I had to go back to the house I mentioned at the top of this thread today.

The owner had moved the antenna (as I discovered later!) and rerouted a lot of the cables, running them down trunking to the sockets in the living room.

Then he discovered a problem. The strong RF signals from the VCR, DT (digital terrestrial) box and satellite receiver were fine all around the house. Two of the four off-air analog stations were of acceptable quality but not perfect, one was quite snowy, and the fourth very bad.

That was without the extra pre-amplifier in the attic on the downlead from the UHF roof antenna. When he'd connected the amplifier in line, the whole band was awash with spurious cross-modulation products, rendering even the strong locally-generated RF signals unwatchable. Not surprising considering there was 20dB gain on that amp, then a route through three diplexers in the various boxes, then into a distribution amp with another 16dB gain to each output.

Simple fact is that the signal strengths aren't too bad here, and with a 13-element Yagi array on the roof, a 20db pre-amp is just not necessary! Out it went.....

I checked that the signal coming in off the roof antenna was good -- It was.

Next step was to fix several bad joints -- Again! (Why do some people find it so difficult to install a coax plug properly?)

That improved the signal strengths no end. I then tossed out the cheap and nasty RF patch cords supplied with each box and installed replacements with decent coax cable. By this time I had all four UHF stations to acceptable quality around the house, along with all the VCR, DT, and satellite signals.

All seemed fine, until I'd began checking out the satellite receiver, then switched back to analog UHF and noticed that one station was now very poor again.

Cut a long story short (Too late! [Linked Image]), disconnecting the satellite dish feeder resulted in perfect UHF reception. Turning off LNB power from the receiver gave the same result.

It was only then then the subject of the mvoed antenna came to light, and when I went out and looked up on the roof, there was the UHF antenna sitting about 18 inches away from the LNB on the satellite antenna! Not surprisingly, the UHF antenna was picking up signals from the LNB which were desensitizing the receivers, especially with the 16dB gain provided by the distribution amplifier.

But why had it been all right earlier?

It was only as I was switching back and forth and turning off the LNB power from the satellite receiver menu that the penny dropped.

The universal LNBs used with most modern Sky Digi-boxes in the U.K. are dual-band. A 22kHz command signal sent up the feeder from the receiver switches the local oscillator in the LNB on the dish from low-band to high-band.

A few tests confirmed it: The UHF reception was poor only when the satellite receiver was on a channel which resulted in the LNB being switched to high band.

Moving the UHF antenna away cleared the problem.