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Old 12-01-2007, 01:59   #1
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GAM Backstay Anetenna ?

Can anyone please provide advice on these guys, who claim to have a clip on backstay SSB Antenna that works? I've never seen these in Europe - and don't seem to be able to get a response from this firm despite one email and a phone message.

Gam Antennas

I don't really want to fit transom mounted antenna for a new DSC SSB set - but equally don't wish to risk integrity (or costs) be putting in backstay insulators.

So if this antenna works - and they are still in business - it might be an alternative. Any comments also really appreciated.
JOHN
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Old 12-01-2007, 06:40   #2
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Snipped from a HAM web site:
The official formula for a half wave antenna in free space is 492/f where f is in Megahertz (MHz) and the length is in feet. For a practical 1/2 wavelength dipole length is feet use general formula of 468/f where f is in megahertz (MHz). A half wave antenna length (in inches) in free space is calculated by the official 5904/f where f is in megahertz (MHz) and the calculated length is in inches. If capacitive loading, end effects, etc. are taken into consideration this formula would then become 5616/f where f is in Megahertz (MHz). These formulas will work fairly well for a dipole mounted at least 1/4 wavelength above ground. Some tweaking might be in order due to capacitive loading from nearby objects. End effect may also change resonance requiring slight adjustment in length. It is best to cut a dipole a few percent longer then the calculated length then prune it for resonance. Starting longer is a LOT better then starting at the calculated length or shorter! It is easy to trim but a bit harder to stretch. Using the official free space formula is likely to be way to long. Proximity to ground, trees, homes, even the feedline greatly influence resonance so make sure you check it. The nominal feedpoint impedance of a dipole is generally 72 ohms but this can vary widely. The lower to ground it is, the lower the impenitence.
For a 1/4 wave vertical the above numbers would be half. Therefore, the height of a simple 1/4 wave vertical can be calculated by 234/f where f is in Megahertz (MHz). Since ground (including radials) act as the other half of the antenna a vertical is really a half wave antenna with a vertical polarization.

You can build your own version of this antenna very easily. I would build a test version and see how it tunes up prior to purchasing theirs. I am a little concerned about capacitive coupling between the antenna and the back stay, but you would have to test and see.

Use the above formula to determine the lenght of your verticle, and cut it a little long as they recommend so you can trim to lenght. Use some solid core copper wire and any type of clip as an offset. You could even use clothes pins to test with. Push the verticle up the backstay. The verticle attaches to the center of your coax, and the shield goes to ground. Hook up your antenna tuner and swr meter and see how it tunes up.

They prob. have worked out he length with some practice, and found a professional looking mounting system, but this will give you and idea how well it will work.
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Old 12-01-2007, 11:52   #3
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Swagman and Doghouse,
This subject brings up so many questions about my own application. I am such a virgin when it comes to radio and HAM especially that I don't even know what the terms are that you are talking about. I'm working on my general license and, unfortunately, spent a lot of time on code but have never been on the net and don't even have a radio yet.
Could you approximate how long an antennae needs to be in feet or do I need to know megahertz before I even do that? I have dual backstays. Could I run an insulated wire antennae between the two backstays?
Sorry to be so basic and if you don't feel like helping someone who knows nothing its really ok. I can start doing more reading and homework.
Thanks,
JohnL
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Old 12-01-2007, 13:13   #4
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Swagman,

There are various positive and negative views re: the Gam antenna you asked about. Gordon West gave it a good review. Others, me included, remain sceptical.

HOWEVER, there's a good alternative. I agree with you completely about not disrupting the integrity of the backstay. Why anyone would wish to take a perfectly good backstay and add 4-6 potential points of failure is beyond me.

Instead, simply rig an "alternate backstay", using s/s lifeline. I've had one on my boat for 17 years, and it works every bit as well as the traditional insulated backstay. Take a length of s/s lifeline about 40' long, put small loops in each end using Nicopress sleeves, hoist one end on a spare halyard, and tie the lower end to the pushpit on either side of the boat using a short length of Dacron line. Put the automatic tuner under the deck near the antenna, feed the antenna with GTO-15 wire, and you've got a robust antenna which will last a very long time in the marine environment.

You can see my installation here:

Gallery :: Born Free 9/26/06 :: SBF062606

The antenna is on the starboard side. You can see the black insulator, and the standoffs I built from teak blocks which are affixed to the pushpit.

