Wright's Aerials

Written early in 1998, when the DTT transmissions were just starting, and we were all having to get used to a new set of problems!

The arrival of Channel Five caused great congestion on the UHF TV bands. It suddenly became difficult to find a clear channel for VCR and satellite receiver outputs. I didn’t think things could get any worse — then digital terrestrial transmissions started!

In South Yorkshire, the only clear channels between 21 and 54 will soon be 36 and 38, and they are both adjacent to Channel Five on 37! I’m sure the situation is much the same over large parts of the country.

Each of the six digital multiplexes occupies a conventional 8MHz channel. On the spectrum analyser screen they look just like a block of noise with a grassy top, extending across the channel exactly to its upper and lower limits. From the point of view of analogue reception, that’s exactly what they are — straightforward noise. We all understand the concept of the signal-to-noise ratio in its familiar manifestation, where the noise floor is more-or-less fixed, and a problem only arises if the signal is not far enough above it — or in other words, is not ‘strong enough’. We must now become familiar with the concept of the varying, or unpredictable, noise floor, since every TV transmitter in the land will soon, in effect, be pumping out six channels of noise.

A lot of engineers have already been out to ‘mystery’ VCR or satellite faults because of this. I was completely taken by surprise by my first one. The customer rung up with the message, “The satellite picture is snowy, but if I unplug the aerial it’s alright”. This sounded unlikely, but turned out to be true. The satellite output was on ch30, and a digital signal on that channel from Belmont was the cause of the trouble. This was not in the Belmont service area.

The effect on the screen is indistinguishable from the snowy picture that results from weak signal. The first thought is that the modulator must be faulty. But when the aerial is unplugged, hey presto, the satellite or VCR picture is perfect.

Since the digital transmissions are about 20dB below the analogue ones, it’s surprising at first sight that they can have such an effect. The reason lies in the analogue signal’s extreme susceptibility to noise. Noise 40dB below the signal will cause visible impairment. Even when the aerial is not directed towards the source of the digital transmission this sort of interference level is quite likely.

I wonder why the digital broadcasters are getting away with it, when Channel Five were made to spend millions re-tuning VCRs….What’s to be done? Where possible abandon RF outputs and fit SCART leads. If this is not possible, for instance where the VCR or satellite receiver output feeds into a distribution system, it might be worthwhile attempting to find a clear channel by re-tuning the RF output of the receiver or VCR. A better idea is to fit a notch filter on the aerial cable. A tuneable notch filter can be adjusted to stop the offending signal whilst passing all other channels.The virtues of this are that it is quick and foolproof, and the customer gets a little shiny thing for his money. We all know how people value even very small physical items above our time and efforts.

I have always been in favour of using a five-channel pass filter/leveller on the aerial input of any distribution system which carries VCR and satellite receiver outputs. I think it’s going to be more-or-less obligatory from now on. A channel pass filter stops everything except the required channels. This is magic, because the whole band is cleared of unwanted signals, allowing VCR and satellite receiver outputs to be placed on virtually any channel. These filters are a bit pricey at £68 + VAT (Taylor Bros of Oldham), but I think we have to see this as a sales opportunity, rather than something sent to try us.

Interference from the digital transmissions will not be confined to VCR and satellite receiver outputs. Off-air analogue reception is sure to suffer. Four of the Sutton Coldfield digital transmissions are on the same channels as Emley Moor analogue transmissions, and vice-versa! It seems incredible that two adjacent main transmitters should share channels like this. The result can only be a worsening of analogue reception for people in fringe areas.

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