Dear J, LF Group,

It is unlikely the distorted waveform is due to saturation of the
transformer core. To have any noticeable effect on the waveform, it would
have to be driven quite deeply into saturation, and at 137kHz, this would
cause it to get smoking hot very quickly due to the core losses. In
practice, it is hard to do this with any reasonably sensible windings. In
any case, the low pass filter should then largely filter out the distortion.

I found that corona discharge makes the oscilloscope display "flicker" with
rapid changes of amplitude, although the waveforms do not look distorted as
such. I would be surprised if you are getting corona with your antenna - the
large length of wire means high capacitance, and so voltage no more than a
few kV.

It would certainly be a good idea to check the output with a 50R load using
the oscilloscope. To make sensible comments, it would help to know:
What kind of amplifier circuit is in use?
What kind of low-pass filter is in use?
At what point in the system is the "scope match" connected?
What do the awful waveforms look like? (a picture or sketch would help a lot
here...)

Cheers, Jim Moritz
73 de M0BMU