[Not] Living with 6m/50MHz interference!
Overhead power line hash (1991)
Sometime in the
summer of 1989 I realised that the S9+ l0dB hash covering the band that peaked up from the northeast was becoming a real nuisance. Fortunately, it was the type of noise that the noise blanker on
my Icom IC-751A could cope with fairly easily.
The difficulty was that as soon as any strong station came up nearby on the band the noise blanker became Ineffective and weak signals were completely blotted out
I've had many instances of being unable to complete a weak F2 QSO because of the hash suddenly coming up in the middle. The hash was at its worst on wet and windy days which, in my opinion,
clearly pointed to be overhead power lines being the culprit. One weekend when 6m was quiet I decided to track down the cause of the interference. I usually tackle this by assembling two elements
of a 4 element Jaybeam and bolting it to a short 4-foot pole sticking up through the sun roof of my car. This is connected to my portable 6m receiver.
After about half an hour of driving around I tacked down the noise to a fenced-in enclosure about two miles away from my house which contained a brick building fed by overhead power lines. This
turned out to be a water pumping station owned by the local water authority. The noise seem to peak when my aerial was pointing to the building from all four compass points and because of this I
was convinced I had located the source of my problems. A letter was immediately sent to the Engineering Director of the local water board which was passed down through several levels of hierarchy
(this was tracked day to day through telephone calls). The procedure took two weeks and I eventually managed to arrange to meet the engineer at the pump site one lunchtime. A quarter of an hour's
testing, which involved switching the pumps on and off and activating the automatic remote data monitors, proved that the pumping station was not the cause of the interference. I must add however
that the engineer was most helpful.
It was then that I decided that the interference must be coming from the power lines feeding the station and half the area's houses. After obtaining permission
from the farmer owning the land over which the power lines "crossed' it only took a couple of hours to track the interference to an individual pole in the field. Another letter was dispatched to
the local electricity board Engineering Director and three weeks followed when the letter was down through several levels of hierarchy yet again. Eventually, when it arrived at the local
engineering manager's desk, I found them most helpful and they even dispatched an engineer with an FM radio to see whether they could trace the fault. They couldn't. After two months of inaction
and a bit of frustration on my part the engineering manager called me to say that they were switching the line off for maintenance and they would take the opportunity to inspect the identified
pole. I only hoped that I had got it right this time.
The offending piece of wire that resonated on 50.110MHz and the burnt through insulation!
The day comes around, I arrive home from work to find a pile of burnt wire on my desk. It seemed a pleased engineering manager had called at my house to show me
that on that very pole I selected the insulation on the wire carrying the 240V AC mains had been eaten through and was shorting to the burnt-through copper wire tying it to the insulator. The tie
wire consisted of about six turns about 2 inches in diameter which I am sure resonated on or near 50MHz which explained the lack of interference on other frequencies. 6M is now quiet to the
northeast. The above efforts took numerous telephone calls over a period of six months but the eventual positive outcome was very satisfying. It was also very interested in locating the problem
and wanting to help.
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