonny, the RF Ground...... Me one who has been around the docks for 40 years...... |
Boy, I hate web-based posting. I replied to this and the computer ate
it, of course, not saving it anywhere for me... Thanks for the info on non-HF, whip, antenna placement. Our nav VHF is mast top, our cockpit VHF is on the arch, as are other antennas. We'll not be using the backstay for an antenna for a variety of reasons, time, money and security in the potential dismasting among them. As further background, we have full rails, with the gates combined electrically with brass straps belowdecks, attached to the arch, the pushpit and pulpit. We have about 110 lineal feet of 1" SS tube rail, unless you count the inner rails, plus the arch. In addition we have the standard 4" copper strapping leading to a sintered bronze Guest plane below the boat, and also connected to a 3x5' plate under the workbench top. I think we have a reasonably good ground. We have an SGC-230 tuner within 2 cable-feet of the whip antenna. We expect to have an Icom I-802 HF for our HF comms. Given that we'll have a whip antenna, is centerline, next to the chain plate, as good as, better or worse than, or something else when compared to near the side as it was before (but before the arch was installed)? Thanks. L8R Skip, refitting as fast as I can Morgan 461 #2 SV Flying Pig http://tinyurl.com/384p2 - The vessel as Tehamana, as we bought her "Twenty years from now you will be more disappointed by the things you didn't do than by the ones you did do. So throw off the bowlines. Sail away from the safe harbor. Catch the trade winds in your sails. Explore. Dream. Discover." - Mark Twain |
http://www.budgetmarine.com/pdf/108.pdf
Ground system. -- The Road Warrior Hobbit no -- it's NOT ok to contact this account with services or other commercial interests "Skip Gundlach" wrote in message ups.com... | Howdy, y'all | | As most of you know, we're in an extensive refit. We're finally to the | point of addressing the stern of the boat. Most of you know we added | an arch, on which we've put wind and solar, satellite, gps and vhf | antennas, davits, and other goodie-hangers. | | Our prior HF antenna location was about midway on the starboard stern | side, attached by a standoff to the stern rail, 24" up. Tuner is very | close to that location, being mounted on the side of the propane | locker. | | As it turns out, my solar module frame is 1" tube and is in such a | position as to allow the standoff from my whip to attach to it (much | more support, of course, being 8' in the air, rather than 2' up, like | the rail was before). However, to make that work, I'd have to put the | whip centerline, or very close to it. The base would be right next to | the chain plate for the aft stay. | | The way I've got the aft entry, now, it's not a problem to do that in | convenience and use terms. The question is, given that it's marginally | closer (don't know that I'd change the end of the cable) to the tuner, | is that a good placement? There's no parallel metal anywhere nearby; | the closest angle is the back stay, going forward, and, 4'+ away, the | arch itself. Everything else is right angles. | | So, is that a good location for the whip? I'd expected to have to put | it very close to the side of the boat in order to keep it out of the | way. That would put it close to the arch itself, and some impediment | to access to some stuff on that side. | | Thanks for any informed opinions :{)) | | L8R | | Skip, refitting as fast as I can in the last two days aboard before | surgery | | Morgan 461 #2 | SV Flying Pig | http://tinyurl.com/384p2 - The vessel as Tehamana, as we bought her | | "Twenty years from now you will be more disappointed by the things you | didn't do than by the ones you did do. So throw off the bowlines. Sail | away from the safe harbor. Catch the trade winds in your sails. | Explore. | Dream. Discover." - Mark Twain | |
Skip
Don't forget, the Icom 802 is also going to want an additional receive-only antenna for the DSC receiver portion. It can't use the main antenna for this. I'm using the backstay for the 802 HF transceiver, a 23 foot whip on one side for the DSC receiver in the 802, and another 23 foot whip on the other side for a separate HF receiver (R-71A). The antenna for the R-71 is also shared with the stereo radio. The backstay uses an AT-140 tuner. The receive-only antennas don't have tuners. Rusty "Skip Gundlach" wrote in message oups.com... Boy, I hate web-based posting. I replied to this and the computer ate it, of course, not saving it anywhere for me... Thanks for the info on non-HF, whip, antenna placement. Our nav VHF is mast top, our cockpit VHF is on the arch, as are other antennas. We'll not be using the backstay for an antenna for a variety of reasons, time, money and security in the potential dismasting among them. As further background, we have full rails, with the gates combined electrically with brass straps belowdecks, attached to the arch, the pushpit and pulpit. We have about 110 lineal feet of 1" SS tube rail, unless you count the inner rails, plus the arch. In addition we have the standard 4" copper strapping leading to a sintered bronze Guest plane below the boat, and also connected to a 3x5' plate under the workbench top. I think we have a reasonably good ground. We have an SGC-230 tuner within 2 cable-feet of the whip antenna. We expect to have an Icom I-802 HF for our HF comms. Given that we'll have a whip antenna, is centerline, next to the chain plate, as good as, better or worse than, or something else when compared to near the side as it was before (but before the arch was installed)? Thanks. L8R Skip, refitting as fast as I can Morgan 461 #2 SV Flying Pig http://tinyurl.com/384p2 - The vessel as Tehamana, as we bought her "Twenty years from now you will be more disappointed by the things you didn't do than by the ones you did do. So throw off the bowlines. Sail away from the safe harbor. Catch the trade winds in your sails. Explore. Dream. Discover." - Mark Twain |
Near the centerline should be fine.
The Icom-802 is a great rig, especially so if you also have a ham license. I highly recommend getting a SCS Pactor III to go with it, along with free Airmail software for your laptop. The combination is very versatile and powerful: Email via HF radio, Navtex, weather fax, position reporting via APRS, etc. Talk to Gary at Dockside Radio if you need help setting it up or getting licensed: http://www.docksideradio.com/ |
Skip Gundlach wrote:
Thanks for any informed opinions :{)) Forget dubbing with an xmting whip, insulate the backstay & light it up for HF. The insulators aren't cheap but neither is another whip-related evolution. The stay is also a good compromise btw vert & horiz polarization. A good tuner'll tune a backstay dowm to 1.5 swr or better almost anywhere you'll operate. Be smart about the feedline (get help if U need it). Elegance, performance, less junk sticking off the boat, tuner below where it belongs. |
In article .com,
"Skip Gundlach" wrote: As further background, we have full rails, with the gates combined electrically with brass straps belowdecks, attached to the arch, the pushpit and pulpit. We have about 110 lineal feet of 1" SS tube rail, unless you count the inner rails, plus the arch. In addition we have the standard 4" copper strapping leading to a sintered bronze Guest plane below the boat, and also connected to a 3x5' plate under the workbench top. I think we have a reasonably good ground. You will never know if you have a "reasonably good ground", unless you get yourself an Impedance Bridge, and check it at the frequencies that you commonly work. Anything that is more than 12" away from the water, isn't going to add "diddley-squat" toward building a Low Impedance Wideband RF Ground System, and anyone who tells you otherwise, is just as uneducated about MF/HF Marine Radio Antenna Systems, as you seem to be. I have seen all kinds of Systems that looked very impresive, untill they were evaluated with real insurmentation. 400 Sq Ft of Copper Screen in the Cabin Overhead was proffered, as a really good RF Ground, by a well known Boat Builder, 20 years ago. It didn't work any better than having nothing at all, when tested, in a real radio enviorment. If you got a Plastic Hull, you are NEVER going to get a Real RF Ground, UNLESS the hull builder was smart, (they never are) and put 200+ Sq Ft of screen under the gellcoat down by the keel. Cellulose hulls are just as bad, and harder to retrofit that Plastic ones. Like I said in my first reply, Autotuners were invented to allow any "Dufus" to think he install an MF/HF Marine Radio System, and save himself all that money he would have paid a Compitant Radioman. SGC Autotuners are some of the worst of the lot, even if they did steal the design from the real inventers. SGC couldn't even copy the design correctly, and "Old PeeAir" couldn't design his way out of a "Wet Paper Bag". Me |
On Sat, 30 Jul 2005 06:12:33 GMT, Me wrote:
In article .com, "Skip Gundlach" wrote: As further background, we have full rails, with the gates combined electrically with brass straps belowdecks, attached to the arch, the pushpit and pulpit. We have about 110 lineal feet of 1" SS tube rail, unless you count the inner rails, plus the arch. In addition we have the standard 4" copper strapping leading to a sintered bronze Guest plane below the boat, and also connected to a 3x5' plate under the workbench top. I think we have a reasonably good ground. You will never know if you have a "reasonably good ground", unless you get yourself an Impedance Bridge, and check it at the frequencies that you commonly work. Anything that is more than 12" away from the water, isn't going to add "diddley-squat" toward building a Low Impedance Wideband RF Ground System, /// If you got a Plastic Hull, you are NEVER going to get a Real RF Ground, UNLESS the hull builder was smart, (they never are) and put 200+ Sq Ft of screen under the gellcoat down by the keel. Cellulose hulls are just as bad, and harder to retrofit that Plastic ones. Like I said in my first reply, Autotuners were invented to allow any "Dufus" to think he install an MF/HF Marine Radio System, and save himself all that money he would have paid a Compitant Radioman.//// Me Hehe...why don't you tell us what you really think, anonymous poster? If it takes 200 sq ft of screen under a gel coat to make a good RF ground, then folks who attempt a similar feat through a hull would need about 0.5 / 0.05 X 200 sq ft of material - That's 2000 sq ft of foil or metal mesh (for a 0.05 in gelcoat, and a mere 0.5 inch hull thickness) Now THAT would be quite a trick - a square about 100 ft by 20 ft. Better not tell the folks who use an antenna coupled through a 1/4 inch glass shield - a coupler that can measure 1.5 inch square. Admitted, this is often for FM radio (say 90 MHz) as opposed to 3MHz (?) on hf. Using these numbers for comparison, 90/3 X 0.5/0.25 X 2.25 sq in = 135 sq in of ground plane, hmmmm that's 1 sq foot in round numbers. Now that *does* look small to me. I expect the truth lies somewhere between 1 sq ft and 2000 sq ft. through a half inch hull section. There! How mealy-mouthed is THAT! :-) OK, let's get serious: how about locating a bronze through hull, and connecting a copper foil externally in contact with it. a square foot THERE, connected internally with good Litz wire might make a serviceable ground.... Brian Whatcott Altus p.s A hint for you: talking about "compitant radiomen" makes prospective customers nervous! :-) |
You seem to be saying that sea water (which is one heck of a lot less
conductive than copper -- I mean orders of magnitude less conductive) is the only rf ground (return path) that works? And that the only way to utilize it in a fiberglass hull is with capacitive coupling? By your reasoning, radio communication from a vessel on fresh water is impossible. Or, as a corollary, radio communication from a vessel on the hard is impossible because the capacitive coupling to the sea is over a distance of more than 12". And of course, for VHF, we all use the equivalent of copper that is not capacitively coupled to the sea. Since we all agree that VHF works fine that way, can you tell us at what frequency the laws of electromagnetic radiation "jump ship" and no longer work the same as at VHF? A reference would be most welcome. The issue here is radiation, not propagation. Can you also provide a reference to the published and repeatable testing of 400 sq. ft. of copper ground that didn't work any better than having nothing at all? Radio stations thousands of miles from the sea would be amazed to learn that their ground systems don't work better than nothing at all. Why not post your theory on rec.radio.amateur.antenna and see how it is received there? Suggest you reef those sails a tad, Me. Chuck Me wrote: In article .com, "Skip Gundlach" wrote: As further background, we have full rails, with the gates combined electrically with brass straps belowdecks, attached to the arch, the pushpit and pulpit. We have about 110 lineal feet of 1" SS tube rail, unless you count the inner rails, plus the arch. In addition we have the standard 4" copper strapping leading to a sintered bronze Guest plane below the boat, and also connected to a 3x5' plate under the workbench top. I think we have a reasonably good ground. You will never know if you have a "reasonably good ground", unless you get yourself an Impedance Bridge, and check it at the frequencies that you commonly work. Anything that is more than 12" away from the water, isn't going to add "diddley-squat" toward building a Low Impedance Wideband RF Ground System, and anyone who tells you otherwise, is just as uneducated about MF/HF Marine Radio Antenna Systems, as you seem to be. I have seen all kinds of Systems that looked very impresive, untill they were evaluated with real insurmentation. 400 Sq Ft of Copper Screen in the Cabin Overhead was proffered, as a really good RF Ground, by a well known Boat Builder, 20 years ago. It didn't work any better than having nothing at all, when tested, in a real radio enviorment. If you got a Plastic Hull, you are NEVER going to get a Real RF Ground, UNLESS the hull builder was smart, (they never are) and put 200+ Sq Ft of screen under the gellcoat down by the keel. Cellulose hulls are just as bad, and harder to retrofit that Plastic ones. Like I said in my first reply, Autotuners were invented to allow any "Dufus" to think he install an MF/HF Marine Radio System, and save himself all that money he would have paid a Compitant Radioman. SGC Autotuners are some of the worst of the lot, even if they did steal the design from the real inventers. SGC couldn't even copy the design correctly, and "Old PeeAir" couldn't design his way out of a "Wet Paper Bag". Me |
