[Not] Living with 6m/50MHz interference!

Burglar alarm hash

For six months I suffered from intermittent hash centred on, you guessed it - 50.110. It peaked up at 315 degrees but it was so strong it could be heard with the aerial at all points of the compass.

The noise appeared mainly at weekends and at random times. Needless to say when it came on it was totally impossible to operate with the aerial anywhere near the northwest. I yet again mounted the aerial in the car and the family went out on a noise hunt. It took me several trips to track the noise down to a private house 1.5 miles away. Outside of the house the noise was end-stopping my S-meter. I knocked on the door and tried to explain my problem. This was not so easy as the occupier was disbelieving and didn't want anything to do with me.

Eventually he agreed that I could telephone him the next time the noise occurred and come to the house to track down the noise. The next time the noise occurred they were out and the next time. One evening at 11 p.m. the noise appeared. I telephone him and it rang for what seemed an eternity before he answered it. As soon as he did the noise went off. It turned out he was in bed. What was I to say? I explained what happened but he was not too pleased and put the telephone down. Within two or three minutes the noise came back up. I decided bravely to call him back again. After a brief conversation he agreed that I could come over to the house and try to clear the problem up. His exact words were "yes come, Iím not going to have this every night!" I drove to the house and went in the gate with my receiver in hand. As soon as I pressed the door bell the noise instantly disappeared! He opened the front door and I tried to explain what happened, but he was obviously incredulous. As I thought it was the doorbell causing the problem we spent 10 minutes fiddling with this to no avail.

Then all of a sudden the noise came on and it only took a few minutes to trace it to a cupboard which contained yes, a burglar alarm! The problem seemed to lie with the mains on/off switch which was arcing. Why did the noise disappear when he answered the telephone and I rang the doorbell? I can only suppose the electrical arc was unstable and when a current pulse was generated by switching on a light or operating the bell, the arcing stopped. Why was I getting such strong interference 1.5 miles away only at 50MHz? I can only assume that the wiring near to the alarm on-off switch resonated on 50MHz and the alarm wiring throughout the house was acting as an efficient aerial. The alarm was eventually replaced because I convinced him that it was a potential fire hazard. I have not suffered any interference from that source since. This noise took a lot of courage to sort out and I have crossed my fingers that I don't get a case like that again. The lesson learnt was to be polite but persistent. If I had given up because of the owners attitude I am sure I would still be suffering that bad interference to this day.

Back to home