| Page 1 of 2 | 1 | 2 |
|
|
SSB Antenna for a Ketch
I am in the final stages of building a 38' steel ketch and am currently
looking at the installation of a SSB system (Icom M802 / AT-140). I have read back through the forum postings and gained some good knowledge, however am wondering if anyone can help out with some the problems specific to a ketch rig. My boat (a Roberts Offshore 38) has a main mast 38' off the deck, and mizzen 28' off the deck. This gives me a total backstay length of about 42', and a triatic about 14' long. My plan is to rig the main mast with twin backstays (i.e. the backstay is split right from the top of the mast). I realise that the longer the antenna the better, however the backstay runs quite close the shrouds of the mizzen mast near the deck, and so I figure I would probably need to place my lower insulator 6' off the deck (what is the minimum clearance between the antenna and another shroud to avoid coupling?). I've seen recommendations saying the top insulator should be 3' from the mast head (any comments??). This would leave me with an effective antenna length of about 33'. I guess what I am trying to gauge is what effect all of the other rigging around it will have on performance (being a steel boat all of this other rigging will be grounded). Is there a gain to be made by insulating the triatic and linking this to the backstay antenna??? What is your judgement on wether this will be an adequate antenna system??? One suggestion that I have had is to rig a whip antenna off the stern and use a change-over switch to manually select either the backstay or whip depending on the frequency for transmission . . . I much prefer the idea of just using the backstay. Any comments would be much appreciated. |
On Tue, 2 Nov 2004 17:02:47 +1030, "David Swindon"
wrote: One suggestion that I have had is to rig a whip antenna off the stern and use a change-over switch to manually select either the backstay or whip ========================================== Having the whip antenna as a backup is not a bad idea for an offshore boat, and you may find the performance to be just as good or better. You could do the change over manually to eliminate the need for a switch. The only drawback I can think of is the possibility that the mizzen boom is long enough that it could contact the whip. |
How about coming down from the main with a 3' insulated penant, then split
the twin backstays with insulators on each at the lower end of each. Using your calculation, that would give you a total length of 66ft. In my own SSB installation on my 38 ft cutter, with twin back stays, the upper insulator is only about a foot from the other rigging (and grounded twin). I haven't noticed any problem (so far). Since my backstays, run only a couple inches from my radar arch, at the lower end, I placed the lower insulator just above the arch. As I side note: I don't fully trust radio insulators (not that I have ever known of one to fail), so I like the idea of leaving the one backstay out of the potential failure equation. My main mast can stand and continue to carry full canvas, even if the insulated backstay were to fail. Just my thoughts, FWIW. Steve s/v Good Intentions |
Steve wrote:
How about coming down from the main with a 3' insulated penant, then split the twin backstays with insulators on each at the lower end of each. Using your calculation, that would give you a total length of 66ft. In my own SSB installation on my 38 ft cutter, with twin back stays, the upper insulator is only about a foot from the other rigging (and grounded twin). I haven't noticed any problem (so far). Since my backstays, run only a couple inches from my radar arch, at the lower end, I placed the lower insulator just above the arch. As I side note: I don't fully trust radio insulators (not that I have ever known of one to fail), so I like the idea of leaving the one backstay out of the potential failure equation. My main mast can stand and continue to carry full canvas, even if the insulated backstay were to fail. Just my thoughts, FWIW. Steve s/v Good Intentions The main thing for good xmission is good counterpoise. krj |
"krj" wrote in message ... The main thing for good xmission is good counterpoise. krj I don't think David (OP) is going to have a counterpoise problem, his boat being steel hull. I think his problem is going to be the lead from the tuner to the insulated backstay (6ft above the deck). Myself, I have no external ballast and all through hulls are non-metalic.. I'm using a 3" foil strips in the bilge and my SWR is very good (I don't remember the exact ratio). Steve s/v Good Intentions |
In article ,
krj wrote: The main thing for good xmission is good counterpoise. krj Counterpoise or RF Grounding System? These two items are NOT the same thing. Bruce in alaska -- add a 2 before @ |
In article ,
"Steve" wrote: I think his problem is going to be the lead from the tuner to the insulated backstay (6ft above the deck). One of the things that most folks just don't get, is the "Classic" RF Thru Bulkhead Insulator. For Metal hulled vessels and houses these are one of the most forgotten items of the installation. On Military Vessels, MF and HF Antenna Feedlines are routed thru no less that 6" Ceramic ThruHouse fittings. On most Commercial SOLAS Vessels, they use the same standard size ThruHouse fittings. If it were me designing for this vessel, I would look at using a vertical wire seperate from any backstay or shroud, and a 28 Ft unloaded vertical whip mounted on the front of the MizzenMast. Since the hull is metal the Autotuner can be mounted anywhere inside the hull, so the vertcal wire can be placed just about anywhere, where it will have a straight shot to the bottom of the whip. This would give you around 24ft of wire under a 28Ft whip for 52ft of total antenna length. That is very respectable in any installation senerio. If you really want to do this right and have GREAT MF capability, move the unloaded whip up to the MainMast and Feed it with a wire that comes from the tuner to an insulator on the top of MizzenMast, than on to another spring dampened insulator on the MainMast just below where the whip is mounted. That will give you, 24Ft Mizzen wire, plus say 30ft between Masts and then 28ft whip, for a total of 82 plus any jumpers or around 85ft. Now your talking with significant MF Antenna radiation effecincy. By doing it this way you are not comprimising any of your Mast Supports, and will have a very effective MF/HF Antenna System. Bruce in alaska -- add a 2 before @ |
In article ,
Wayne.B wrote: Having the whip antenna as a backup is not a bad idea for an offshore boat, and you may find the performance to be just as good or better. You could do the change over manually to eliminate the need for a switch. The only drawback I can think of is the possibility that the mizzen boom is long enough that it could contact the whip. Another drawback to any switching of antennas external to the autotuner is that you defeat the purpose of the autotuner antenna memory function, in that you now have two different antennas the tuner is trying to tune for the same frequency, and the tuner can only store one set of data/Freq tuned. Makes for a lot of Relay chattering and the relay points have ALWAYS been the weakest link in autotuners. Something most noncommercial users never think about. Don't try that on a SGC Tuner, or you will have it InTransit to the factory, more than installed on the boat, especially in installations on metal hulled vessels with short (less than 35ft) antennas on MF Frequencies. Bruce in alaska -- add a 2 before @ |
Don't get hung up on the length issue. I use a 23' whip and get as good
as perfomance and any of my friends with insulated backstays. I believe the specs say that 23' is the min length that can be tuned by the AT-140. Any length beyond that doesn't gain that much additional performance. Doug, k3qt s/v Callista "David Swindon" wrote in message ... I am in the final stages of building a 38' steel ketch and am currently looking at the installation of a SSB system (Icom M802 / AT-140). I have read back through the forum postings and gained some good knowledge, however am wondering if anyone can help out with some the problems specific to a ketch rig. My boat (a Roberts Offshore 38) has a main mast 38' off the deck, and mizzen 28' off the deck. This gives me a total backstay length of about 42', and a triatic about 14' long. My plan is to rig the main mast with twin backstays (i.e. the backstay is split right from the top of the mast). I realise that the longer the antenna the better, however the backstay runs quite close the shrouds of the mizzen mast near the deck, and so I figure I would probably need to place my lower insulator 6' off the deck (what is the minimum clearance between the antenna and another shroud to avoid coupling?). I've seen recommendations saying the top insulator should be 3' from the mast head (any comments??). This would leave me with an effective antenna length of about 33'. I guess what I am trying to gauge is what effect all of the other rigging around it will have on performance (being a steel boat all of this other rigging will be grounded). Is there a gain to be made by insulating the triatic and linking this to the backstay antenna??? What is your judgement on wether this will be an adequate antenna system??? One suggestion that I have had is to rig a whip antenna off the stern and use a change-over switch to manually select either the backstay or whip depending on the frequency for transmission . . . I much prefer the idea of just using the backstay. Any comments would be much appreciated. |
