"Rick " ) writes:
I have a so-called "modified sine wave" inverter that is connected to 200+
amp hours of deep discharge battery located in about the center of the
house in the basement. About a 30-foot length of #14 wire goes to an
outlet box in the shack, down on one end of the house, and provides 110VAC
(of a sort) when the power is off and I don't feel like hauling out the
generator. :-)

Trouble is, whenever I turn on the inverter it spews harmonics all up and
down the HF band and makes it difficult to impossible to hear weak signals.

I realize that a pure sine wave inverter would mostly alleviate this but
that's not gonna happen any time soon... meanwhile is there a filter I can
apply to the output of the inverter to round off the corners of the
modified sine wave (which is really nothing but a stairstep square wave)
and eliminate the worst of the harmonic noise?

Thanks...

Look in old ARRL Handbooks, or even their old Mobile Handbook.

A lot of the things about alternator noise would seem to apply
to such inverters, and even switching supplies. But it seems like
such coverage is much smaller nowadays than thirty or so years ago.

Figure out whether the signal is radiating from the chassis, or from
the output wire. If it's the former, it needs better shielding. If
it's the latter, add feedthrough capacitors in a box that shields
the output, and maybe put an LC circuit in series with the output
line, and tune it for a null. The primary of the inverter may also
be a source of noise, you need to figure that out first.

Michael VE2BVW