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Old 15 August 2010, 18:25   #1
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How to re-wire VHF antenna?

Been one of those days...managed to pull the cable out of my antenna today. Having never wired one of these before can someone please let me know how it should go back together? The central pin on the cable seems to just push back in but the braiding looks a bit oxidised and not sure how it should sit within the mount?
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Old 15 August 2010, 22:13   #2
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Judging by the photos and speaking with a fair amount of amateur radio experience under my belt, if it was mine I would dump everything back to the radio and start again with a new aerial and new run of coax and seal everything up with sealant and/or self-amalgamating tape.

Corrosion indicates probable water ingress which is the No. 1 enemy of coax and kills VHF radio performance doesn't matter if it is bolted to a boat, a vehicle or the outside of your house. A corroded sh*tty looking connection won't work well with teeny weeny radio signals and trying to clean it up is just a bodge to delay the inevitable replacement IMHO as the corrosion is probably right down the inside of the braiding and will soon reappear.

I found a similar mess in the base of the aerial that was fitted to my Humber when I bought it (it had never been sealed properly) dumped it and put a Vtronix on and performance was vastly improved, and everybody says those aerials are rubbish also found when I pulled the old one out and cut it up that the corrosion was right back to the radio end of the coax (5 metres or so) and inside the plug at the radio end. Yours may well be the same.
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Old 15 August 2010, 23:20   #3
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V H F radio

Sound advice Steve if in doubt chuck it out you know it makes sence
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Old 16 August 2010, 03:36   #4
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Hi Stephen - thanks very much. I've not wired this type of cable before - can you help with how it should be done please (ie finished at each end and where the braiding goes etc) or an idiot's guide online maybe?
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Old 16 August 2010, 03:42   #5
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Try a Metz antenna!

Stuff the connectors full of silicone grease.
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Old 16 August 2010, 06:35   #6
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Quote:
Originally Posted by Max... View Post
Hi Stephen - thanks very much. I've not wired this type of cable before - can you help with how it should be done please (ie finished at each end and where the braiding goes etc) or an idiot's guide online maybe?
Basically on most vertical whip antennas (there are some odd exceptions among my ham radio aerial collection that I've never quite figured out how/why they work but usually these seem to work less well than the conventional ones) the signal is carried by the core which is therefore connected to the actual whip part of the aerial, and the outer braiding forms the shielding so the centre core needs a good solid connection to the antenna and preferably soldered at both ends.

Have a look here http://www.saltyjohn.co.uk/cruisingresources.htm about half way down there is a guide to fitting a PL259 plug to coax which sums up the correct technique very well also another link to aerial installation.

If you buy a pre-wired aerial with a plug on the end it is usually easier to cut the PL259 plug off the radio end and solder a new one on afterwards, this makes it much easier feeding coax through whatever you are installing it in (I have done quite a few vehicle installations over the years). New plugs are only a couple of quid and soldering it is a 5 minute job.
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Old 16 August 2010, 11:30   #7
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That's brilliant - thanks Stephen.
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Old 17 August 2010, 12:17   #8
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[QUOTE=BogMonster;365356]the signal is carried by the core which is therefore connected to the actual whip part of the aerial, and the outer braiding forms the shielding so the centre core needs a good solid connection to the antenna and preferably soldered at both ends.[quote]

The transmission signal is an AC signal. The array within the antenna (or for the base loaded jobs, I think it's just a coil type thingie) is designed to radiate the signal out in an all-around pattern.

To have voltage, you need two points of reference: usually referred to as a "ground" or "return", and a "hot" or "signal". In the case of a VHF, the signal itself is carried on the core wire, and the return is the braided shield. The shield does double-duty as a return path and, well, a shield, to prevent the signal itself from affecting other electronics, and vice-versa (fishfinders are the usual culprit, notorious for both leaking into VHF transmissions or picking up the signal when transmitting.) Any resistance in either of these will affect the performance of the VHF system.

To get a good connection, all connections should be soldered (and soldered well.) It takes a bit of practice to be able to do a decent job on terminating a PL-259, so make sure you've got a bit of spare cable available. Soldering the braid is the hard part (you need enough heat to get the solder to flow to the braid and connector, but not so much that you melt the insulator around the core wire.)

There are a few "solderless" connectors on the market (Shakespeare makes one that actually seems to work pretty well.) Be aware that corrosion will be the problem with these, so greasing or otherwise sealing them up (and probably regular disaassembly and inspection) would be a good idea. One other note is that (especially with the Shakespeare connector) corrosion will form, not from the connector's materials, but from the cable conductors themselves, generally as a result of salt water or salt moisture ingress. Good quality co-ax cable, made of tinned copper conductors will help slow this problem.


jky
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Old 18 August 2010, 07:22   #9
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Stephen was right - whole thing was fooked to be honest so it's in the bin and a new antenna/cable on the way - thanks to Mike at allgadgets.
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