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Re: [RFI] New RFI

To: Don Kirk <wd8dsb@gmail.com>
Subject: Re: [RFI] New RFI
From: K9MA <k9ma@sdellington.us>
Date: Wed, 13 Jan 2021 08:21:11 -0600
List-post: <mailto:rfi@contesting.com>
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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