Analogue interference to DTT

I had a seemingly inexplicable problem with the ch68 mux from Belmont. This was in a place where Belmont reception was reasonably good, but the ch68 mux had a very poor BER, despite the signal strength being quite decent. I suspected local interference, so at ground level I connected an aerial to the analyser and swung it round, and was rewarded with a very faint suggestion of an analogue picture on ch68. In ‘analyser mode’ I could see a bit of a spike exactly where an analogue vision carrier would be.

It turned out that Granny, who lived in an upstairs flat at one end of the house, had a Sky box with the RF output set (as they usually are) to ch68. Granny's aerial was on Emley Moor, and was producing very poor signal levels. The output from the satellite box went to a one-in one-out 'booster' which fed into a 'one-in four-out' booster, and then from that to two bedrooms and the kitchen. In one of the loft bedrooms — near the aerial I was working on — was a home-made flylead which had the outer conductor not connected to the plugs.

I replaced the flylead and moved Granny's skybox output to ch67 which pleased her massively because the Sky picture had been snowy before, due to the Belmont mux. This also restored a normal ch68 BER to the feed to her son's IDTV.

I asked about the 'boosters' in Granny’s flat and it turned out that the man from the local TV shop had originally fitted the four way one, together with the cables. When the results were poor he had responded by fitting the additional booster, which of course made no difference. He then blamed the trees, which were in the way of the signal apparently, and departed. The total gain of the boosters was about 30dB, so the signal in the flylead would have been about 37dBmV. This was evidently enough for the signal to radiate sufficiently for it to compete at the nearby Belmont aerial with the DTT mux on ch68.