Jim Nye wrote:
?? I'm not aware of ANY HF transceivers that contain nulling-type noise
reduction circuitry. (and as an active contester with 30 years in ham
radio, I'm aware of pretty much every transceiver available)

Many transceivers do include a "noise blanker". This works by detecting

The Icom 746PRO contains such a circuit, which is separate from its
noise blanker function. Basically, the 746 samples slightly out of
band signals in order to determine the probable phase and amplitude
of coherent interference, and then it subtracts them from the in-band
signals. I believe that several of the Yaesu FT series transceivers
can do the same thing.

How does it determine which slightly out of band signals are broadband
noise and which are actual signals?

I can see a system where, for example, when tuned to 7006KHz the noise
reduction samples 6999KHz and subtracts what it finds there from the
signals on 7006. *Assuming* that the noise is consistent in phase and
amplitude across frequency (I'm not so sure that's a valid assumption
with BPL) I can see that cancelling the interference.

*Unless*...

there's a real live *signal* on 6999. Say, a MARS station running RTTY
morale traffic or a LSB drug-trafficing pirate or someone who wasn't
watching his VFO dial close enough or whatever. Haven't you now
frequency-converted that signal right on top of the guy you're listening
to on 7006? Admittedly 180 degrees out of phase, not that that makes
any difference...

The MFJ and similar systems work by sampling the *same* frequency
spectrum, but using a *different antenna*. This antenna (hopefully) has
a much worse signal-to-noise ratio - or better put for this application,
a much better noise-to-signal ratio! - than the normal station antenna.
In other words, it puts out mostly noise and very little signal.

You can then subtract the second antenna's output from that of the main,
reducing and hopefully completely nulling the noise while only slightly
if at all weakening the desired signal.

In any case, that's not the primary point of the post. The point is
that the ARRL has conveniently neglected the coherency property of BPL
leakage signals.

The coherency property is meaningless to the vast majority of amateur
radio operation.

Virtually none of the equipment is designed to take advantage of it.
And even if MFJ-type cancellation circuitry *was* present in most ham
receivers, the necessity for readjusting every time you change frequency
would make it useless for many hams.

--
Doug Smith W9WI
Pleasant View (Nashville), TN EM66
http://www.w9wi.com