Hi Dave,
That's a nice touch, using your phone to monitor the SDR spectrum.
One of the defining characteristics of line noise is that it's very
broadband, unlike switching supplies, etc. There are also 120 Hz
harmonics in the AM audio, but other sources can do that.
73,
Scott K9MA
On 6/23/2020 08:12, Dave wrote:
Nice write up Scott.
Once the pandemic winds down I have to have JCP&L (just crummy power and light)
come back out. I am 99% sure it is the insulator on the pole in front of my house.
It is the end of a high voltage run. I have videos using an RS390 receiver and
tapping on the pole with a small sledge hammer (I did not hit it hard enough to
dent wood). With my Flex 6700 I can listen and watch the panadapter noise floor
from my iPhone (SmartSDR for IOS). So it is easy to listen to the 390 and watch the
SDR noise floor at the same time.
My noise is the same. Wide band looks like whole noise floor rises. 10 to 20 dB
jump and most pronounced on dry breezy days. Goes away with rain.
Dave wo2x
Sent from my waxed string and tin cans.
On Jun 22, 2020, at 11:53 PM, K9MA <k9ma@sdellington.us> wrote:
For many months, I've been plagued by line noise, often 25 dB above my
"normal" urban noise level. I tracked it down to a 60+ year old power pole about a
quarter mile (350 m) from my QTH. I tracked it down initially with the MFJ-856, a 135 MHz
receiver with a 3 element yagi. I have two shorting straps on it to act as attenuators, one
across the antenna and another across the receiver input. The noise nearly pegged the meter with
both straps in place. To verify it was the correct source, I relayed the HF noise from my
station receiver, so I could listen to both simultaneously. There are usually fluctuations in
the noise; when they match, I know I have the right one. Unfortunately, when it was active, this
source was often very steady. It frequently went silent in wet weather, so I made numerous trips
over there. It was always silent at 135 MHz when it was silent on HF, and vice versa. Finally
one day it was erratic, so I walked over there and indeed heard matching patterns. Later I took
the MFJ-5008 ultrasonic receiver over when it was active, and heard noise from a specific area
near some insulators on the pole.
MFJ-856 Shunts <http://sdellington.us/hr/MGE/shunt1.jpg>
The pandemic gave my local utility an excuse to be even more intransigent than
usual, but after a couple months a crew actually showed up. Four guys, two
trucks. One guy in the bucket truck did all the work. To make matters worse,
the streets in the area were all torn up for utility work, and there was a
constant stream of dump trucks, front end loaders, and excavators going by. The
pole in question was a rat's next of HV wires, coming from several directions.
Working on it was clearly very dangerous. Although the crew called me when they
arrived, they had very little interest in communicating when I got there. (All
the noise didn't help.) I could have saved them a lot of trouble if the lineman
had just poked around with a pole until I heard the noise change. Instead, he
just started replacing insulators, starting on the opposite side from where I
heard the ultrasonic noise. Sure enough, changing the first 4 insulators had no
effect. As soon as he started working on the other side, it affected the noise.
I couldn't tell which of the 3 insulators it was, but after they were replaced,
the noise was almost entirely gone. I could hear just a little at 135 MHz with
both shorting straps open, much, much weaker than earlier. Because of that
residual noise, and the fact that I couldn't identify the specific component
that caused it, I wasn't entirely sure the problem had been solved.
Nearly a week of monitoring, in both wet and dry weather, makes it pretty clear the this particular
problem has been solved. Here are the "Before" and "After" spectra:
Before <http://sdellington.us/hr/MGE/Before.jpg>
After <http://sdellington.us/hr/MGE/After.jpg>
Note that the signal in the "After" plot would have been completely inaudible
"Before". That signal was about S5. Now I can again hear all the routers and switching
supplies in the neighborhood. And sometimes DX, too.
The method I used to track down this source is the same one I've been using
here for about 30 years. (It's a constant battle; such is the lot of the urban
ham.) I've only been using the ultrasonic receiver for a few years, but it's
been very useful. In every case of a strong line noise source, I've been able
to hear acoustic noise. It can localize the source within a foot or so at a
range of 50 feet. The ultrasonic noise fluctuations usually match the HF ones,
another sure sign it's the correct source. Conversely, when I point that thing
at other utility poles, I never hear anything. It's a useful tool.
Hopefully, this experience will help other surrounded by overhead power lines.
73,
Scott K9MA
--
Scott K9MA
k9ma@sdellington.us
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Scott K9MA
k9ma@sdellington.us
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