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  #71  
Old 03-17-2008
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There are no empirical data that I'm aware of that attempt to correlate the efficiency of grounding devices...keel, engine, through-hulls, coppercoating or Dynaplate, etc to the depth they happen to be at. Engines and (sailboat) keels are usually way deeper than where one would find a Dynaplate.(s). It's RF coupling to the water that's critical.
Howard Keiper
Sea Quest
Berkeley
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  #72  
Old 03-17-2008
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Howard,

It's a deep and complex subject (no pun intended).

And, as you say, empirical data are hard to come by. But, evidence-based experience isn't. There's tons of that around. And, the science is pretty good re: attenuation of RF signals in various mediums, including salt water.

Here are a few "facts", followed by some experience-based opinion.

Facts

1. Attenuation of RF signals in seawater is acute...on the order of 70-80db per foot at the HF frequencies used for marine SSB.

2. Therefore, you cannot communicate from one station to another by sending RF signals through seawater.

3. RF energy pumped into seawater at depths greater than a foot or so is almost entirely consumed by heating the surrounding water.

4. There's little if any basis in fact to support the notion of "100 square feet of copper" to form a good RF ground, however much it's been touted by "authoritative" sources including engineers, radio consultants, authors, and manufacturers. Even Gordon West now says you don't need that; he recommends just running a wide strap to the nearest bronze thru-hull and forget it. And Stan Honey says much the same thing about RF grounds in his piece for West Marine.

5. You don't necessarily need to couple an antenna system on a boat to the seawater in order to have an efficient radiator of RF energy. If you don't believe this, consider the lowly dipole...especially the vertical dipole. You will look long and hard to find a more effective antenna, and it works well with NO separate RF ground, NO connection to seawater, NO connection to the hull or tanks or engine or keel, etc.

6. There are several ways to create an effective RF ground on a boat. Among these are the traditional "coupling to seawater" route...with it's many variants...and, just as effective, the "pseudo-ground", e.g., radials...tuned or not.

Opinions

1. It may well be that the apparent efficiency of dynaplates, engine connections, tanks, keel, etc. comes not from the contact with water or even necessarily from the proximity and capacitative coupling to water, but rather from the copper strips and heavy wires used in such connections actually acting as radials.

2. The "seawater as the perfect ground" statement is often misinterpreted and/or misrepresented. In fact, the reflective quality of the surface of seawater is what's most important, to serve as a springboard for the RF emissions as they take off for the ionosphere and "hop" to their destination.

3. Mobile installations aboard aircraft, in vehicles, and fixed installations ashore can be very efficient without bonding to seawater. There are lots of ways to construct a suitable counterpoise.

4. The best antenna system is one which works well on your boat, and may take a variety of forms depending on the boat, the rigging and equipment, and other factors.

Bill

Last edited by btrayfors; 03-17-2008 at 10:06 PM.
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Bill
I don't have the in depth (ha ha) of knowledge or experience that you do. What experience I do have is largely gleaned from others. I install this stuff... according to the recipes derived from their collective experiences. I think I said in an earlier response to something or other in this thread that the installations I do have to produce not just good results some of the time, but predictably good, measurable results all the time. I was a copper strap guy for ages...and still could be swayed, having done it that way for a long time, if I were sure that the end justified the means. As an independent contractor...a technical gypsy, so to say, (and a good one, I might add), mistakes, in the form of poor or disappointing performance come back to haunt me.
I accept all and everything you say about most everything you say. Given my 'druthers however, left to my own devices, the skipper who's got to have his boat ready and certified for The Pac Cup in a couple of months will on doubt get the Keiper special...the subtleties of a discussion such as this forgotten before he gets to the Gate. That is, of course assuming that the performance is at least equal to that of a first-rate strapping job...radials...ok.
Howard
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Please overlook the spelling errors.
Howard
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Old 03-18-2008
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It is indeed a complex subject..

Since the Marine HF SSB frequency band is so wide (anywhere between 2182kHz and 12365kHz for typical voice calls) all of the marine installations I have ever seen are compromises limited mainly by the length of the backstay.

Certainly, a good antenna tuner can help but you can't get away from the fact that the antenna will be optimal ("tuned") around one particular frequency (usually a distress frequency) and less than optimal on the others. This is made worse when you consider that marine SSB transmissions are either vertically- or horizontally-polarised and the backstay isn't either!

Quote:
Originally Posted by btrayfors View Post
5. You don't necessarily need to couple an antenna system on a boat to the seawater in order to have an efficient radiator of RF energy. If you don't believe this, consider the lowly dipole...especially the vertical dipole. You will look long and hard to find a more effective antenna, and it works well with NO separate RF ground, NO connection to seawater, NO connection to the hull or tanks or engine or keel, etc.
True, you could build a vertical dipole - but it would need to be blooming huge to work properly on 2182.. bigger than most people's yachts..

..but that doesn't mean it will necessarily work better than a properly-tuned SSB antenna up the backstay that uses the entire boat as the ground.
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Last edited by Classic30; 03-18-2008 at 02:00 AM.
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Howard,

I'm with you, guy! If you want to install something that works reasonably well and is very simple, just follow Gordon West's advice and connect a copper ground strap from the tuner ground lug to the nearest bronze thru hull. No question, this is a simple, straightforward way to go. It will produce acceptable results, and most any owner or installer can do it without screwing up too badly.

But for better results, you gotta build a better RF ground, IMHO.

Hartley18,

Yes, a dipole for 2mHz would be a pretty big beastie...suitable only for large ships! But if I were there in Aussie land and wanted to contact someone in North America or Europe, say, I'd build a vertical dipole for 20 meters. That's only 33' long. I keep one mounted on my boat all the time.