"David Swindon" wrote in message ... My boat (a Roberts Offshore 38) has a main mast 38' off the deck, and mizzen 28' off the deck. This gives me a total backstay length of about 42', and a triatic about 14' long. My plan is to rig the main mast with twin backstays (i.e. the backstay is split right from the top of the mast). We use a tuner mounted on the rail of the pushpit. The antenna is tied off to the rail beside the tuner and basically looks like and inverted 'L'. We use a flag hoist on the mizzen and then the antenna is run to the peak of the main mast. The mainmast end, ends about three feet from the main and the antenna is adjusted to clear the mizzen about the same as it runs down to the tuner. The overall length is 28 feet. Leanne s/v Fundy |
A number of issues have been raised here
that deserve some further comment. Regarding RF coupling to the standing rigging, don't be overly concerned. You will be coupled to the rigging no matter where you place the antenna. Some geometries will doubtless be worse than others, but it will be difficult to predict in advance. Moreover, such coupling is not necessarily a bad thing. It is just a difficult thing to model and thus more of an unknown. Regarding antenna length, more is not necessarily better and may be worse! But whether worse or better, changing the length of an antenna may make it different. For example, if you are crossing oceans and want reliable skip communication over great distances, you want low radiation angles. A quarter-wave or 5/8-wave vertical will be your best choice. That would be about 16 feet in length at 14 MHz. Make your antenna 32 feet long and you have a half-wave vertical with very little low-angle radiation at 14 MHz, but at 7 MHz and below, low-angle radiation will be plentiful. Which is better depends on your objectives. Operating near the coast, you may find that higher radiation angles produce shorter skip zones to your advantage. Were your boat fiberglass or wood instead of steel, it is possible that a horizontal antenna laid on the deck would outperform any vertical antenna for high radiation angle communications with a range of say 400 miles. Think about maintaining solid ssb contact with boats scattered throughout the Bahamas, for example. With vertical antennas such a task would be quite difficult. Start out your planning with some consideration of which distances are most important. From that, move to which frequencies and radiation angles provide the appropriate skip zones to achieve those distances, and from there, consider antenna options that further those objectives. Good luck Chuck |
"Chuck" wrote in message ink.net... A number of issues have been raised here that deserve some further comment. Regarding RF coupling to the standing rigging, don't be overly concerned. You will be coupled to the rigging no matter where you place the antenna. Some geometries will doubtless be worse than others, but it will be difficult to predict in advance. Moreover, such coupling is not necessarily a bad thing. It is just a difficult thing to model and thus more of an unknown. This is my experience. Regarding antenna length, more is not necessarily better and may be worse! But whether worse or better, changing the length of an antenna may make it different. For example, if you are crossing oceans and want reliable skip communication over great distances, you want low radiation angles. A quarter-wave or 5/8-wave vertical will be your best choice. That would be about 16 feet in length at 14 MHz. Make your antenna 32 feet long and you have a half-wave vertical with very little low-angle radiation at 14 MHz, but at 7 MHz and below, low-angle radiation will be plentiful. I think the OP mentioned an automatic tuner. From my experience both the ICOM and SGC tuners require at least 23'. Not sure how a longer antenna fairs. Shorter will definitely not tune well. Good luck Chuck |
In article . net,
Chuck wrote: Regarding antenna length, more is not necessarily better and may be worse! But whether worse or better, changing the length of an antenna may make it different. For example, if you are crossing oceans and want reliable skip communication over great distances, you want low radiation angles. A quarter-wave or 5/8-wave vertical will be your best choice. That would be about 16 feet in length at 14 MHz. Make your antenna 32 feet long and you have a half-wave vertical with very little low-angle radiation at 14 MHz, but at 7 MHz and below, low-angle radiation will be plentiful. Also be aware that autotuners CAN"T tune antennas that are within 50Khz of 1/2 wavelength. So inlight of this one must pick a length of antenna that puts the 1/2 wavelength point on a frequency band that will never be used for transmitting. Also understand that short antennas preform very BADLY, as the Input Capacitance on the L tuner model is increased. So your preformance below 4 Mhz will be drastically reduced with antennas of less than 50 ft of electrical length. If your using MF Frequencies for comms of less than 400 miles, which is what they are there for, you will need a longer antenna. Bruce in alaska -- add a 2 before @ |
In article ,
"Doug Dotson" wrote: I think the OP mentioned an automatic tuner. From my experience both the ICOM and SGC tuners require at least 23'. Not sure how a longer antenna fairs. Shorter will definitely not tune well. As I posted eslewhere, autotuners have some very specific flaws that keep them from having optimum preformance. 23' isn't near long enough for ANY reasonable comm's below 8Mhz. The tuners get get VERY lossy as input capacitance is increased, in order to tune short antennas. Bruce in alaska -- add a 2 before @ |
Antennas are really a lot like boats: No
boat will do everything well and no antenna will either. Boats and antennas that try to do everything usually fail across the board. FWIW, SGC-237, -230, and -231 tuners need 23 feet only to tune from 1.6 MHz to 3.3 MHz. Above 3.3 MHz, these SGC tuners require only eight (8) feet. The Icom AH-4, for example, needs 23 feet only to tune down to 3.5 MHz, but will tune from 7 MHz up with Icom's AH-2b whip (8.2 feet long). But it doesn't matter what lengths the tuners require if there is no desire to operate in that frequency range, and chances are excellent that recreational boaters will not be found at the very low frequencies. As has been pointed out, some antenna lengths will be more taxing for an autotuner than other lengths. Your objective is not to make life easier for your tuner, especially when doing so may move you farther from your real needs. You may not even need a tuner! Your objective is to achieve your communication goals. You might give some thought to posting on one of the cruising newsgroups to ask experienced cruisers for their thoughts on things like "if you had only one frequency to operate on, what would it be? Among other things, that might be the basis for an antenna you can stow for emergencies. But tell them where and how you'll be cruising and what you want the ssb for (email, emergencies, boat-to-boat communication, etc.) Then return to the antenna design questions. Keep to it! Chuck |
I've been able to do reasonably well on 40 and 80 meters with my
23' whip. I do get more side effects like twinkling lights and sometimes the LectraSan activates itself :) Doug, k3qt s/v Callista "Bruce in Alaska" wrote in message ... In article , "Doug Dotson" wrote: I think the OP mentioned an automatic tuner. From my experience both the ICOM and SGC tuners require at least 23'. Not sure how a longer antenna fairs. Shorter will definitely not tune well. As I posted eslewhere, autotuners have some very specific flaws that keep them from having optimum preformance. 23' isn't near long enough for ANY reasonable comm's below 8Mhz. The tuners get get VERY lossy as input capacitance is increased, in order to tune short antennas. Bruce in alaska -- add a 2 before @ |
"Bruce in Alaska" wrote in message ... So your preformance below 4 Mhz will be drastically reduced with antennas of less than 50 ft of electrical length. If your using MF Frequencies for comms of less than 400 miles, which is what they are there for, you will need a longer antenna. Bruce in alaska -- add a 2 before @ Theres been some really good discussion here. In my experiance cruising we used a whole range of frequencies as sometimes we were communicating with boats in the same area, and other times with boats back in home port. With regard to the need for a longer antenna for short range (definatly required) what are your thoughts on tying the triatic into the backstay as part of the antenna system (the triatic is 14' long - although as the mizzen is shorter than the main mast, the angle between the triatic and backstay is only about 30 degrees). |
As Bruce says, "tuners get very lossy with short antennas". But that
