[RFI] New RFI

[K9MA]

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 at sdellington.us