Marine VHF Installations
The installation looks relatively simple right?
How hard can it be?
It’s amazing that some of the systems we have come across actually work. The problem is that you can’t really tell how well your system is operating unless you connect a meter to it and perform a health check.
The health check you may have heard before is called a VISWAR (VSWR). It stands for Voltage Standing Wave Ratio. When an antenna is not matched to the VHF radio, power is reflected. This causes a "reflected voltage wave", which creates standing waves along the transmission line. If the VSWR = 1.0, there would be no reflected power and the voltage would have a constant magnitude along the transmission line. Reflected power produces peaks and valleys as per the diagram below.
In general, if the VSWR is under 2 the antenna match is considered very good and little would be gained by performing additional work in order to reduce it.
As the VSWR increases, there are 2 main issues. The first being that more power is reflected from the antenna and therefore not transmitted hence reducing overall transmitted range of your VHF. However, another problem arises. As VSWR increases, more power is reflected to the radio, which is transmitting. Large amounts of reflected power can damage the radio. In addition, radios have trouble transmitting the correct information when the antenna is poorly matched.
So you have a high VSWR, what could the problem be?
Common faults include connector, cable and antenna faults. When looking for faults, it’s important to know that most faults are connector related. This includes loose connectors, corroded connectors, and poorly installed connectors. Most remaining faults are cable related. This includes water in the cable, loose weather wrap, pinched cables and even screws in the cable! A small portion of the faults are antenna related.
The diagram above is an excellent example of a poor installation.
Coax connectors aren’t waterproof and should be sealed using self amalgamating tape if exposed to the weather or located in wet areas such as the bilge. Cables should have no sharp bends and ensure the proper cable is used for the length of run required. Larger diameter cables such as RG213 are used for lengths exceeding 5m due to the degree of attenuation (cable losses) experienced over the VHF frequency band.
At the end of the day, the object is to have the most efficient system so your transmission travels the maximum distance. Don’t let a poor installation let you down when you least expect it!