[RFI] MFJ-856 oddity

N1BUG paul at n1bug.com
Wed Aug 15 06:51:17 PDT 2012


My total attenuation (series connected) is 50 dB. There are places 
even that is not enough - where the meter will sit pegged throughout 
a ~400 foot area. These are *not* areas where the attenuator anomaly 
is happening, but places where the attenuators are working yet 50 dB 
is just not enough.

Note that I seem to have some rather strong noise sources. One I can 
hear from one mile away with the 856 pointed toward the offending 
pole, trees in the path but *no power lines, poles or man made 
structures of any kind* between the 856 and noise source. Perhaps 
this is not unusual, but this level of intensity surprised me. The 
particular source, later narrowed down by an ultrasonic device, 
seems to be either a cutout or pin insulator on a ~13kV line.


On 08/14/2012 10:15 PM, Missouri Guy wrote:
> Paul,
> 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.
> 73,
> 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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