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Author Topic: Antenna for fiberglass boat  (Read 1807 times)
KB6WIJ
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Posts: 19




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« on: October 27, 2004, 04:40:19 PM »

Hi! I'd like to install a 2M radio in my fiberglass boat. What sort of antenna will work well without much of a ground plane?

Thanks and 73! Joe KB6WIJ
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KC8AXJ
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Posts: 303




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« Reply #1 on: October 27, 2004, 07:52:47 PM »

I found this:


http://campus.murraystate.edu/org/msuarc/lakeops.htm


73
Steve
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KC8AXJ
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Posts: 303




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« Reply #2 on: October 27, 2004, 08:08:37 PM »


http://www.radialllarsen.com/products_mobile_2.htm

MHW: Half-wave, no ground plane antenna for SO239 type mounts (144-174 MHz only)


I believe their 5/8 wave requires very LITTLE ground plane. I think you could put a couple of copper straps under the fiberglass.
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KB6WIJ
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Posts: 19




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« Reply #3 on: October 27, 2004, 09:32:19 PM »

I should have mentioned that this boat is only used in freshwater, so I don't need anything made to withstand offshore use.

I've also thought about lining the cuddy overhead with copper foil and putting upholstery over it. Does the ground plane have to be physically connected to the antenna ground?

What about on on-glass antenna? Would one work through fiberglass?

Thanks again!  Joe  KB6WIJ
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N3ZKP
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Posts: 2008




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« Reply #4 on: October 28, 2004, 12:47:52 PM »

Joe:

Forget the glass mount. They are pretty poor performers on glass; I can't imagine them being worth a darn on fiberglas.

I like the idea of lining the cabin roof with copper foil. Anything over about 6 to 8 square feet would work and yes, you need to connect the groundplane to the ground side of the antenna. Do the foil first, them drill the mounting hole through both the roof and the foil.

A very good and robust antenna is the Larsen NMO270B. It is   about as indestructable an antenna as you can find  and performs very well. Forget those 5-foot tall antennas from Comet and Diamond. They do not perform any better than the Larsen.

Lon
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WB6BYU
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Posts: 13028




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« Reply #5 on: November 04, 2004, 12:52:51 PM »

A j-pole or coaxial dipole wouldn't need a ground plane,
though they are more likely to be mounted on a mast than
on the roof of a cabin.

Do consider the effect of the low frequency wave bounces
on antenna behavior - they can set up a mechanical
oscillation if they happen to match the resonant
frequency of the antenna.  I'd probably stick with
a quarter wave whip, though up to 5/8 wave is probably
usable.  You can put a sheet of aluminum on the top of
the fibreglass for a ground plane, then cover it with
a layer of resin (or another layer of fiberglass to
get the color to match.)
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KB6WIJ
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Posts: 19




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« Reply #6 on: November 09, 2004, 11:22:28 AM »

WB6BYU said: "A j-pole or coaxial dipole wouldn't need a ground plane..." and "Do consider the effect of the low frequency wave bounces on antenna behavior - they can set up a mechanical oscillation if they happen to match the resonant frequency of the antenna."

I'd already considered the mechanical stresses, having lost a 2M loop antenna in exactly this fashion.

So... how about a twin-lead J-pole inside of a PVC pipe housing? I can envision a number of ways to mount it so that it could easily be stowed.I'd usually be using it at low power levels (10 watts or less), but the $64,000 question is how much power will a twin-lead J-pole handle in an emergency?

Thanks again!  Joe  KB6WIJ
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WB6BYU
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Posts: 13028




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« Reply #7 on: November 09, 2004, 01:50:17 PM »

There have been a number of articles about mounting
a twinlead J-pole inside PVC pipe.  The main thing to
remember is that the PVC will affect the velocity factor
of the matching section.  So test it inside the pipe
when you are tuning it up (slide it out, adjust the
feedpoint tap and shorting bar, slide it back in, check
the SWR, etc.)  And some foam inside to hold the antenna
centered in the pipe will help, too, since having the
antenna flat against one side of the pipe will change
the tuning from having it centered.  (Hint:  look for
the foam covers for insulating pipes and see if you
can get a size that will fit inside your PVC pipe.
Otherwise a lenght of weatherstripping for around doors
could be added to each side of the antenna to help keep
it centered.)

How much power?  Certainly 50 watts, probably a lot more
if you make sure the notch at the top of the matching
section is large enough and there are no sharp wire
ends to arc over.  You might want to make it from the
window line instead of 300 ohm ribbon if this is of
concern.  How much power are you planning to be able to
run in an emergency???
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KB6WIJ
Member

Posts: 19




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« Reply #8 on: November 10, 2004, 08:17:16 AM »

WB6BYU said: "How much power are you planning to be able to run in an emergency?"

Maybe 50 or 60 watts... whatever the radio will produce on high power. I'm not planning to install an amplifier!

I found plans for a dual-band J-pole in February 2003 issue of QST, and I think that will be my starting point. It'll be relatively rugged, easy to mount, and inexpensive to replace when it falls overboard.  Smiley

Thanks again, and 73!  Joe  KB6WIJ
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N4ZOU
Member

Posts: 340




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« Reply #9 on: November 19, 2004, 12:29:52 PM »

A Radio Shack 102 inch stainless steel whip provides the material required to make a J-pole except the required spacer to mount the 1/4 wave section to the 3/4 wave section plus a little for the 3/8 thread mount. An old ARRL antenna handbook details building one. Just leave off the top section. I used it on a Bass Boat for many years and still use it mobile on my truck.
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KC2MMI
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Posts: 621




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« Reply #10 on: November 21, 2004, 09:10:39 PM »

A marine VHF is pretty much sitting closer to two meters than a ham 2m radio is, in the range of 157-162MHz. You might find that a good marine 2m antenna, already designed to be (salt?) water resistant, is your best best. And, for 2m or marine VHF, the higher your antenna is the better your range will be. So a marine VHF, preferably mounted up on a "stick" on an aft corner, is probably going to be your simplest solution. I'd set one up for marine VHF first and then check the SWR on the ham radio, I suspect it will be SO close that you won't want anything more. If you want to be able to use both the marine and ham radios at the same time, twin whips on the quarters are the best way to go. With a powerboat, expecting it to be flat most of the time (compared to a keel boat), you can go for the highest gain you can find, i.e. a flat radiation pattern, without worry.

In terms of counterpoise or ground...the marine whips usually have a counterpoise built in, but taking the radio power/antenna to a commonly bonded ground plate under the boat is probably your most effective bang for the buck. (Bearing in mind there are always issues of galvanics, bonding versus lightning grounding, etc.)

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K8KAS
Member

Posts: 569




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« Reply #11 on: November 22, 2004, 11:58:20 AM »

I used a J-Pole for 5 years on my 27 foot trawler and it was the best of anything I ever tried. The antenna was made out of copper tube and then sprayed with clear Krylon paint. I think I had $10 or 15 bucks in the thing. 50 watts and I could cover a good size section of western Lake Erie, I could always work fixed stations 30 to 50 miles from my boats location on 2 FM.
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