[RFI] MFJ-856 oddity

Dave Cole dave at nk7z.net
Mon Aug 6 06:12:29 PDT 2012

Hi again Paul,
Unplug anything from the the headphone jack and try that test...  :)

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On Mon, 2012-08-06 at 07:06 -0400, 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?
> 73,
> Paul N1BUG
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