So, you’ve bought a new marine VHF antenna and you want to buy coaxial cable to connect it to your radio or AIS transceiver. What should you look for?
Here’s the super-quick check-list:
Stranded centre conductor
RG58 up to 15m, RG8X up to 25m, RG213 for super-yachts.
Braid coverage over 90%
Tinned copper if you can get it. If not, make sure you seal outside connections to prevent water intrusion using silicone, self-fusing compression tape. In fact, do this even with tinned cable although it isn’t as vital.
When you fit the connectors make sure they aren’t shorted. Use a multi-meter set to 2k ohms. Should read 1 (infinite resistance) between centre pin and outer case of the connector. Do this with the cable disconnected at both ends.
Here’s the slightly longer guide:
First, it has to be 50ohm coaxial cable. Television cable is 75 ohm – forget it, you can’t use that coil left over from your satellite dish installation.
Beyond the requirement for 50ohm resistance you have several choices, determined by how much you’re prepared to spend, the length of the cable run and availability.
To make size selection simple, my advice is that for cable runs over 12m and up to about 25m use RG8X (sometimes called mini8) which is nominally a 7mm diameter cable, in reality a little over 6mm. For runs below 12m it’s ok to use RG58, nominally 6mm diameter, actually closer to 5mm.(With low loss RG58 it is possible to stretch to 15m at which point the loss will be 3dB which is 50%. We don’t recommend more than 50% loss in the cable run).
The next important requirement relates to the construction of the cable. Coaxial cable comprises a central conductor running inside a plastic sleeve (called a dielectric) which is wrapped in a braid sheath, then the outer jacket. The centre conductor can be a single solid wire or a stranded wire. You will want a stranded wire conductor because it is less likely to break due to metal fatigue. Lots of movement on a boat, lots of cable flexing – get a stranded centre conductor. Get braid coverage of more than 90%.
Tinned cable is always the most desirable choice on a boat – exposed to the marine environment, copper turns black and corrodes quickly. If you can’t get tinned coax, make sure your connections are carefully water proofed. Wrap external joints with non-adhesive silicone compression tape. Leave a few centimetres of cable at each end of the run so, if you get corrosion, you can trim the cable back to good clean copper and fit a new connector.
Be aware, tinned coax, particularly RG8X, is difficult to find in the UK and it’s quite expensive because it’s mainly imported from the USA. For some reason white jacketed cable is also rare – I don’t know why – but it should only really be an issue if you’re running cable externally and want to keep it discrete.
When you check you have fitted the PL259 connectors properly, using a multi-meter to test for a short circuit between pin and outer shell, do it with the cable disconnected at both ends. Many antennas show a short circuit to a multi-meter because they are internally grounded as protection against lightning.
There you have it. Check out saltyjohn.com for Metz antennas and marine coaxial cable.