[RFI] Line Noise Fixed!

[K9MA]

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 at sdellington.us