[RFI] MFJ-856 oddity
RFI Services' Michael Martin
mike at rfiservices.com
Tue Aug 7 05:07:45 PDT 2012
I have an 856 and have not experienced this issue.
Never stand under the power line with the receiver. The antenna for the
noise will be over the antenna for the receiver and the yagi will be like a
ball of aluminum instead of a well-designed directional antenna.
When looking at signal levels from utility poles remember the antenna for
the noise is the poles. Maintain equal distance from the "Two" poles you are
comparing. Don't stand under the power line while performing this RF level
test or the power line and every other wire will become the antenna for the
Maintain equal distances between 2 poles and as close to the same distance
away from being under the wires. In the search for power line noise make
your measurements a comparison between two poles and reduce your gain until
you can easily determine which of the two poles has the highest signal from
the arcing source. If the higher signal is from the pole to the left, the
move to the left between the next two poles and repeat the process.
Most importantly is to tell your power company to call me! I had to throw
that in there.
One thing I did to my receiver, and I really don't know if it made a
difference, was I lined the inside with aluminum foil tape to help shield
it. It just made sense to do this due to the fields.
I just tried out a new antenna for DFing power line noise and I can't
believe how affective it is. It's made by a company called Telonics and it
looks like 4 quarter wave rubber duck antennas on a boom. They refer to it
as an H design antenna (for some reason). I have never purchased an antenna
that performed as well without modification. I'm in the field and don't have
the model number with me but it will soon be on my web site. If you call
them ask for Phil and tell him I sent you. He'll know the antenna. They are
designing one for me that is centered at 143MHz. This allows max gain on a
frequency. that won't have traffic.
Michael C. Martin
6469 Old Solomons Island Rd
Tracys Landing, MD 20779
From: rfi-bounces at contesting.com [mailto:rfi-bounces at contesting.com] On
Behalf Of N1BUG
Sent: Monday, August 06, 2012 7:07 AM
To: rfi at contesting.com
Subject: [RFI] MFJ-856 oddity
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?
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