11 December 2013

Simple test results of a 7MHz Halfwave Power Output Filter

As shown in the attached schematic and test results. Using my MFJ259B antenna analyzer I was able to obtain a view of the VSWR of this filter over the HF spectrum.

At the very least this illustrated 2 observations to me.

1) A reasonable match to 50 Ohms is obtained over the 40m band.

2) A mismatch exists for frequencies other than the 40m band. Both above and below this band.

The obvious benefit, and the whole point of the filter, is the fact that harmonics are attenuated by the filter. This is all good.

The more interesting Questions to me are :-

1) What is the effect of the reflected harmonic energy when it is absorbed by the Class C Non Linear Power amplifier?

2) Is the Return Loss of the harmonic energy so great (>80 dB) that it is insignificant or does the reflected energy generate significant IMD?

3) Why are diplexers mostly implemented on Mixer outputs but not on output filter circuits?

All good :)

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