is not the only problem with short antennas. The antenna and ground system become very lossy with short antennas. Below 1/4 wavelength the radiation resistance of the antenna drops drastically. It can be less than an ohm. That equates to very high losses. The antenna system in those cases may be only a few percent efficient. It is far better to have a longer antenna that gives a much higher radiation resistance even if it may not be the optimum length as far as radiation pattern is concerned. If you can't get the power to the antenna the radiation pattern doesn't much matter. You still won't get out very well. On a typical boat the radiation pattern is going to be far from ideal with whatever length antenna you have due to all the surrounding objects on the boat. The difference in radiation patterns between a 1/2 wavelength and 5/8 wavelength antennas are minimal. About the only real difference is the feed point impedance they present. As far as antennas greater in length than a quarter wavelength, they start to produce multiple lobes in the pattern. Which on a boat may not be a bad thing. As you mention, sometimes higher angles are desired depending on the distance trying to be covered. A longer antenna on a typical boat is most always going to be more efficient than a short antenna even if the longer antenna produces multiple pattern lobes. Regards Gary On Tue, 09 Nov 2004 00:29:53 GMT, Chuck wrote: Antennas are really a lot like boats: No boat will do everything well and no antenna will either. Boats and antennas that try to do everything usually fail across the board. FWIW, SGC-237, -230, and -231 tuners need 23 feet only to tune from 1.6 MHz to 3.3 MHz. Above 3.3 MHz, these SGC tuners require only eight (8) feet. The Icom AH-4, for example, needs 23 feet only to tune down to 3.5 MHz, but will tune from 7 MHz up with Icom's AH-2b whip (8.2 feet long). But it doesn't matter what lengths the tuners require if there is no desire to operate in that frequency range, and chances are excellent that recreational boaters will not be found at the very low frequencies. As has been pointed out, some antenna lengths will be more taxing for an autotuner than other lengths. Your objective is not to make life easier for your tuner, especially when doing so may move you farther from your real needs. You may not even need a tuner! Your objective is to achieve your communication goals. You might give some thought to posting on one of the cruising newsgroups to ask experienced cruisers for their thoughts on things like "if you had only one frequency to operate on, what would it be? Among other things, that might be the basis for an antenna you can stow for emergencies. But tell them where and how you'll be cruising and what you want the ssb for (email, emergencies, boat-to-boat communication, etc.) Then return to the antenna design questions. Keep to it! Chuck |
In article ,
Gary Schafer wrote: As Bruce says, "tuners get very lossy with short antennas". But that is not the only problem with short antennas. The antenna and ground system become very lossy with short antennas. Below 1/4 wavelength the radiation resistance of the antenna drops drastically. It can be less than an ohm. That equates to very high losses. The antenna system in those cases may be only a few percent efficient. It is far better to have a longer antenna that gives a much higher radiation resistance even if it may not be the optimum length as far as radiation pattern is concerned. If you can't get the power to the antenna the radiation pattern doesn't much matter. You still won't get out very well. On a typical boat the radiation pattern is going to be far from ideal with whatever length antenna you have due to all the surrounding objects on the boat. The difference in radiation patterns between a 1/2 wavelength and 5/8 wavelength antennas are minimal. About the only real difference is the feed point impedance they present. As far as antennas greater in length than a quarter wavelength, they start to produce multiple lobes in the pattern. Which on a boat may not be a bad thing. As you mention, sometimes higher angles are desired depending on the distance trying to be covered. A longer antenna on a typical boat is most always going to be more efficient than a short antenna even if the longer antenna produces multiple pattern lobes. Regards Gary Exactly, Doug says he does fairly well on 80 and 40 Meters with a 23' whip and an autotuner. We take him at his word, but if he would figure out how to increase that to 50' or 75', there is a GOOD chance that he would do better, and even in poorer band conditions. It doesn't take much power or antenna to communicate if the band is open, to where you want to talk, on the frequency that your using. Try that if the bands isn't so hot and the time of day is against you, with a Very Marginal antenna system. Yea, I know most pleasure boaters have never heard of Marine Frequencies below 4Mhz, but up here in Alaska we have been using 1.6Mhz and 2.0Mhz - 4Mhz Marine frequencies for years, and very suscessfully, even on Poor Band Years. I have used 3261Khz for Maritime Comms for 35 years, and worked my Fleet Vessels, with 65% completion of Comms, rate on a daily basis. You will not get that kind of connectitvity, with a 23' antenna, on your Maritime Mobile Stations. In the "Good Old Days", the previous generation of Alaskan RadioMen used to work 1630Khz consistantly every night for intercompany Comms. Just because SGC says their tuner only needs 23ft of wire, doesn't mean that you can actually talk to anyone with that type of system. By the way SGC didn't do the design of that autotuner, themselves, they stole it from SEA, and didn't even change the "CopyWrite Statement" in the firmware code. All autotuners on the market today, come from the original design work of Bill Schillb, an engineer for Motorola MF/HF Systems, at the time. He worked out the basics of the tuning code and the hardware design. When he left Motorola and came out west to Seattle, he landed at Northern Radio for s short while, and while there passed on the basic technology to Bill Forgey, who was Chief Engineer at Nothern at the time. Both Bill's left Northern Radio just before it went under, with Forgey taking the whole Design Team with him, and with Dick Stephens started SEA. (Stephens Engineering Asscoiates) Dick was the Chief Engineer at Northern before Bill, and his mentor. Bill Forgey along with Mark Johnson (an ex Northern Tech) designed the first truely Marine Radio Autotuner which was the SEA1600, using the basics that Bill Schillb had imparted, and on which, they improved and expanded. The first autotuner that had Frequency Memory and Instant Band Switching was the SEA1612, and the B version is what SGC copied for their 23x series tuners. Icom, Kenwood, Furuno, and the rest are "Johnnie Come Latelys" in the world of Marine Autotuner design, and basically they reverse engineered the SEA design and firmware, for their systems. I was closely associated with these folks as a Traveling Radio Tech for Northern Radio, before leaving to become Comm Supt. for the largest Salmon Canner in Alaska. I count all these folks a close friends, even after all these years, and also while working the Dark Side (I was a Field Agent for the FCC for five years) of the industry. I also did a pile of beta testing over the years for SEA, that included most of their designs for autotuners. Some of the prototypes are still in use today, in various places in alaska. I designed and installed the first Marine Autotuner feeding a Dipole Antenna, and that system is still in use today. Bill, Mark and I designed and built a 1Kw Maritime Mobile Coast Station that has 8 Control Points, and uses a SEA1612B Autotuner, and one of a kind Dipole Antenna for MF/HF Frequencies from 1630Khz thru 30Mhz at the 150W PEP level, and 1Kw on 4Mhz, 6Mhz, 8Mhz, 12Mhz 16Mhz, 22Mhz Marine Frequencies, using another special Dipole Antenna, from Morad Electronics. This system is still in use today as well, and has been around for more than 10 years. I don't usually "Toot my own horn", but I do have considerable practical experience in this field, as well as a long history in the industry. Bruce in alaska onetime Fed, and long time Radioman.......... -- Bruce (semiretired powderman & exFCC Field Inspector for Southeastern Alaska) add a 2 before @ Bruce Gordon * Debora Gordon R.N. Bruce's Trading Post P.O. Box EXI Excursion Inlet South Juneau, Alaska 99850 Excursion Inlet, Alaska 99850 www.btpost.net www.99850.net |
In article ,
"David Swindon" wrote: "Bruce in Alaska" wrote in message ... So your preformance below 4 Mhz will be drastically reduced with antennas of less than 50 ft of electrical length. If your using MF Frequencies for comms of less than 400 miles, which is what they are there for, you will need a longer antenna. Bruce in alaska -- add a 2 before @ Theres been some really good discussion here. In my experiance cruising we used a whole range of frequencies as sometimes we were communicating with boats in the same area, and other times with boats back in home port. With regard to the need for a longer antenna for short range (definatly required) what are your thoughts on tying the triatic into the backstay as part of the antenna system (the triatic is 14' long - although as the mizzen is shorter than the main mast, the angle between the triatic and backstay is only about 30 degrees). Is that the rigging that goes between the mizzenmast and the mainmast near their tops? Bruce in alaska -- add a 2 before @ |
I've seen quite a few rigged this way. Must be worth something.
