I'm still pursuing my power line noise. I have a few questions,
starting with this one:
A while ago I modified an MFJ-856 power line noise sniffer. The mods
consisted of adding a T match to the driven element, moving the
receiver back behind the reflector, and adding a 3 step attenuator
(25, 15, 10 dB).
Most of the time this behaves as expected, but every once in a while
I find a pole where the attenuators do nothing whatsoever. These are
not always situations where the noise is so strong that the meter is
full scale, but sometimes that is the case. At first I was paranoid
about bad switches on the attenuator, but this pattern is repeatable
at certain poles and happens at less than 5% of noisy poles. When
this happens the sound is more like a hum than other poles. By that
I mean less like a buzz, frying or other variations. So what is
going on? The only two theories I have - and I fail to really fathom
how either could be happening - is that 1) Noise is coming down the
coax through the attenuator and into the receiver common mode in
rather than differential mode or 2) a strong induction field
coupling to something in the receiver or headphones. Sounds crazy?
Any thoughts?
73,
Paul N1BUG
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