VHF Radio Antennas
By Mark Corke
For any marine VHF radio to work correctly a suitable antenna is necessary. Your choice of antenna will play a large role in how well your radio's signal will be heard by other stations. Success in transferring energy from the radio's radio frequency power amplifier to the atmosphere depends on both the type of antenna you chose, and the specific coaxial cable used to connect the radio to the antenna. At the frequencies used for marine VHF communication (centered on the two meter band, 155 MHz) a considerable amount of the transmitter energy can be lost during its passage through the antenna cable.
The cable connecting the antenna to the radio is always a coaxial cable, which is in essence a center conductor, covered in a plastic insulating sleeve around which is braided copper wire. The signal is carried to and from the antenna via the center conductor, while the shield is there to prevent any unwanted electrical interference. This cable will be terminated with a connector at each end, one connecting to the antenna and the other to the back of the radio. It is important to bear in mind that there is always some power loss between the radio and the antenna due to inefficiencies in the cable itself. All connections must be of the highest order with good electrical continuity, and sealed to prevent moisture entering. This is especially important where routine inspections are not possible, at the top of a sailboat mast for example.
Power loss in the antenna cable may not be of great concern on a small powerboat, where the length of cable from the radio to the antenna is often only 10-20 feet. Conversely, a sailboat's antenna, mounted on the top of the main mast may require a cable length in excess of 100 feet, making cable loss a major concern. Generally, the larger the diameter of the cable, the lower the loss per foot. Successful marine use requires the highest quality coaxial cable. The insulation must be of a type that will not absorb water. The woven shield layer must have a high shielding density, should be tin-plated to prevent corrosion, as should the inner conductor wires. Although marine grade coaxial cable may cost more than cable made for land use, it is well worth the added expense.
The coaxial connectors used to connect to the cable must be marine grade if they are to provide satisfactory service over time. Although they require some skill in application, crimp type connectors installed with the proper crimping tools are best for this application. Soldering the connector to the cable is a good choice, if you know how to solder. Citizens band and closed circuit TV connectors are not suitable for this service. Buying the best quality connectors is a wise investment.
The strength of the transmitted signal available to a distant receiver depends largely on the performance of the antenna. Since the boat is free to maneuver, the antenna must radiate equally in all directions. VHF antennas are often described as having a certain amount of "gain", an indication that they can increase the strength of the transmitted signal. They cannot increase the amount of energy received via the coaxial cable from the transmitter, but they can redirect the available energy in a way that makes it most effective in reaching a distant receiver.
A truly omni-directional antenna would radiate energy equally in all directions. The radiation pattern would resemble the light emitted from a spherical light bulb. Sending energy directly upward or downward would be wasteful as there are no marine receivers above or below the boat. Properly designed, an antenna can redirect some of the energy that might have gone up or down, and thereby increase the amount directed toward the horizon. A short vertical antenna, usually about three feet long will radiate little energy upward or downward, thereby increasing the amount radiated horizontally, toward the distant horizon. The short antenna will typically have a gain of 3 db (3 db equals a doubling of signal power). This gain effect can compensate for some of the energy lost in the passage of radio frequency power from the transmitter to the antenna. Making the antenna longer increases the directional effect; the signal is further "squashed downward", radiating more energy in the direction of the horizon and still less in both upward and downward directions. Depending on antenna length and design, this type of antenna can have a power gain of 6 or even 9 db.
The law against free lunch also applies to antennas. The greater the gain of the antenna the more directional it becomes. Everything is fine as long as a 6 or 9 db antenna is vertical and stable. However, as the vessel rolls and pitches, the antenna will no longer be vertical. The signal radiated from the more directional 6 or 9 db antenna may then be directed upward into the sky or downward into the surrounding sea, rather than in the direction of the horizon. A good general rule for antenna selection is to use 3 db antennas for sailboats and either 3 or 6 db antennas for powerboats. In general, 9 db antennas should be reserved for use on land. The directional characteristics of the antenna operate in the receive mode as well as when the antenna is used for reception of incoming signals.
— Published: December 2015