When you talk of capacitive coupling, frequency does matter. (Xc =
1/[2*pi*F]) There's two orders of magnitude difference between HF at 1.8 MHz and VHF at 180 MHz. "Ground" is one of those elusive concepts that get more magic/conundrum (aka BS) than it deserves. A full dipole needs no ground. The whip or backstay needs a ground plane so that its "virtual image" creates a full dipole. Note that aircraft use HF communications with a half dipole antenna (trailing wire) with no ground plane. Of course they do have an excellent antenna height. (Don't hold the end in your fingers to test on the surface. When your boss hits the transmit key, it hurts, for weeks.) Antennae are magic. Roger http://home.earthlink.net/~derbyrm "chuck" wrote in message ink.net... You seem to be saying that sea water (which is one heck of a lot less conductive than copper -- I mean orders of magnitude less conductive) is the only rf ground (return path) that works? And that the only way to utilize it in a fiberglass hull is with capacitive coupling? By your reasoning, radio communication from a vessel on fresh water is impossible. Or, as a corollary, radio communication from a vessel on the hard is impossible because the capacitive coupling to the sea is over a distance of more than 12". And of course, for VHF, we all use the equivalent of copper that is not capacitively coupled to the sea. Since we all agree that VHF works fine that way, can you tell us at what frequency the laws of electromagnetic radiation "jump ship" and no longer work the same as at VHF? A reference would be most welcome. The issue here is radiation, not propagation. Can you also provide a reference to the published and repeatable testing of 400 sq. ft. of copper ground that didn't work any better than having nothing at all? Radio stations thousands of miles from the sea would be amazed to learn that their ground systems don't work better than nothing at all. Why not post your theory on rec.radio.amateur.antenna and see how it is received there? Suggest you reef those sails a tad, Me. Chuck Me wrote: In article .com, "Skip Gundlach" wrote: As further background, we have full rails, with the gates combined electrically with brass straps belowdecks, attached to the arch, the pushpit and pulpit. We have about 110 lineal feet of 1" SS tube rail, unless you count the inner rails, plus the arch. In addition we have the standard 4" copper strapping leading to a sintered bronze Guest plane below the boat, and also connected to a 3x5' plate under the workbench top. I think we have a reasonably good ground. You will never know if you have a "reasonably good ground", unless you get yourself an Impedance Bridge, and check it at the frequencies that you commonly work. Anything that is more than 12" away from the water, isn't going to add "diddley-squat" toward building a Low Impedance Wideband RF Ground System, and anyone who tells you otherwise, is just as uneducated about MF/HF Marine Radio Antenna Systems, as you seem to be. I have seen all kinds of Systems that looked very impresive, untill they were evaluated with real insurmentation. 400 Sq Ft of Copper Screen in the Cabin Overhead was proffered, as a really good RF Ground, by a well known Boat Builder, 20 years ago. It didn't work any better than having nothing at all, when tested, in a real radio enviorment. If you got a Plastic Hull, you are NEVER going to get a Real RF Ground, UNLESS the hull builder was smart, (they never are) and put 200+ Sq Ft of screen under the gellcoat down by the keel. Cellulose hulls are just as bad, and harder to retrofit that Plastic ones. Like I said in my first reply, Autotuners were invented to allow any "Dufus" to think he install an MF/HF Marine Radio System, and save himself all that money he would have paid a Compitant Radioman. SGC Autotuners are some of the worst of the lot, even if they did steal the design from the real inventers. SGC couldn't even copy the design correctly, and "Old PeeAir" couldn't design his way out of a "Wet Paper Bag". Me |
Thanks for the comments, Roger.
While I don't disagree with what you have said (except that antennae are magic), I don't believe your remarks touch on the issues here. The typical marine VHF antenna, for example, does not depend upon the sea for its operation. I am assuming the usual quarter-wave ground plane vertical atop a mast. Do you believe that because VHF and HF antennas involve different frequencies the underlying theory is different? A backstay "vertical" antenna may or may not depend on the sea for its return path. Me has unequivocally asserted that it always depends on the sea and thus its ground must be coupled to the sea. (400 sq. ft. of copper is no better than nothing at all he believes) Conventional antenna theory suggests that a backstay "vertical" could utilize a counterpoise or other return path element (like a horizontal dipole half) with no coupling to the sea at all. Me asserts this is false: that such an antenna will not work. I attempted to present a "reductio ad absurdum" argument showing that if Me is correct, his reasoning leads to absurd results. There is no magic here. I would be pleased to reconsider any of my comments if they appear incorrect or incomprehensible. Regards, Chuck Roger Derby wrote: When you talk of capacitive coupling, frequency does matter. (Xc = 1/[2*pi*F]) There's two orders of magnitude difference between HF at 1.8 MHz and VHF at 180 MHz. "Ground" is one of those elusive concepts that get more magic/conundrum (aka BS) than it deserves. A full dipole needs no ground. The whip or backstay needs a ground plane so that its "virtual image" creates a full dipole. Note that aircraft use HF communications with a half dipole antenna (trailing wire) with no ground plane. Of course they do have an excellent antenna height. (Don't hold the end in your fingers to test on the surface. When your boss hits the transmit key, it hurts, for weeks.) Antennae are magic. Roger http://home.earthlink.net/~derbyrm "chuck" wrote in message ink.net... You seem to be saying that sea water (which is one heck of a lot less conductive than copper -- I mean orders of magnitude less conductive) is the only rf ground (return path) that works? And that the only way to utilize it in a fiberglass hull is with capacitive coupling? By your reasoning, radio communication from a vessel on fresh water is impossible. Or, as a corollary, radio communication from a vessel on the hard is impossible because the capacitive coupling to the sea is over a distance of more than 12". And of course, for VHF, we all use the equivalent of copper that is not capacitively coupled to the sea. Since we all agree that VHF works fine that way, can you tell us at what frequency the laws of electromagnetic radiation "jump ship" and no longer work the same as at VHF? A reference would be most welcome. The issue here is radiation, not propagation. Can you also provide a reference to the published and repeatable testing of 400 sq. ft. of copper ground that didn't work any better than having nothing at all? Radio stations thousands of miles from the sea would be amazed to learn that their ground systems don't work better than nothing at all. Why not post your theory on rec.radio.amateur.antenna and see how it is received there? Suggest you reef those sails a tad, Me. Chuck Me wrote: In article .com, "Skip Gundlach" wrote: As further background, we have full rails, with the gates combined electrically with brass straps belowdecks, attached to the arch, the pushpit and pulpit. We have about 110 lineal feet of 1" SS tube rail, unless you count the inner rails, plus the arch. In addition we have the standard 4" copper strapping leading to a sintered bronze Guest plane below the boat, and also connected to a 3x5' plate under the workbench top. I think we have a reasonably good ground. You will never know if you have a "reasonably good ground", unless you get yourself an Impedance Bridge, and check it at the frequencies that you commonly work. Anything that is more than 12" away from the water, isn't going to add "diddley-squat" toward building a Low Impedance Wideband RF Ground System, and anyone who tells you otherwise, is just as uneducated about MF/HF Marine Radio Antenna Systems, as you seem to be. I have seen all kinds of Systems that looked very impresive, untill they were evaluated with real insurmentation. 400 Sq Ft of Copper Screen in the Cabin Overhead was proffered, as a really good RF Ground, by a well known Boat Builder, 20 years ago. It didn't work any better than having nothing at all, when tested, in a real radio enviorment. If you got a Plastic Hull, you are NEVER going to get a Real RF Ground, UNLESS the hull builder was smart, (they never are) and put 200+ Sq Ft of screen under the gellcoat down by the keel. Cellulose hulls are just as bad, and harder to retrofit that Plastic ones. Like I said in my first reply, Autotuners were invented to allow any "Dufus" to think he install an MF/HF Marine Radio System, and save himself all that money he would have paid a Compitant Radioman. SGC Autotuners are some of the worst of the lot, even if they did steal the design from the real inventers. SGC couldn't even copy the design correctly, and "Old PeeAir" couldn't design his way out of a "Wet Paper Bag". Me |
We're not discussing theory, we're discussing implementation. VHF and HF
differ significantly due to near-field considerations as well as physical dimensions. A ground plane for a VHF whip can live on the same pole. For HF, you'd need a MUCH bigger boat. We once asked the lab to do a finite element analysis of a hunk of beef being roasted. They came back later and said there wasn't enough computer power in the world to do the job due to the various elements (water, fat, protein, bone, etc.) By the same token, HF antenna theory is useless because so many things on the boat are part of the "near field." One can't ignore the rails, the other antennae, the people walking about, ... In general, I tend to agree with "me" that the sea is the significant ground plane if we're talking small boats (under 65'). 400 sq.ft. of copper is 20' x 20' or so. The wavelength of 4215 KHz is 75 meters = 277 feet. The magic comes in when the various elements interact. Even small dish antenna are tested for their radiation pattern before going into production. Theory, hah. Google antenna "near field" and you'll get sufficient theory, mostly applicable to small dishes, to convince you. As Arthur C. Clarke stated, "Any technology which is sufficiently advanced (or complex) is indistinguishable from magic." Roger http://home.earthlink.net/~derbyrm "chuck" wrote in message ink.net... Thanks for the comments, Roger. While I don't disagree with what you have said (except that antennae are magic), I don't believe your remarks touch on the issues here. The typical marine VHF antenna, for example, does not depend upon the sea for its operation. I am assuming the usual quarter-wave ground plane vertical atop a mast. Do you believe that because VHF and HF antennas involve different frequencies the underlying theory is different? A backstay "vertical" antenna may or may not depend on the sea for its return path. Me has unequivocally asserted that it always depends on the sea and thus its ground must be coupled to the sea. (400 sq. ft. of copper is no better than nothing at all he believes) Conventional antenna theory suggests that a backstay "vertical" could utilize a counterpoise or other return path element (like a horizontal dipole half) with no coupling to the sea at all. Me asserts this is false: that such an antenna will not work. I attempted to present a "reductio ad absurdum" argument showing that if Me is correct, his reasoning leads to absurd results. There is no magic here. I would be pleased to reconsider any of my comments if they appear incorrect or incomprehensible. Regards, Chuck Roger Derby wrote: When you talk of capacitive coupling, frequency does matter. (Xc = 1/[2*pi*F]) There's two orders of magnitude difference between HF at 1.8 MHz and VHF at 180 MHz. "Ground" is one of those elusive concepts that get more magic/conundrum (aka BS) than it deserves. A full dipole needs no ground. The whip or backstay needs a ground plane so that its "virtual image" creates a full dipole. Note that aircraft use HF communications with a half dipole antenna (trailing wire) with no ground plane. Of course they do have an excellent antenna height. (Don't hold the end in your fingers to test on the surface. When your boss hits the transmit key, it hurts, for weeks.) Antennae are magic. Roger http://home.earthlink.net/~derbyrm "chuck" wrote in message ink.net... You seem to be saying that sea water (which is one heck of a lot less conductive than copper -- I mean orders of magnitude less conductive) is the only rf ground (return path) that works? And that the only way to utilize it in a fiberglass hull is with capacitive coupling? By your reasoning, radio communication from a vessel on fresh water is impossible. Or, as a corollary, radio communication from a vessel on the hard is impossible because the capacitive coupling to the sea is over a distance of more than 12". And of course, for VHF, we all use the equivalent of copper that is not capacitively coupled to the sea. Since we all agree that VHF works fine that way, can you tell us at what frequency the laws of electromagnetic radiation "jump ship" and no longer work the same as at VHF? A reference would be most welcome. The issue here is radiation, not propagation. Can you also provide a reference to the published and repeatable testing of 400 sq. ft. of copper ground that didn't work any better than having nothing at all? Radio stations thousands of miles from the sea would be amazed to learn that their ground systems don't work better than nothing at all. Why not post your theory on rec.radio.amateur.antenna and see how it is received there? Suggest you reef those sails a tad, Me. Chuck Me wrote: In article .com, "Skip Gundlach" wrote: As further background, we have full rails, with the gates combined electrically with brass straps belowdecks, attached to the arch, the pushpit and pulpit. We have about 110 lineal feet of 1" SS tube rail, unless you count the inner rails, plus the arch. In addition we have the standard 4" copper strapping leading to a sintered bronze Guest plane below the boat, and also connected to a 3x5' plate under the workbench top. I think we have a reasonably good ground. You will never know if you have a "reasonably good ground", unless you get yourself an Impedance Bridge, and check it at the frequencies that you commonly work. Anything that is more than 12" away from the water, isn't going to add "diddley-squat" toward building a Low Impedance Wideband RF Ground System, and anyone who tells you otherwise, is just as uneducated about MF/HF Marine Radio Antenna Systems, as you seem to be. I have seen all kinds of Systems that looked very impresive, untill they were evaluated with real insurmentation. 400 Sq Ft of Copper Screen in the Cabin Overhead was proffered, as a really good RF Ground, by a well known Boat Builder, 20 years ago. It didn't work any better than having nothing at all, when tested, in a real radio enviorment. If you got a Plastic Hull, you are NEVER going to get a Real RF Ground, UNLESS the hull builder was smart, (they never are) and put 200+ Sq Ft of screen under the gellcoat down by the keel. Cellulose hulls are just as bad, and harder to retrofit that Plastic ones. Like I said in my first reply, Autotuners were invented to allow any "Dufus" to think he install an MF/HF Marine Radio System, and save himself all that money he would have paid a Compitant Radioman. SGC Autotuners are some of the worst of the lot, even if they did steal the design from the real inventers. SGC couldn't even copy the design correctly, and "Old PeeAir" couldn't design his way out of a "Wet Paper Bag". Me |
