From G3PLX:
Uwe:
I am not an expert on LF power-amplifiers (I worked on 10kW HF
power-amplifiers 40 years ago but not recently), but it looks to me that the
voltage and current waveforms of your home-brew PA have some harmonics. I
took the image of the homebrew waveform and did a mirror-image (flip
vertically) then superimposed it on the original. This shows clearly that
the voltage waveform has different rising and falling slopes, which means
there is some even-order harmonic present. Maybe this is due to some
unbalance between the two halves of a push-pull circuit. The current
waveform is more confused but that is probably because the harmonic currents
are higher. This is some effect of the low-pass filtering and the antenna
tuning.
There is also some broadening of the scope trace at some parts of the
waveform, which shows there is some signal there which is not
harmonically-related. It could be HF/VHF parasitic oscillation, but it's not
possible to tell from the scope trace for sure. It could be just some
power-line-frequency effect. Only a spectrum analyser could show this
properly, but you may get a clue if you can hook the scope ahead of the PA
low-pass filter. If this broadening of the trace is worse here, it may mean
that there is some HF parasitic oscillation.
73
Peter
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