QRM eliminators can work surprisingy well at times and appear next to
useles at others. It's rarely the unit, rather the situation. All of
these similar units, of which the MFJ noise cancelling signal enhancer
is but one operate by combining two essentially identical signals, the
unwanted noise. With the phase of one 180 degree from the other and the
amplitude matched total cancellation should occur, and it will, almost
certainly.
The difficulties are two fold. First is that whilst phase adjustment can
be made over a reasonable range the amplitudes must be the same. That's
not so easy and the noise antenna often needs to hear a lot more of the
noise signal than you might think necessary. The second problem is that
these units can only eliminate one noise source. If you are surrounded
by plasma and switch mode devices removing one just leaves a bunch more.
The problem here, other than amplitude matching is that all of the
arriving noise signals from different sources have a different phase
relationship between your main and noise antenna. So adjusting for one
noise signal to have 180 degree relationship between main and noise
antenna will make every other signal have a different phase
relationship, the signals come from different directions and different
distances so reach the two antennas at different times, and hence
different phases. Cancelling one doesn't do much for the other. If you
have one noise source coming from one direction and you can collect
enough of it then the so-called QRM eliminators work rather well, until
something shifts and the phase changes and your tweaking again.
Of course once Wayne gets done with all the K4 trickery he will have
time to develop the K-null that will eliminate all unwanted noise from
all sources on all frequencies at all times at the push of a button.
Join the queue :-)
Martin, HS0ZED
On 30/6/63 00:18, [email protected] wrote:
Hello all,
In a recent rush to eliminate urban noise I bought an QRM eliminator.
The working principle is to null out signals by shifting the phases of
two antennas (one main/tx and one rx). I can see the effect sometimes,
but it is not the holy grail for all types of QRM. I still don't get rid
of the PSU/LED/plasmaish noise here, sometimes the S/N is a bit better
but maybe thats my inner impression to justify buying this thing.
Now, I do wonder how this differs from diversity receive, effectively
versus local QRM.
Note I have a 10m vertical as the 2nd ant, while the main ant is
horizontal.
Any thoughts or hints ?
tnx 73 Gernot DF5RF
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