A look at Emley Moor’s output, on a rainy day in April 2010

This little investigation was done in a quick and casual manner on a cold and miserable afternoon when I didn’t feel like doing anything ‘sensible’. It isn’t particularly scientific, but I think it illustrates a few points about the pre-switchover situation, where digital and analogue TV run side by side. It was provoked by the appearance of the temporary HD mux on Emley Moor. For a while this was 9dB below the adjacent mux, which would have been pretty useless, but after a few days it settled down at a level roughly the same as mux D, which makes sense because both are the same ERP.

In the past we have been accustomed to mux D being the troublesome one; now we have two troublesome ones! The main analyser screenshot shows the full spectrum used by Emley Moor.

You can see how the 4kW muxes are significantly lower than the rest, which are 10kW. The analogue video carriers are all about 870kW, which comes out at about 20dB above the muxes. You can’t see the ones on channels 41 and 51 very well because I carelessly left markers over them. This screenshot was taken with the analyser storing the maximum levels for about a minute, and this does tend to exaggerate analogue carriers by about 8dB.

The screenshot with channels 39, 40, and 41 shows that there is only a tiny gap between the muxes on channels 39 and 40. The ch40 mux is offset downwards by 167kHz to move it away from the not-quite-perfectly-suppressed lower sideband of the ch41 analogue video carrier, but there is no offset on ch39, so the two muxes are almost touching. Let’s hope the receivers can cope.

The table of signal levels was derived by averaging the levels received by two aerials. One was a Blake log periodic; the other an Antiference 18 element Group B. There were no startling discrepancies between the two aerials. Both have a reasonably flat response across the relevant frequency band, and both had perfect line-of-sight to Emley Moor. Analogue ch37 (Channel Five) is about 6dB below the other analogue signals. This could be expected at an over-the-horizon site because the transmission aerial for ch37 is much lower, but in fact there is nearly always a discrepancy between ch37 and the other analogues, varying from place to place. At locations beyond clear line-of-sight the signal on ch37 can be disastrously low, and added to the ever-present threat of co-channel interference from Lichfield this means that its coverage is significantly less than the other analogue channels.

As you can see most of the muxes are about 20dB below the average level of the analogue signals. That’s how it should be because digital TV will work with a signal to noise ratio that’s about 20dB lower than analogue needs. So, although the signal strength is much lower, the coverage is about the same.

This little bit of nerdy fun was inspired by Justin Smith, who has dedicated himself to the investigation of aerial characteristics, and has brought it to a fine art. It’s heroic work that takes all his spare time, yet his wife still stands by him! Justin has done what the aerial manufacturers don’t want anyone to do: he has compared aerials objectively and has stripped away what he calls the ‘marketing bollocks’.

See http://www.aerialsandtv.com/tvaerialtests.html