Most boat owners are aware that electrical cable for marine use should be tinned to avoid the corrosion problems inherent with copper cable. They are also aware that the wiring should have stranded conductors so that it doesn’t fatigue and break as can happen with solid conductors continuously flexed by the motion of the boat.
It is often overlooked, however, that these same principles apply to antenna coaxial cables. Your VHF radio system comprises a radio, an antenna and the cable which connects the two. Even the most exquisite radio and top class antenna will under perform, or fail completely, if they are connected by the wrong size and type of coaxial cable.
There are three sizes of cable commonly used with marine VHF radio and AIS systems: RG58, RG8X and RG213. (RG8U is also seen, it’s the same diameter as RG213 and with similar performance). These are 50 ohm cables. By the way, TV cable is 75 ohm cable and can’t be used for this application.
The size of cable you choose will depend on the length of run between radio and antenna. The radio signal loses power as it travels through the cable: The smaller the cable diameter, the higher the loss. This power loss is called attenuation and is quoted as dB (decibels) per unit of length – usually dB per 100’ at a particular frequency – in our case we’re concerned with a frequency of 150MHz.
A loss of 3dB over the cable run is a halving of the power. We only have 25w available at high power, so we want to preserve as much of this as we can. It is usually recommended that you aim to lose no more than 3dB, or 50% of the signal power. In fact the ISAF, who regulate offshore sailboat races, recommend you lose no more than 40%, but for most of us 50% is a worthy target.
Manufacturers give attenuation figures for their cable so you can calculate from this the length at which a 3 dB loss occurs. If you don’t want to be bothered with that, let me offer some simpler guidance:
For cable runs up to 6m (20’) – RG58 For cable runs up to 20m (65’) – RG8X
For cable runs over 20m use RG213.