The maximum power or high power, which can be legally transmitted by a marine VHF radio is 25 watts. When reliable VHF communication is available, that is for ranges less than 10 NM, it is perfectly satisfactory to use less transmitter power. The advantage for the user is that there is less drain on the radio’s power source, that is battery supply. However, the main advantage of using less power is that by reducing the range over which one’s vessel transmissions can be heard, it allows others a few nautical miles away, to utilize the same channel. Another important advantage is the possibility to avoid capture effect explained below.

For these reasons, all marine VHF radios must be able to transmit on a reduced or low power setting of 1 watt. With one’s VHF antenna sited as high as possible, good communication ranges in excess of 5 to 10 NM on even the smallest boat are possible.

When selecting VHF CH 15 or VHF CH 17 for on board communications only, the power setting should automatically switch to low power.

Place the mouse on the vessel and check who hears the broadcast through VHF radios on the chosen vessel

Capture effect

When a radio is not being used as a transmitter, it is simply a receiver and it will lock on to the strongest signal it receives. For example, 1 NM away from our vessel are two vessels which are transmitting on the same channel at the same time. One of them is transmitting on high power and another on low power. Only the signal from vessel that is transmitting on high power can be heard by our vessel.

For this reason you should first try to transmit on low power and only if you are unsuccessful you may try on high power.

Last modified: Saturday, 25 April 2020, 7:50 PM