We evaluate marine VHF radios through a series of bench tests to check performance of the transmitter section, receiver section, and audio system.
To get accurate results we use a communications service monitor.
FCC Regulations limit marine VHF transmitter maximum power output to 25 watts, they also specify the radio must have a low power setting, typically 1 watt, for harbor use.
Watts of power output leaving the radio translates into radio frequency (RF) energy coming out of the back of the radio and eventually through your antenna. We take all of our transmitter power measurements directly off the radio antenna port on the back of the radio. In the real world several other factors could limit the actual power that reaches the antenna.
Each transmitter test is done on channel 16 at room temperature (74F), as well as at temperature extremes. To reach the cold extreme, each radio was put in a freezer (at 15F) for four hours prior to testing. To get the radio to a high-temperature extremes we use an environmental chamber. Each radio is left to cook for two hours at 122F and then immediately run through another transmitter power and frequency test.
We also test marine VHF radio at two different input voltages. First, we use a 70-amp power supply to produce 13.8 volts DC to simulate operations in a boat with the engine running and the alternator powering the boat’s electrical system. Next, we step down the voltage to 11.0 volts DC to simulate trying to transmit with a nearly dead battery. Amperage draw measurements are recorded using clamp-on ammeter at both voltages. We also check each marine VHF radios low-power setting, measuring both power input and output.
Receiver sensitivity is the ability of the radios receiver section to hear a weak signal. Normally stated in microvolts, typical marine VHF receiver sensitivities run from .22 to .35 with industry groups recommending a minimum .50 microvolts. Each radio receiver was tested for the minimum signal it could receive at a specific industry standard setting between background noise and generated signal (12 dB SINAD).
Another receiver standard is selectivity; it is the ability of the receiver to reproduce only the signals you want to hear and not others even though they may be strong and nearby. Our test equipment does not allow us to test each radio for this characteristic. Information on sensitivity is provided by the manufacturer, a higher number is better.
Audio System Testing
One important part of the marine VHF radio often overlooked is the audio amplifier and speaker. Boats can be noisy places and if you can’t hear the output, it doesn’t really matter how well the transmitter or receiver work. To rate the audio system of each radio, we measured the sound pressure at maximum volume while generating a 1 KHz tone with our COM3010 and inputting that signal into the radio. Sound pressure measurements, in dBA, are taken at a distance of 1 meter. Our tester also rated each audio system with a voice input by monitoring a weather channel and rating the quality of sound reproduction.
The display unit on each radio was rated by our tester based on the size of the channel number display, the amount of other information shown, the size of the screen, and the quality of the backlighting.
We rate each marine VHF radio on the level of features incorporated into the unit beyond the basic ability to transmit and receive. Units with more sophisticated DSC capabilities, superior channel comments (channel usage descriptions on the radio display), hailers, and alphanumeric keypads received higher ratings.
To facilitate hailer testing on radios with that capability we connected a PA horn. Then we operated the hailer system in manual and automatic modes to make sure everything worked as stated in the manual. Hailer/foghorn systems are rated for ease of operation and the extent of their capabilities, more capable systems received higher ratings. To actually use the hailer/foghorn aboard a vessel equipped with one of these radios would require the purchase and installation of a PA horn matched to the radio.