In article ,
Me wrote: In article .com, "Skip Gundlach" wrote: As further background, we have full rails, with the gates combined electrically with brass straps belowdecks, attached to the arch, the pushpit and pulpit. We have about 110 lineal feet of 1" SS tube rail, unless you count the inner rails, plus the arch. In addition we have the standard 4" copper strapping leading to a sintered bronze Guest plane below the boat, and also connected to a 3x5' plate under the workbench top. I think we have a reasonably good ground. You will never know if you have a "reasonably good ground", unless you get yourself an Impedance Bridge, and check it at the frequencies that you commonly work. Anything that is more than 12" away from the water, isn't going to add "diddley-squat" toward building a Low Impedance Wideband RF Ground System, and anyone who tells you otherwise, is just as uneducated about MF/HF Marine Radio Antenna Systems, as you seem to be. I have seen all kinds of Systems that looked very impresive, untill they were evaluated with real insurmentation. 400 Sq Ft of Copper Screen in the Cabin Overhead was proffered, as a really good RF Ground, by a well known Boat Builder, 20 years ago. It didn't work any better than having nothing at all, when tested, in a real radio enviorment. If you got a Plastic Hull, you are NEVER going to get a Real RF Ground, UNLESS the hull builder was smart, (they never are) and put 200+ Sq Ft of screen under the gellcoat down by the keel. Cellulose hulls are just as bad, and harder to retrofit that Plastic ones. Like I said in my first reply, Autotuners were invented to allow any "Dufus" to think he install an MF/HF Marine Radio System, and save himself all that money he would have paid a Compitant Radioman. SGC Autotuners are some of the worst of the lot, even if they did steal the design from the real inventers. SGC couldn't even copy the design correctly, and "Old PeeAir" couldn't design his way out of a "Wet Paper Bag". Me Geeze Louise "Me" give the guy a break...... He was just asking for an opinion.... Bruce in alaska -- add a 2 before @ |
In article . net,
chuck wrote: You seem to be saying that sea water (which is one heck of a lot less conductive than copper -- I mean orders of magnitude less conductive) is the only rf ground (return path) that works? And that the only way to utilize it in a fiberglass hull is with capacitive coupling? By your reasoning, radio communication from a vessel on fresh water is impossible. Or, as a corollary, radio communication from a vessel on the hard is impossible because the capacitive coupling to the sea is over a distance of more than 12". And of course, for VHF, we all use the equivalent of copper that is not capacitively coupled to the sea. Since we all agree that VHF works fine that way, can you tell us at what frequency the laws of electromagnetic radiation "jump ship" and no longer work the same as at VHF? A reference would be most welcome. The issue here is radiation, not propagation. Can you also provide a reference to the published and repeatable testing of 400 sq. ft. of copper ground that didn't work any better than having nothing at all? Radio stations thousands of miles from the sea would be amazed to learn that their ground systems don't work better than nothing at all. Why not post your theory on rec.radio.amateur.antenna and see how it is received there? Suggest you reef those sails a tad, Me. Chuck Well Chuck, now lets look at the subject a bit and see what the difference is between your VHF analogy, and MF/HF RF Ground systems. Copper vs Salt Water at VHF....Very significant difference in conductivity, but it isn't conductivity that we are dealing with here. At VHF the RF Ground side of the antenna is built into the antenna, and needs only to be .5 meters long. (roughly) Also consider that at VHF an antenna can operate over a very wide Frequency Range (155-159 Mhz) without any major RF Impedance changes. This is NOT even close to being true at MF/HF frequencies. Move your frequency 250Khz and your antenna tuning is totally blown away. Copper vs Salt Water at MF/HF.....Still some what significant in Conductivity, but again it isn't conductivity we are talking about here. When was the last time you saw a vessel dragging around a pile of copper so as to have a Good RF Ground, no matter what the hull was made of? At MF/HF Frequencies the RF Impedance of the Ground System is inversely proportional to coupling to the Salt water, if the vessel is floating in salt water. Obviously you have never tried to communicate using MF/HF Frequencies in the Great Lakes and Mississippi River Systems from plastic or wood hulled vessels. It is a bitch to design an effective RF Ground for such vessels, as there just isn't enough there to make it work, like in Salt Water. Also remember that RF Impedance changes with Frequency, a rather LARGE point to be made here. When you are designing a Marine RF Ground system it must work across one order of magnitude, or better in frequency span, and provide a Low Impedance across the whole span. Now tell us all just how you accomplish this feat of magic with a plastic or cellulose hull sitting in fresh water? Like I said, it is a bitch, and a compromise in the best situation. The Laws of Physics aren't the problem here, it is how the Frequency interacts with RF Ground System Impedance that determine how well things work. No "Jump ship" about it, just simple Physics, that you apparently don't have a good handle on. Radio Stations in the MF Frequency Band use Marconi Antennas, and or Phased Marconi Antennas, for Transmission. These have EXTENSIVE RF Ground Systems, usually made up of over 100 1/4 Lambda length radial wires buried 6 to 10" below the surrounding surface, depending on the Radiation Pattern for that station, as designated on the License issued by the FCC. These RF Grounds are engineered specifically for the site and Radiation Pattern needed. The RF Ground for such Systems covers significantly more area than 400 Sq Ft, and the addition of such a small area of copper screen, wouldn't make any significant difference to the Impedance of such a Ground System. What few HF Stations that are licensed in the US these days, mostly use Balanced Antennas instead of Marconi Antennas and the RF Ground isn't nearly as significant to their operation as it is for Marconi Antennas. The significance is basically limited to Takeoff Angles. Also of some significance here is that all these situations are single frequency antennas, and therefor don't need to be Low Impedance across a wide spread of frequencies (an order of magnitude or better when opertional. If you had some practical knowledge in the field, you would notice that if a wet, swampy field, or marshland could be found, that is where these antenna systems are built. Can you guess why? Having been in both the Bradcast Engineering, and Marine Radio Engineering end of things for MANY years, and having designed, installed, and maintained both classes of systems, I have dealt with these issues for MANY years, and the moral to the story IS, " It's the RF Ground, Sonny, the RF Ground". Me been there, done that......for a long long time..... |
In article ,
"Roger Derby" wrote: When you talk of capacitive coupling, frequency does matter. (Xc = 1/[2*pi*F]) There's two orders of magnitude difference between HF at 1.8 MHz and VHF at 180 MHz. "Ground" is one of those elusive concepts that get more magic/conundrum (aka BS) than it deserves. A full dipole needs no ground. The whip or backstay needs a ground plane so that its "virtual image" creates a full dipole. Note that aircraft use HF communications with a half dipole antenna (trailing wire) with no ground plane. Of course they do have an excellent antenna height. (Don't hold the end in your fingers to test on the surface. When your boss hits the transmit key, it hurts, for weeks.) Antennae are magic. Roger Actually the aircraft skin, if metal, or in the case of doped fabric covered planes, the tube frames, becomes the RF Ground system for aircraft MF/HF antenna systems. Thats why you almost NEVER see an MF/HF Radio installed in a spruce framed, fabric coverd, aircraft, and if you did see one it wouldn't work very well. Me who actually has an Aircraft Endorsement on his First Graph ticket..... |
In article . net,
chuck wrote: Thanks for the comments, Roger. While I don't disagree with what you have said (except that antennae are magic), I don't believe your remarks touch on the issues here. The typical marine VHF antenna, for example, does not depend upon the sea for its operation. I am assuming the usual quarter-wave ground plane vertical atop a mast. Do you believe that because VHF and HF antennas involve different frequencies the underlying theory is different? A backstay "vertical" antenna may or may not depend on the sea for its return path. Me has unequivocally asserted that it always depends on the sea and thus its ground must be coupled to the sea. (400 sq. ft. of copper is no better than nothing at all he believes) Conventional antenna theory suggests that a backstay "vertical" could utilize a counterpoise or other return path element (like a horizontal dipole half) with no coupling to the sea at all. Me asserts this is false: that such an antenna will not work. I attempted to present a "reductio ad absurdum" argument showing that if Me is correct, his reasoning leads to absurd results. There is no magic here. I would be pleased to reconsider any of my comments if they appear incorrect or incomprehensible. Regards, Chuck Best you go back to Elementary Physics, and relearn everything that you didn't get the first time, on Antenna Theory, Frquency vs Impedance, and Smith Charting of basic Marconi Style Antennas..... Me |
In article et,
"Roger Derby" wrote: We're not discussing theory, we're discussing implementation. VHF and HF differ significantly due to near-field considerations as well as physical dimensions. A ground plane for a VHF whip can live on the same pole. For HF, you'd need a MUCH bigger boat. We once asked the lab to do a finite element analysis of a hunk of beef being roasted. They came back later and said there wasn't enough computer power in the world to do the job due to the various elements (water, fat, protein, bone, etc.) By the same token, HF antenna theory is useless because so many things on the boat are part of the "near field." One can't ignore the rails, the other antennae, the people walking about, ... In general, I tend to agree with "me" that the sea is the significant ground plane if we're talking small boats (under 65'). 400 sq.ft. of copper is 20' x 20' or so. The wavelength of 4215 KHz is 75 meters = 277 feet. The magic comes in when the various elements interact. Even small dish antenna are tested for their radiation pattern before going into production. Theory, hah. Google antenna "near field" and you'll get sufficient theory, mostly applicable to small dishes, to convince you. As Arthur C. Clarke stated, "Any technology which is sufficiently advanced (or complex) is indistinguishable from magic." Roger Roger has it right on....Not much Moss growing under hs feet..... Me |
In article ,
Brian Whatcott wrote: Hehe...why don't you tell us what you really think, anonymous poster? If it takes 200 sq ft of screen under a gel coat to make a good RF ground, then folks who attempt a similar feat through a hull would need about 0.5 / 0.05 X 200 sq ft of material - That's 2000 sq ft of foil or metal mesh (for a 0.05 in gelcoat, and a mere 0.5 inch hull thickness) Now THAT would be quite a trick - a square about 100 ft by 20 ft. Better not tell the folks who use an antenna coupled through a 1/4 inch glass shield - a coupler that can measure 1.5 inch square. Admitted, this is often for FM radio (say 90 MHz) as opposed to 3MHz (?) on hf. Using these numbers for comparison, 90/3 X 0.5/0.25 X 2.25 sq in = 135 sq in of ground plane, hmmmm that's 1 sq foot in round numbers. Now that *does* look small to me. I expect the truth lies somewhere between 1 sq ft and 2000 sq ft. through a half inch hull section. There! How mealy-mouthed is THAT! :-) OK, let's get serious: how about locating a bronze through hull, and connecting a copper foil externally in contact with it. a square foot THERE, connected internally with good Litz wire might make a serviceable ground.... Brian Whatcott Altus p.s A hint for you: talking about "compitant radiomen" makes prospective customers nervous! :-) Nice thought Brian.... However what you seem to forget is that capacative couping at RF Frequencies, is determined by three things. 1. Area of the Inside the hull Plate. 2. Distance between the two Plates. 3. Conductivity of the Sea Water Plate. A 200 Sq Ft Screen in the cabin overhead isn't near (Orders of magnitude) big enough to be an RF Ground for a MF/HF Antenna System, by itself. Being located far (relative in capactive terms) from the other plate (Salt water) means that the Rf couping into the water is what, can you guess? Less than a few Picofarads. Now calculate the RF Impedance for such a system at ANY, and All MF/HF Marine Frequencies that you like to use, if you have enough computer power in your supercomputer. then come back and explain it all to the rest of the world. Me |
Well Bruce/Me, I think you need to pull your two "selves" together!