And, sorry, you can't compare the performance of a backstay antenna....any backstay antenna with any conceivable/practical RF ground....with that of a vertical dipole properly tuned and rigged. The dipole will outperform the backstay most every time. The reason has to do with vertical angle of radiation....a vertical dipole puts most of its energy out very near to the horizon, which is exactly where you want it for DX (long distance) communication. Radiation from a backstay antenna will be at higher angles, depending on the frequency.

On the other hand, when you need to transmit on lower bands you can't build a practical dipole...they're too long, traps are problematic, etc., etc.

That's why I believe that the traditional backstay antenna -- or an 'alternative backstay' equivalent -- belongs on all boats with ham or SSB communications aboard. It's flexibility is unparalleled, even if its performance isn't :-)

Bill
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Quote:
Originally Posted by btrayfors View Post
Yes, a dipole for 2mHz would be a pretty big beastie...suitable only for large ships! But if I were there in Aussie land and wanted to contact someone in North America or Europe, say, I'd build a vertical dipole for 20 meters. That's only 33' long. I keep one mounted on my boat all the time.

And, sorry, you can't compare the performance of a backstay antenna....any backstay antenna with any conceivable/practical RF ground....with that of a vertical dipole properly tuned and rigged. The dipole will outperform the backstay most every time. The reason has to do with vertical angle of radiation....a vertical dipole puts most of its energy out very near to the horizon, which is exactly where you want it for DX (long distance) communication. Radiation from a backstay antenna will be at higher angles, depending on the frequency.

On the other hand, when you need to transmit on lower bands you can't build a practical dipole...they're too long, traps are problematic, etc., etc.
Bill, I was talking about practical installations and used the same reasoning you use above. On marine SSB bands a dipole antenna might be possible on shore, but is not practical on a typical yacht simply because it would be too big.. so big even ships can't use them! On a higher band it's a different story.

BTW, I'm not convinced that the angle of polarisation has as much of an effect as physical size (ie. gain) and suspect that a large, properly grounded, backstay antenna on a large yacht would have a greater gain than a 1/4-wave dipole, offsetting any losses due to backstay angle... but I'm too lazy to do the calcs!!

Quote:
Originally Posted by btrayfors View Post
That's why I believe that the traditional backstay antenna -- or an 'alternative backstay' equivalent -- belongs on all boats with ham or SSB communications aboard. It's flexibility is unparalleled, even if its performance isn't :-)
Agreed 100%.
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Last edited by Classic30; 03-18-2008 at 06:08 PM.
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Cameron,

You don't need to "do the calcs". The exceptional performance of 1/2-wave (not 1/4-wave) vertical dipoles when rigged near the ground or water has been documented in lots of places, including the ARRL antenna handbook. It is the choice of well-funded DX-peditions which can afford any kind of equipment; some have documented "gain" as high as 18-20db due to the very low angle of radiation and consequent number of reduced hops.

On my boat, I've been using both a well-installed backstay antenna with a first-class antenna coupler, and vertical dipoles on the foredeck for many years. Have been using and writing about the properties of vertical dipoles for over 30 years. Those who have paid attention, and have bothered not only to "do the research" but to actually build and try one out have found -- as I and many others have found -- that they are dynamite antennas. On bands for which you can physically carry them, they outperform the backstay antenna hands-down.

Re: size, obviously how large a dipole you can erect depends on the height of your mast. On my 42' sloop, I am able to comfortably erect vertical dipoles for the 8mHz marine band and all bands at higher frequencies: 10, 14, 21 mHz ham bands and 12, 16, 22 mHz marine bands. In a week or so, I intend to try out an idea for rigging a vertical dipole for 40 meters with a bent back portion of the lower leg. I hope to be able to compare it on-the-air with my backstay antenna on 40-meter nets.

Bill
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Old 03-18-2008
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Quote:
Originally Posted by btrayfors View Post
Re: size, obviously how large a dipole you can erect depends on the height of your mast. On my 42' sloop, I am able to comfortably erect vertical dipoles for the 8mHz marine band and all bands at higher frequencies: 10, 14, 21 mHz ham bands and 12, 16, 22 mHz marine bands.
So Bill, 42' sloop but you didn't tell us the height of your mast!

Also, some questions: why do you use vinyl-coated SS lifeline wire rather than plain uncoated SS wire to make up your marine dipoles? Is there anything special about the wire diameter (e.g. 1/8" or perhaps even smaller?) or construction (1x19, 7x7, etc?)? Could I use the same kind of wire as above for making an "alternate backstay" antenna? Would this just be a length of wire with an eye nicro-pressed onto each end, suspended from rope leaders (acting as insulators), with some sort of tensioning (e.g spare halyard or topping lift), with the GTO-15 from the antenna tuner attached to the lower nicro-press?

Thanks,

Tim
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Reluctantly, I write this only to lend some objectivity to the discussion - i say reluctantly as the last time I tried, it was not well received...

A vertical dipole has been around for ever and is nothing special in terms of it's radiation effectiveness or it's practical use aboard a boat. Virtually every ham contester, as an example of people who have tried every conceivable form of antenna to gain an advantage, much perfer either sloper antennas or phased quater wave verticals. As the latter is impractical on a boat, the conventional wisdom is the sloper fed against the mast represents to all around better compromise. The major problem wuth a vertical dipole is it's design requirement that it be fed in the middle such that the feed coax is parallel to the antenna thereby inducing both pattern distrubance and radiation analomies on the feed.

This is largely an academic exercise since all antennas practical for a boat are a compromise of some sort and it's not my intent or desire to argue academic issues; only to point out that one not need to get too wrapped up in any one design as they all work fairly well if fed properly.
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