Wright's Aerials
Strange cable fault

It was a very large private house set in an ancient quarry. The owner was on holiday and he had left instructions that the four large TV aerials were to be removed. Accordingly a steel mast 7m high had been fixed to the cliff face about 80m from the house, and out of sight. The estate workers had installed a 54mm plastic duct all the way from the mast to the house, via a river bridge. I was asked what type of cable to pull in and I suggested that they put at least six lengths of CT125DB (direct burial) in the duct. One for UHF, four for satellite, and one spare. There was a lot of grumbling about this, because they thought one cable would be enough, but they did as they were told.

When I went to have a look at the job I saw that the place they had chosen for the mast was behind a big tree, but I was told not to worry. The offending branches would be removed. I fixed the aerial at the top of a 4m mast clamped to the top of their 7m mast. Analogue signals were all approximately –2dBmV (quite good for the area). Since cable losses worked out at about 10dB I thought that it would be better to use a single stage masthead amp and add further amplification at the bottom end rather than risk having a high gain masthead overloaded by out of band signals such as Tetra. Having connected one of the CT125 cables (they were actually a cheap looking ‘equivalent’) I went to the distribution amplifier in the house and checked the input. These were all low, equating to cable losses as follows: ch22 14dB, ch25 18dB, ch28 21dB, ch32 22dB. The higher channels had massive losses, with ch56 losing about 38dB. Of course I wasted time checking the masthead amp output. Finally I tried one of the other cables and that worked fine. I picked up the offcut from the first cable and cut a few feet off and looked at the end. It looked OK, so I did it again. I was looking for moisture, but instead I found that the inner did not run centrally in the dielectric but was at the very edge, occupying one of the ‘air space’ cells. It turned out that this fault occurred intermittently along the length of the cable. It was very lucky that we had a spare cable.

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