[Elecraft] Diversity receive versus QRM eliminators

[Martin Sole]

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, gt-i at gmx.net 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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