Sifting through the humorous postings, I think your bottom line is that HF/MF vertical antennas will not work well (sometimes I think you mean will not work at all) unless they are (1) over sal****er with a return path capacitively coupled to the sea (at least for nonmetallic vessels); or (2) over land with 100 quarter-wave radials in marshland. You have labored to persuade us that less-than-perfect marine RF ground systems are certain to disappoint. It will surprise you, perhaps, to learn that there are many thousands of vertical HF and MF transmitting antennas in operation in the world today that satisfy none of those conditions, and yet enable effective communications activities. Some on land and some over water. These installations are supported by rigorous theory as well as by on-the-air performance data. If you would like to learn more about how this is being done, often with losses of only a few dB below ideal conditions, drop in at rec.amateur.radio.antenna and "read the mail." You'll find some bombastic assertions and opinions to be sure, but also many reasoned analyses and even quantitative experiments. Hope to see you there, Bruce. Regards, Chuck Bruce in Alaska wrote: In article , Me wrote: In article .com, "Skip Gundlach" wrote: As further background, we have full rails, with the gates combined electrically with brass straps belowdecks, attached to the arch, the pushpit and pulpit. We have about 110 lineal feet of 1" SS tube rail, unless you count the inner rails, plus the arch. In addition we have the standard 4" copper strapping leading to a sintered bronze Guest plane below the boat, and also connected to a 3x5' plate under the workbench top. I think we have a reasonably good ground. You will never know if you have a "reasonably good ground", unless you get yourself an Impedance Bridge, and check it at the frequencies that you commonly work. Anything that is more than 12" away from the water, isn't going to add "diddley-squat" toward building a Low Impedance Wideband RF Ground System, and anyone who tells you otherwise, is just as uneducated about MF/HF Marine Radio Antenna Systems, as you seem to be. I have seen all kinds of Systems that looked very impresive, untill they were evaluated with real insurmentation. 400 Sq Ft of Copper Screen in the Cabin Overhead was proffered, as a really good RF Ground, by a well known Boat Builder, 20 years ago. It didn't work any better than having nothing at all, when tested, in a real radio enviorment. If you got a Plastic Hull, you are NEVER going to get a Real RF Ground, UNLESS the hull builder was smart, (they never are) and put 200+ Sq Ft of screen under the gellcoat down by the keel. Cellulose hulls are just as bad, and harder to retrofit that Plastic ones. Like I said in my first reply, Autotuners were invented to allow any "Dufus" to think he install an MF/HF Marine Radio System, and save himself all that money he would have paid a Compitant Radioman. SGC Autotuners are some of the worst of the lot, even if they did steal the design from the real inventers. SGC couldn't even copy the design correctly, and "Old PeeAir" couldn't design his way out of a "Wet Paper Bag". Me Geeze Louise "Me" give the guy a break...... He was just asking for an opinion.... Bruce in alaska |
On Sat, 30 Jul 2005 19:58:46 GMT, Me wrote:
In article , Brian Whatcott wrote: Hehe...why don't you tell us what you really think, anonymous poster? If it takes 200 sq ft of screen under a gel coat to make a good RF ground, then folks who attempt a similar feat through a hull would need about 0.5 / 0.05 X 200 sq ft of material - That's 2000 sq ft of foil or metal mesh (for a 0.05 in gelcoat, and a mere 0.5 inch hull thickness) Now THAT would be quite a trick - a square about 100 ft by 20 ft. Better not tell the folks who use an antenna coupled through a 1/4 inch glass shield - a coupler that can measure 1.5 inch square. Admitted, this is often for FM radio (say 90 MHz) as opposed to 3MHz (?) on hf. Using these numbers for comparison, 90/3 X 0.5/0.25 X 2.25 sq in = 135 sq in of ground plane, hmmmm that's 1 sq foot in round numbers. Now that *does* look small to me. I expect the truth lies somewhere between 1 sq ft and 2000 sq ft. through a half inch hull section. There! How mealy-mouthed is THAT! :-) OK, let's get serious: how about locating a bronze through hull, and connecting a copper foil externally in contact with it. a square foot THERE, connected internally with good Litz wire might make a serviceable ground.... Brian Whatcott Altus p.s A hint for you: talking about "compitant radiomen" makes prospective customers nervous! :-) Nice thought Brian.... However what you seem to forget is that capacative couping at RF Frequencies, is determined by three things. 1. Area of the Inside the hull Plate. 2. Distance between the two Plates. 3. Conductivity of the Sea Water Plate. A 200 Sq Ft Screen in the cabin overhead isn't near (Orders of magnitude) big enough to be an RF Ground for a MF/HF Antenna System, by itself. Being located far (relative in capactive terms) from the other plate (Salt water) means that the Rf couping into the water is what, can you guess? Less than a few Picofarads. Now calculate the RF Impedance for such a system at ANY, and All MF/HF Marine Frequencies that you like to use, if you have enough computer power in your supercomputer. then come back and explain it all to the rest of the world. Me Oh my! Anonymous poster, it was *YOUR* suggestion that an RF ground of 200 sq ft of mesh under the (external hull) gel-coat was required for a satisfactory RF ground at HF. It was the original poster's suggestion of an elevated mesh that caught your interest, not mine. As you asked about antenna testing, I should mention that a supercomputer is not really necessary: there is a handy dandy gadget ( from MFJ ) which combines several RF test functions like antenna bridge, SW ratio etc. It ran about $200 as I recall. I satisfy myself with an LC meter these days - which gets one into the ball park at $100. eBay has an MFJ noise bridge at $25 currently. Hmmm...the capacitance to ground of a few objects in my vicinity runs about 45 pF per sq ft. Like me, standing on carpet It is just possible a hi level ground screen of 200 sq ft might get you 200 X 45 pF = 9000 pF At 3 MHz that would put the capacitive reactance at 1/2pi.f.C ohms = 6 ohms. Not that great. The actual value might well be quite a bit higher than that. But that's just me measuring with an instrument, rather than you guessing how poor it is. Another thing: the conductivity of sea water does not vary all that much - it doesn't have much impact on capacitance. But there I go again, actually measuring things! Regards Brian Whatcott p.s. I have an FCC GROL+rdr. You? |
On Sat, 30 Jul 2005 19:35:36 GMT, Me wrote:
In article , "Roger Derby" wrote: When you talk of capacitive coupling, frequency does matter. (Xc = 1/[2*pi*F]) There's two orders of magnitude difference between HF at 1.8 MHz and VHF at 180 MHz. "Ground" is one of those elusive concepts that get more magic/conundrum (aka BS) than it deserves. A full dipole needs no ground. The whip or backstay needs a ground plane so that its "virtual image" creates a full dipole. Note that aircraft use HF communications with a half dipole antenna (trailing wire) with no ground plane. Of course they do have an excellent antenna height. (Don't hold the end in your fingers to test on the surface. When your boss hits the transmit key, it hurts, for weeks.) Antennae are magic. Roger Actually the aircraft skin, if metal, or in the case of doped fabric covered planes, the tube frames, becomes the RF Ground system for aircraft MF/HF antenna systems. Thats why you almost NEVER see an MF/HF Radio installed in a spruce framed, fabric coverd, aircraft, and if you did see one it wouldn't work very well. Me who actually has an Aircraft Endorsement on his First Graph ticket..... Ah, well, well! How much area does a light aircraft tube fuselage etc., subtend? Could it possibly be, like, 200 sq feet? And it can transmit successfully, using this as its ground plane? You mean, like a 200 ft ground plane in a boats cabin overhead? :-) Brian Whatcott |
On Sat, 30 Jul 2005 19:29:39 GMT, Me wrote:
/// Also consider that at VHF an antenna can operate over a very wide Frequency Range (155-159 Mhz) without any major RF Impedance changes. This is NOT even close to being true at MF/HF frequencies. Move your frequency 250Khz and your antenna tuning is totally blown away. /// Me been there, done that......for a long long time..... Oh my, anonymous poster, you compare a frequency deviation of 2.5% at VHF (135 - 139 MHz) with a frequency change of 250kHz at (say) 3 MHz But wait: that's a 8.3% change. And you say that the effect of a 250kHz difference at HF is much greater than 4 MHz at VHF? Well yes! 8.3% is much greater than 2.5% How long did you say you have been doing this sort of thing? :-) Brian W |
On Sat, 30 Jul 2005 18:41:54 GMT, Bruce in Alaska
wrote: Geeze Louise "Me" give the guy a break...... He was just asking for an opinion.... Bruce in alaska Hehe...a blow hard is not silenced quite that easily! :-) Brian W |
On Sat, 30 Jul 2005 21:45:27 GMT, chuck wrote:
Well Bruce/Me, /// It will surprise you, perhaps, to learn that there are many thousands of vertical HF and MF transmitting antennas in operation in the world today that satisfy none of those conditions, and yet enable effective communications activities. Some on land and some over water. These installations are supported by rigorous theory as well as by on-the-air performance data. /// Regards, Chuck I see that he realises that airborne trailing wire antennas can work well at HF with just a tube fuselage as a ground reference - miles away from ground! This just about completely contradicts his earlier posts about ground references needing to be in close proximity to the ground for successful HF work, wouldn't you say? Brian Whatcott Altus OK |
|
On Sat, 30 Jul 2005 19:41:03 GMT, Me wrote:
In article et, "Roger Derby" wrote: In general, I tend to agree with "me" that the sea is the significant ground plane if we're talking small boats (under 65'). 400 sq.ft. of copper is 20' x 20' or so. The wavelength of 4215 KHz is 75 meters = 277 feet. Roger Roger has it right on....Not much Moss growing under hs feet..... Me I just had a wild guess that if Roger was agreeing with anonymous poster 'me' and me was agreeing with Roger, then Roger was probably going to be in error too. So I checked a Roger fact: that 4215 kHz = 75 meters = 277 ft. Here's the scoop: 4215 kHz = 71.2 meters = 233.5 ft Waddaya know! :-) Brian Whatcott |
Hey, like I said, magic. Bent coat hangers also radiate. The fuselage
gives you a place to terminate the coax shield. Wiggle the connection to the radiator until the SWR is not too bad. Also, don't get hung up on "conductivity." Both soil and water have less than silver, but the resistance is what counts and that's the product of conductivity and conductor cross-section. If it floats your boat, it has a LARGE cross-section. Roger (changing radar detector diodes in 1955) http://home.earthlink.net/~derbyrm "Brian Whatcott" wrote in message ... On Sat, 30 Jul 2005 19:35:36 GMT, Me wrote: In article , "Roger Derby" wrote: When you talk of capacitive coupling, frequency does matter. (Xc = 1/[2*pi*F]) There's two orders of magnitude difference between HF at 1.8 MHz and VHF at 180 MHz. "Ground" is one of those elusive concepts that get more magic/conundrum (aka BS) than it deserves. A full dipole needs no ground. The whip or backstay needs a ground plane so that its "virtual image" creates a full dipole. Note that aircraft use HF communications with a half dipole antenna (trailing wire) with no ground plane. Of course they do have an excellent antenna height. (Don't hold the end in your fingers to test on the surface. When your boss hits the transmit key, it hurts, for weeks.) Antennae are magic. Roger Actually the aircraft skin, if metal, or in the case of doped fabric covered planes, the tube frames, becomes the RF Ground system for aircraft MF/HF antenna systems. Thats why you almost NEVER see an MF/HF Radio installed in a spruce framed, fabric coverd, aircraft, and if you did see one it wouldn't work very well. Me who actually has an Aircraft Endorsement on his First Graph ticket..... Ah, well, well! How much area does a light aircraft tube fuselage etc., subtend? Could it possibly be, like, 200 sq feet? And it can transmit successfully, using this as its ground plane? You mean, like a 200 ft ground plane in a boats cabin overhead? :-) Brian Whatcott |
I have to agree with Chucky on at least one point. The postings on
