Convert backstay to aerial - how long?

[Talulah]

I am about to convert the backstay into an aerial for the SSB. The question is what distance should I have between the two insulators? (There will be about 2m of aerial cable from the auto tuner to just above the first insulator? Opinions please.

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[Talbot]

sounds to me like the bottom insulator is too low. It should be high enough so that it cant be reached from standing position at top of cockpit, 150watts through the hand will give you a nasty burn. The top insulator should be about a metre from the mast. The ATU will then manage the aerial length as necessary.

further details are available from <A target="_blank" HREF=http://www.icomamerica.com/support/documents/antena_grounding.pdf>ICOM</A>

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[bruce]

you prob should do some homework and find out what length the freq you are using requires an antenna to be, then measure your back stay, then look at the antenna tuner and see what lengths it can tune in relation to your back stay, and as already mentioned, bottom insulator should be high enough to keep hands away.

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[jimboaw]

I am about to replace my rigging and am going with Calder on this one. No lower insulator. It's a weak point that is not needed on a plastic boat. Cable run from AT can be much shorter. Crew will soon learn not to hang on to backstay when skipper is using SSB

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[William_H]

Hello Jimboa The idea of using the fibreglass boat structure may or may not be a good idea. It depends on the length of path to the water from the backstay metal parts and if this area is likely to become damp enough to conduct. Of course a specialised insulator has about 3 inches of path for leakage so you should be able to better that however beware of carbon fibre in the boat structure near the path from backstay to water. You could try the resistance with a multimeter (should be way over 2megohms) or better still use a megger high voltage resistance checker. I don't think that RF burns would be a real problem in practice.
regarding the length of wire for the antenna longer is better however the range around 30 feet is good.(including the lead in) The antenna coupler will put a capacitor in series to electrically shorten the antenna for frequencies above 7mhz while fitting an inductor in series to electrically lengthen the antenna for frequencies below 7mhzto make it look electrically like a 1/4 wavelength long. As you will use frequencies both above and below you need a manual or automatic coupler (tuner). regards will

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[peterbringloe]

Height of lower insulator

I made the mistake of having the lower insulator high enough to make it impossible for even the most determined person to get hold of the backstay above the insulator. But it makes maintenance of the connection much more difficult. And if it's too high it's out of sight and out of mind. Obviously this connection must be good, and make sure you use the correct cable (not coax) and also use standoffs.

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[Talulah]

I understand that if you make the aerial as long as possible the aerial becomes directional even if the ATU can cope. I had previously thought that about 8m was about right which would only be about half the length of the backstay. This is why some boats who have their aerial as long as possible find they get better connection when they are pointing towards the receiving station. I have no technical background on this. Hence the posting on here for others opinion.

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[Talbot]

with your LOA of 11.75m, I would suspect your mast will be abt 13m, and your backstay abt 14m. delete 3m for bottom insulator, and 1m for the top, and your length of aerial becomes 10m. Icom recomendation is that the section between the isolators (see my previous reference) should be between 120ft and 23ft. Your 8m is very close to the absolute minimum, and will probably result in poor reception of the lower frequencies (i.e. 2182 KHz).

I suspect that the boats that have a problem with directional aerials, have made their upper insulator too close to the mast.

The biggest problem with SSB installations is not having a large enough area of backplane for the ATU. Again this has an impact on the lower frequencies. To achieve the best results you will need to spend £200-300 in just this area, preferably with two large plates , again there is more detail on the reference in the previous post.

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[PaulJ]

I too am in the throes of installing an SSB and have been researching this topic, including a question on this forum two or three weeks ago, following which I found an excellent article on the Sailmail website under "Sailmail Primer" and then "Aplication notes". Hopefully this will take you direct to it though you will have to cut/paste it because I don't know how to insert a link.....

http://www.sailmail.com/smprimer.htm#Overview of the Network

I suggest that you read the whole thing but I have lifted this directly from it.....

Common mistakes are a ground strap that is too small, ground straps that don't connect to enough metal, or an antenna lead wire that is run right alongside a backstay below the backstay insulator. The latter problem is very common and shunts a large part of your signal to the ocean, so put the lower antenna insulator at the extreme lower end of the backstay right above your turnbuckle or backstay adjustor. Then use plastic tubing to insulate the lower part of the backstay (above the insulator) that is within the crew's reach. Radio waves propagate just fine through the plastic tubing, but the plastic tubing will protect your crew just as well as the insulation on the feed wire would have. The fact that you don't have the feed wire running near a grounded wire will dramatically improve your radio's efficiency.

Some folks ask if there is any magic length for a backstay antenna. The answer is "yes and no." In general longer is better, and unnecessary wire or rod terminations are bad, so often the best approach is to have the top insulator right at the masthead, and the bottom insulator right at the top of the backstay adjuster. This eliminates four wire terminations (which are sources of rig failure) and gives you the longest antenna. There is a wrinkle (as always). Base loaded verticals (e.g. backstay) antennas are hard to efficiently load if their length is an even multiple of 1/4 wavelength (e.g. 1/2 wave, or 1 wave, or 3/2 wave ...). Therefore if you are also a ham, and 14.313 MHz is really important to you, avoid those lengths. A 1/4 wavelength in feet is 234/(freq in MHz), so a 1/4 wavelength at 14.313 MHz is 16.35 ft. Therefore, if you are a ham, avoid having a backstay antenna that is 32.7 feet or 65.4 feet long. If you are not a ham and just use SailMail, then just go with the "longer is better", and "fewer swedge fittings is better" principles. There are enough SailMail frequencies so that if your tuner has trouble tuning your backstay at some frequency because you happened to end up at exactly 1/2 wavelength, there will be other SailMail frequencies on which it will work fine. Remember when working out the length of your backstay antenna, you start measuring right at the tuner, and you include in your measured length the length of the feed wire.

'Hope this helps,

Paul.

<hr width=100% size=1>I used to have an open mind but my brains kept falling out.....

 

[ParaHandy]

interested in why you've chosen ssb .... i examined the technical bits (ground plane, aerial etc) and thought better of attempting it whereas a sat phone might be more reliable ......

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[Gunfleet]

Where are you going para... we should be told!

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[snowleopard]

ssb v satphone is like forum v email- you can't join in a general network chat with a phone.

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[ParaHandy]

if i find where and G Brown can't get his grasping paws on my siller, d'you think i'm going to tell anyone?!

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[Talulah]

Many thanks to all respondants.

What do people use to make the standoffs between the backstay and the aerial lead?

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[PaulJ]

If you do it as suggested in the Sailmail article (ie. have the insulator at the bottom of the backstay and use a plastic sleeve to prevent determined crew from burning themselves) then the feeder wire will be right at the bottom and you won't need standoffs.......
Paul.

<hr width=100% size=1>I used to have an open mind but my brains kept falling out.....

 

[jerryat]

Hi Talulah,

You are correct re the directionality. Your aerial should be not less than 11 metres (between insulators) if possible. Most ATU's e.g. the Icoms, will tune that length superbly to all frequencies you are likely to require. Do not worry about the height of the lower backstay insulatorif it means reducing the aerial length below the above. Frankly, whilst the warnings about burns should be heeded, most people I know find it perfectly satisfactory to warn others that they are about to transmit, to avoid this. On passage, who is hanging onto the backstay anyway?!!

As others have also mentioned, it is FAR more important to be able to readily access the backstay connection for regular maintenance, as this is by far the most neglected 'link' in the radio chain. So keep it within sensible reach.

Cheers

Jerry

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[Gunfleet]

siller

My education continues. Siller....

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