VHF comms antennae on gaffers

lenseman

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Hiya Runningdog, welcome to the YBW forums.

Hollow mast, nice, do you have access, a hole about 1 inch (25mm) into the hollow section at the base of the mast about 4" - 6" above the tabernacle? Is there any other mechanical facility using the inside of the hollow mast (halyards etc)? Will anything using the inside of the mast rub, chafe any co-axial cable?

Does the mast have a similar hole at the top of the mast which you can use? Forget about the extra height which might be gained by fitting the VHF antenna on the gaff, it won't serve you much better and you won't have any hassle with cables as you raise and lower the main.

Once the correct type of antenna is installed at the top of the mast and connected and sealed correctly to the co-ax cable, the cable is passed through the top hole and lowered through the mast and installed, the top hole needs to be permanently sealed with a silicone sealer.Enough cable must pass down the mast to allow it to exit a 'service hole' near the tabernacle.

A RF plug needs to be fitted to this end and the hole in the lower end of the mast needs to be large enough to be able to accept this plug passing back into the mast when fully installed.

From the navigation table VHF set, you will need a similar VHF co-ax fed forward until it can exit through the coach roof/deck-head as close to the tabernacle and on the correct side of the mast nearest to the lower hole in the mast. Where it passes through the deck-head, you will need to seal thoroughly will a sealant. This hole needs to be the same size as the co-ax cable as this will be easier to seal. The larger the hole in the deck, the harder to seal.

Once outside on deck, you will need enough co-ax to allow you to fit a mating RF plug and still allow enough cable to just disappear inside the hole in the base of the mast.

Once the two sections of RF cable are successfully connected together, the two connectors are sealed with amalgamating tape and pushed into the hole in the base of the mast.

Once the RF plugs are tucked inside, the hole is temporarily sealed 'for your seasons sailing'. If using RG58 (thin co-ax cable) then BNC plug and socket will be ideal at the VHF frequency and work well as the connectors to join the two RF cables at the tabernacle.. If using RG8 or RG213 (thick cable) then 'N'-Type connectors are preferable over the PL259/SO239 types as they are slightly less 'lossy' at VHF.

As mentioned above, be mindful of any mechanical services using the inside of the hollow mast and act and plan accordingly.

Hope this helps?
 

William_H

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I would suggest an antenna mounted on the stern rail or similar. Mine is thus fitted and works well. Yes you will get better range with a mast head mounted antenna but this must be weighed against its loss if the mast comes down and susceptibility to failure in service with joins and mast removal in winter. Ideally both antennae should be available.

Note with a wooden mast that you will need the type of antenna that does not need an earth connection at the mast top. ie the steel rod type are no good as they rely on a mass of metal to be a groundplane to the antenna. The longer fibreglass antenna with cable fitted is what you need.
good luck olewill
 

Salty John

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You can run the cable inside your hollow mast, as others have said. If you don't want to do that you must either chase a groove in the mast to hide the cable from your lacing etc, or you will have to put the antenna at deck level.
Most stainless whip VHF antennae work fine on a wooden mast, they don't require a groundplane.
Various rail mount brackets are available for fitting the antenna to the pushpit rail. Your boat to boat communication will be somewhat limited; communication with another yacht with a mast mounted antenna might be 12 - 15nm, with another yacht with a rail mount antenna it might be 7 - 8 nm.
Communication with the CG or large ships will be somewhat better because of the altitude and quality of their equipment.
 

William_H

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If you can be confident the mast wood will always be dry then the antenna should be OK inside the mast. I don't think I would risk it however.
Re Salty John's assertion that SS rod antenna work well on wooden mast with0out a groundplane. I have think he has something different in mind to what I have. Certainly a 17 inch 1/4 whip antenna connected straight to the coax inner will need a groundplane connected to the outer of the cable for proper operation at low VSWR. Without a ground plane it may still appear to work OK but will not be correct. Likewise a simple 1/4 wave whip on fibreglass cabin top. It needs a bulk of metal for a counterpoise or groundplane. olewill
 

Beadle

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My vhf aerial is on top of the mizzen on my gaff rig.

The feeder runs up one of the shrouds and into the base of the antenna. Works fine.

It may be better to use one of the 5/8ths whips rather than a 1/4 wave. They avoid the "ground plane" issue.

I think its perhaps better to avoid drilling holes in masts if one can avoid it.
 
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