VHF Antenna

[pappaecho]

I have just taken delivery of a tired Evasion 32 ketch, which has no VHF radio. The question is, looking at Swindlery catalogues, there seems to be GRP whip aerials which are 4-8ft long, as against mast mounted aerials, which are much shorter.
Do the shorter aerials give worse reception/transmission? Second question is about coax connections. If I mount the aerial at the top of the mizzen mast, I will need a plug and socket arrangement to allow the mast be raised and lowered. Does the plug and socket result in much attenuation of the signal? How water proof are they?
Finally on th subject of radar reflectors, there seems to be "tube types" quoting a reflective area or either 2 or 4 sq metres. How do they compare with the larger canister types ?

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[Robin]

Height of aerial is more important than transmit power for VHF range (line of sight) so the best place is on the mainmast. I would go for a S/S whip with a windex as well, the grp extended length aerials are really for lower mounting on powerboats and concentrate the transmit signal, not good on a heeling sailboat. Ideally take the aerial feed below decks either straight to the set or via a below deck plug/socket in a waterproof box, but there are waterproof deck plugs/sockets available for deck mounting if you wish, they do work OK.

The Mobri radar tube reflectors are worthless IMO, use a Firdell or Gillie Firth type or a simple octahedron open cheapie.

<hr width=100% size=1><font size=1>Sermons from my pulpit are with tongue firmly in cheek and come with no warranty!</font size=1>

 

[snowleopard]

the longer aerials are high-gain type which output more of their power at right angles to the axis. they manage to do this by emitting less above and below the horizontal.

a short antenna puts out more power above and below the horizontal plane so it is less effective horiziontally but still works when the boat is heeling. if you use a high gain type on a sailing boat it will send very little of its power parallel to the sea when heeling.

high gain types are only suitable for powerboats and multihulls. incidentally you will often see powerboats with their aerials raked back in line with their windscreens instead of vertical. in that configuration they are almost useless!

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[dickh]

The usual aerial for a sail boat is the ss whip type at the masthead. Presumably because it has much less wind resistance than the longer grp ones - which are usually fitted to motorboats. Don't forget that vhf is 'line of site' communication so the higher the aerial is the better transmission & reception. For the bottom end, I fit a deck gland - the sort that is big enough to pass an 'inline' coax connector. Then make the connection internally to prevent seawater access and corrosion. Then it's easy to disconect, open the deck gland , pass the connctor thro'. and lower the mast. Do the same for any lighting cables etc, but with a connection box below. Works well.
Radar reflectors - DO NOT USE the 'tube' type - all the tests show THEY DO NOT WORK!! Use the large canister type - Firdell Blipper or Echomax or even the octahedral type.

<hr width=100% size=1>dickh
I'd rather be sailing... /forums/images/icons/smile.gif