You do, of course, need a good RF ground...just as you would with a traditional backstay antenna.

RF grounding is a subject which evokes a lot of emotion and invective. Some folks believe that you need a football-field-sized copper groundplane underneath the antenna. Others, like Gordon West, say it's only necessary to run a wide copper strip from the tuner to the nearest thru hull.

Still others, like me and a legion of savvy hams, say that neither is necessary. The aluminum toe rail, if you have one, can serve as a perfectly good RF ground. So, too, can the pushpit/lifelines/pulpit complex. And, failing the availability or the practicality of those, you can use multiple radials, laid inside the hull below the deck. These will work very well, and will avoid the undesirable situation of "ground loops" which can occur if you attempt to tie your RF ground to the boat's DC ground, lightning ground, keelbolts, etc.

Whatever your philosophy re: RF grounds, it's best to conduct a few tests on your boat to see what works best for you. Each boat is different, so there's no "one size fits all" solution -- unless, that is, you're satisfied to put up with the mediocre performance one can expect from the more traditional RF grounding solutions (bond to everything, at least 100 square feet, lots of wide copper straps, rf grounding plates under the boat, keelbolts, metal tanks, engine, etc., etc., ad nauseum).

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Old 12-01-2007, 13:31   #5
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Aloha Bill,
I, for one, definitely like your antennae idea and will do that. How did you come up with "about 40' long?" Just curious. If it works that's great but how about all the formulas that Doghouse provided?
JohnL
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Old 12-01-2007, 13:49   #6
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Hi, John...

Welcome to the "simple and sane alternate backstay" crowd!

The 40' is an approximation. As Greg has mentioned on this and other Boards, the length of a backstay antenna must be considered in relation to the desired bands of operation.

Since I'm a ham, and I carry very efficient vertical dipole antennas for 15m, 20m, and 30m, I'm mostly interested in a backstay antenna which can deliver the goods below 13 MHz (so as to cover the 12MHz marine band and both ham and marine bands below 10 MHz).

IMO, you can do all the figurin' and calculatin' and modelin' you want, but in the end you're left will some experimentin' :-)) Especially since the electrical length of the antenna is a function of the antenna length, the length of the GTO-15 wire to the tuner, the velocity factor of the materials used, and god knows what else.

I'd say, if you want to use the backstay antenna for all bands, start with about 40-45' overall, including the GTO-15 wire, and see if your tuner can handle the tuneup on all bands easily. If you have trouble with one or more bands, lop off a foot or so and try again. In the end you want to wind up with a system which will tune easily on all desired bands.

I know that Greg is gonna cringe at this method, as his models will show all manner of effect on radiation patterns. But, in practice, I don't really believe these distortions make that much difference across-the-board. Especially since there are a lot of other factors which intervene, like rigging, proximity to other vessels, salt vs. fresh or brackish water, propagation conditions, etc.

How come I can be so cavalier about this? Because I KNOW, from many years of use and testing and comparisons with hundreds of boats and thousands of QSOs, that I have extremely efficient vertical dipoles for the most-used bands. And, the alternate backstay antenna works reasonably well on ALL bands, from 2-30 MHz.

Best,

Bill
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Old 12-01-2007, 14:14   #7
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As I stated, with the forumulas, leave them long and tune it up. Bill has explained a lot of the other things involved, and the fact that you will be having to tune your rig to the antenna so just use the rough figures. That is why I am saying experiment cheeply then go with the good stuff. You can always experiment with pulling up the antenna with the topping lift just to see how it will tune up before you play with putting it on your backstay.
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Old 12-01-2007, 14:22   #8
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d = Delay in nanoseconds
L = Length of the cable in feet
VOP = dielectric constant expressed in percent
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Old 12-01-2007, 14:31   #9
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Doghouse,


"That is why I am saying experiment cheeply then go with the good stuff."

Yes, thanks for that. I forgot to mention that it's a very good idea to experiment with the quick and cheap stuff before building a permanent antenna.

Same is true of the ground system.

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Old 12-01-2007, 15:27   #10
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Thanks Bill, Greg, Gord. I don't know what the letters in the formula are that you sent Gord but thanks anyway. Can I find those in my radio books? Currently I'm on a beer break from aligning my engine (for the second time) so can't reach them at this very moment.
I feel like such a newbie. Been sailing since '72 and always talked about doing the radio thing but never have. You guys are a big help.
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Old 12-01-2007, 15:37   #11
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The letters in the formula are:
d, l, and VOP
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Old 13-01-2007, 11:20   #12
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Hi Guys,
Appreciate all the advice, although I must confess much of what has been 'explained' went right over the top of my head.

