OK, so you have a superb radio and a top class antenna. What do you use to connect one to the other without losing the potential of these fine pieces of kit? Coaxial cable is the answer, but not all coax was created equal.
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.
Furthermore, marine cable needs to work in a hostile and constantly moving environment so conductor and braid need to be tinned copper to resist corrosion and the conductor needs to be stranded so it can bend without breaking. A good PVC jacket will keep sunlight degradation at bay.
But what size cable should you choose?
You should aim at a transmission loss of no more than 50% in the run from radio or AIS engine to antenna. In fact, the requirement of the ISAAF for offshore racing is 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, 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 nominally 10 mm diameter, nearly half an inch in old money, so it’s heavy and doesn’t like to go around tight corners. It’s difficult to work with.
RG8X is nominally 7mm diameter, although actually about 6.5mm diameter unless it has a particularly thick outer jacket. This cable is much lighter and easier to work with than 10mm cable. Good quality RG8X will lose a little less than 50% in a 20m run.
You may also see RG58 cable, it sometimes comes with cheap aerials. It is nominally a 6mm cable, but usually closer to 5mm diameter, and is very lossy. OK for very short runs, perhaps, 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. It’s scandalous that this cable is offered in ever increasing lengths, factory crimped to an aerial. It used to be offered in 5m lengths for power boats but just recently I’ve seen it offered in 25m lengths. Don’t use it for anything more than 6m runs or you’ll lose more than half your signal power.
So, make sure your cable is of marine quality with good shielding and for a cable run of up to about 20m use RG8X, for much longer runs you’ll need to wrestle with RG213/ RG8U. Don’t use RG58 except for very short runs, up to 6m.
And remember: “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.