SSB antenna feed

[Klaus]

I'm about to mount a SSB transceiver in my boat using an isolated backstay as antenna. Can I run the feed wire up inside the mast and connect to top end of backstay, and what kind of wire would then be the best choice?

Klaus

 

[snowleopard]

the feed wire is part of the radiating line so should not go up then down again. The normal route is to put the ATU in the lazarette or below decks at the foot of the backstay.

Be careful where you route the wire as it will be pushing out 150w of RF current which will induce current in any nearby cables. For example it might cook your masthead instruments!

 

[jerryat]

Hi Klaus,

Agree with Snowleopard. The best professional advice is to keep the ATU-Aerial feeder length as short as possible for all the reasons he mentions. Most locate their ATU underdeck as near the transom as possible (perhaps boxing it in if it intrudes into the aft cabin) then pass the feeder through a deck gland to the backstay.

A call to any Ham radio supplier will elicit the 'best' feeder, but we used RG 8 (or summink like that - not on boat at the moment so can't confirm) which is very heavy duty co-ax. However, several experts I consulted prior to installing my set-up some years ago, recommended cutting back and isolating the INNER core, then teasing out, twisting the screen wires together and soldering them into a 'solid' wire. This is then clamped to the backstay and weather protected. Apparently this gives a far greater mass of conducter and it's certainly proved excellent in our installation. Many just use a heavy sq mm multi strand cable and get super results, so yer pays yer money ...............

Ok, this method does mean the feeder radiates a little more energy, but as the length from the ATU is so short (about five feet in our case) there is no problem.

Hope this helps,

Cheers Jerry

 

[Benbow]

From the ATU to the antenna you MUST NOT use coax. Many people strip the outer screen off coax and use the inner. This is surprisingly easy to do (and strangely satisfying, a bit like popping bubble-wrap !).

As others have said, you should not run the feed up the mast or you will get very poor performance and maybe fry someone leaning on the mast. If you really wanted to feed from the top, you would have to put the ATU at the top of the mast and feed it with coax.

 

[mdrifter]

Agree with BenBow - no co-ax. If you can have a look at the SGC web site, they have excellent .pdf guide. If possible keep the feed from the ATU as short as possible and use GTO15 (c.6mm diameter) which is a heavily insulated tinned multi-strand. You can get backstay insulators with an eye for the bottom one; on a c.35 foot backstay position the top insulator about 4 feet down from the top of the mast. Having said all that on a recent canal trip with the mast down an improvised antenna with tinned multistrand spiral wrapped around a redundant 8 foot fibreglass dan buoy appeared to work quite well. Overall length of wire was about 15 feet and diameter c.2.5mm. Using a shroud cover for the bottom 6 feet of backstay should reduce risk of rf burn - good idea to make sure this cover isn't sealed at its bottom end - let the water out...

 

[William_H]

For a fibreglass or wooden boat it is not so critical to have a short lead from the ATU to the antenna. Basically the feed wire is part of the antenna if it radiates into the hull on a steel or aluminium boat it can't get out so is lost but on F/G it call escape (radiate) although not so well being close to the water. The antenna including feed wire should be well clear of other conductors ie mast or pulpit as this will absorb rdiated power and or alter the radiation pattern. The wire useed provided it is suitable to carry sebveral amps really is dictated by good insulation requirement and resistance to damage and corrosion. On a steel boat this should be only a few inches if possible. regards olewill

 

[jerryat]

Hi mdrifter and Benbow,

Yes, entirely agree re not using co-ax, BUT as I mentioned earlier, expert advice to me was to use a heavy duty co-ax (similar to that used from the ATU to the transceiver) but NOT use the inner core which MUST be isolated at both ends. Using the outer (screen) braiding only, which on heavy duty high quality co-ax is very heavy, provides an excellent method of connection. I must stress again, the inner core is not used in any way and must be cut back and completely insulated from the screen. On no account join the two together in the belief that this 'doubles' the effect - it doesn't, and can have quite the contrary effect!!

I must confess to some doubts about this method at the time (some eight years ago) but it has proven over many, many thousands of miles to be totally sound and reliable. Somewhat surprisingly, and as noted by William_h, the degree of unwanted radiation using this method is tiny, providing the total length of the feeder (as several have advised) is kept to the absolute minimum length.

As a complete contrast to the foregoing, we sailed a Pond circuit on one of our friend's boats a few years ago, where he used a basic bit of 'car type' wire from his ATU as a feeder. Transmission was perfect the whole way round, with Herb's Net still being worked till we were little short of the Scillies!!! Gratifying that the 'experts' don't know everything all the time, isn't it?!! /forums/images/graemlins/smile.gif /forums/images/graemlins/smile.gif

Cheers Jerry

 

[Benbow]

Hi

Yes, apologies, I did misread your message as saying that coax could be used. I have not heard of the screen being used as the conductor as you suggest but I guess it should work at least as well as using just the inner.

Years ago I read that GTO15 was the only way to feed an antenna. I spent ages wandering around shops and electricians asking for this stuf, but no-one had ever heard of it so I resorted to stripping coax. In practice, as you say any old bit of wire would probably do provided it has reasonable diameter and is reasonably well insulated.

 

[iangrant]

Klaus - I reckon your question has been answered but as I spent last night with all the boards up checking my installation (no Transmit) I can describe what works:

10mm wire from back of set (Icom 710) about one metre long soldered to a copper strip. Big low loss coax on UHF solder connector. Power connections straight to the battery, ATU control cables aft to the ATU.

Copper strip installed under the floor and connected to the hull anode, up through the transom to the ATU where it is nut and bolt connected to the ATU.

The wire from the ATU to the insulated backstay is a 10mm multistrand bolted on with a u bolt clamp. It is encased in a plastic tube.

Tests - 2182kHz on J3E whistle into the mike this should draw 20 - 25 amps.

Rx - I got Genoa Radio and Yarmouth Coastguard from Chichester Harbour last night after reconnecting a copper strip joint that had come apart..

Anyway for what it's worth.. hop eit helps



Ian

 

[Birdseye]

I used some copper cored spark plug wire for the short run to the backstay. Really well insulated and fitted a standard deck gland. Cheap too.

Seemed to work OK