It is then technically simple to use the electrical signal from the microphone to vary the amplitude (wave “height”) of radio waves, while keeping their wavelength and frequency constant. This is known as Amplitude Modulation, or AM. It is used for MF broadcasting and a variation of it (Single Sideband, or SSB) is used in marine MF communications.


A more complex, but technically superior alternative is to vary the frequency of the radio waves. This is known as Frequency Modulation, or FM, and is used for VHF broadcasts, and for VHF and UHF communications.


Antennas 


Almost all electrical and electronic equipment transmits and receives electromagnetic waves, whether it is intended to or not. A radio, however, is specifically intended to transmit and receive radio waves as efficiently as possible. To do so, it requires an antenna — sometimes called an aerial or occasionally a radiator.


 


The science behind how an antenna works is complex, but a key point is that a vertical antenna is most efficient if it is very slightly shorter than half the wavelength of the radio waves it is intended to transmit. The exact length varies slightly, depending on the diameter of the antenna, but for marine VHF, the optimum length is about 93cm. Longer or shorter antennas will work, so long as they are an exact and simple ratio of the ideal length.

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