Tuesday, August 24, 2010

A DAB of interference

I think it is important when operating a ham radio station from home to ensure that none of your own domestic equipment suffers from interference. It's a sign that everything is OK, and if a neighbour should complain then it helps to prove the point that it is not your fault if you can demonstrate that your own TV, phone, stereo etc. are not affected by your transmissions. I've been surprised at the amount of power I've been able to run using antennas in the attic without experiencing any problems.

Last night I decided to use the digital DAB band of my radio tuner to listen to the Promenade Concert on BBC Radio 3 because Olga complained of hearing some high pitched noise (inaudible to me) on the analogue FM band, when played through a stereo amplifier and a pair of large 40 year old IMF monitor speakers. For a few minutes it was fine, then there were a couple of interruptions to the broadcast which I guessed were caused by the transmission of my APRS beacons. I think that the DAB transmissions are quite close in frequency to 2m so I shut down the 2m APRS gateway and we enjoyed the rest of the concert without interruptions. I imagine that the neighbours, if they listen to radio at all, will use analogue FM just as we normally do, but it is a bit of a concern that just 10W of 2m FM can cause interference to anything.

The only other known problem caused by my radio transmissions is a neighbour's security lights. I imagine this is a common problem. I installed an identical looking PIR controlled security light at the front of the house a few years ago after a couple of drunk youths wandered up our cul de sac one night and decided to uproot some of the plants in a neighbour's garden. I found that I could turn the lights on with as little as 5W on 20m. The solution was to leave our lamp turned off and hope that the neighbour thought it was the wind blowing the bushes around that was triggering his ones. Fortunately it is often windy here and I didn't used to go on the air in the evening all that often.

But my APRS gateway runs from morning to night and runs 50W output on 30m so the problem will become more evident as the nights draw in unless I adopt the simplest solution which is to shut it down (or switch to receive only) at nightfall. Breaking cover by admitting to a neighbour that I have been causing this to happen risks opening a can of worms that could put me off the air entirely, and remedying the problem would be expensive as his lights are part of a professional security installation that I would not be allowed to tamper with even if I wanted to.
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