this thread are humorous. That must be frustrating to those who were trying to follow the thread in the hope of learning something useful. The reason I find it amusing is that both the con's and the pro's are to some extent correct in their reasoning all be it flowed to one extent or the other. As I am really old and lacking any substantial short-term memory, re-reading the entire thread to glean every single nuance of every attempted point would be counter productive for me because I would probably forget what I was doing. I will however make some assumption, which I will hold true, for the sake of simplicity. By the way Chucky, the proper use of reduction to the absurd (reductio ad absurdum) logic would require a model theory which is based on the law of excluded middle (tertium non datur) which clearly does not apply to the statements put forth in this thread. My assumptions: 1. The usual antenna system used with marine transmitters in the intermediate and high-frequency bands is the Marconi or grounded radiator, in contrast to the Hertz or ungrounded radiator. Technically the most basic of antenna, it is an "isotropic radiator". This is a mythical antenna, which radiates in all directions, as does the light from a lamp bulb. This assumption is somewhat modified by assumption #5 below, as a reflection (pardon the witticism) of reality. 2. Depending upon height above ground, the influence of surrounding objects and other factors, our quarter wave antenna with a near perfect ground exhibits a nominal input impedance of around 36 ohms. 3. Ground losses affect the feed point impedance and antenna efficiency. When mounted on a real ground, the input impedance can range from 38 ohms for a well-designed antenna mounted over a specially prepared ground, to over 100 ohms for a Marconi mounted above poor, unprepared ground that has no radials. 4. Ground loss reduces the antenna's efficiency, because part of the power being delivered to the antenna is being dissipated in the ground rather than being radiated. The efficiency can be computed from the measured value of input resistance by using the formula; Efficiency equals 36 ohms divided by antenna impedance. 5. The radiation pattern of the Marconi antenna is a half doughnut. There is no radiation straight up in the direction of the antenna. The bulk of the radiation occurs at a low elevation angle, which is what is needed to launch a ground wave. 6. Finally yet importantly, the HF installation shall be used for communications in excess of line of sight. In other words, the skipper will want this installation to work when s/he needs to contact S&R in a life-threatening situation, not just to collect the email while birthed in some marina. Ok, now the model part. This antenna is to be mounted on a boat. So what is all this talk about ground? Well let us look at the Marconi antenna. It is actually one half of a dipole antenna. Trust me on this, explaining why and how will just get us lost, but the other half of the dipole is needed and is provided by the image produced by the previous mentioned ground. There is that word ground again. Now Chucky, this is were some clarification needs to be. This is not the same ground we all have come to know and love when we talk about the ground in an electrical system like your house (safety ground), your car (negative return) or your boat (negative return) this is terra firma, real honest to goodness dry land, and for the time being we will assume its perfect. Now, by perfect I don't mean a perfect conductor, hell it's not even close. By perfect I mean a low impedance to RF currents. The RF ground currents are greatest in the vicinity of the feed point at the base of the antenna up to a distance of ¼ wave length from the antenna. Now we all know there isn't any perfect ground left, the Aztecs used it all to build their pyramids. So how do we make it perfect or at least acceptable, we install a counterpoise, a conductor or system of conductors used as a substitute for perfect ground in an antenna system. That's were Me's radials or RF ground system comes in. It should have the effective radius equal to the height of the antenna (1/4 wave length). I say should have, but in reality the radials do not all have to be the same length and that losses may be decreased by adding extra radials near the feed point. These extra radials can be as short as 1/40 wave length and still be effective. Now, with this added counterpoise, you can pick up this antenna and move it anywhere and it will still function very well with a relatively high efficiency. That's a new term I snuck in without you seeing it coming. Cool! Remember assumption #2 above? I said a Marconi had an input impedance of 36 ohms on perfect ground, well it turns out that with a counterpoise it has an impedance of 38 ohms. Trust me, it does. Now let's drag this sucker over to the marina and hoist it onto that boat we have all been talking about. We get it mounted, counterpoise and all. Not easy considering its size. A Marconi antenna for a 2 MHz system is 117 feet high (234/2) with a 117 foot diameter counterpoise. Big sucker isn't it? How are we going to keep the counterpoise level? We're not. Let's let it droop, say 45 degrees. I'm cheating here. Some of you may know that a Marconi with a counterpoise set at a 45 degree down slop has an input impedance of 50 ohms and just by chance, that exactly matched the radio set's output impedance, thus maximum RF power transfer between antenna and radio. Cool! However, the antenna's efficiency has dropped to 72% and we have this honking big antenna messing up the aesthetics of our nice boat. Let's scale it down. Instead of 2 MHz, let's go to the other end of the band to 30 MHz. That makes the antenna 7.8 feet high with a 7.8 foot radius counterpoise. Before you start screaming about my math, I allow for a "velocity factor" of 5%. It's not as big, but it's still ugly and we need more power to be able to raise S&R when need them. For aesthetics, let's drop the counterpoise. What happens? The antenna's impedance goes up to about 100 ohms and its efficiency drops to 36%, but that isn't the worst of it, the power transfer is not maximum because the impedances are not matched. The reflective coefficient will be 0.3333 ((100-50)/50)/((100+50)/50), thus a VSWR of 2.0, a return loss of 9.5 dB which means the power actually reaching the antenna is 11.1% so now I can't reach S&R and I'm going down. Me is thinking "I'm vindicated" and Chucky is thinking "Another nut case". You're both right. So why does it work, simple. The antenna isn't a Marconi; it's an industry standard Marine HF band antenna, 2-30 MHz bandwidth, 10.8 MHz resonant frequency, 23 feet high and when connected to an HF radio set configured to its manufacturer's specifications it will perform admirably. If that were not the case, we would have had to have had at the very least five quarter wave Marconi antennae ranging in height from 7.8 to 117 feet and we don't. That Chucky is the proper use of reductio ad absurdum logic. "chuck" wrote in message ink.net... | Well Bruce/Me, I think you need to pull your two "selves" together! | | Sifting through the humorous postings, I think your bottom line is that | HF/MF vertical antennas will not work well (sometimes I think you mean | will not work at all) unless they are (1) over sal****er with a return | path capacitively coupled to the sea (at least for nonmetallic vessels); | or (2) over land with 100 quarter-wave radials in marshland. | | You have labored to persuade us that less-than-perfect marine RF ground | systems are certain to disappoint. | | It will surprise you, perhaps, to learn that there are many thousands of | vertical HF and MF transmitting antennas in operation in the world today | that satisfy none of those conditions, and yet enable effective | communications activities. Some on land and some over water. These | installations are supported by rigorous theory as well as by on-the-air | performance data. | | If you would like to learn more about how this is being done, often with | losses of only a few dB below ideal conditions, drop in at | rec.amateur.radio.antenna and "read the mail." You'll find some | bombastic assertions and opinions to be sure, but also many reasoned | analyses and even quantitative experiments. Hope to see you there, Bruce. | | Regards, | | Chuck | | | | | | | | Bruce in Alaska wrote: | In article , | Me wrote: | | | In article .com, | "Skip Gundlach" wrote: | | | As further background, we have full rails, with the gates combined | electrically with brass straps belowdecks, attached to the arch, the | pushpit and pulpit. We have about 110 lineal feet of 1" SS tube rail, | unless you count the inner rails, plus the arch. In addition we have | the standard 4" copper strapping leading to a sintered bronze Guest | plane below the boat, and also connected to a 3x5' plate under the | workbench top. I think we have a reasonably good ground. | | You will never know if you have a "reasonably good ground", unless | you get yourself an Impedance Bridge, and check it at the frequencies | that you commonly work. Anything that is more than 12" away from the | water, isn't going to add "diddley-squat" toward building a Low Impedance | Wideband RF Ground System, and anyone who tells you otherwise, is just as | uneducated about MF/HF Marine Radio Antenna Systems, as you seem to be. | I have seen all kinds of Systems that looked very impresive, untill they | were evaluated with real insurmentation. 400 Sq Ft of Copper Screen in | the Cabin Overhead was proffered, as a really good RF Ground, by a well | known Boat Builder, 20 years ago. It didn't work any better than | having nothing at all, when tested, in a real radio enviorment. If | you got a Plastic Hull, you are NEVER going to get a Real RF Ground, | UNLESS the hull builder was smart, (they never are) and put 200+ Sq | Ft of screen under the gellcoat down by the keel. Cellulose hulls | are just as bad, and harder to retrofit that Plastic ones. | Like I said in my first reply, Autotuners were invented to allow any | "Dufus" to think he install an MF/HF Marine Radio System, and save | himself all that money he would have paid a Compitant Radioman. | SGC Autotuners are some of the worst of the lot, even if they did steal | the design from the real inventers. SGC couldn't even copy the design | correctly, and "Old PeeAir" couldn't design his way out of a "Wet Paper | Bag". | | Me | | | Geeze Louise "Me" give the guy a break...... He was just asking for | an opinion.... | | | Bruce in alaska |
So you're saying that IF the resistance of the sea return path is less
than the resistance of copper, the sea will provide a better return path. Not much value there. IS the resistance of the sea return path greater than the resistance of copper? How do you know that? Because boats float? You should be aware that at HF, the skin depth is a mere fraction of a foot so the "conductor cross-section" is fairly small. You might want to consult an expert. The readers of this newsgroup are probably aware that, ceteris paribus, lower resistance grounds are better than higher resistance grounds. What is at issue here is whether 400 sq. ft. of copper is unusable (no better than nothing at all) as an RF ground on a vessel, and whether only a return patch capacitively-coupled to the sea can work. By the way, your formula for capacitive reactance is in error through the omission of a variable for Capacitance. Roger Derby wrote: Hey, like I said, magic. Bent coat hangers also radiate. The fuselage gives you a place to terminate the coax shield. Wiggle the connection to the radiator until the SWR is not too bad. Also, don't get hung up on "conductivity." Both soil and water have less than silver, but the resistance is what counts and that's the product of conductivity and conductor cross-section. If it floats your boat, it has a LARGE cross-section. Roger (changing radar detector diodes in 1955) http://home.earthlink.net/~derbyrm "Brian Whatcott" wrote in message ... On Sat, 30 Jul 2005 19:35:36 GMT, Me wrote: In article , "Roger Derby" wrote: When you talk of capacitive coupling, frequency does matter. (Xc = 1/[2*pi*F]) There's two orders of magnitude difference between HF at 1.8 MHz and VHF at 180 MHz. "Ground" is one of those elusive concepts that get more magic/conundrum (aka BS) than it deserves. A full dipole needs no ground. The whip or backstay needs a ground plane so that its "virtual image" creates a full dipole. Note that aircraft use HF communications with a half dipole antenna (trailing wire) with no ground plane. Of course they do have an excellent antenna height. (Don't hold the end in your fingers to test on the surface. When your boss hits the transmit key, it hurts, for weeks.) Antennae are magic. Roger Actually the aircraft skin, if metal, or in the case of doped fabric covered planes, the tube frames, becomes the RF Ground system for aircraft MF/HF antenna systems. Thats why you almost NEVER see an MF/HF Radio installed in a spruce framed, fabric coverd, aircraft, and if you did see one it wouldn't work very well. Me who actually has an Aircraft Endorsement on his First Graph ticket..... Ah, well, well! How much area does a light aircraft tube fuselage etc., subtend? Could it possibly be, like, 200 sq feet? And it can transmit successfully, using this as its ground plane? You mean, like a 200 ft ground plane in a boats cabin overhead? :-) Brian Whatcott |
Hello Mungo,
