You might try a common mode choke on the coax.
On 8/6/2012 5:06 AM, N1BUG wrote:
> 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?
> Paul N1BUG
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