Doug s/v Callista "David Swindon" wrote in message ... "Bruce in Alaska" wrote in message ... So your preformance below 4 Mhz will be drastically reduced with antennas of less than 50 ft of electrical length. If your using MF Frequencies for comms of less than 400 miles, which is what they are there for, you will need a longer antenna. Bruce in alaska -- add a 2 before @ Theres been some really good discussion here. In my experiance cruising we used a whole range of frequencies as sometimes we were communicating with boats in the same area, and other times with boats back in home port. With regard to the need for a longer antenna for short range (definatly required) what are your thoughts on tying the triatic into the backstay as part of the antenna system (the triatic is 14' long - although as the mizzen is shorter than the main mast, the angle between the triatic and backstay is only about 30 degrees). |
A 50' or 75' whip would be a bit ungainly :)
Doug s/v CAllista "Bruce Gordon" wrote in message ... In article , Gary Schafer wrote: As Bruce says, "tuners get very lossy with short antennas". But that is not the only problem with short antennas. The antenna and ground system become very lossy with short antennas. Below 1/4 wavelength the radiation resistance of the antenna drops drastically. It can be less than an ohm. That equates to very high losses. The antenna system in those cases may be only a few percent efficient. It is far better to have a longer antenna that gives a much higher radiation resistance even if it may not be the optimum length as far as radiation pattern is concerned. If you can't get the power to the antenna the radiation pattern doesn't much matter. You still won't get out very well. On a typical boat the radiation pattern is going to be far from ideal with whatever length antenna you have due to all the surrounding objects on the boat. The difference in radiation patterns between a 1/2 wavelength and 5/8 wavelength antennas are minimal. About the only real difference is the feed point impedance they present. As far as antennas greater in length than a quarter wavelength, they start to produce multiple lobes in the pattern. Which on a boat may not be a bad thing. As you mention, sometimes higher angles are desired depending on the distance trying to be covered. A longer antenna on a typical boat is most always going to be more efficient than a short antenna even if the longer antenna produces multiple pattern lobes. Regards Gary Exactly, Doug says he does fairly well on 80 and 40 Meters with a 23' whip and an autotuner. We take him at his word, but if he would figure out how to increase that to 50' or 75', there is a GOOD chance that he would do better, and even in poorer band conditions. It doesn't take much power or antenna to communicate if the band is open, to where you want to talk, on the frequency that your using. Try that if the bands isn't so hot and the time of day is against you, with a Very Marginal antenna system. Yea, I know most pleasure boaters have never heard of Marine Frequencies below 4Mhz, but up here in Alaska we have been using 1.6Mhz and 2.0Mhz - 4Mhz Marine frequencies for years, and very suscessfully, even on Poor Band Years. I have used 3261Khz for Maritime Comms for 35 years, and worked my Fleet Vessels, with 65% completion of Comms, rate on a daily basis. You will not get that kind of connectitvity, with a 23' antenna, on your Maritime Mobile Stations. In the "Good Old Days", the previous generation of Alaskan RadioMen used to work 1630Khz consistantly every night for intercompany Comms. Just because SGC says their tuner only needs 23ft of wire, doesn't mean that you can actually talk to anyone with that type of system. By the way SGC didn't do the design of that autotuner, themselves, they stole it from SEA, and didn't even change the "CopyWrite Statement" in the firmware code. All autotuners on the market today, come from the original design work of Bill Schillb, an engineer for Motorola MF/HF Systems, at the time. He worked out the basics of the tuning code and the hardware design. When he left Motorola and came out west to Seattle, he landed at Northern Radio for s short while, and while there passed on the basic technology to Bill Forgey, who was Chief Engineer at Nothern at the time. Both Bill's left Northern Radio just before it went under, with Forgey taking the whole Design Team with him, and with Dick Stephens started SEA. (Stephens Engineering Asscoiates) Dick was the Chief Engineer at Northern before Bill, and his mentor. Bill Forgey along with Mark Johnson (an ex Northern Tech) designed the first truely Marine Radio Autotuner which was the SEA1600, using the basics that Bill Schillb had imparted, and on which, they improved and expanded. The first autotuner that had Frequency Memory and Instant Band Switching was the SEA1612, and the B version is what SGC copied for their 23x series tuners. Icom, Kenwood, Furuno, and the rest are "Johnnie Come Latelys" in the world of Marine Autotuner design, and basically they reverse engineered the SEA design and firmware, for their systems. I was closely associated with these folks as a Traveling Radio Tech for Northern Radio, before leaving to become Comm Supt. for the largest Salmon Canner in Alaska. I count all these folks a close friends, even after all these years, and also while working the Dark Side (I was a Field Agent for the FCC for five years) of the industry. I also did a pile of beta testing over the years for SEA, that included most of their designs for autotuners. Some of the prototypes are still in use today, in various places in alaska. I designed and installed the first Marine Autotuner feeding a Dipole Antenna, and that system is still in use today. Bill, Mark and I designed and built a 1Kw Maritime Mobile Coast Station that has 8 Control Points, and uses a SEA1612B Autotuner, and one of a kind Dipole Antenna for MF/HF Frequencies from 1630Khz thru 30Mhz at the 150W PEP level, and 1Kw on 4Mhz, 6Mhz, 8Mhz, 12Mhz 16Mhz, 22Mhz Marine Frequencies, using another special Dipole Antenna, from Morad Electronics. This system is still in use today as well, and has been around for more than 10 years. I don't usually "Toot my own horn", but I do have considerable practical experience in this field, as well as a long history in the industry. Bruce in alaska onetime Fed, and long time Radioman.......... -- Bruce (semiretired powderman & exFCC Field Inspector for Southeastern Alaska) add a 2 before @ Bruce Gordon * Debora Gordon R.N. Bruce's Trading Post P.O. Box EXI Excursion Inlet South Juneau, Alaska 99850 Excursion Inlet, Alaska 99850 www.btpost.net www.99850.net |
Hello Gary:
Seems I've simply lost the ability to communicate any more. My point was that seeking the most efficient antenna (as defined by maximum power transfer into space) ought not to be the guiding principle. We want maximum power delivered to the other station. All other things the same, a higher radiation resistance would mean lower ohmic losses. But all other things are not the same when antenna length is increased. Yes, radiation patterns on real boats will differ from radiation patterns in space. But even on a real boat, a high percentage of energy is radiated at quite high angles when the antenna is a half-wavelength. Yes, the half-wave will be more "efficient" in getting energy off the boat because the radiation resistance is higher than with a shorter antenna. But if it doesn't get the signal to the other station because the radiation angle is too high, then it's not really optimal. Look at some numbers on vertical radiation patterns. You can easily lose 6 db at useful, low radiation angles by going from a quarter-wave to a hslf-wave. There is no way you'll ever recover that through the half-wave's higher radiation resistance, although that high-angle stuff could actually be a good thing if you're not in the middle of the Pacific trying to work Europe. And we have not even addressed the consequences of a sloping antenna on both horizontal and vertical patterns. No quarrel, of course, with your observation that as the length of an antenna falls below a quarter-wave, the radiation resistance (and thus radiated efficiency) falls. Those losses are one of the parameters that one needs to weigh against other considerations. At the same time, a reduction of length to say, .2 wavelengths would probably not even be detectable (i.e., 1 dB). Also, even as the length decreases, the radiation pattern remains basically that of a quarter-wave antenna. I probably need to repeat that I have not advocated "shorter" antennas, "longer" antennas, quarter-wave antennas, half-wave antennas, vertical antennas, horizontal antennas or much of anthing other than an analysis of the desired signal paths and the basing of an antenna design and frequency combination on that analysis. Well, I have also cautioned against blindly increasing antenna length. Sort of struck me as a motherhood kind of thing. Onward . . . Chuck Gary Schafer wrote: As Bruce says, "tuners get very lossy with short antennas". But that is not the only problem with short antennas. The antenna and ground system become very lossy with short antennas. Below 1/4 wavelength the radiation resistance of the antenna drops drastically. It can be less than an ohm. That equates to very high losses. The antenna system in those cases may be only a few percent efficient. It is far better to have a longer antenna that gives a much higher radiation resistance even if it may not be the optimum length as far as radiation pattern is concerned. If you can't get the power to the antenna the radiation pattern doesn't much matter. You still won't get out very well. On a typical boat the radiation pattern is going to be far from ideal with whatever length antenna you have due to all the surrounding objects on the boat. The difference in radiation patterns between a 1/2 wavelength and 5/8 wavelength antennas are minimal. About the only real difference is the feed point impedance they present. As far as antennas greater in length than a quarter wavelength, they start to produce multiple lobes in the pattern. Which on a boat may not be a bad thing. As you mention, sometimes higher angles are desired depending on the distance trying to be covered. A longer antenna on a typical boat is most always going to be more efficient than a short antenna even if the longer antenna produces multiple pattern lobes. Regards Gary On Tue, 09 Nov 2004 00:29:53 GMT, Chuck wrote: Antennas are really a lot like boats: No boat will do everything well and no antenna will either. Boats and antennas that try to do everything usually fail across the board. FWIW, SGC-237, -230, and -231 tuners need 23 feet only to tune from 1.6 MHz to 3.3 MHz. Above 3.3 MHz, these SGC tuners require only eight (8) feet. The Icom AH-4, for example, needs 23 feet only to tune down to 3.5 MHz, but will tune from 7 MHz up with Icom's AH-2b whip (8.2 feet long). But it doesn't matter what lengths the tuners require if there is no desire to operate in that frequency range, and chances are excellent that recreational boaters will not be found at the very low frequencies. As has been pointed out, some antenna lengths will be more taxing for an autotuner than other lengths. Your objective is not to make life easier for your tuner, especially when doing so may move you farther from your real needs. You may not even need a tuner! Your objective is to achieve your communication goals. You might give some thought to posting on one of the cruising newsgroups to ask experienced cruisers for their thoughts on things like "if you had only one frequency to operate on, what would it be? Among other things, that might be the basis for an antenna you can stow for emergencies. But tell them where and how you'll be cruising and what you want the ssb for (email, emergencies, boat-to-boat communication, etc.) Then return to the antenna design questions. Keep to it! Chuck |
Well do I have egg on my face!
