co-ax for v.h.f

[mickshep]

Am in the process of re wiring the boat ready for the off. the co-ax cable which runs inside the mast is in a pretty bad way. I have looked at some some cable in one of the local chandlers and to be honest it appears pretty cheap 'n' nasty. I have on the other hand access to a length of the co-ax used to connect my computer to the broad band system, which in my ignorance I assume will be of very good quality. Is this likely to affect the performance of the Sailor RT 2048 if I use this cable instead??? Cheers in advance. Mick

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[Roberto]

For the masthead antenna, if at all possible install RG8 coax (the thicker one) instead of rg58 which is usually sold at chandleries (if they are good, otherwise it is not rare to even find 75ohm tv coax). Loss in dB for a masthead antenna can easily double with thinner rg58.

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[mickshep]

Thanks for that. the stuff I have is (according to the N.T.L chap I met outside after placing the post) is IG11. as opposed to RG. He seems to think it would be fine but 2nd opinion welcome. Regards Mike.

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[jfkal]

Don't go for the flimsy stuff at the chandlers. Go to an electronics or HAM radio shop and get the "fat" cable. Note the connectors are also different for the fat cables. If you are not so handy let them do the splicing and connectors. A bad connection reuins the job. The fatter the cable the better the performance.
Besides no one goes uop the mast more times than necessary ;-))


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[wooslehunter]

Whatever you do stay away from TV coax. It'll reduce receive sensitivity on receive and may even damage the set on transmit.

I'm not sure what the numbers are for RG58 and RG8 but equivalents are URM76 and URM67. URM76 is specified at around 2dB loss per 10m of cable at 100MHz. This translates to around 37% power loss. Marine VHF is higher frequency than 100MHz so the loss will be more. For URM67 its 0.68dB for the same length and frequency. That translates to around 15% loss. Connectors will add a little extra loss. Stay away from the nasty PL259 type connectors and use BNC or N-Type if possible. They're far better. You can get PL259 to BNC adaptors that screw into the back of the radio so you never need to use a PL259 on the cable. Also I prefer solder type connectors rather than crimp. The crimp tools are expensive and the joints are prone to corrosion.

So what does it all add up to? With a mast head antenna and a 25W transmitter you have enough power to cover the line of sight distance over sea water with either cable. On receive you may have a slight improvement with the fatter cable.

Check out places like www.farnell.co.uk and www.maplin.co.uk. They will both accept credit cards and deliver the next day.

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[fluffc]

I'd recommend using RG58, unless you own a super-tanker and are running very long distances. Reasoning behind this is that it is readily available and replaceable. The connectors are also the easiest to get hold of.

There are many types of 50 Ohm cable available, and for the more obscure types getting the right connectors that will fit can be challenging.

So, Get RG58 cable - and buy yourself a few extra connectors as spares, as these are the weak point in the installation.

Regarding connector types; thoroughly agree with your concerns. I do use crimp connectors a lot, but the tools are expensive and getting a good connection takes experience.

Avoid using interseries adaptors - its' another link in the chain to fail. If you need to connect to a PL259 socket; buy a PL259 plug.

Regarding corrosion - vaseline rules! Coat the outside of the completed joint with an excess of the stuff so that no metalwork is exposed to air. (Even better is non-setting silicon gel)

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[Strathglass]

I am just trying to decide whether to use 50 ohm Heliax on me new mast. a bit of an overkill but I have it. Lower loss than virtually anything but very rigid and difficult to install.
If I can engineer some way of fitting it then I will have virtually no loss between the VHF and the aerial. The mast allows me to bring the connection below decks. I am very tempted but I have the 50 ohm coax supplied with the aerial.

everything in life is a compromise.

Iain


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[fluffc]

If you have cable supplied with your new aerial, then do make use of it. The losses associated with using a low-loss cable aren't particularly important on a normal yacht installation. More important is to make sure your joints are well made, and mechanically supported.

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