I'd say the first part rates a C-, mostly for credit in summarizing a great deal of material. But did you say that the input impedance of a quarter-wave vertical antenna at 30 MHz is 100 ohms without a counterpoise? NO ground return path at all? Sorry, but I think maybe you have fallen prey to Roger's "magic." Your understanding of what the 23' antenna is and how it works is sadly wanting. It also does not comport with the material in the earlier paragraphs. You have not even hinted at the sleight-of-hand introduction of an antenna tuner and why it is needed, and you have introduced an antenna without any mention of an RF return path! You are posting to a newsgroup where the readers are generally familiar with the 23' whip and the need for an antenna tuner and an RF return path. Do yourself a favor: find a book on antennas and read it. An elementary text is a good place to start. And in regard to the excluded middle, I think I understand now why you have not found it. Regards, Chuck Mungo Bulge wrote: I have to agree with Chucky on at least one point. The postings on this thread are humorous. That must be frustrating to those who were trying to follow the thread in the hope of learning something useful. The reason I find it amusing is that both the con's and the pro's are to some extent correct in their reasoning all be it flowed to one extent or the other. As I am really old and lacking any substantial short-term memory, re-reading the entire thread to glean every single nuance of every attempted point would be counter productive for me because I would probably forget what I was doing. I will however make some assumption, which I will hold true, for the sake of simplicity. By the way Chucky, the proper use of reduction to the absurd (reductio ad absurdum) logic would require a model theory which is based on the law of excluded middle (tertium non datur) which clearly does not apply to the statements put forth in this thread. My assumptions: 1. The usual antenna system used with marine transmitters in the intermediate and high-frequency bands is the Marconi or grounded radiator, in contrast to the Hertz or ungrounded radiator. Technically the most basic of antenna, it is an "isotropic radiator". This is a mythical antenna, which radiates in all directions, as does the light from a lamp bulb. This assumption is somewhat modified by assumption #5 below, as a reflection (pardon the witticism) of reality. 2. Depending upon height above ground, the influence of surrounding objects and other factors, our quarter wave antenna with a near perfect ground exhibits a nominal input impedance of around 36 ohms. 3. Ground losses affect the feed point impedance and antenna efficiency. When mounted on a real ground, the input impedance can range from 38 ohms for a well-designed antenna mounted over a specially prepared ground, to over 100 ohms for a Marconi mounted above poor, unprepared ground that has no radials. 4. Ground loss reduces the antenna's efficiency, because part of the power being delivered to the antenna is being dissipated in the ground rather than being radiated. The efficiency can be computed from the measured value of input resistance by using the formula; Efficiency equals 36 ohms divided by antenna impedance. 5. The radiation pattern of the Marconi antenna is a half doughnut. There is no radiation straight up in the direction of the antenna. The bulk of the radiation occurs at a low elevation angle, which is what is needed to launch a ground wave. 6. Finally yet importantly, the HF installation shall be used for communications in excess of line of sight. In other words, the skipper will want this installation to work when s/he needs to contact S&R in a life-threatening situation, not just to collect the email while birthed in some marina. Ok, now the model part. This antenna is to be mounted on a boat. So what is all this talk about ground? Well let us look at the Marconi antenna. It is actually one half of a dipole antenna. Trust me on this, explaining why and how will just get us lost, but the other half of the dipole is needed and is provided by the image produced by the previous mentioned ground. There is that word ground again. Now Chucky, this is were some clarification needs to be. This is not the same ground we all have come to know and love when we talk about the ground in an electrical system like your house (safety ground), your car (negative return) or your boat (negative return) this is terra firma, real honest to goodness dry land, and for the time being we will assume its perfect. Now, by perfect I don't mean a perfect conductor, hell it's not even close. By perfect I mean a low impedance to RF currents. The RF ground currents are greatest in the vicinity of the feed point at the base of the antenna up to a distance of ¼ wave length from the antenna. Now we all know there isn't any perfect ground left, the Aztecs used it all to build their pyramids. So how do we make it perfect or at least acceptable, we install a counterpoise, a conductor or system of conductors used as a substitute for perfect ground in an antenna system. That's were Me's radials or RF ground system comes in. It should have the effective radius equal to the height of the antenna (1/4 wave length). I say should have, but in reality the radials do not all have to be the same length and that losses may be decreased by adding extra radials near the feed point. These extra radials can be as short as 1/40 wave length and still be effective. Now, with this added counterpoise, you can pick up this antenna and move it anywhere and it will still function very well with a relatively high efficiency. That's a new term I snuck in without you seeing it coming. Cool! Remember assumption #2 above? I said a Marconi had an input impedance of 36 ohms on perfect ground, well it turns out that with a counterpoise it has an impedance of 38 ohms. Trust me, it does. Now let's drag this sucker over to the marina and hoist it onto that boat we have all been talking about. We get it mounted, counterpoise and all. Not easy considering its size. A Marconi antenna for a 2 MHz system is 117 feet high (234/2) with a 117 foot diameter counterpoise. Big sucker isn't it? How are we going to keep the counterpoise level? We're not. Let's let it droop, say 45 degrees. I'm cheating here. Some of you may know that a Marconi with a counterpoise set at a 45 degree down slop has an input impedance of 50 ohms and just by chance, that exactly matched the radio set's output impedance, thus maximum RF power transfer between antenna and radio. Cool! However, the antenna's efficiency has dropped to 72% and we have this honking big antenna messing up the aesthetics of our nice boat. Let's scale it down. Instead of 2 MHz, let's go to the other end of the band to 30 MHz. That makes the antenna 7.8 feet high with a 7.8 foot radius counterpoise. Before you start screaming about my math, I allow for a "velocity factor" of 5%. It's not as big, but it's still ugly and we need more power to be able to raise S&R when need them. For aesthetics, let's drop the counterpoise. What happens? The antenna's impedance goes up to about 100 ohms and its efficiency drops to 36%, but that isn't the worst of it, the power transfer is not maximum because the impedances are not matched. The reflective coefficient will be 0.3333 ((100-50)/50)/((100+50)/50), thus a VSWR of 2.0, a return loss of 9.5 dB which means the power actually reaching the antenna is 11.1% so now I can't reach S&R and I'm going down. Me is thinking "I'm vindicated" and Chucky is thinking "Another nut case". You're both right. So why does it work, simple. The antenna isn't a Marconi; it's an industry standard Marine HF band antenna, 2-30 MHz bandwidth, 10.8 MHz resonant frequency, 23 feet high and when connected to an HF radio set configured to its manufacturer's specifications it will perform admirably. If that were not the case, we would have had to have had at the very least five quarter wave Marconi antennae ranging in height from 7.8 to 117 feet and we don't. That Chucky is the proper use of reductio ad absurdum logic. "chuck" wrote in message ink.net... | Well Bruce/Me, I think you need to pull your two "selves" together! | | Sifting through the humorous postings, I think your bottom line is that | HF/MF vertical antennas will not work well (sometimes I think you mean | will not work at all) unless they are (1) over sal****er with a return | path capacitively coupled to the sea (at least for nonmetallic vessels); | or (2) over land with 100 quarter-wave radials in marshland. | | You have labored to persuade us that less-than-perfect marine RF ground | systems are certain to disappoint. | | It will surprise you, perhaps, to learn that there are many thousands of | vertical HF and MF transmitting antennas in operation in the world today | that satisfy none of those conditions, and yet enable effective | communications activities. Some on land and some over water. These | installations are supported by rigorous theory as well as by on-the-air | performance data. | | If you would like to learn more about how this is being done, often with | losses of only a few dB below ideal conditions, drop in at | rec.amateur.radio.antenna and "read the mail." You'll find some | bombastic assertions and opinions to be sure, but also many reasoned | analyses and even quantitative experiments. Hope to see you there, Bruce. | | Regards, | | Chuck | | | | | | | | Bruce in Alaska wrote: | In article , | Me wrote: | | | In article .com, | "Skip Gundlach" wrote: | | | As further background, we have full rails, with the gates combined | electrically with brass straps belowdecks, attached to the arch, the | pushpit and pulpit. We have about 110 lineal feet of 1" SS tube rail, | unless you count the inner rails, plus the arch. In addition we have | the standard 4" copper strapping leading to a sintered bronze Guest | plane below the boat, and also connected to a 3x5' plate under the | workbench top. I think we have a reasonably good ground. | | You will never know if you have a "reasonably good ground", unless | you get yourself an Impedance Bridge, and check it at the frequencies | that you commonly work. Anything that is more than 12" away from the | water, isn't going to add "diddley-squat" toward building a Low Impedance | Wideband RF Ground System, and anyone who tells you otherwise, is just as | uneducated about MF/HF Marine Radio Antenna Systems, as you seem to be. | I have seen all kinds of Systems that looked very impresive, untill they | were evaluated with real insurmentation. 400 Sq Ft of Copper Screen in | the Cabin Overhead was proffered, as a really good RF Ground, by a well | known Boat Builder, 20 years ago. It didn't work any better than | having nothing at all, when tested, in a real radio enviorment. If | you got a Plastic Hull, you are NEVER going to get a Real RF Ground, | UNLESS the hull builder was smart, (they never are) and put 200+ Sq | Ft of screen under the gellcoat down by the keel. Cellulose hulls | are just as bad, and harder to retrofit that Plastic ones. | Like I said in my first reply, Autotuners were invented to allow any | "Dufus" to think he install an MF/HF Marine Radio System, and save | himself all that money he would have paid a Compitant Radioman. | SGC Autotuners are some of the worst of the lot, even if they did steal | the design from the real inventers. SGC couldn't even copy the design | correctly, and "Old PeeAir" couldn't design his way out of a "Wet Paper | Bag". | | Me | | | Geeze Louise "Me" give the guy a break...... He was just asking for | an opinion.... | | | Bruce in alaska |
Hello Brian,
Yes I believe it does contradict Bruce's own position as he stated it. The relative performances of the various marine RF return path techniques is a fascinating, but complex, area in which virtually all the data are either theoretical or anecdotal. The theoretical materials don't focus much on RF grounds on small, non-metallic cruising boats. Imagine the problems one would encounter in order to objectively and quantitatively compare two competing RF ground systems. An RF impedance bridge could be used to measure the ground loss resistance of the competing systems, but the mesurement would have to be made first with one system on the boat, and then with the other system on the same boat. If the system being mesured involves copper mesh in the hull while the boat is built, that is obviously going to be difficult. The measurements would have to be made in the same location and with the same antenna and at the same frequencies. Then there is the problem of knowing what is being measured. For example, suppose the RF ground system is connected electrically to the prop, or the rudder post, or a metal keel. How much of the ground loss reduction is attributable to these items and how much to the RF ground system itself? It matters, because these are techniques being advocated. As you can see, it becomes very difficult to make generalizations (except this one, of course). But boaters want to know these things. They want to know what RF ground system performs best, and they want to know trade-offs. If A performs 5% better than B, how much more does it cost? Most non-ham boaters are not comfortable with the idea of a one decibel change in transmitted or received signal strength. It is difficult for them to translate radio performance into those terms so even if we know how much better A is than B, it is difficult to communicate that. Even more so when we recognize that there are several dimensions to performance: galvanic issues, lightning protection, noise, frequency, etc. Amazing as it seems, I am not aware of any serious, repeatable, quantitative comparison of ANY two RF ground systems for cruising boats! What has been published in boating/cruising magazines does not qualify. So boaters wind up instead subjected to a barrage of opinion of questionable quality. One constant emerges from all of this: many different approaches are in use and some measure of operational success seems to be obtained from most of them. My belief is that the better one understands the general idea of an RF return path (and this applies in the air, on land, and on the water) the more likely it is that a "better" (cheaper?) RF ground system can be developed. Unfortunately, this discussion has revealed that this is the one ingredient most conspicuously absent from the strong overstatements of opinion we are finding. Keep up the good work, Brian. Regards, Chuck Brian Whatcott wrote: On Sat, 30 Jul 2005 21:45:27 GMT, chuck wrote: Well Bruce/Me, /// It will surprise you, perhaps, to learn that there are many thousands of vertical HF and MF transmitting antennas in operation in the world today that satisfy none of those conditions, and yet enable effective communications activities. Some on land and some over water. These installations are supported by rigorous theory as well as by on-the-air performance data. /// Regards, Chuck I see that he realises that airborne trailing wire antennas can work well at HF with just a tube fuselage as a ground reference - miles away from ground! This just about completely contradicts his earlier posts about ground references needing to be in close proximity to the ground for successful HF work, wouldn't you say? Brian Whatcott Altus OK |