Gary, you are correct, of course, in stating that there is not a lot of difference between the vertical radiation patterns of half-wave and quarter-wave antennas. Surely not the differences I was alluding to. And so my statements to the contrary were just plain wrong. While I was writing half-wave, I was thinking of something longer, like 3/4 wave. I should have been more careful and I do apologize. My point, however, is just as valid. Many sailboats sport 45' backstay antennas and that is close to 3/4 wavelength in the 15 MHz range. A 3/4 wave antenna has maximum vertical radiation at 45 degrees! I would say a 16- or even an 8-foot whip would be very competitive with such a backstay antenna at the lower radiation angles needed for transoceanic communication. At higher marine frequencies, 3/4 wavelength is obviously even less than 45 feet. Of course, the 3/4 wave will be efficient and easy on the autotuner. I'll try to keep my brain in synch with my typing, henceforth. Chuck |
"Bruce in Alaska" wrote in message ... Is that the rigging that goes between the mizzenmast and the mainmast near their tops? Bruce in alaska -- add a 2 before @ Yes, thats the one |
On Wed, 10 Nov 2004 03:16:47 GMT, Chuck wrote:
Well do I have egg on my face! Gary, you are correct, of course, in stating that there is not a lot of difference between the vertical radiation patterns of half-wave and quarter-wave antennas. Surely not the differences I was alluding to. And so my statements to the contrary were just plain wrong. While I was writing half-wave, I was thinking of something longer, like 3/4 wave. I should have been more careful and I do apologize. My point, however, is just as valid. Many sailboats sport 45' backstay antennas and that is close to 3/4 wavelength in the 15 MHz range. A 3/4 wave antenna has maximum vertical radiation at 45 degrees! I would say a 16- or even an 8-foot whip would be very competitive with such a backstay antenna at the lower radiation angles needed for transoceanic communication. At higher marine frequencies, 3/4 wavelength is obviously even less than 45 feet. Of course, the 3/4 wave will be efficient and easy on the autotuner. I'll try to keep my brain in synch with my typing, henceforth. Chuck Hi Chuck, That 3/4 wavelength antenna pattern you are looking at I will bet is for a horizontal antenna 3/4 wave high. The pattern for a vertical antenna is different. Also when you see antenna patterns that show main lobe radiation angles you need to look closely at them to see how many db down the signal really is at the desired angle. It does not disappear entirely at any angle. Although there sharp notches in the pattern at times where the signal is highly attenuated it is rare that the signal is completely eliminated at that small angle. Also with longer antennas, multiple lobes are created rather than a single lobe as seen with a shorter antenna. Many times those multiple lobes can be a help in filling in angles that may be otherwise missed. Sometimes the nulls can work against you too. With a sloping antenna such as a backstay, while the radiation angle may be raised in one direction because of a long antenna it also is lowered in the opposite direction because of the higher angle lobe. On a boat you usually have little control of where the antenna goes and the angle at which it runs. The lowest radiation angle may not always be the best for the path you are trying to work either. For very long distances low angles are usually better but medium and shorter distances may be better with a little higher radiation angle. A note about the low frequencies: If you are working surface wave communications below 3 mhz a vertical antenna is essential. Only vertical polarization works in that mode and is very reliable night and day over the given range. Horizontally polarized signals cancel out and you get no surface wave with them. AM broadcast stations are an example of this type of propagation. Surface waves follow close to the earth on the low frequencies. On higher frequencies they are quickly attenuated. I am sure that Bruce can attest to the reliable communication on the low band. Doug's 23 foot whip may work very well on the higher bands as it is more vertical than a backstay and probably more in the clear. But it will not be a good performer on the lower bands. Another note on short antennas: That 23 foot whip that Doug uses is less than an 1/8 wavelength on 4 mhz. A quarter wavelength vertical has a radiation resistance of around 36 ohms. Shorten it to an 1/8 wavelength and the radiation resistance does not drop in half but goes down to around 6 ohms! That antenna radiation resistance is in series with the ground system resistance which is usually quite high. It may be in the order of 20 to 30 ohms in many cases. Guess where most of the power goes. Regards Gary |
Hello again, Gary. Thanks for your reply.
There are a few points still not settled. But first we need to separate the objective importance of radiation at various vertical angles from the objective reality of how much energy different antennas radiate at these angles. There seems to be a reluctance among some in the group to acknowledge that vertical radiation angles are important enough to warrant an influence on antenna design decisions. I'm going to leave that alone and just talk about how various antenna designs radiate. My first "exhibit" is figures 54, 59, and 60 from the ARRL Antenna Book (20th edition), Chapter 16. Vertical radiation patterns for a quarter-wave, transom-mounted whip are compared with those for a typical backstay antenna at 20 meters. Figure 60 shows the backstay at 15 meters but there is no corresponding quarter-wave whip figure for that frequency. Fortunately, however, we are on fairly solid ground by assuming the vertical pattern of a quarter-wave 15 meter whip will be quite similar to that of a quarter-wave 20 meter whip. Thus, we can compare figure 54 to figure 60. I believe that the ARRL patterns show the whip to be an unambiguously better low-angle radiator than the backstay. From these patterns, I can easily imagine situations in which the backstay would nonetheless be a better choice. I can just as easily imagine situations in which the quarter-wave whip would be a better choice. Just based on the vertical radiation patterns alone. But that's not all! The versatility of the backstay antenna at other frequencies and the attendant complication of a tuner could be compared to the simplicity of the whip, its physical independence from the mast, and its lack of need for a tuner. These are other considerations that might affect one's choice. Heck, they're not even mutually exclusive! Just measurably different. Regarding the alleged necessity of a vertical antenna for "surface wave type communications," please consider the US Marine Corps' take on this in their Antenna Handbook (MC RP 3-40.3C, page 4-40): "NVIS propagation is simply sky wave propagation that uses antennas with high-angle radiation and low operating frequencies. Just as the proper selection of antennas can increase the reliability of a long- range circuit, short-range communications also require proper antenna selection. NVIS propagation is one more weapon in the communicator’s arsenal. To communicate over the horizon to an amphibious ship on the move, or to a station 100 to 300 kilometers away, the operators should use NVIS propagation. The ship’s low take-off angle antenna is designed for medium and long-range communications. When the ship’s antenna is used, a skip zone is formed. This skip zone is the area between the maximum ground wave distance and the shortest sky wave distance where no communications are possible. Depending on operating frequencies, antennas, and propagation conditions, this skip zone can start at roughly 20 to 30 kilometers and extend out to several hundred kilometers, preventing communications with the desired station. NVIS propagation uses high take-off angle (60° to 90°) antennas to radiate the signal almost straight up. The signal is then reflected from the ionosphere and returns to Earth in a circular pattern all around the transmitter. Because of the near-vertical radiation angle, there is no skip zone. Communications are continuous out to several hundred kilometers from the transmitter. The nearly vertical angle of radiation also means that lower frequencies must be used. Generally, NVIS propagation uses frequencies up to 8 MHz." Sorry for the poor formatting. NVIS is what you get with a horizontal dipole on the deck of a non-metal hull that I had mentioned. I really doubt that you can get reliable daytime 3 MHz communication using 150 watt transmitters and antennas connected to 50 foot masts and at distances of hundreds of kilometers. But with NVIS, it is routine. The other point has to do with the vertical radiation pattern of a 3/4-wave vertical. You will agree, I believe, that the VERTICAL pattern of the 3/4-wave vertical over perfect ground is "one-half" of the HORIZONTAL pattern of a 1.5 wavelength dipole in free space. (Split the dipole with a plane perpendicular to the wire's axis and then rotate the plane through 90 degrees so the wire is vertical. You can throw away the image beneath the plane to make it look like the usual patterns.) It follows, then, that the lobe of the 3/4 wave antenna in the vertical plane will peak at 45 degrees. Of course, over real ground the pattern will be different. I doubt though that real ground will LOWER the vertical radiation pattern. In any case, my statement has nothing to do with the the height of a horizontal dipole above ground. As an "exhibit" on this point, I offer a meager quote from Low Band DXing (3rd edition), page 9-51: Note that going from a 1/4 wave vertical to a 1/2 wave vertical drops the radiation angle from 26 degrees to 21 degrees. More important, however, is that the 3-dB vertical beamwidth drops from 42 degrees to 29 degrees. Going to a 5/8 vertical drops the radiation angle