[600MRG] Noise cancelers

[Rudy Severns]

I have used both the MFJ1026, modified for MF, and the DXengineering NCC 1. 
I used them with two identical 30' whips, separated 200'.  I found that both 
were useful for rejecting signals/noise originating at distance greater than 
a few miles but absolutely useless for noise originating in the near field 
especially when the noise was not coming from a point source.

At 475 kHz a wavelength is about 2000'.  For a signal originating at a 
distance less than this, your rx antenna will be in the near-field of the 
offending radiator.  If you model the near-field patterns for a pair of 
verticals you'll see they are utterly different from the classical far-field 
patterns we usually assume as the basis for the phasing noise canceling 
schemes.  My noise sources are within 400' of my antennas and distributed 
around a wide arc, clearly within the near-field.  The problem with 
near-fields, especially for top-loaded short verticals, is that they are not 
1/r but 1/r^3.  The phase and amplitude of the signal from the noise source 
changes very rapidly over the spacing of the whips.  I found there was a 
difference in amplitude of 15 dB between the two whips for the same noise 
source!  At 630m utility lines, with periodic vertical wires for ground 
connections and service drops, are basically just heavily top loaded 
vertical arrays radiating garbage!

I found that my terminated loop and BOG antennas are much better which 
agrees with the NEC modeling prediction.  I have begun to write up this work 
in some detail but right at the moment I'm taking a math course at the 
University of Oregon and the little grey cells are totally occupied with 
trying to survive.  It's not clear if they will!  This will go on for 
another seven weeks so any outputs from me will not happen until after 
Christmas.  Except for monitoring WSPR I'm pretty much QRT.

73, Rudy N6LF