Sorry, Chuck. If you don't understand what I'm saying, then I've either
expressed myself badly or you haven't read it accurately. I do have both the academic and practical background and experience to express an opinion and despite your prejudice, it is not automatically wrong. Roger http://home.earthlink.net/~derbyrm "chuck" wrote in message link.net... So you're saying that IF the resistance of the sea return path is less than the resistance of copper, the sea will provide a better return path. Not much value there. IS the resistance of the sea return path greater than the resistance of copper? How do you know that? Because boats float? You should be aware that at HF, the skin depth is a mere fraction of a foot so the "conductor cross-section" is fairly small. You might want to consult an expert. The readers of this newsgroup are probably aware that, ceteris paribus, lower resistance grounds are better than higher resistance grounds. What is at issue here is whether 400 sq. ft. of copper is unusable (no better than nothing at all) as an RF ground on a vessel, and whether only a return patch capacitively-coupled to the sea can work. By the way, your formula for capacitive reactance is in error through the omission of a variable for Capacitance. Roger Derby wrote: Hey, like I said, magic. Bent coat hangers also radiate. The fuselage gives you a place to terminate the coax shield. Wiggle the connection to the radiator until the SWR is not too bad. Also, don't get hung up on "conductivity." Both soil and water have less than silver, but the resistance is what counts and that's the product of conductivity and conductor cross-section. If it floats your boat, it has a LARGE cross-section. Roger (changing radar detector diodes in 1955) http://home.earthlink.net/~derbyrm "Brian Whatcott" wrote in message ... On Sat, 30 Jul 2005 19:35:36 GMT, Me wrote: In article , "Roger Derby" wrote: When you talk of capacitive coupling, frequency does matter. (Xc = 1/[2*pi*F]) There's two orders of magnitude difference between HF at 1.8 MHz and VHF at 180 MHz. "Ground" is one of those elusive concepts that get more magic/conundrum (aka BS) than it deserves. A full dipole needs no ground. The whip or backstay needs a ground plane so that its "virtual image" creates a full dipole. Note that aircraft use HF communications with a half dipole antenna (trailing wire) with no ground plane. Of course they do have an excellent antenna height. (Don't hold the end in your fingers to test on the surface. When your boss hits the transmit key, it hurts, for weeks.) Antennae are magic. Roger Actually the aircraft skin, if metal, or in the case of doped fabric covered planes, the tube frames, becomes the RF Ground system for aircraft MF/HF antenna systems. Thats why you almost NEVER see an MF/HF Radio installed in a spruce framed, fabric coverd, aircraft, and if you did see one it wouldn't work very well. Me who actually has an Aircraft Endorsement on his First Graph ticket..... Ah, well, well! How much area does a light aircraft tube fuselage etc., subtend? Could it possibly be, like, 200 sq feet? And it can transmit successfully, using this as its ground plane? You mean, like a 200 ft ground plane in a boats cabin overhead? :-) Brian Whatcott |
"Rusty" wrote in message
... Skip Don't forget, the Icom 802 is also going to want an additional receive-only antenna for the DSC receiver portion. It can't use the main antenna for this. I'm using the backstay for the 802 HF transceiver, a 23 foot whip on one side for the DSC receiver in the 802, and another 23 foot whip on the other side for a separate HF receiver (R-71A). The antenna for the R-71 is also shared with the stereo radio. The backstay uses an AT-140 tuner. The receive-only antennas don't have tuners. Rusty Sorry, I missed this one on the boat, relying on the web-based info and the digest versions sent to me. I'm not familiar with the 802 at all, yet, so didn't know of the separate DSC antenna receive function. The inference is, then, that my similarly equipped VHFs will need some separate antenna? I'd thought they'd connect to a GPS... Thanks for any insights - and the renaming will try to recapture the thread to placement rather than esoteric discussions of antenna frequencies and unrelated theory. Not that I'm not enjoying them - but it's not what I asked, and I have very limited time to sort, witness missing this post. L8R Skip, cleaning up in prep for surgery and being off-line for several weeks -- Morgan 461 #2 SV Flying Pig http://tinyurl.com/384p2 - The vessel as Tehamana, as we bought her "Twenty years from now you will be more disappointed by the things you didn't do than by the ones you did do. So throw off the bowlines. Sail away from the safe harbor. Catch the trade winds in your sails. Explore. Dream. Discover." - Mark Twain |
As usual Chucky, you are defending your position by denial of fact.
Did I say the input impedance of a Marconi quarter wave was 100 ohms if it was without a counterpoise? Assumption #3, so I guess I did. Not just at 30 MHz, but at any single frequency for which it was designed to operate. And I wish you'd stop using that "ground return path" term you keep throwing around as if it was something you understood or with which you even had a passing acquaintance. A conductor placed above a ground plane forms an image in the ground plane such that the resulting pattern is a composite of the real antenna and the image antenna. This phenomenon remains in effect until you have spacing between the conductor and the ground plane approaching a quarter wavelength of the operating frequency. Above this, you get into a whole new field of wave propagation that is outside the scope of this discussion, unless Chucky wants to go there if he feels threatened. Actually Chucky, I'm not going to try to explain this to you, why don't you grab a copy of the US Army Field Antenna Handbook and read it. No disrespect intended towards the US Marine Corps, but whoever writes their manuals seems to address the lowest common denominator when deciding to what level of intelligence its audience is operating, therefore Chucky even you can understand the theory put forth in the handbook (it even has pictures). Again, I apologize to any US Marine current or past who may be reading this thread, you are in no way, at least in my mind, to be considered functioning at Chuckey's level. It's just that the DEPARTMENT OF THE NAVY opted not to exclude the likes of Chucky from its audience when publishing this handbook. As for the matching networks (antenna tuner) most practical antennae require some form of impedance matching between the transmission line and the radiating elements. The implementation of a matching network can take on many forms, depending on the operating frequency and output power. There was no sleight of hand Chucky, in this case, it has always been present in the equipment package of the boat in question, remember? This was a functioning system and the OP just wanted to relocate the antenna. I do apologise for not quoting my sources and resources, which was rude of me, but then Chucky never does so I thought it was ok. However, as you insist: ARRL Handbook, Chapter 17, sec "The Vertical Antenna"; W9UCW "The Minooka Special" Dec 1974 QST; VE2CV "Technical Correspondence" Feb 1991 QST; ARRL Handbook, Chapter 17, sec "Ground Systems"; The W1GHZ Online Microwave Antenna Book; The HAM Radio Operator's Antenna Manual, by Buck Rogers K4ABT; W5DXP's No-Tuner, All-HF-Band, Horizontal, Center-Fed Antenna; TM 11-5985-379-14&P, Operator's, Organizational, Direct Support, And General Support Maintenance Manual; TM 11-5985-370-12, Operator's And Organizational Maintenance Manual; The RF Transmission Systems Handbook, Ch 15, Radio Wave Propagation - Gerhard J. Straub; Ch 16, Antenna Principles - Pingjuan L. Werner, Anthony J. Ferraro, and Douglas H. Werner; Ch 17, Practical Antenna Systems - Jerry C. Whitkaer I could go on, but Chucky, you're not really interested in me, you're just interested in saying "you're wrong" to everyone until you have had the last post and can claim victory. Therefore, to that end, I say you win, go back under your bridge and wait for the next traveler, Chucky. That is what Trolls do isn't it? While your there, with nothing to do, try reading some theory, http://ftp.21ic.com.cn/RFDesign/ is a good place to start, you will find a copy of the US Army Field Antenna Handbook, although maybe too advanced, as you would have to get you mind around the concepts. My daddy once told me to never try to argue with an imbecile, "They drag you down to their level, then beat you with experience" so I guess I will leave it at that. P.S. The reason I have all this reference material, is that the Marconi is my "weapon of choice", you see I am a Road Warrior, one of those sleazy WarDrivers who use laptops equipped with wireless Ethernet cards and remote antennae to acquire internet access over unsecured Wireless Access Points. My antenna is a quarter-wave Marconi with a 30° counterpoise. I use 30° because unlike 45°, the 30° slopping counterpoise gives a slight upward tilt to the radiation pattern's maximum lob without affecting impedance that adversely. The increase in effective radiated power more than cancels the loss due to reflection and power transfer losses due to impedance mismatch. -- "chuck" wrote in message nk.net... | Hello Mungo, | | I'd say the first part rates a C-, mostly for credit in summarizing a | great deal of material. But did you say that the input impedance of a | quarter-wave vertical antenna at 30 MHz is 100 ohms without a | counterpoise? NO ground return path at all? Sorry, but I think maybe you | have fallen prey to Roger's "magic." | | Your understanding of what the 23' antenna is and how it works is sadly | wanting. It also does not comport with the material in the earlier | paragraphs. You have not even hinted at the sleight-of-hand introduction | of an antenna tuner and why it is needed, and you have introduced an | antenna without any mention of an RF return path! You are posting to a | newsgroup where the readers are generally familiar with the 23' whip and | the need for an antenna tuner and an RF return path. | | Do yourself a favor: find a book on antennas and read it. An elementary | text is a good place to start. | | And in regard to the excluded middle, I think I understand now why you | have not found it. | | Regards, | | Chuck | | | | Mungo Bulge wrote: | I have to agree with Chucky on at least one point. The postings on | this thread are humorous. That must be frustrating to those who were | trying to follow the thread in the hope of learning something useful. | | The reason I find it amusing is that both the con's and the pro's are | to some extent correct in their reasoning all be it flowed to one | extent or the other. | | As I am really old and lacking any substantial short-term memory, | re-reading the entire thread to glean every single nuance of every | attempted point would be counter productive for me because I would | probably forget what I was doing. I will however make some assumption, | which I will hold true, for the sake of simplicity. By the way Chucky, | the proper use of reduction to the absurd (reductio ad absurdum) logic | would require a model theory which is based on the law of excluded | middle (tertium non datur) which clearly does not apply to the | statements put forth in this thread. | | My assumptions: | | 1. The usual antenna system used with marine transmitters in | the intermediate and high-frequency bands is the Marconi or grounded | radiator, in contrast to the Hertz or ungrounded radiator. Technically | the most basic of antenna, it is an "isotropic radiator". This is a | mythical antenna, which radiates in all directions, as does the light | from a lamp bulb. This assumption is somewhat modified by assumption | #5 below, as a reflection (pardon the witticism) of reality. | | 2. Depending upon height above ground, the influence of | surrounding objects and other factors, our quarter wave antenna with a | near perfect ground exhibits a nominal input impedance of around 36 | ohms. | | 3. Ground losses affect the feed point impedance and antenna | efficiency. When mounted on a real ground, the input impedance can | range from 38 ohms for a well-designed antenna mounted over a | specially prepared ground, to over 100 ohms for a Marconi mounted | above poor, unprepared