N9NEO wrote:
So the multipath distortion causes fading of the carrier only?? This
makes some sense to me. A small set of the lower sideband frequencies
would also cause phase cancellation, but since the audio spectrum is
moving around so fast no one notices. I think I'm on the right track

No, a simple 2-path is effectively a comb filter with the separation
between
frequencies of constructive and destructive interference determined by
the
difference in propagation delay. With enough delay, this separation can
be
much less than the bandwidth of the desired signal, causing multiple
cancellations within the passband. The delay difference is not a
constant
due to the roiling ionosphere so the frequencies at which destructive
interference occurs and their separations are constantly changing. The
severest form of distortion is when a cancellation occurs at the
carrier
frequency but if you have ever heard of 'flanging' in the recording
industry, you'll know what the interference effect can be when a
cancellation occurs in the sidebands. It is is very noticeable.

So use another carrier slaved to the received carrier and you
get better reception during fade. Even if it wanders a few cycles
during fade you probably don't hear anyway. I guess that is how a sync
detector works.

That's basically it.

Tom