Syllabus Sections:-

External power amplifier

4i.1 Understand the need to drive external power amplifiers with the minimum power required for full output and how overdriving may cause harmonics and/or spurious intermodulation products.

The onset of the generation of unwanted components is relatively sudden.

Driving the amplifier with the minimum required output power results in a "clean" signal.

There should be a simple linear relationship between input and output powers  for instance :-

input is 10 Watts, output 80 Watts,

input 20 Watts, output 160 Watts, etc.

input 40 Watts, output 320 Watts, etc.

When the linear relationship starts to fail for example :-

input 40 watts gives only an output of 300 watts in stead of the expected 320 watts

then you have reached the point of over driving the amplifier which may cause harmonics and/or spurious intermodulation products.

Also the result of not reducing the drive is an increase in spurious intermodulation products and harmonics products and harmonics, using unnecessary bandwidth, and possible damage to the amplifier.

Do not to overdrive but operate below maximum ratings.

For those who know a thing or two about power - and that should be all readers - a small reduction in output power level will give a negligible variation to the strength of received signal. A doubling of power is usually needed to raise the "S" meter by half a point (but this does depend upon the calibration of the "S" meter on your rig. So dropping power to 350W from 400W and having a clean signal will lose you only a fraction of an "S" point.

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