[RFI] MFJ-856 oddity

Missouri Guy n0tt1 at juno.com
Tue Aug 14 19:15:05 PDT 2012


I haven't experienced the same problem as yours.
I use a homebrew Moxon antenna for my RFI chasing.  I put several #43
beads on the coax right at the antenna driven element and at the
receiver.   The homebrew *series-connected* step attenuators in my unit
are 6, 10 
and 20db (maximum total of 36db) but sometimes that is not
enough!!   Is the attenuation on your unit at *total* of 25db or 50db?
I would think 50db would be adequate.  25db is not enough.

Charlie, N0TT

On Mon, 06 Aug 2012 07:06:45 -0400 N1BUG <paul at n1bug.com> writes:
> 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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