By Tom Burden
Coaxial cable (coax) is designed to transfer your radio transmitter’s energy to your antenna with as little loss as possible. It is precisely made, with the correct impedance for VHF installations.
We offer three kinds of 50-ohm coax cable for VHF radio installations: RG-58CU, RG-8X, and RG-213, the mil-spec replacement for RG-8U. Use the largest coax you can fit to reduce signal loss. Here is the reason: Almost all VHF radios start with about 25 watts of transmit power. As soon as the signal starts down the coax cable, it begins to lose strength. This loss is measured in dB. High gain antennas cannot recover this lost energy, but they can focus the remaining energy across the water so you come through with as much punch as possible.
Bigger coax reduces signal losses between the radio and the antenna, which increases power at the antenna. This chart shows the effect of using different sizes of coax:
|Coax Size vs. Signal Loss at 200 MHz Frequency|
We strongly recommend that you use Ancor coax cable unless your antenna is supplied with cable. Ancor cable has a very tightly woven shield braid and uses tinned wire for greater corrosion resistance.
One of the most common ways to negatively impact your radio’s performance is with a poor splice to your antenna cable. Splices made without the correct connectors can reduce your transmit power by a factor of ten or more. When it is necessary to splice a coax cable, like at the base of a mast, use PL-259 connectors and a PL-258 double female connector. This method will reduce losses compared to simply soldering or crimping the conductors together.
One drawback of connectors is that they can allow water to get into the cable. Water tends to “wick” inside the braided shield on the cable, which quickly corrodes the copper into a green crust. You must protect all coax connections from water intrusion, especially in the bilge area, masthead, or where exposed to water.
For connections that are open to the elements, we recommend Ancor heat shrink tubing, after coating the connection with silicone grease. Many turns of waterproof tape is an acceptable, but less desirable, alternative.
The most difficult part of a marine antenna installation may be attaching the PL-259 connector. This Shakespeare video explains how to do it in three different ways: no-solder compression, the traditional solder method, and the Centerpin® easy way.