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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 |
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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 |
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