to 15 degrees with a 3-dB beamwidth of only 23 degrees. But notice the high-angle lobe showing up with the 5/8 wave vertical. If we make the vertical still longer, the low-angle lobe will disappear and be replaced by a high-angle lobe. A 3/4 wave vertical has a radiation angle of 45 degrees. So the humble contribution I've been trying to make is that longer antennas are not always better than shorter ones. They are sometimes better and sometimes worse. But they are always different. Whether the difference is worth considering pretty much depends on the nature of the difference. Time to move on, I think. Regards, Chuck Gary Schafer wrote: On Wed, 10 Nov 2004 03:16:47 GMT, Chuck wrote: Well do I have egg on my face! Gary, you are correct, of course, in stating that there is not a lot of difference between the vertical radiation patterns of half-wave and quarter-wave antennas. Surely not the differences I was alluding to. And so my statements to the contrary were just plain wrong. While I was writing half-wave, I was thinking of something longer, like 3/4 wave. I should have been more careful and I do apologize. My point, however, is just as valid. Many sailboats sport 45' backstay antennas and that is close to 3/4 wavelength in the 15 MHz range. A 3/4 wave antenna has maximum vertical radiation at 45 degrees! I would say a 16- or even an 8-foot whip would be very competitive with such a backstay antenna at the lower radiation angles needed for transoceanic communication. At higher marine frequencies, 3/4 wavelength is obviously even less than 45 feet. Of course, the 3/4 wave will be efficient and easy on the autotuner. I'll try to keep my brain in synch with my typing, henceforth. Chuck Hi Chuck, That 3/4 wavelength antenna pattern you are looking at I will bet is for a horizontal antenna 3/4 wave high. The pattern for a vertical antenna is different. Also when you see antenna patterns that show main lobe radiation angles you need to look closely at them to see how many db down the signal really is at the desired angle. It does not disappear entirely at any angle. Although there sharp notches in the pattern at times where the signal is highly attenuated it is rare that the signal is completely eliminated at that small angle. Also with longer antennas, multiple lobes are created rather than a single lobe as seen with a shorter antenna. Many times those multiple lobes can be a help in filling in angles that may be otherwise missed. Sometimes the nulls can work against you too. With a sloping antenna such as a backstay, while the radiation angle may be raised in one direction because of a long antenna it also is lowered in the opposite direction because of the higher angle lobe. On a boat you usually have little control of where the antenna goes and the angle at which it runs. The lowest radiation angle may not always be the best for the path you are trying to work either. For very long distances low angles are usually better but medium and shorter distances may be better with a little higher radiation angle. A note about the low frequencies: If you are working surface wave communications below 3 mhz a vertical antenna is essential. Only vertical polarization works in that mode and is very reliable night and day over the given range. Horizontally polarized signals cancel out and you get no surface wave with them. AM broadcast stations are an example of this type of propagation. Surface waves follow close to the earth on the low frequencies. On higher frequencies they are quickly attenuated. I am sure that Bruce can attest to the reliable communication on the low band. Doug's 23 foot whip may work very well on the higher bands as it is more vertical than a backstay and probably more in the clear. But it will not be a good performer on the lower bands. Another note on short antennas: That 23 foot whip that Doug uses is less than an 1/8 wavelength on 4 mhz. A quarter wavelength vertical has a radiation resistance of around 36 ohms. Shorten it to an 1/8 wavelength and the radiation resistance does not drop in half but goes down to around 6 ohms! That antenna radiation resistance is in series with the ground system resistance which is usually quite high. It may be in the order of 20 to 30 ohms in many cases. Guess where most of the power goes. Regards Gary |
I believe that the ARRL patterns show the whip to be an unambiguously
better low-angle radiator than the backstay. This might explain why I have had such good luck with a whip compared to the backstay antenna I had on my previous boat. But that's not all! The versatility of the backstay antenna at other frequencies and the attendant complication of a tuner could be compared to the simplicity of the whip, its physical independence from the mast, and its lack of need for a tuner. How do you get away without a tuner? Doug s/v Callista |
On Wed, 10 Nov 2004 19:07:19 GMT, Chuck wrote:
Hello again, Gary. Thanks for your reply. There are a few points still not settled. But first we need to separate the objective importance of radiation at various vertical angles from the objective reality of how much energy different antennas radiate at these angles. There seems to be a reluctance among some in the group to acknowledge that vertical radiation angles are important enough to warrant an influence on antenna design decisions. I'm going to leave that alone and just talk about how various antenna designs radiate. They may be important but there is usually little you can do about it on a boat. My first "exhibit" is figures 54, 59, and 60 from the ARRL Antenna Book (20th edition), Chapter 16. Vertical radiation patterns for a quarter-wave, transom-mounted whip are compared with those for a typical backstay antenna at 20 meters. Figure 60 shows the backstay at 15 meters but there is no corresponding quarter-wave whip figure for that frequency. Fortunately, however, we are on fairly solid ground by assuming the vertical pattern of a quarter-wave 15 meter whip will be quite similar to that of a quarter-wave 20 meter whip. Thus, we can compare figure 54 to figure 60. I believe that the ARRL patterns show the whip to be an unambiguously better low-angle radiator than the backstay. From these patterns, I can easily imagine situations in which the backstay would nonetheless be a better choice. I can just as easily imagine situations in which the quarter-wave whip would be a better choice. Just based on the vertical radiation patterns alone. They only show the vertical angle at an azmuth of 90 degrees. Don't know if that is the best or worse direction for the vertical angle. If you look closely at those patterns you willl see that the backstay vertical pattern is much broader than the whip. That is an advantage when working various distances that require different take off angles. Very low angles are usually only good for very long haul communications. Want to talk to China or Japan? Shorter range, around the US, usually require higher angles to do the job. Talk to some of the hams that have stacked beams on tall towers. Often the lower antenna, with it's higher take off angle is superior to the higher antenna on shorter paths. Also remember that just because the maximum of the lobe may be at 30 degrees, dosn't mean that it is dead at 15 or even 10 degrees. It may only be down a couple of db at lower angles. But that's not all! The versatility of the backstay antenna at other frequencies and the attendant complication of a tuner could be compared to the simplicity of the whip, its physical independence from the mast, and its lack of need for a tuner. These are other considerations that might affect one's choice. Heck, they're not even mutually exclusive! Just measurably different. A whip with no tuner is good for only one frequency. And then it requires some sort of matching network to make it work. Might just as well put in a tuner and make use of it on other frequencies too. Regarding the alleged necessity of a vertical antenna for "surface wave type communications," please consider the US Marine Corps' take on this in their Antenna Handbook (MC RP 3-40.3C, page 4-40): "NVIS propagation is simply sky wave propagation that uses antennas with high-angle radiation and low operating frequencies. Just as the proper selection of antennas can increase the reliability of a long- range circuit, short-range communications also require proper antenna selection. NVIS propagation is one more weapon in the communicator’s arsenal. To communicate over the horizon to an amphibious ship on the move, or to a station 100 to 300 kilometers away, the operators should use NVIS propagation. The ship’s low take-off angle antenna is designed for medium and long-range communications. When the ship’s antenna is used, a skip zone is formed. This skip zone is the area between the maximum ground wave distance and the shortest sky wave distance where no communications are possible. Depending on operating frequencies, antennas, and propagation conditions, this skip zone can start at roughly 20 to 30 kilometers and extend out to several hundred kilometers, preventing communications with the desired station. NVIS propagation uses high take-off angle (60° to 90°) antennas to radiate the signal almost straight up. The signal is then reflected from the ionosphere and returns to Earth in a circular pattern all around the transmitter. Because of the near-vertical radiation angle, there is no skip zone. Communications are continuous out to several hundred kilometers from the transmitter. The nearly vertical angle of radiation also means that lower frequencies must be used. Generally, NVIS propagation uses frequencies up to 8 MHz." Sorry for the poor formatting. NVIS is what you get with a horizontal dipole on the deck of a non-metal hull that I had mentioned. I really doubt that you can get reliable daytime 3 MHz communication using 150 watt transmitters and antennas connected to 50 foot masts and at distances of hundreds of kilometers. But