[RFI] Noise resolution at N5BG/NI5T
Tim Duffy K3LR
k3lr at k3lr.com
Tue Feb 16 20:29:05 PST 2010
This is a followup to the report in our 3830 posting for the CQ 160 CW
contest. It is intended as a help for others that might be plagued with a
long term, or suddenly appearing, power line noise source. With proper
tools and a knowledge of what you are looking for, most often positive
results can be achieved.
Here is how N5BG and I approached this GIGANTIC noise problem which pretty
much wiped out all the first night western direction contacts. The 2nd
night we used a work around noise cancelling configuration which helped a
lot, but still kept us from a number of contacts and multipliers.
To set the stage, here are the factors involved. 1. We have never had any
significant noise on the Beverages over the past 10 years. 2. The noise
was BIG, and Broad band. 3. It was the strongest on the JA Beverages,
which from our station location is ~315 degrees. 4. From the Beverage farm
it is four (4) miles to the nearest power lines in the JA direction. 5. I
retired in January 2009 after 40 years of service at the local power
company. I know the circuits and all of the things to look for that might
cause RF interference
N5BG and I spent some time the week after the contest, searching for the
general source of the noise generator. This past Tuesday Larry and I spent
a couple of hours narrowing down the area.
I hooked up the K-3 to a VHF mag mount antenna and set it at 1.810 mHz AM.
We also had the AM broadcast radio set to 1710 kHz and the Yaesu FT-8800
dual band mobile set on 123.5 mHz AM aircraft band on the VHF side. The
intent here was not to be super sensitive. We knew that when we got close
to the source, that these radio setups would give us a definitive indication
of our proximity to the noise generator.
With the S meter on the K-3 we were able to narrow the area down to one
particular 3 phase lateral line on the a distribution circuit headed away
from our area. This circuit originates at the a substation about 5 miles
from our location, goes north across the river, and then down the valley
northwesterly for about 15 miles. This location is about 3 miles from where
we had previously thought the strongest noise was located, just from
listening on the AM broadcast radio. All of this area is in the direction
of JA from our station location.
The S meter on the K-3 did its job. With one trip down the valley it was
determined that the strongest signal was closer towards our area and the
substation than previously suspected.
The S Meter on the FT-8800 confirmed we were near the source, because it
actually came up and registered 2 segments when we were right under the line
as it crossed the highway we were traveling. It had never registered
anywhere before, even though we could hear the noise quite often, and
strong, in the aircraft band.
We inspected the poles on the lateral line and zeroed in on the last pole.
It has mounted on it a three phase transformer cluster for powering an
irrigation pump. In daylight I saw a suspect pole ground wire in close
proximity to one of the two steel X-arm braces supporting the top two
crossarms where the lines are dead ended.
This still didn't compute as the noise is so broad banded, and so strong
over such a wide area, that I didn't think it could be generated by a simple
hardware/ground wire situation.
It was just at sundown, and we both agreed that if there was an arc of the
magnitude I expected, we could see it after dark. We returned to the site
at near dark and sure enough, it was quickly spotted by Larry while I was
trying to listen for an arcing sound.
The hot line clamp which connects the lead from the top of the lightning
arrester/cutout combo to one of the phase conductors was arcing. It was
VERY visible. The irrigation pump was not operating, and probably hasn't
since last fall some time. However, the excitation current required for
energizing the primary coil and core of the transformer is just enough to
create a constant arc at 7.2 kV.
It makes sense now. The arc is on a primary phase line, which in turn is
VERY long antenna. You just have to LOVE those spark gap transmitters.
This one is probably in the nanoWatts of PO, but the ERP is a whole lot
more. So, with the lengths of multiple lateral taps and the main circuit
added together, there are perhaps 40 miles of antenna wire attached directly
to the transmitter.
I have a jpeg shot off Google Earth which shows the relationship of Larry's
place, the Beverage farm and the noise source location. It is 6.5 miles at
300 degrees from the Bevs to the noise source.
The two JA Bevs, at 315 degrees, always had the strongest noise signal level
(about S-5 average). They consistenly told us the noise was in that
direction. I will send the jpeg shot to anyone who contacts me directly.
We prepared a letter describing the problem, suspect location and equipment.
Larry submitted the letter to the power company people this morning.
In less than 3 hours, the BIG NOISE went away. The power company line crew
replaced the hot line clamp, and all is well on the N5BG/NI5T Beverages.
This morning the noise was an S-6 level on Topband in the AM mode. Now the
level is background hiss.
In most cases if the amateur will search out the probable noise source and
present it to the power company operations people, they will clean it up.
Most small, rural power company's do not have the in-house expertise, nor
the equipment, to search for and find an RF interference problem.
Your mileage may vary, but it works for us. CU all in CQ 160 SSB next week.
73 de Milt, N5IA
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