Guess it will all come with time.

Btrayfors - love your idea of the independently rigged antenna and suspect I'll try to replicate this - but please - some more guidance is needed.

I don't have any form of metal toerail - and have simply followed advice in the ICOM installation and planned to fit a Dynaform plate to the hull. This has already been purchased but not fitted as yet.

Please treat me as as an idiot and explain if the ground wave issues you are talking about are something I can forget if I link back to this Dynaform plate - or if you are talking about another issue I am ignorant of!

Sorry to sound so dim witted - but in matters of radio installation - that's exactly what I am!

Cheers
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Old 13-01-2007, 12:42   #13
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John,

You don't sound dimwitted at all. IMO, the real dimwits are those who don't know what they don't know :-)

Glad you like the alternate backstay idea. I can tell you that it works extremely well and, if you build it from s/s lifeline, it is very robust in a hostile marine environment.

Re: the RF grounding system, as I noted, there's lots of controversy. The simplest solution of all is probably Gordon West's idea of running a SHORT wide copper strap from the tuner's ground lug to the nearest bronze thru-hull. Indeed, this will work. It just isn't the most efficient solution in terms of the transmitted signal strength.

I've done some long posts on various rf grounding systems. You can see one here:

SSCA Discussion Board :: View topic - RF GROUNDS IN THE MARINE ENVIRONMENT

This explains in simple terms what you're actually trying to do in constructing an RF ground, and the different types of RF grounding systems.

On my boat, the aluminum toerails are enough. The lifeline/pushpit/pulpit complex works well, too. In your case, with no metal toerail, you might be able to use the pushpit/lifelines/pulpit complex -- assuming they're all s/s and all tied together -- or you might be better off using radials.

If, after reading the above-cited post, you have further questions I'd be happy to try to answer them.

Keep in mind that all this stuff about RF grounding is an admixture of science, experience, voodoo, and luck. Don't be afraid to dive right in with your own thoughts and intuitions, since there are lots of cockeyed ideas floating about, and you know your boat and situation better than others.

Bill
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Old 13-01-2007, 13:20   #14
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Aloha Bill,

This is yours: "Still others, like me and a legion of savvy hams, say that neither is necessary. The aluminum toe rail, if you have one, can serve as a perfectly good RF ground. So, too, can the pushpit/lifelines/pulpit complex. And, failing the availability or the practicality of those, you can use multiple radials, laid inside the hull below the deck. These will work very well, and will avoid the undesirable situation of "ground loops" which can occur if you attempt to tie your RF ground to the boat's DC ground, lightning ground, keelbolts, etc."

This sounds good for me in that I am installing aluminum toerail during my refit. Do you tie the whole port and starboard toerail together? Any chance of shock or burn while transmitting if a bare body is on the toerail? Please excuse my simple questions but I've got to start simple before I understand complex.

Thanks again, JohnL

P. S. Easy to reply to the two newbies since both our names are John
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Old 13-01-2007, 13:41   #15
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Hawaii John:

Yes, tie the toe rails together. Usually, it's easy to do this at the bow. It wouldn't be a bad idea to do it at the stern, too, if that's easy for you (like if you have a double-ender or can easily run a copper strap or heavy wire across the stern.

Also, you don't have to wait for the toe rail installation to try this out. If you like, just stretch a long length of insulated wire...any kind...all round the deck where the toerails are to go, hook it to your tuner's ground lug, and give it a try! The toerails will be better, but this would give you an idea of how it will work.

Yes, in theory it's possible to get an RF burn by touching any part of the antenna or ground system while actually transmitting. The chances are not great on a small yacht, though, since you can control who's where when you transmit. I've been an active ham for 40 years, with installations on numerous sailboats beginning in 1969 and haven't ever had an RF burn. Nor have my family or crew members.

The small but real potential for RF burns is the likely reason why most "authorities" specify what are really DC grounds (to the seawater) and antenna feedlines of heavily insulated GTO-15 wire attached to the antenna way above the deck level. These reduce the chance of getting an RF burn to a minimum.

To me, though, it's a little like asking, "can't that knife in the drawer cut off your finger?". Well, yes it can, but I'm gonna be a bit careful when I use it to avoid such an outcome :-)

Bill
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