When UHF TV transmissions started we all changed over to ‘low loss’ coax, and very good cable it was, with a dense braid that provided virtually 100% screening. Although good quality low loss is still available, the vast majority of the cable now on the market has quite sparse braiding, with a coverage of only 20% or so.

For some time I’ve used Raydex CT100 (or equivalent) for aerial and distribution system downleads. The only exceptions have been rock bottom ‘economy’ aerial jobs, of which fortunately I do very few. CT100 has a copper foil wrap in contact with the copper braiding, so the screening is 100%. But does the low screening percentage of the cheaper cables matter? CT100-type cables cost twice as much as cheap brown low loss, so there has to be a valid reason for using them. I’ve had a sneaking suspicion in the past that CT100 is a waste of money for some applications, but two recent incidents have reassured me that this is not the case.

I was asked to install a distribution amplifier in a large private house. The electricians had already installed eleven downleads from the loft to the wall outlets, and they told me proudly that they had obtained the cable for £7.50 per 100 metres. As I fitted the coax plugs in the loft I noticed that it was difficult to find enough braid in the cable to make a good connection. For reasons irrelevant to this letter it was necessary to identify all the cables. To trace coax cables I have a battery-powered UHF modulator. The output level is high (30dB/mV) because this lets us use the modulator to trace cables that have high loss due to damage or moisture ingress. With the modulator connected to an outlet downstairs I sat in the loft and connected the UHF signal strength meter to each cable in turn. I’ve carried out this procedure many times, and the result should be an unmeasurably tiny signal — less than –50dB/mV — on all the cables except the correct one. In this case, every ‘wrong’ cable produced about –6dB/mV. The correct cable gave +23dB/mV. I followed the cables and found that all eleven went down to bedroom floor level together in a loose group, after which they went their separate ways. The bunch of cables was not taped together – the cables were close, but loose at the back of a cupboard. They ran together for about 3 metres, and it seems that this was sufficient to allow crosstalk at about –30dB. Such a cable must be very susceptible to interference if it passes through an RF field. Think about the bottom end of a downlead, with rather low signal levels on it, passing near a computer, satellite receiver, or whatever. Incidentally, signal loss on the ‘correct’ cable (on channel 38) was about twice what I would have expected for CT100. Later, I accidentally left one of the downleads disconnected at the amplifier. The result was merely a snowy TV picture, even on channels derived from modulators.

A few days later I installed a new aerial for an existing distribution system. Afterwards all the TV sets had perfect reception except for a new widescreen Sony. This TV set had a Sky digital box, a VCR, and a DVD player sitting beneath it. The ITV picture, on channel 29, had about ten straight vertical lines roughly 200mm long in the background. These lines were about 20mm apart and formed a rectangular grid that drifted slowly across and down the picture. I called in the lady of the house and asked if the fault had been there before. “Oh yes” she said, “and much worse than it is now. It don’t bother me, but it drives ‘im mad!” It seems that the TV shop had told them that a new aerial might cure the fault, and that was why I was there!

Pulling the VCR forward affected the intensity of the interference. Connecting the aerial directly to the TV set removed it completely. With the spectrum analyser in ‘TV’ mode, and using a screwdriver as an aerial, the analyser displayed the interference, but only when the TV set was turned on. The VCR, etc, could be turned on and off with no effect. If the VCR-to-TV flylead was connected to the TV set and the aerial signal attenuated at the VCR end, the interference, as seen on the Sony, would fade in and out in inverse proportion to the signal applied. With a very weak signal the interference would dominate the picture. It seems that the TV set itself was radiating interference, and that this was getting into the VCR-to-TV flylead. The flylead was made from cheap coax. I fitted a CT100 flylead and this provided a complete cure. Not only that, but there was a subtle but definite improvement of the other terrestrial channels. The pictures looked ‘cleaner’.