Hi Don,
My experience is very similar to yours, using the MFJ-856. To make sure
I have the right source, I relay the audio from the HF receiver at home,
and listen for a matching pattern on 135 MHz. My crude attenuator
consists of two shorting straps, but it works. In this urban area, with
its high background noise level, I find I can usually hear a bothersome
source on 135 MHz right out the front door. The ones I've found have
generally been within a half mile or so.
I've also found that I often have to rotate the VHF antenna to account
for polarization. The initial bearing I get with the HF antenna at home,
however, is usually pretty close.
The MFJ-856 does have an S meter, which is useful. Using a receiver
without one, you really have to use an attenuator. What you're doing
there is using the receiver's minimum discernible signal as a reference.
That can work. However, just walking around with a receiver with AGC
tells you very little unless it has an S-meter. Even then, often the
S-meter resolution is very poor.
I just built a 20 meter shielded loop, which I'll try out next time.
On 1/13/2021 7:22 AM, Don Kirk wrote:
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)
--
Scott K9MA
k9ma@sdellington.us
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