with NVIS, it is routine. The "surface wave" propagation that I was talking about, that requires vertical polarization, is not the same thing. NVIS is still dependent on ionosphere reflections and is at the mercey of the ionosphere. Daytime may kill the signal. With surface wave propagation it is there all the time, night or day. It is what broadcast stations depend on. It is very usefull on the 2 mhz marine band with proper antennas. The signal follows the surface of the earth rather than being reflected from the ionosphere. The other point has to do with the vertical radiation pattern of a 3/4-wave vertical. You will agree, I believe, that the VERTICAL pattern of the 3/4-wave vertical over perfect ground is "one-half" of the HORIZONTAL pattern of a 1.5 wavelength dipole in free space. (Split the dipole with a plane perpendicular to the wire's axis and then rotate the plane through 90 degrees so the wire is vertical. You can throw away the image beneath the plane to make it look like the usual patterns.) It follows, then, that the lobe of the 3/4 wave antenna in the vertical plane will peak at 45 degrees. Of course, over real ground the pattern will be different. I doubt though that real ground will LOWER the vertical radiation pattern. In any case, my statement has nothing to do with the the height of a horizontal dipole above ground. No, you can not just split the pattern of a horizontal antenna and rotate it to get a vertical pattern. In free space yes, on the ground no. The earth has a large effect on it. Reflections from the earth add and subtract to determine the pattern. If you look at the vertical patterns from a horizontal antenna at different heights above ground you will see drastic changes in the vertical pattern. There is not a lot of information printed on vertical radiators of different lengths. Folks often confuse the horizontal patterns with what a vertical pattern would be. As an "exhibit" on this point, I offer a meager quote from Low Band DXing (3rd edition), page 9-51: Note that going from a 1/4 wave vertical to a 1/2 wave vertical drops the radiation angle from 26 degrees to 21 degrees. More important, however, is that the 3-dB vertical beamwidth drops from 42 degrees to 29 degrees. Going to a 5/8 vertical drops the radiation angle to 15 degrees with a 3-dB beamwidth of only 23 degrees. But notice the high-angle lobe showing up with the 5/8 wave vertical. If we make the vertical still longer, the low-angle lobe will disappear and be replaced by a high-angle lobe. A 3/4 wave vertical has a radiation angle of 45 degrees. So the humble contribution I've been trying to make is that longer antennas are not always better than shorter ones. They are sometimes better and sometimes worse. But they are always different. Whether the difference is worth considering pretty much depends on the nature of the difference. I agree. Large ships usually have several different types of antennas. However, in most boat installations you usually only have one shot at it. One antenna is all there is room for. I would opt for as much wire as I could get up in that case. More wire will give much improved performance on the low bands with a moderate compromise on the high bands. Time to move on, I think. Regards, Chuck Regards Gary Gary Schafer wrote: On Wed, 10 Nov 2004 03:16:47 GMT, Chuck wrote: Well do I have egg on my face! Gary, you are correct, of course, in stating that there is not a lot of difference between the vertical radiation patterns of half-wave and quarter-wave antennas. Surely not the differences I was alluding to. And so my statements to the contrary were just plain wrong. While I was writing half-wave, I was thinking of something longer, like 3/4 wave. I should have been more careful and I do apologize. My point, however, is just as valid. Many sailboats sport 45' backstay antennas and that is close to 3/4 wavelength in the 15 MHz range. A 3/4 wave antenna has maximum vertical radiation at 45 degrees! I would say a 16- or even an 8-foot whip would be very competitive with such a backstay antenna at the lower radiation angles needed for transoceanic communication. At higher marine frequencies, 3/4 wavelength is obviously even less than 45 feet. Of course, the 3/4 wave will be efficient and easy on the autotuner. I'll try to keep my brain in synch with my typing, henceforth. Chuck Hi Chuck, That 3/4 wavelength antenna pattern you are looking at I will bet is for a horizontal antenna 3/4 wave high. The pattern for a vertical antenna is different. Also when you see antenna patterns that show main lobe radiation angles you need to look closely at them to see how many db down the signal really is at the desired angle. It does not disappear entirely at any angle. Although there sharp notches in the pattern at times where the signal is highly attenuated it is rare that the signal is completely eliminated at that small angle. Also with longer antennas, multiple lobes are created rather than a single lobe as seen with a shorter antenna. Many times those multiple lobes can be a help in filling in angles that may be otherwise missed. Sometimes the nulls can work against you too. With a sloping antenna such as a backstay, while the radiation angle may be raised in one direction because of a long antenna it also is lowered in the opposite direction because of the higher angle lobe. On a boat you usually have little control of where the antenna goes and the angle at which it runs. The lowest radiation angle may not always be the best for the path you are trying to work either. For very long distances low angles are usually better but medium and shorter distances may be better with a little higher radiation angle. A note about the low frequencies: If you are working surface wave communications below 3 mhz a vertical antenna is essential. Only vertical polarization works in that mode and is very reliable night and day over the given range. Horizontally polarized signals cancel out and you get no surface wave with them. AM broadcast stations are an example of this type of propagation. Surface waves follow close to the earth on the low frequencies. On higher frequencies they are quickly attenuated. I am sure that Bruce can attest to the reliable communication on the low band. Doug's 23 foot whip may work very well on the higher bands as it is more vertical than a backstay and probably more in the clear. But it will not be a good performer on the lower bands. Another note on short antennas: That 23 foot whip that Doug uses is less than an 1/8 wavelength on 4 mhz. A quarter wavelength vertical has a radiation resistance of around 36 ohms. Shorten it to an 1/8 wavelength and the radiation resistance does not drop in half but goes down to around 6 ohms! That antenna radiation resistance is in series with the ground system resistance which is usually quite high. It may be in the order of 20 to 30 ohms in many cases. Guess where most of the power goes. Regards Gary |
Doug, it's difficult to generalize but in many cases the radiation
resistance of a whip will be in the range of 20 to 35 ohms (assuming the whip is a quarter-wave or somewhat shorter) and the ground resistance in series with that may be another 25 ohms or so. What you get is a feedpoint impedance of about 45 to 60 ohms (could be more or less) which will match 50 ohm coax very nicely without a tuner. Most transmitters will feed loads of 25 to 100 ohms (2:1 swr) without complaining. For a short run of coax, your total losses will probably be less than if you used a tuner. It is true that you can only use such an antenna for a single marine or ham band. Even then, at the lower frequencies, you will experience a limited band of frequencies that you can use without a tuner. On 8 MHz and above, you will probably find that an antenna cut for the middle of the band will cover the whole band nicely. A lot of cruisers keep a 14 MHz "Hamstick" on board as an emergency antenna they can use if their tuner fails or if (heaven forbid) they are dis-masted and can't use their backstay antenna. In an emergency you can check in to the Maritime Mobile Service Net on 14.300 MHz even if you're not a ham. It is one of the few frequencies monitored almost continuously by experienced operators. The Hamstick is easy to store, easy to install, and once adjusted, should be trouble-free. To switch bands, you switch Hamsticks. They even make a quick-connect gizmo. There are other makes as well. But if you don't use something like a Hamstick, and just use a longer whip like a 16 foot whip, it will be good for just one band. Actually, 16 feet is close to a quarter-wave on 20 meters so you may be able to use it as-is without a tuner on that band. If you're not comfortable doing the hookup, find a local ham to advise you. They're usually glad to help. Good luck! Chuck Doug Dotson wrote: I believe that the ARRL patterns show the whip to be an unambiguously better low-angle radiator than the backstay. This might explain why I have had such good luck with a whip compared to the backstay antenna I had on my previous boat. But that's not all! The versatility of the backstay antenna at other frequencies and the attendant complication of a tuner could be compared to the simplicity of the whip, its physical independence from the mast, and its lack of need for a tuner. How do you get away without a tuner? Doug s/v Callista |
On Tue, 9 Nov 2004 20:39:06 -0500, "Doug Dotson"
wrote: A 50' or 75' whip would be a bit ungainly :) ================================ A lot of backstays on larger sailboats are that long. Definitely a problem on a powerboat however. |
A 50' to 75' whip on any boat would be a problem. A backstay
is not a whip. "Wayne.B" wrote in message ... On Tue, 9 Nov 2004 20:39:06 -0500, "Doug Dotson" wrote: A 50' or 75' whip would be a bit ungainly :) ================================ A lot of backstays on larger sailboats are that long. Definitely a problem on a powerboat however. |
Comments below.
