Selecting coaxial cable

If you have a top class radio and a top class antenna you’ll want the right type of cable in between or you won’t get the maximum performance from them – simple as that. For a cruising boat, selecting the right coax is as important as selecting the right anchor chain, so let us begin:

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

Marine coax needs to work in a hostile and constantly moving environment so both outer braid and centre conductor should be tinned copper to resist corrosion and the centre conductor must be stranded so it can bend without breaking. A good PVC jacket will protect the working bits and keep sunlight degradation at bay.

Transmission loss in the cable between antenna and radio (or AIS engine) is a significant factor, and if the cable is undersized and of inferior quality this loss will be unacceptably high. I think a leisure boat should aim to lose no more than 50% of the transmission strength between radio and antenna. In fact, the ISAAF, who manage offshore racing events, specify that there should be no more than 40% loss in the radio antenna cable.

A loss of 3 decibels (dB) halves the signal so you’ll want to restrict the line loss to no more than that. Signal loss in the cable, known as attenuation and measured in dB loss per unit of length, is determined by the size and construction of the conductor, the quality of the shielding and the operating frequency.

Good quality RG213 will lose about 33% of the signal strength in a 20m run, about 45% in a 30m run, so for very big boats it’s the way to go. However, RG213 (and its slightly lower spec but similarly stiff cousin, RG8U) is around 9.5 mm diameter so it’s heavy and doesn’t like to go around tight corners. It’s difficult to work with.

RG8X is nominally 6.5mm diameter, although actually about 6.0mm diameter unless it has a particularly thick outer jacket. This cable is much lighter and easier to work with than 9.5mm cable. Good quality RG8X will lose a little less than 50% of the signal in a 20m run.

You may also encounter RG58 cable; it sometimes comes with cheap aerials. It’s a 5mm cable and it’s OK for runs up to 6m but certainly not for masthead installations. It loses a whopping 65% of the signal in a 20m run. That means 15 watts of your 25 watts maximum power is lost just in the cable run.

So, make sure your cable is of marine quality with good shielding. For a cable run of up to about 20m or so use RG8X, for much longer runs you’ll need to wrestle with RG213/ RG8U. Don’t use RG58 for runs over 6m.

A penny in the antenna system is worth a pound in the radio, so don’t skimp on your antenna, cable and connectors if you want to unlock the full potential of your radio or AIS unit.