Great job Scott
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Michael Martin
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On Jun 22, 2020, 11:53 PM, 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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