Wright's Aerials

  This was written in 1998, before DTT became the focus of attention. The comments about wideband aerials are probably even more relevant now than they were in 1998!  

When the transmission details for Channel Five were announced in the early nineties, many of us in this trade were appalled that the four-channel transmission plan which had served us so well in the UK was to be so seriously compromised. The four-channel system was well thought out and carefully engineered, and at the vast majority of locations it allowed good reception of four TV services on one small aerial. All that has now changed. It wouldn’t be so bad if the only problem was poor Channel Five reception, but what annoys me is that the mere existence of Channel Five often spoils reception of the other channels. At first sight this seems surprising, but it often happens.

Use of wideband aerials
Before Channel Five came along, almost every TV transmitter site in the country radiated four signals that could be received on an aerial designed for one group of channels. A ‘grouped’ aerial has a relatively narrow bandwidth, which allows higher gain and directivity. Despite the attempts at obfuscation by those who sell wideband aerials, the relationship between gain/directivity and bandwidth is set by the laws of physics, not by the advertising department, and thus can’t be altered. A wideband aerial will always perform much less well than its grouped equivalent. That, after all, is why the four-channel plan contrived to have all four signals from each transmitter on fairly close channels — why we have channel groups, in fact.

The pressure is now often on the installer to supply a wideband aerial, against his better judgement, for the sake of Channel Five reception. In fringe areas, or places where ghosting is a problem, the likely result is that the customer is left with unnecessarily poor reception of the other channels.

To take the Crosspool transmitter at Sheffield, S. Yorkshire, as an example, the channels in use are 21, 24, 27, 31, and, would you believe it, 67! Four on Group A, and one—Channel Five of course—near the top of Group C/D, and on permanent half power at that! I was incredulous when this channel allocation was announced, and I still am. Can anyone explain why ch34 couldn’t be used?

Even the better quality, properly designed wideband aerials don’t perform nearly as well as their grouped equivalent. Wideband arrays based on the traditional yagi design are available from the large manufacturers, who often quote the performance figures next to those for the grouped version, where they tell their own story. Log-periodics are once again becoming fashionable, all these years after Antiference had a brief fling with them in the ‘70s. These arrays are genuinely wideband, and they have good directional properties, but the gain is very poor. The stacked bowtie, or ‘fireguard’ has its adherents, but I’m not one of them. I’ve always found the directional qualities leave something to be desired, especially when the aerial is used to receive horizontally polarised signals.

Local aerial riggers who always use the cheapest possible ‘contract’ arrays are not going to suddenly start using good quality wideband aerials. They are going to use the cheapest possible wideband aerials. The reaction of one manufacturer to the Crosspool channel allocation has been to produce what is probably the worst UHF aerial I have ever encountered. This item is basically a ‘contract’ 18 element array, but whilst the folded dipole is the correct length for Group A, the director chain is about right for Group C/D. To make matters worse, the flat plate reflector is not long enough to function below channel 25. There’s no attempt at impedance matching or efficient signal transfer from director chain to dipole. The grouped aerials from this firm aren’t exactly brilliant, but they perform very much better than this wideband effort, which is, well, staggeringly bad. I haven’t tested this aerial thoroughly, because frankly I can’t be bothered with such rubbish, but I played about with one before I replaced it, and found that on ch21 it had no useful directional properties whatsoever. I guess that the polar response plot on the lower channels would resemble a deformed starfish! On ch67 the gain was about 3dB, rather than the 13dB of a half-decent grouped aerial. The sad thing is, these aerials are selling like hotcakes, because for a lot of aerial riggers in Sheffield they ‘solve’ the Channel Five problem.

Choice of transmitter
Again, this is best explained by example. The coverage areas of the Emley Moor and Belmont transmitters have a very large overlap. Before Channel Five came along we would, of course, use whichever transmitter provided the best reception. But at many locations we now have a dilemma. The Channel Five transmission from Belmont is so low powered that in most parts of my area we can’t use it. In places where we would automatically use Belmont we now have to consider Emley Moor, for the sake of Channel Five. A ridiculous situation frequently arises: we have to use an indifferent Emley Moor signal despite the presence of a good four-channel Belmont signal. This must be a common problem, and I’d be interested to hear from aerial installers in other parts of the country. The low-powered relays and the south coast main stations don’t carry Channel Five, so this situation must arise frequently.

Was it worth it?
We have paid a high price for Channel Five. A nonsense has been made of the four-channel transmission plan, and this has, insidiously, caused a general reduction of TV reception quality. This is a hidden cost that is borne by the public.

What are the benefits of Channel Five? Has it lead to a wealth of brilliant, innovative programming? Has it increased viewing choice in any real sense? I don’t need to answer these questions, do I? Whatever the reasons were for starting Channel Five, the interests of the man in the street were not considered.

We now have digital terrestrial and digital satellite. Both of these developments were in the pipeline when Channel Five started. They show up the terrestrial analogue Channel Five transmissions as the anachronism that they undoubtedly are. In years to come it will seem incredible that a brand new analogue network was set up, at such cost, at the dawn of the digital revolution. What a pity it is that the moneymen and politicians have a louder voice than those of us who are technically competent, in what are, after all, fundamentally technical matters.

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