[Elecraft] RF Noise canceller ?

[Ron D'Eau Claire]

> Would this type of noise canceler not work on "white" noise also ? 73,
> Marinos, ki4gin
> 
 
That depends a lot on the source of the noise, broadband (white) or
otherwise. These circuits depend upon a second antenna picking up the noise
better than the signal. Then the signal from the "noise" antenna is inverted
and added to the signal from the main antenna. Adjustment of both phase and
the signal levels in the circuit is required so that when a pulse of noise
from one antenna hits, say +10 uV, the same pulse from the other antenna
hits - 10 uV at the same instant so they add together and produce an output
of zero volts. The result is that the noise is cancelled out while,
hopefully, the signal remains. 

Even a wire on the ground will pick up a distant signal. Indeed, a wire on
the ground sometimes picks up the signal with less noise that a huge wire in
the air! One of the most effective "low noise" receiving antennas known is a
wire on the ground or sometimes even buried a few inches below the surface
of the earth! (And insulated). So the noise canceling system works in some
situations and not in others. As one person pointed out, it often results in
significant attenuation of the desired signal too when the desired signal is
being picked up by both antennas.

How well the system works on broadband noise depends upon how stable the
phase relationship remains between them. Some noise will arrive via ground
wave, multiple reflections from the ionosphere and even reflections from
nearby objects that may shift constantly. That will cause the phase
relationship, and the degree of cancellation, to shift considerably as well.
For that reason these blankers have been most effective on locally-generated
noise arriving by stable ground wave. Sky wave noise simply shifts too much
to provide a useful null.

I have what many Hams call (for the lack of a better word) a "counterpoise"
that I often use with tinkering with end-fed radiators. It's simply a wire
about a meter or so off of the ground that runs from the shack window in two
directions: along a porch in one direction and along a fence in another
direction. I have found that this low, random wire sometimes makes a superb
receiving antenna. Often I can hear signals better using it than using my
doublet overhead, especially on the lower bands, thanks to a much lower
noise level it picks up. That fits with regular reports over the decades
that noise is often attenuated far more by the earth than the signals
arriving via sky wave. 

Sure, signals overall are weaker with an antenna near the ground, but that
is of absolutely no consequence. In receive, antenna gain is unimportant as
long as the receiver signal-to-noise ratio is such that the
internally-generated receiver noise won't mask the signals. Any of the HF
Elecraft rigs has an internal noise level that will allow even a very lossy,
small antenna to be used to maximum effectiveness when receiving. Just turn
on the preamplifier or turn up the volume to make up for the low signals
from the antenna. Using my "counterpoise" for a receiving antenna often
gives me huge improvement in signal to noise levels, especially on 160 and
80 meters. 

Some Hams have reported excellent results receiving on a length of old coax
thrown on the ground! 

Bottom line: The noise canceling system seems to be most effective when the
noise antenna can be positioned so as to pick up the noise much better than
it can the signal, and the noise source is nearby and providing a stable
phase relationship at both antennas. A purpose-built receiving antenna close
to the ground may be more effective, especially when trying to pull weak DX
out of the noise level. 

Ron AC7AC