ground that has no radials. | | 4. Ground loss reduces the antenna's efficiency, because part | of the power being delivered to the antenna is being dissipated in the | ground rather than being radiated. The efficiency can be computed from | the measured value of input resistance by using the formula; | Efficiency equals 36 ohms divided by antenna impedance. | | 5. The radiation pattern of the Marconi antenna is a half | doughnut. There is no radiation straight up in the direction of the | antenna. The bulk of the radiation occurs at a low elevation angle, | which is what is needed to launch a ground wave. | | 6. Finally yet importantly, the HF installation shall be used | for communications in excess of line of sight. In other words, the | skipper will want this installation to work when s/he needs to contact | S&R in a life-threatening situation, not just to collect the email | while birthed in some marina. | | Ok, now the model part. This antenna is to be mounted on a boat. So | what is all this talk about ground? Well let us look at the Marconi | antenna. It is actually one half of a dipole antenna. Trust me on | this, explaining why and how will just get us lost, but the other half | of the dipole is needed and is provided by the image produced by the | previous mentioned ground. There is that word ground again. Now | Chucky, this is were some clarification needs to be. This is not the | same ground we all have come to know and love when we talk about the | ground in an electrical system like your house (safety ground), your | car (negative return) or your boat (negative return) this is terra | firma, real honest to goodness dry land, and for the time being we | will assume its perfect. Now, by perfect I don't mean a perfect | conductor, hell it's not even close. By perfect I mean a low impedance | to RF currents. The RF ground currents are greatest in the vicinity of | the feed point at the base of the antenna up to a distance of ¼ wave | length from the antenna. | | Now we all know there isn't any perfect ground left, the Aztecs used | it all to build their pyramids. So how do we make it perfect or at | least acceptable, we install a counterpoise, a conductor or system of | conductors used as a substitute for perfect ground in an antenna | system. That's were Me's radials or RF ground system comes in. It | should have the effective radius equal to the height of the antenna | (1/4 wave length). I say should have, but in reality the radials do | not all have to be the same length and that losses may be decreased by | adding extra radials near the feed point. These extra radials can be | as short as 1/40 wave length and still be effective. Now, with this | added counterpoise, you can pick up this antenna and move it anywhere | and it will still function very well with a relatively high | efficiency. That's a new term I snuck in without you seeing it coming. | Cool! | | Remember assumption #2 above? I said a Marconi had an input impedance | of 36 ohms on perfect ground, well it turns out that with a | counterpoise it has an impedance of 38 ohms. Trust me, it does. Now | let's drag this sucker over to the marina and hoist it onto that boat | we have all been talking about. We get it mounted, counterpoise and | all. Not easy considering its size. A Marconi antenna for a 2 MHz | system is 117 feet high (234/2) with a 117 foot diameter counterpoise. | Big sucker isn't it? How are we going to keep the counterpoise level? | We're not. Let's let it droop, say 45 degrees. I'm cheating here. Some | of you may know that a Marconi with a counterpoise set at a 45 degree | down slop has an input impedance of 50 ohms and just by chance, that | exactly matched the radio set's output impedance, thus maximum RF | power transfer between antenna and radio. Cool! | | However, the antenna's efficiency has dropped to 72% and we have this | honking big antenna messing up the aesthetics of our nice boat. Let's | scale it down. Instead of 2 MHz, let's go to the other end of the band | to 30 MHz. That makes the antenna 7.8 feet high with a 7.8 foot radius | counterpoise. Before you start screaming about my math, I allow for a | "velocity factor" of 5%. It's not as big, but it's still ugly and we | need more power to be able to raise S&R when need them. | | For aesthetics, let's drop the counterpoise. What happens? The antenna's | impedance goes up to about 100 ohms and its efficiency drops to 36%, | but that isn't the worst of it, the power transfer is not maximum | because the impedances are not matched. The reflective coefficient | will be 0.3333 | | ((100-50)/50)/((100+50)/50), thus a VSWR of 2.0, a return loss of 9.5 | dB which means the power actually reaching the antenna is 11.1% so now | I can't reach S&R and I'm going down. | | | | Me is thinking "I'm vindicated" and Chucky is thinking "Another nut | case". You're both right. So why does it work, simple. The antenna isn't | a Marconi; it's an industry standard Marine HF band antenna, 2-30 MHz | bandwidth, 10.8 MHz resonant frequency, 23 feet high and when | connected to an HF radio set configured to its manufacturer's | specifications it will perform admirably. If that were not the case, | we would have had to have had at the very least five quarter wave | Marconi antennae ranging in height from 7.8 to 117 feet and we don't. | | That Chucky is the proper use of reductio ad absurdum logic. | | | | | | | | "chuck" wrote in message | ink.net... | | Well Bruce/Me, I think you need to pull your two "selves" together! | | | | Sifting through the humorous postings, I think your bottom line is | that | | HF/MF vertical antennas will not work well (sometimes I think you | mean | | will not work at all) unless they are (1) over sal****er with a | return | | path capacitively coupled to the sea (at least for nonmetallic | vessels); | | or (2) over land with 100 quarter-wave radials in marshland. | | | | You have labored to persuade us that less-than-perfect marine RF | ground | | systems are certain to disappoint. | | | | It will surprise you, perhaps, to learn that there are many | thousands of | | vertical HF and MF transmitting antennas in operation in the world | today | | that satisfy none of those conditions, and yet enable effective | | communications activities. Some on land and some over water. These | | installations are supported by rigorous theory as well as by | on-the-air | | performance data. | | | | If you would like to learn more about how this is being done, often | with | | losses of only a few dB below ideal conditions, drop in at | | rec.amateur.radio.antenna and "read the mail." You'll find some | | bombastic assertions and opinions to be sure, but also many reasoned | | analyses and even quantitative experiments. Hope to see you there, | Bruce. | | | | Regards, | | | | Chuck | | | | | | | | | | | | | | | | Bruce in Alaska wrote: | | In article , | | Me wrote: | | | | | | In article | .com, | | "Skip Gundlach" wrote: | | | | | | As further background, we have full rails, with the gates | combined | | electrically with brass straps belowdecks, attached to the arch, | the | | pushpit and pulpit. We have about 110 lineal feet of 1" SS tube | rail, | | unless you count the inner rails, plus the arch. In addition we | have | | the standard 4" copper strapping leading to a sintered bronze | Guest | | plane below the boat, and also connected to a 3x5' plate under | the | | workbench top. I think we have a reasonably good ground. | | | | You will never know if you have a "reasonably good ground", unless | | you get yourself an Impedance Bridge, and check it at the | frequencies | | that you commonly work. Anything that is more than 12" away from | the | | water, isn't going to add "diddley-squat" toward building a Low | Impedance | | Wideband RF Ground System, and anyone who tells you otherwise, is | just as | | uneducated about MF/HF Marine Radio Antenna Systems, as you seem | to be. | | I have seen all kinds of Systems that looked very impresive, | untill they | | were evaluated with real insurmentation. 400 Sq Ft of Copper | Screen in | | the Cabin Overhead was proffered, as a really good RF Ground, by a | well | | known Boat Builder, 20 years ago. It didn't work any better than | | having nothing at all, when tested, in a real radio enviorment. If | | you got a Plastic Hull, you are NEVER going to get a Real RF | Ground, | | UNLESS the hull builder was smart, (they never are) and put 200+ | Sq | | Ft of screen under the gellcoat down by the keel. Cellulose hulls | | are just as bad, and harder to retrofit that Plastic ones. | | Like I said in my first reply, Autotuners were invented to allow | any | | "Dufus" to think he install an MF/HF Marine Radio System, and save | | himself all that money he would have paid a Compitant Radioman. | | SGC Autotuners are some of the worst of the lot, even if they did | steal | | the design from the real inventers. SGC couldn't even copy the | design | | correctly, and "Old PeeAir" couldn't design his way out of a "Wet | Paper | | Bag". | | | | Me | | | | | | Geeze Louise "Me" give the guy a break...... He was just asking | for | | an opinion.... | | | | | | Bruce in alaska | | |
"Skip Gundlach on wifi"
skipgundlach.sez.remove.this.part@earthlink.(fish catcher)net wrote in : I'm not familiar with the 802 at all, yet, so didn't know of the separate DSC antenna receive function. I've got Lionheart's DSC antenna hooked to the handrail right over the transceiver's location at the chart table. Seems to work fine. -- Larry |
In article ,
Brian Whatcott wrote: Oh my! Anonymous poster, it was *YOUR* suggestion that an RF ground of 200 sq ft of mesh under the (external hull) gel-coat was required for a satisfactory RF ground at HF. It was the original poster's suggestion of an elevated mesh that caught your interest, not mine. As you asked about antenna testing, I should mention that a supercomputer is not really necessary: there is a handy dandy gadget ( from MFJ ) which combines several RF test functions like antenna bridge, SW ratio etc. It ran about $200 as I recall. I satisfy myself with an LC meter these days - which gets one into the ball park at $100. eBay has an MFJ noise bridge at $25 currently. Hmmm...the capacitance to ground of a few objects in my vicinity runs about 45 pF per sq ft. Like me, standing on carpet It is just possible a hi level ground screen of 200 sq ft might get you 200 X 45 pF = 9000 pF At 3 MHz that would put the capacitive reactance at 1/2pi.f.C ohms = 6 ohms. Not that great. The actual value might well be quite a bit higher than that. But that's just me measuring with an instrument, rather than you guessing how poor it is. Another thing: the conductivity of sea water does not vary all that much - it doesn't have much impact on capacitance. But there I go again, actually measuring things! Regards Brian Whatcott p.s. I have an FCC GROL+rdr. You? I am not so "Anonymous" as you would think. There are, certainly, folks who know who "Me" really is. Some even post here. I didn't "Say or State" that the above WAS required. I stated that "200 sq ft" would certainly provide a "Low Impedance Wideband RF Ground, on plastic hulled vessles floating in Salt Water." I also introduced the discreditied concept of "copper screen in the overhead" into the thread, if you would go back and actually read the whole thread. Your testing tools seem to be of the consumer variety. Some one should teach you a bit about modern RF Antenna Design & Testing Tools, one of these days. Most compitant folks use both RF Network Analysers, and, or an Antenna Impedance Bridge feeding a Spectrum Analyser with a Tracking Sweep Generator. Best you come back after you learn to use the tools, that "the Big Boys" use. It is just possible that you don't have much of a clue about MF/HF Marine Antenna Systems and RF Grounds aboard Vessles. The above statement about some mythical capacitive reactance at 3 Mhz really shows that your way out of your league in this dicussion. There are few 3 Mhz Marine Frequencies, (Mostly in alaskan waters) and most non-commercial MF/HF Marine Radio Users rarely use any below the Maritime Mobile 4 Mhz Band. There are many 1.6Mhz, 2.0 - 3.3 Mhz, Marine Frequnecies used in alaska, by commerical users, and a daily basis, and have been for many years. When was the last time you actually operated, or for that matter installed, a Private Coast Station, using any frequency at all, or for that matter any Maritime Mobile Station of any kind. I operate a Private Coast/ Alaska Public Fixed Station, on a daily basis, that I designed and installed 20 years ago, and communicate with vessels all over the North Pacific. Tell us all, about your great experience in Marine Communications. The conductivity of Seawater isn't in dispute in this thread on it's own, what is in dispute seems to be how it compares to fresh water, RF Grounds used in MF Commercial Radio Stations, and other mediums. What you fail to understand is that capacative coupling to SeaWater is extremely Frequency Sensitive, and to design an effective Low Impedance, "Wideband" RF Ground on a plastic, or cellulose hulled vessel, requires a very effective coupling to the Seawater over a Wide Frequency Range. That is the crucks of the problem. Again, "It is the RF Ground, sonny, the RF Ground"... Me 1st Class RadioTelegraph, with Seatime Endorsement, Radar Endorsement, and, wait for it.... Aircraft Endorsement... |
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