Ham Radio Frequency Modulation Use

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Frequency modulation provides many advantages when used for amateur radio communications.

Frequency modulation tends to be used where mobile and portable operation has been more widely used. The reason for this is its greater resilience to noise and signal strength variations when compared to AM.

In view of this, FM has gained very widespread acceptable on the VHF and UHF amateur radio bands, and in addition to this it is also used on the Ten Metre ham radio band as well.

Frequency modulation technology

The technology behind frequency modulation is the same whether it is used for amateur radio or any other application.

Instead of carrying the information, i.e. sound in most cases as changes in amplitude, for frequency modulation, the frequency of the signal is changed in line with the modulating waveform.

As all the modulating information is carried as frequency variations and no amplitude changes are required, the signal can be passed through a limiter stage. This not only removes the major source of noise which is amplitude noise, but it also removes the major changes in signal strength resulting from mobile operation.

In view of these advantages, frequency modulation is an ideal mode of modulation for many amateur radio applications.

Specifications for amateur radio FM operation

One of the key parameters associated with any form of modulation is its bandwidth and for amateur radio FM operation there is no exception.

The form of FM used for amateur radio communications is narrowband FM and for the VHF and UHF bands. A deviation of ± 5 kHz is normally the maximum and the channel spacing is dependent upon location. In the USA it can be 15, 20 or 30kHz dependent upon the region. In Europe channels are spaced every 12.5 kHz and the bandwidth must be less than 12.5 kHz to prevent undue adjacent channel interference.

For VHF and UHF operation, FM uses vertically polarised signals. Vertical antennas like single element ground planes and other single element antennas radiate vertically polarised signals. These are used because of their omnidirectional radiation pattern in the horizontal plane and mobile stations can operate without any need to realign antennas as they move.

On Ten Metres FM amateur radio operation is a little different. It typically takes place between 29.1 and 29.3 MHz for simplex operation and with repeater inputs between 29.5 and 29.6 MHz and repeater outputs between 29.6 and 29.7 MHz. The simplex calling channel is 29.600MHz.

For this operation, the permitted bandwidth is 6 kHz against a previous maximum permitted bandwidth of only 2.7 kHz.

FM power amplifier ratings

One of the points to note when utilising or designing transmitters for FM use, is that when transmitting, the carrier is present all the time and as a result the heat-sinking requirements must be able to accommodate this.

Additionally the battery must be able to accommodate the drain from a continuous carrier during transmission periods.

Modes like single sideband have a much lower average to peak power ratio.

The advantage of a mode like FM is that the final amplifier can be run in compression, i.e. class C and this gives a much greater level of efficiency than a class A or class AB amplifier that would be needed for SSB.

Frequency modulation is a mode of transmission that has many advantages for amateur radio use. It offers effective transmission properties in terms of reducing noise and signal variations, and it also allows the final amplifiers to be run in class C which enable high efficiency levels to be achieved.

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