Coaxial cable – what size?

I’m often asked what size and type of coaxial cable is suitable for a marine VHF antenna system. Well, the first thing to say is, it must be 50 ohm cable. Bits left over from your TV installation won’t do, that’s 75 ohm cable. Next, think about the environment in which it must work: Lots of movement – anyone who’s been up the mast in a seaway will know all about that – so a stranded centre core is essential to avoid breaking due to metal fatigue. Then there’s all that salt spray and rain about, so tinned copper is better than plain copper to avoid corrosion. If you use non-tinned cable you must be meticulous about sealing the end fittings.

Then we come to cable size. The smaller the cable, the higher the power loss (attenuation) between radio and antenna. For cruising boats a good guideline is to aim for not much more than a 50% loss in the cable run, this is 3db and limits the popular 5.5mm RG58 to about 10m and RG8X to about 20m. If you race under World Sailing rules the requirement is a little tougher. They want no more than 40% loss, or 2.2db:

World Sailing ( International Sailing federation) regulations.

3.29.1 The following shall be provided:  

       a) A marine radio transceiver (or if stated in the Notice of Race, an installed satcom terminal.

      b) an emergency antenna when the regular antenna depends upon the mast. 

  2)When the marine radio transceiver is VHF

      i) it shall have a rated output power of 25W

      ii) it shall have a masthead antenna, and co-axial feeder cable with not more than 40% power loss.

     iii) the following types and lengths of co-axial feeder cable will meet the requirements of OSR 3.29.1

(a) up to 15m (50ft) – type RG8X;

(b) 15-28m (50-90ft) – type RG8U;

(c) 28-43m (90-140ft) – type 9913F (uses conventional connectors, available from US supplier Belden);

(d) 43-70m) 140-230ft – type LMR600 (uses special connectors)

Coaxial cable is an important component of the VHF antenna system, choose wisely to get the very best out of your VHF radio or AIS transceiver.