Corroded vhf cables, sockets

[MM5AHO]

Having decided to replace the coax from tranceiver to antenna, I was amazed at how bad some of them were, and yet they were working (after a fashion).
http://www.bebo.com/PhotoAlbumBig.jsp?Me...toId=6381659300 shows what the cable looks like inside the deckmount N type socket. Otherwise the socket looked in very good condition. Under the deck flange of that socket looked like this http://www.bebo.com/PhotoAlbumBig.jsp?Pa...oId=6381659330
The coax inner was no longer soldered to the centre pin in the socket, and the braid went nowhere electrically. Wonder how often I should look at this? It must have been like this for years!

edit: can't get this image insertion right, must read the instructions!

 

[wotayottie]

Yuk! Problems like that can occurr because of a leak into the cable at the top of the mast - watyer then flows down between the outer and the inner insulations. Once had a mysterious water leak into the cabin, not a lot but enough to irritate and just onto the chart table of course. Turned out to be water coming dowt the inside of the coax

 

[tillergirl]

I don't actually understand why anyone would want a connection at the masthead. I used the opportunity of a similar failure to replace the aerial with a moulded connector. I also banished deck connectors by having a swan neck. I now have a neat mahogany box underneath the deckhead in which all connections are made.

 

[MM5AHO]

There is a need to connect the coax to the antenna at the mast head.
During the winter, the mast is stored about 50m from the hull, hence the need for a deck connector at the other end of the mast.

 

[tillergirl]

Don't agree. I drop my mast every year and never stress the moulded connection. When fitting leave a small loop sufficient to permit undoing of the holding nut, loop the aerial back and tie to mast. Its safe and you don't need a connector in the most inaccessible part of the boat. There is no danger to the loop, nor does it look untidy as its quite small.

 

[MM5AHO]

I can't see how you have an antenna at the end of a length of coax up the mast without a connection?
I agree that the antenna need not be seperated from the mast (I was referring above to mast being seperate from boat), but how does coax cable suddenly become antenna at the mast head?

 

[tillergirl]

Vtronic make a range of aerials with coax moulded in at the bottom rather like one of those moulded 3 pin plugs you get on some appliances nowadays. Of course you have to feed the cable down the mast on fitting but that's no real problem. It's all one piece having been made in the factory.

I know you didn't mean separate aerial from mast but undoing it is a sound thing to do to prevent damage.

 

[mikejames]

I had rain run down through an aerial with moulded on cable once.

It was only when I unplugged the Buccaneer connector at the mast foot , found it full of water , and then I found water running down inside the cable.

At about the same time , the wind had hit 90mph and the NASA wind instrument had disintegrated. (all in the marina...)

 

[Salty John]

Unfortunately the moulded connection does not allow you to select the right coax for the job; you are stuck with RG58. The coax cable size and quality is important to the perfomance of the aerial system. RG58 has attenuation (loss of transmitted power) of about 5db per 100' at 100MHz. 3db represents a power loss of 50%. RG8X is the best compromise with attenuation of 3.7 db per 100'. RG8U has a loss of about 1.9db.
This may or may not be important to you but you should be aware that offshore racing rules specify the acceptable level of attenuation in the VHF antenna cable and there is no masthead application for which RG58 complies.
Also, consider the quality of the cable. Cheap cable will have an open cell foam dielectric that will wick moisture into the cable; it should have a close cell or solid polyethylene dielectric.
The moulded connector means the cable goes with the antenna - you can't detach the antenna and take it home for the winter.
Deck plugs should be avoided and a swan neck is a good method whereby all connections are made in the boat. Another method is a deck gland through which the connector can pass without detaching it from the cable.

Important! I declare a commercial interest in vhf antennas and cable so readers should treat this advice with due scepticism.