"Chuck" wrote in message link.net... Doug, it's difficult to generalize but in many cases the radiation resistance of a whip will be in the range of 20 to 35 ohms (assuming the whip is a quarter-wave or somewhat shorter) and the ground resistance in series with that may be another 25 ohms or so. What you get is a feedpoint impedance of about 45 to 60 ohms (could be more or less) which will match 50 ohm coax very nicely without a tuner. Most transmitters will feed loads of 25 to 100 ohms (2:1 swr) without complaining. For a short run of coax, your total losses will probably be less than if you used a tuner. Agreed. It is true that you can only use such an antenna for a single marine or ham band. And as such is an inconvenient situation on a boat. Even then, at the lower frequencies, you will experience a limited band of frequencies that you can use without a tuner. On 8 MHz and above, you will probably find that an antenna cut for the middle of the band will cover the whole band nicely. SOP for single band antennas unless one is interested in only a subsection of the band in which case one cuts it for the center of the segment of interest. A lot of cruisers keep a 14 MHz "Hamstick" on board as an emergency antenna they can use if their tuner fails or if (heaven forbid) they are dis-masted and can't use their backstay antenna. I have Hamsticks for 80, 40, 20, and 15M. They were my only antennas for a long time before I insulated the backstay and got an SGC auto tuner. I still hang on to the Hamsticks as backups though. When I got another boat that already had a 23' whip, I found that it worked better than the backstay on my previous boat, so I have stuck with it. In an emergency you can check in to the Maritime Mobile Service Net on 14.300 MHz even if you're not a ham. It is one of the few frequencies monitored almost continuously by experienced operators. I have been checking into the MMSN for years now. Actually, the MMSN is only in operation during certain hours (1200 to 2000 Eastern time). Other nets are in operation during other hours (Coast Guard Net, InterCon, etc) In an emergency you can check into anything anywhere. The Hamstick is easy to store, easy to install, and once adjusted, should be trouble-free. To switch bands, you switch Hamsticks. They even make a quick-connect gizmo. Been using them for years. There are other makes as well. But if you don't use something like a Hamstick, and just use a longer whip like a 16 foot whip, it will be good for just one band. Actually, 16 feet is close to a quarter-wave on 20 meters so you may be able to use it as-is without a tuner on that band. If you're not comfortable doing the hookup, find a local ham to advise you. They're usually glad to help. I am a local ham :) I help myself alot :) Good luck! Thanks! Doug, k3qt s/v Callista |
Hey there, Doug,
I guess I took your question of How do you get away without a tuner? too literally! Obviously you've been doing it for years. 73, Chuck Doug Dotson wrote: Comments below. "Chuck" wrote in message link.net... Doug, it's difficult to generalize but in many cases the radiation resistance of a whip will be in the range of 20 to 35 ohms (assuming the whip is a quarter-wave or somewhat shorter) and the ground resistance in series with that may be another 25 ohms or so. What you get is a feedpoint impedance of about 45 to 60 ohms (could be more or less) which will match 50 ohm coax very nicely without a tuner. Most transmitters will feed loads of 25 to 100 ohms (2:1 swr) without complaining. For a short run of coax, your total losses will probably be less than if you used a tuner. Agreed. It is true that you can only use such an antenna for a single marine or ham band. And as such is an inconvenient situation on a boat. Even then, at the lower frequencies, you will experience a limited band of frequencies that you can use without a tuner. On 8 MHz and above, you will probably find that an antenna cut for the middle of the band will cover the whole band nicely. SOP for single band antennas unless one is interested in only a subsection of the band in which case one cuts it for the center of the segment of interest. A lot of cruisers keep a 14 MHz "Hamstick" on board as an emergency antenna they can use if their tuner fails or if (heaven forbid) they are dis-masted and can't use their backstay antenna. I have Hamsticks for 80, 40, 20, and 15M. They were my only antennas for a long time before I insulated the backstay and got an SGC auto tuner. I still hang on to the Hamsticks as backups though. When I got another boat that already had a 23' whip, I found that it worked better than the backstay on my previous boat, so I have stuck with it. In an emergency you can check in to the Maritime Mobile Service Net on 14.300 MHz even if you're not a ham. It is one of the few frequencies monitored almost continuously by experienced operators. I have been checking into the MMSN for years now. Actually, the MMSN is only in operation during certain hours (1200 to 2000 Eastern time). Other nets are in operation during other hours (Coast Guard Net, InterCon, etc) In an emergency you can check into anything anywhere. The Hamstick is easy to store, easy to install, and once adjusted, should be trouble-free. To switch bands, you switch Hamsticks. They even make a quick-connect gizmo. Been using them for years. There are other makes as well. But if you don't use something like a Hamstick, and just use a longer whip like a 16 foot whip, it will be good for just one band. Actually, 16 feet is close to a quarter-wave on 20 meters so you may be able to use it as-is without a tuner on that band. If you're not comfortable doing the hookup, find a local ham to advise you. They're usually glad to help. I am a local ham :) I help myself alot :) Good luck! Thanks! Doug, k3qt s/v Callista |
I supose that one could make a trap-vertical to avoid a tuner, but
I've never seen one for marine bands. I do recall a version of the OutBacker that was for marine use but have never known anyone that had one. Doug, k3qt s/v Callista "Chuck" wrote in message ink.net... Hey there, Doug, I guess I took your question of How do you get away without a tuner? too literally! Obviously you've been doing it for years. 73, Chuck Doug Dotson wrote: Comments below. "Chuck" wrote in message link.net... Doug, it's difficult to generalize but in many cases the radiation resistance of a whip will be in the range of 20 to 35 ohms (assuming the whip is a quarter-wave or somewhat shorter) and the ground resistance in series with that may be another 25 ohms or so. What you get is a feedpoint impedance of about 45 to 60 ohms (could be more or less) which will match 50 ohm coax very nicely without a tuner. Most transmitters will feed loads of 25 to 100 ohms (2:1 swr) without complaining. For a short run of coax, your total losses will probably be less than if you used a tuner. Agreed. It is true that you can only use such an antenna for a single marine or ham band. And as such is an inconvenient situation on a boat. Even then, at the lower frequencies, you will experience a limited band of frequencies that you can use without a tuner. On 8 MHz and above, you will probably find that an antenna cut for the middle of the band will cover the whole band nicely. SOP for single band antennas unless one is interested in only a subsection of the band in which case one cuts it for the center of the segment of interest. A lot of cruisers keep a 14 MHz "Hamstick" on board as an emergency antenna they can use if their tuner fails or if (heaven forbid) they are dis-masted and can't use their backstay antenna. I have Hamsticks for 80, 40, 20, and 15M. They were my only antennas for a long time before I insulated the backstay and got an SGC auto tuner. I still hang on to the Hamsticks as backups though. When I got another boat that already had a 23' whip, I found that it worked better than the backstay on my previous boat, so I have stuck with it. In an emergency you can check in to the Maritime Mobile Service Net on 14.300 MHz even if you're not a ham. It is one of the few frequencies monitored almost continuously by experienced operators. I have been checking into the MMSN for years now. Actually, the MMSN is only in operation during certain hours (1200 to 2000 Eastern time). Other nets are in operation during other hours (Coast Guard Net, InterCon, etc) In an emergency you can check into anything anywhere. The Hamstick is easy to store, easy to install, and once adjusted, should be trouble-free. To switch bands, you switch Hamsticks. They even make a quick-connect gizmo. Been using them for years. There are other makes as well. But if you don't use something like a Hamstick, and just use a longer whip like a 16 foot whip, it will be good for just one band. Actually, 16 feet is close to a quarter-wave on 20 meters so you may be able to use it as-is without a tuner on that band. If you're not comfortable doing the hookup, find a local ham to advise you. They're usually glad to help. I am a local ham :) I help myself alot :) Good luck! Thanks! Doug, k3qt s/v Callista |
On Wed, 10 Nov 2004 23:35:22 -0500, "Doug Dotson"
wrote: A 50' to 75' whip on any boat would be a problem. A backstay is not a whip. =================================== True but it functions the same way electrically. When I was a kid it was common to see continuously loaded whips with wire loosely wound around a tapered form, frequently a varnished bamboo pole. Is anyone making anything like that in fiberglass? |
It's hard to believe the marine version of the Outbacker was very
efficient, but it probably was better than nothing at all. Yeah, other than traps, the only other reliable technique I can think of for a multiband (as opposed to simply a "wideband") antenna is to use switched, tuned feeders (open wire line stubs actually, cut to tune a dipole to each band). I have a difficult time imagining that on a sailboat! Anyway, that stretches the idea of "no tuner", but at least there are no moving parts other than the band switch. Well, now that I think of it, there are some common antennas that work on more than one band without a tuner or traps. A simple 40 meter dipole (or quarter-wave vertical) ought to work on 15 meters, for example. And the G5RV, and some sky loop antennas are multiband. A vertical (cut the horizontal one in half) G5RV would be interesting to consider. I don't think I've encountered that before and matching might be interesting, but it may have potential for backstay antennas. These antennas usually benefit from a rig with a built-in tuner, so maybe they should be "disqualified" as not tuner-less. Chuck, NT3G s/v Sans Serif Doug Dotson wrote: I supose that one could make a trap-vertical to avoid a tuner, but I've never seen one for marine bands. I do recall a version of the OutBacker that was for marine use but have never known anyone that had one. Doug, k3qt s/v Callista |
I believe that is what my whip is. Although I'm not sure that it is
wound around the pole though. It is made by Shakepeare. Doug "Wayne.B" wrote in message ... On Wed, 10 Nov 2004 23:35:22 -0500, "Doug Dotson" wrote: A 50' to 75' whip on any boat would be a problem. A backstay is not a whip. =================================== True but it functions the same way electrically. When I was a kid it was common to see continuously loaded whips with wire loosely wound around a tapered form, frequently a varnished bamboo pole. Is anyone making anything like that in fiberglass? |
In article ,
"Doug Dotson" wrote: A 50' to 75' whip on any boat would be a problem. A backstay is not a whip. Doug, what I was talking about was a backstay that feed a whip side mounted to the top of the mast. that way a 35' backstay feeding a 28' whip would be 60+ft electrical antenna. Bruce in alaska -- add a 2 before @ |
In article ,
"Doug Dotson" wrote: I believe that is what my whip is. Although I'm not sure that it is wound around the pole though. It is made by Shakepeare. Doug Nope, Helical wound antennas exist but Shakespear doesn't make them. At least not the ones that I am familiar with. Bruce in alaska -- add a 2 before @ |
| All times are GMT +1. The time now is 04:02 AM. |
|
Powered by vBulletin® Copyright ©2000 - 2025, Jelsoft Enterprises Ltd.
Copyright ©2004 - 2014 BoatBanter.com