Hi Scott,
The Moxon sounds interesting and I will have to build one for my tool box
of DF antennas, nevertheless let me share my experience with my 136 MHz 4
element portable Yagi that has very deep nulls off the side that I use with
a portable AM receiver for locating power line noise. While a deep null on
136 MHz is indeed very helpful, even with a deep null I find that I need to
switch in a lot of attenuation when close to the suspect pole and
especially to make sure I have identified the faulty pole. I typically
switch in 20 to 30 dB of attenuation but in a few cases I have had to use
40 dB of attenuation (40 dB of attenuation when the poles were very close
to each other as an example). When I feel I have located the faulty pole I
will stand between the suspect pole and the adjacent pole but off to the
side of the power line (maybe 25 to 50 feet off to the side of the power
line as an example) and then I just use the main lobe of the beam to
determine which pole is the faulty pole (really very simple). I say I'm
using the main lobe but of course the deep null is coming into play as
that's part of the antennas directivity. Also vertical versus horizontal
orientation of the Yagi sometimes is helpful in determining the suspect
pole versus the adjacent pole.
When using 136 MHz and my 4 element portable yagi I can typically hear the
suspect pole from at least 0.1 to 0.2 miles away and in some cases more
than 0.5 miles away when not using attenuation, and then I just add in
attenuation as I approach the suspect pole. My 136 MHz receiver does not
have an S-meter but as long as I use adequate attenuation an S meter is
absolutely not needed.
I have had cases where the power lines more than 2 miles away were
radiating the RFI so strong on MF and HF that I swore I was very close to
the suspect pole but then when using 136 MHz nothing was heard (thankfully).
When approaching the suspect pole if needing to walk along power lines when
listening on 136 MHz I can sometimes hear the RFI a good distance down the
line (many poles down the line) from the suspect pole (can sometimes hear
the standing waves too), so I once again switch in attenuation to help
reduce what I am hearing. I will also get away from the power lines to get
a new bearing to make sure I have not walked past the suspect pole, etc. I
used to do a lot of intentional triangulation when close but now often just
walk toward the RFI using the peak and null to determine which way to go.
I use MF and HF direction finding (and intentional triangulation when
needed) to get me within range where I can then start hearing the RFI on
136 MHz, and on MF and HF I have to often stay very far away from existing
power lines in order to get a true heading on the source of RFI versus
getting tricked by radiation of the nearby lines.
I also built my own very simple 437 MHz Yagi that I use with an SDR
receiver running on my laptop and will occasionally use it to double check
that I have indeed identified the correct pole, but only use it for
confirmation as I find 136 MHz much more useful for direction finding in
conjunction with the use of attenuators since I don't hear the RFI on 437
MHz unless I'm right on top of the suspect pole (lets say within 100 feet
and sometimes less as an example based on my limited experience using 437
MHz).
P.S. my 4 element 136 MHz Yagi is just an old cushcraft 2 meter beam in
which I lengthened the tip of each element by approximately 1.4 inches
using alligator clips based on modeling I did using 4NEC2, and it works
great (no need to change element spacing).
Just FYI based on my experience tracking down power line RFI for myself and
others.
Don (wd8dsb)
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