Marine coaxial cable

VHF antenna systems comprise an antenna, a feeder cable and a few connectors. If you have a really good quality antenna such as the Metz and use good quality connectors, you’ll want the correct coaxial cable to complete the system.

Coaxial cable for VHF radio is 50 ohm – your TV cable is 75 ohm so you can’t just use some coax left over from that satellite dish installation.

Furthermore, marine cable needs to work in a hostile and constantly moving environment. That’s why it needs to be tinned copper to resist corrosion and the centre core needs to be stranded so it can bend without breaking. Really low loss cable will also include a foil barrier between the braid and the centre conductor.  A good PVC jacket will keep sunlight degradation at bay. 

The top picture shows the anatomy of a good, marine quality, RG8X cable.

And what size cable should you choose? 

You’ll want to restrict the line loss (known as attenuation) to about 2.5 decibels (dB) in the run from radio to antenna. This will ensure you don’t lose more than about 40% of the signal power in the antenna system – a requirement, by the way, of the offshore racing authorities.

RG213 (or its slightly lower spec but similarly stiff cousin, RG8U) is a whopping 10.3mm diameter, nearly half an inch in old money. So it’s heavy and doesn’t like to go around tight corners, but it only loses 2.2 dB per 30m length. Alongside it is its less uptight little brother – RG8X. This flexible fellow is only 6.5mm diameter, much lighter and easier to work with – it loses about 3.5 dB per 30m. 

(RG58, which sometimes comes with cheap aerials is 5.5mm diameter and is very lossy – about 5 dB per 30m. OK for short runs, perhaps, but not for masthead installations).

So, if you have a cable run of up to about 25m or so you can go with RG8X, for much longer runs you’ll need to wrestle with RG213/ RG8U.