[RFI] MFJ-856 oddity

Sam Morgan k5oai.sam at gmail.com
Tue Aug 7 06:35:35 PDT 2012

GB & 73
Sam Morgan

On 8/7/2012 7:07 AM, RFI Services' Michael Martin wrote:
> 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
> noises.
> 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.
> Best wishes,
> Michael C. Martin
> RFI Services
> 6469 Old Solomons Island Rd
> Tracys Landing, MD 20779
> www.rfiservices.com
> 240-508-3760
> -----Original Message-----
> 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?
> Any thoughts?
> 73,
> Paul N1BUG

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