The lenght of any ariel for radio equipment for maximum efficiency needs to be half of the wavelength of the radio waves that the radio uses.
The Wave-length of a VHF is about 1m, so therefore a 50cm "whippy" ariel at the top of your mast is perfect for getting maxium range. Your backstay is too long and reception and transmitions will be of poor quality.
Hope this hopes.
'High gain' aerials, mostly used on mobos for VHF, are about twice the length of a yachtie one and get the apparent increase in power by pushing out the signal as a flat disc rather than a sphere (oversimplification). This works on mobos because they stay fairly level so the disc of radiation stays parallel to the sea. On a heeling yacht most of the power would go into the sky or sea.
If you used a section of backstay its angle would make it inefficient for the same reason.
Insulators to insert in a stay for aerial use are surprisingly expensive, far more than a whip antenna.
Snowleopard's comments are valid and a whip antenna is the easiest way to go. However to answer your question. The actual antenna should be 1/4 wavelength (or about 5% shorter to allow for end effect). However a multiple of 1/4 wave is also Ok so 3/4 wavelength is good, Multiple wavelength antennas provide gain however as stated they sharpen the horozontal lobe but that means that at an angle to the horizon they have less ( negative) gain. You would need insulators in the backstay to give you precisely the correct dimension to which you attach the centre of the coax cable. However the antenna needs a groundplane. That is a similar antenna or large conductive surface to connect to the braid of the cable at that same point. So you could connect another similar length of backstay (insulated) to the braid or make a coaxial antenna by fitting another piece of braid (insulated) over the outside of the coax and connected to the cable braid. Or just rely on a large amount of backstay below the actual antenna. (As happens when an Al mast is used as groundplane)
as the feed coax must connect to the backstay well up the backstay I think you would find that a bit of an eyesore and susceptible to damage.
If youu do decide to try these ideas remember that every cetimetre of coax both centre and braid that are separated become part of the antenna.
You would need a VSWR bridge to check the tuning of the antenna. My own preference would be for a free standing antenna mounted on the pulpit or transom that is still operable if the mast conmes down.
Will, with all due respect might I suggest that a better solution is to have the VHF on top of the mast (vastly increased range from the height) and an emergency antenna you can plug in and tape to the boathook if the mast comes down? Most masts don't come down very often, and why lose the height and range advantage whilst cluttering up the pushpit.
Here's another angle- have your main antenna at the masthead, of course, for maximum range. Fit a second to pushpit or goalposts. the second aerial is your reserve in case of dismasting but until then it is plugged into your DSC box as receive-only.
The great benefit of this is that you only receive DSC alerts from the immediate area. DSC calls from further away are a lot less likely to be relevant to you. It cuts out a lot of the irritating traffic that makes people switch off their DSC sets.
Yes John I wouldn't disagree that one on top and one on the transom would be best. As for masts coming down well i might disagree there. I am on my 4th mast 25 years but have 3 friends inn the local club who has lost mast in last 2 years all in 21ft or thereabouts siized boats. Largely due to shroud failure due old age but in my case the last 2 due to bad boat management. honest, the post just jumped right out in front of us. say no more. Anyway I wonder if the original poster was completely ignorant or whether he really will try a backstay as a VHF antenna. regards olewill PS whats all this due respect
thing? Actually while I have a trade in radio I don't have a VHF as here we all have these elcheapo 27 mhz modified Cb radios as a marine band. It is cheap ( a new radio for 50 quid)but certainly no DSC. if you have a VHF no one will talk to you as it is all commercial. will
The aerial length should be full, half or quarter wave dependant on what gain you want. The wavelength is equal to the propogation speed of the radio wave (300,000 kilometers per second) divided by the frequency. The marine VHF starts around 156MHz so the sum is 300,000,000 divided by 156,000,000. You notice that the 0's conveniently cancel each other out, so the length of a full wave is approx 2 meters. The standard VHF whip is a half wave aerial and as such is approx 1 meters long while a motor boats aerial will be a three quarter or full wave, up to 2 meters long. A medium/long frequency such as 2182 KHz will be a much longer wave length, 137.4 meters, so a backstay will give a much stronger signal than a short whip. The small stubby aerials from a handheld are actually springs and appear to be much longer electrically to the radio waves than their actual length, if that makes sense? It gets more complicated with base loaded or centre loaded aerials. You have to match the length for what you want to receive.
If you decide to fit an aerial to the pushpit, please mount it on the stb side because (God forbid) should you need the services of a helo they come in over your port rail and the diver doesn't like singing saprano after the event<s>
If your mast should come down on a breezy day in a chop, it will be mayhem on deck. Wire, string, sail, spars will be flapping all over the place. I think there is an excellent chance that an aerial on the pushpit will be taken out. That is why my emergency aerial lives below in its original packing.
The great advantage of this piece of safety advice is that to follow it requires no effort, just a little inactivity!