We repainted and unstepped our mast a couple of years ago. The mast was easy to handle and horizontal, so I checked the wiring. The wiring was relatively new for some of the components, such as the radar and windspeed, but it was original equipment and over 25 years old for the lighting and coaxial cable. It was time to replace the wiring. It was simple to find a replacement cable for the light. However, there are many types of coaxial cables, so deciding on the right one for VHF took some research.
The coaxial cable (pronounced COE-ax) is a cable with a central conductor enclosed within a shield and protected by a plastic outer jacket. A thick layer of insulation separates the conductor from the shield. The conductor and the shield are connected by a common axis. This cable is designed to carry high-frequency radio waves and comes in various varieties, depending on its application.
A radio frequency wave is sent along the wire connecting the VHF to the antenna when it transmits. This high-frequency signal then tries to radiate from the conductor. A wire without a protective shield would result in a very unreliable antenna that would lose most of its output signal before it reached the antenna. The concentric shield of the coaxial cable prevents this from happening by reflecting the output signal back to the center conductor instead of letting it radiate.
The VHF receives a weak signal at the antenna, which is then transmitted along the coax cable to the radio. The coaxial shielding stops RF noises from other sources, such as motors, generators, or HF radios, from interfering with the received signal.
The choices of coaxial cables for marine VHF applications are limited. The majority of fixed-mount marine VHFs operate between 156MHz and 164MHz, with a maximum output of 25 watts. These specifications apply to three types of coaxial cable that are commonly found in marine stores: RG-58U (which has replaced the older RG-8U), RG-8X, and RG-213 (which has replaced the older RG-8U). The choice of cable is determined by signal loss, cost, and wire size.
RG-8X should be used for shorter runs of up to 15 or 20 meters, and RG-8U is best for longer runs. This is also recommended by the IOC and ISAF for their Cat 1 offshore race requirements of an antenna feeder cable that has less than 40% loss.
A copper braid is required for coaxial cable, preferably tinned. Braids are described in percentages; for example, 98% is best and 96% is acceptable. The dielectric (the insulating material surrounding the core of the cable) must be solid polyethylene. Foam-core dielectrics can draw moisture from the cable through the connections, causing the core and the braid to corrode.
This post was written by Justin Tidd, Director at Becker Mining Communications! For over 15 years, Becker Communications has been the industry’s leader in mine supplies and electrical mining communication systems. As they expanded into surface mining, railroads, and tunneling they added wireless communication systems, handheld radios, tagging, and tracking systems, as well as gas monitoring.