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Author Topic: Sailboat -- backstay & counterpoise ?  (Read 1526 times)
VA7CPC
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Posts: 2831




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« on: May 14, 2004, 10:59:45 AM »

 I'm installing an IC-706 and SG-230 on a Morgan Out Island 36 -- a center-cockpit sailboat.  The backstay falls at the aft end of the aft cabin.    Most of my operating will be on 20 and 40 meters, with occasional forays down to 80m.

I'm considering a "pure counterpoise" antenna:
   (a) Insulate a section of the bottom of the backstay
    about 15' long,  and put the SG-230 close to its base;

   (b) Put a fan of counterpoise wires (16 of them,
   14 gauge) (two are 15', 2" copper foil) on the  
   _ceiling of the aft cabin_, radiating from the SG-230;

    (c) mount the transceiver in the forward cabin, about
       20' forward from the tuner;
 
    (c) Use Radioworks "line isolators"
      (common-mode chokes) at the
        tuner _and_ at the transceiver,
      to avoid common-mode RF getting back
      to the transceiver.
         
Since the aft cabin is roughly rectangular, the lengths of the counterpoise wires will range from 10 - 15 feet.

I _think_ this will give me decent radiation (the antenna and radials are both about 1/8 wave on 40m).   But it doesn't have any common "RF Ground" between the tuner and the transceiver, and it doesn't have foil in the bilge.  

Some of my friends are predicting disastrous RF feedback and RFI problems, along with weak signals.

Any comments or suggestions?    Thanks.
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K0BG
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« Reply #1 on: May 15, 2004, 09:27:48 AM »

If you do things right, you won't need any isolaters. I have a good friend with a 48 foot cat. I installed almost the exact setup, but I used an SG235.

The "Y" backstay was insulated by the boat manufacturer just for the express use as an antenna. The SG235 is mounted just below the port bottom attachment point. The ground side of the 235 is attached to a keel grounding plate about 12 feet away with a 2" wide monel braid. The 235 easily matches the backstay from 80 through 6 meters, although it is not rated for 6 meters.

Alan, KØBG
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VA7CPC
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« Reply #2 on: May 15, 2004, 09:46:41 PM »

Alan --

Do you remember, roughly, how long the insulated section of backstay was?

Thanks.
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KC2MMI
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« Reply #3 on: May 17, 2004, 04:46:26 PM »

Charlie, are you the same Charlie I've spoken with some time ago, out of Vancouver way?

I can't think of why anyone would want to use couterpoise wires on a boat, when you've got such a perfect "earth" ground right under the keel. Foils in the bilge are usually there only because the bilge gives you the longest run in a straight line. Limiting yourself to 10-15' in the cabin roof can't help. Better to go with a Dynaplate as a radio ground through the hull. From what I've heard they are sintered bronze, not just bronze plate, to give more effective surface area and better ground contact.

Of course the inevitable nasties (is your boat equipment all bonded /vs/ grounded?) will affect what you can do with best effect. But I'd also wonder, if your radio is going to be in the forward cabin, and if you don't have a roller furler, wouldn't it be simpler to use the headstay instead of the backstay for the vertical?

It seems that the particulars of rigs are so variable that everyone has a different "best" configuration for a boat. Might be more effective to give yourself the best ground (to the salt water beneath you<G>) and then run a temporary wire here and there, see what part of the rig you can best use for the antenna before making a choice and cutting the rigging. The 2004 ARRL Handbook has a section on sailboat antennas...basically, anything goes if it works on your rig.
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VA7CPC
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« Reply #4 on: May 19, 2004, 02:35:13 PM »

Thanks for the reply.  I'll try to explain why I'm trying something out of the ordinary.

> Charlie, are you the same Charlie I've spoken with some time ago, out of Vancouver way?

Yes, I'm in Vancouver.  We probably talked, and I have you in the IC-706 Yahoo group.

> I can't think of why anyone would want to use couterpoise wires on a boat, when you've got such a perfect "earth" ground right under the keel. Foils in the bilge are usually there only because the bilge gives you the longest run in a straight line.

The problem is _reaching_ the saltwater.  My keel casting is "excapsulated" _inside_ the fiberglass hull, and it's not easy to get to; fiberglass encases it on all sides.   I'd need a 10' run of copper foil from the tuner, forward to the aft end of the keel.  That's a substantial fraction of a wavelength on 20m.   [It would be coupling to the water along the way, though -- and that might be save the situation.  Nobody treats a foil run underwater as it _should_ be treated -- it's a lossy, unbalanced transmission line, copper on one side, salt water on the other, fiberglass dielectric.]

Foils in the bilge bring RF "inside the boat".  They're grounding conductors, and they carry current.  I've always been suspicious of this "solution" to RFI.

> Limiting yourself to 10-15' in the cabin roof can't help. Better to go with a Dynaplate as a radio ground through the hull. From what I've heard they are sintered bronze, not just bronze plate, to give more effective surface area and better ground contact.

You're absolutely right about the short counterpoise.  I'd like a bigger one.  But there's a lot of wire and strip in it -- about 150'.  I've been studying the ARRL Antenna Book, and it _might_ do the job if the SGC tuner is a low-loss device.   We'll see . . .

The Dynaplate, AFAIK, behaves exactly the same as a plain bronze plate of equal size.  I asked Guest (the maker) if there was any test data showing otherwise -- they said "No".  We could argue about this, but only test data will convince me.  Any exterior ground (according to SGC and other) must be kept _clean_, a non-trivial job.

> Of course the inevitable nasties (is your boat equipment all bonded /vs/ grounded?) will affect what you can do with best effect. But I'd also wonder, if your radio is going to be in the forward cabin, and if you don't have a roller furler, wouldn't it be simpler to use the headstay instead of the backstay for the vertical?

RFI is a major issue.  The electronics (and other gear) _is not_ bonded.  I expect to be adding ferrite chokes to every wire out of every piece of electronics.  Putting insulators on the forestay is a no-no unless one is at the very top, and the other is at the bottom -- you can't raise and lower jibs, otherwise.

> It seems that the particulars of rigs are so variable that everyone has a different "best" configuration for a boat. Might be more effective to give yourself the best ground (to the salt water beneath you<G>) and then run a temporary wire here and there, see what part of the rig you can best use for the antenna before making a choice and cutting the rigging. The 2004 ARRL Handbook has a section on sailboat antennas...basically, anything goes if it works on your rig.
 
Several "wise voices" (including SGC rep) suggested that I should try this out with some pieces of wire, before cutting the backstay.   Sounds like a good idea! <g>

If it doesn't work, I can revert to the "conventional solution", which will give me the conventional problems -- RFI in the boat, and poor antenna performance.
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