[RFI] New RFI

[David Eckhardt]

In working with our STEM students last year (before COVID), I ran into the
same problem in using online Yagi designers.  User beware on ALL these
online antenna designers!!!  I don't know where they came from, but they
are polluting the www resources with bad designs.  Plug them into 4NEC2,
EZNEC, or MMANA before committing to hardware.

Dave - WØLEV

On Fri, Jan 22, 2021 at 2:52 PM Don Kirk <wd8dsb at gmail.com> wrote:

> 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)
> >
> >
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-- 
*Dave - WØLEV*
*Just Let Darwin Work*