[RFI] New RFI

[Don Kirk]

As a follow up to the Moxon antenna suggestion a week or so ago, I built a
couple of Moxon antennas this week (one for 146 MHz and one for 136 MHz)
but ran into some problems with some online design calculators that I
thought I should warn folks about.  I stumbled upon some online calculators
that generate dimensions for the Moxon antenna (KG4ZOW and W4/VP9KF
calculators), and they both generated values that did not model correctly
when using 4NEC2.  I then downloaded the program MoxGen (by AC6LA) and the
dimensions generated by this program model correctly (proper pattern, SWR,
etc.), and it provides dimensions similar to the QST article by K1BQT.  I
did more digging and found an error in one line of the source code that
KG4ZOW said he uses, and I have notified KG4ZOW and W4/VP9KF (should have
been a multiply function versus the divide function that was used).

I suggest folks only build the Moxon antenna based on the K1BQT article or
the MoxGen program.

Initial testing of my 136 and 146 MHz Moxons look good, and now I will test
my 136 MHz Moxon under real life conditions versus my 4 element Yagi when
locating faulty power lines to see what benefit they might offer (I
definitely like the smaller size of the Moxon).

Just FYI,
Don (wd8dsb)

On Wed, Jan 13, 2021 at 8:22 AM Don Kirk <wd8dsb at gmail.com> 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)
>
>