Hi Don,
Thanks for sharing! It was good to see you using the FLAG... I may
build one after seeing the nice null... I like the choke you have on
the handle!! :)
73, and thanks,
Dave (NK7Z)
https://www.nk7z.net
ARRL Volunteer Examiner
ARRL Technical Specialist, RFI
ARRL Asst. Director, NW Division, Technical Resources
On 3/9/21 4:27 PM, Don Kirk wrote:
Just thought I should share something that was a first for me when tracking
down power line RFI and man it was frustrating, and I have tracked down a
lot of powerline RFI.
I noticed some weak RFI at Dans (KB9AX) on 160 meters earlier this year but
did not have time to track it down. Dan also complained about the RFI and
he mentioned this week that it was not strong but had become very
consistent. It definitely looked and sounded like powerline noise (120 Hz
spikes observed on audio scope and SDR receiver, etc.) Today I decided to
follow up on this RFI and using both my portable flag as well as a tuned
loop the direction indicated the noise was North from Dans QTH. I did some
tracking and both antennas kept pointing perpendicular to some power lines
a long distance North from Dans that were running East/West along a major
road and I was standing a good distance from the lines out in farm fields.
No matter where I walked my antennas kept indicating the signal was coming
from the lines with my antennas always pointing perpendicular to the lines
for peak signal even when I was hundreds of feet away from the lines (like
500 feet or more away from the lines). Dans 4 square receive antenna
system (DX Engineering 4 square receive array) indicated the signal was
about equal when pointing NE and NW which pretty much agreed with the North
direction my portable flag and tuned loop indicated and the East/West power
lines are 0.75 miles North of his receive array. I started to get the
feeling that the ground wave signal from the pole generating the signal was
weaker than the signal propagating down the line in our direction, and
therefore I could not detect the RFI directly from the faulty pole with my
ground based antennas.
We therefore looked for another frequency where the powerline RFI might be
heard from Dans, and using his horizontal HF beam (4 element beam up at 112
feet) we found the signal up on 24.9 MHz and it was strongest when pointing
NNE. Again my portable flag and tuned loop used on 24.9 MHz indicated the
signal was coming from the North (antennas perpendicular to the power
lines) when standing at Dans house very far from these lines (0.75 miles
from the lines). I then decided traditional direction finding was not
going to work. I then used my tuned loop adjusted for 24.9 MHz and with
receiver gain backed way down I drove the East/West lines with my small
tuned loop sticking out the window until I could hear the signal on 24.9
MHz. I then got out of my car and used my portable 4 element beam on 135
MHz and quickly found the faulty pole but the faulty pole was different
than all other poles I have located in the past from an RFI level
standpoint. I normally have to use 20 to 30 dB of attenuation with my 135
MHz receiver and 4 element portable beam to isolate a faulty pole, but I
actually was able to isolate the pole from nearby poles using no
attenuation at all but 10 dB of attenuation was ideal. This pole was
definitely generating RFI and I could even see low level of RFI coming from
it up on 437 MHz with a portable 4 element beam with SDR receiver, but it
was generating the weakest amount of RFI than I have ever encountered with
a power pole and I believe this is what caused traditional radio direction
finding to be useless (propagated signal along the lines was stronger than
the ground wave signal when viewed from Dans house). The faulty pole
turned out to be located 0.84 miles from Dan at a heading of 44 degrees
which really is not very far compared with other faulty poles we have
encountered at his QTH and some have been double that distance while
generating much stronger RFI. When close to the pole (few hundred feet
away as an example) my traditional HF direction finding techniques using
both my portable flag as well as tuned loop worked great and pointed
directly at the faulty pole which I was happy to see but this sure had me
scratching my head as I spent many hours tracking down this faulty pole.
I just wanted to share this experience as it was definitely frustrating,
but I'm glad I didn't give up. I suspect others may unknowingly run into a
similar situation but hopefully it will be few and far between.
P.S. I had my easiest powerline RFI find just a few days ago at Kens (KJ9B)
and the utility company responded today and said the problem was a bad
insulator. You can see my youtube video of this find at
https://youtu.be/X_5h-gSc6Ns
73,
Don (wd8dsb)
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