Wright's Aerials

Aerial Topics

This is a miscellany of comment and information, mostly concerning the aerial trade. It’s just stuff that’s cropped up over the years, either in newsgroup discussions or in conversations with other installers. Some of the items are in the form of letters to magazines. Some items are quite old, so please bear this in mind, because some things change quickly in this trade!

Misapprehensions about Analogue Switch-off
I'm very sad that I won't be able to watch my films soon."
"How do you mean?"
"All my tapes, I love them. I've got all the old films. But they're switching it off. It's really not fair."
I didn't speak. I thought hard...

Regarding the misreporting of symptoms
in a recent instance it turned out that the report was 100% accurate, although I was highly sceptical...
In this instance the report was that the TV would only work when a person was sitting in a particular armchair.
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Interference from DTT
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!
Analogue interference to DTT
I had a seemingly inexplicable problem with the ch68 mux from Belmont. This was in a place where Belmont reception was reasonably good, but the ch68 mux had a very poor BER, despite the signal strength being quite decent. I suspected local interference...
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Channel Five: the Aftermath
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.

Coaxial Cable screening
When UHF TV transmissions started we all changed over to 'low loss'coax, and very good cable it was, with a dense braid that provided virtually 100% screening. Although good quality low loss is still available, the vast majority of the cable now on the market has quite sparse braiding, with a coverage of only 20% or so.
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Wideband rubbish
I’ve noticed these odd looking aerials before, but today I finally got my hands on one. The thing is obviously supposed to be a wideband UHF aerial. The design is terrible, though. The 16 directors are about the correct length for Group C/D. In fact there are just two lengths of director, 140mm and 145mm. The folded dipole is just that, a folded piece of tube with no balun or attempt at impedance matching. It is 255mm across, about right for Gp A.

Red box scam
I recently visited a new customer to discuss the positioning of her proposed satellite dish. Noticing a shiny red plastic box fitted to the aerial mast, I assumed that it would be a splitter. Guessing from this that the house must have two or more TV sets, despite the fact that the customer was a fairly elderly lady living alone, I asked if Sky reception would be required in other rooms. The customer replied that she only had one set, so I had to explain why I’d thought there were others.

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Quick and easy TV tuning
Campers intending to take a TV set on holiday should contact BBC Engineering Information for details of transmitter locations, channels, and polarity. Thus armed, it becomes much easier to adjust the aerial for good reception. From the fun and games that I’ve seen on campsites, though, for many people this is only half the battle. They also have to tune in the correct channels on the TV set. If the TV set does not display the ‘real channel’ number, but simply scan tunes along an uncalibrated green line on the screen, as many do, it is impossible to know just what channel you are tuned in to.

Stacking Aerials to improve Bit Error Rate?
This item is concerned with large TV distribution systems and their conversion for DTT. Since c/n inevitably degrades with every successive stage of amplification, it seems a good idea to start off with the best c/n ratio possible. This is especially the case when there is a lot of amplification in the system, for instance 45dB at the head-end and 40dB in repeaters. I should mention that I'm thinking of a broadband rather than channelised system.
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Channel planning for distribution systems - the n+9 and n+5 conundrums explained
When planning a distribution system you should, if possible, avoid having signals on channels five or nine channels apart. Thanks to modern TV receiver design the problems caused by these channel combinations are far less important than they used to be, but nevertheless it’s worth avoiding these channel spacings if you can, especially if the system will feed any older TV sets. The channel plan for systems that carry both analogue and digital signals should also avoid these channel relationships where possible, treating both analogue and digital signals as potential interferers.

The Lesser Spotted Decibel
Unfortunately we use one word, ‘decibel’ for several units that have different uses.
The decibel is a ratio – nothing more, nothing less. In practice it is usually accepted as a ratio of powers measured logarithmically.
As it is a ratio it must have a reference, but this is where usage falls down. Sound is measured in dB relative to 'the threshold of hearing' which I believe actually has been defined as an absolute sound pressure level. It has become practice to measure sound levels in dB relative to that threshold but just to give the reading in 'dB' without nominating the reference.
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Terrestrial TV - Do we need it?
I've been on holiday in the Scottish Highlands. Like many other areas of the UK, the terrestrial television network there has countless low-powered transmitters, some serving only a few hundred houses. Some of these transmitters are at the end of a very long retransmission chain, and it shows.

Vandalism by alarm contractor
Today a Housing Association rung up. "Can you go to Sunset Villas urgently? There's two things wrong. First, the alarm company say there's a serious fault with the aerial system, and secondly some of the residents have really poor reception since Tuesday."

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Playing about with a DAB aerial
Digital Audio Broadcasting uses roughly the same frequencies as some of the old 405-line VHF TV transmissions.
For some time I have been on the lookout for an old Band III TV aerial so that I could experiment with DAB. Yesterday I found one. It was a 9-element yagi and it had spent the last forty years in a loft, so it was in good condition. I connected it to a length of coax, and believe me it seemed funny working on a Belling

Paul came in the other day, aghast. He had been approached on the street by another aerial installer, an older man, who wanted to buy a few bits and pieces having been 'caught short'. Since Paul was just in the process of lifting his analyzer out of the van the conversation had turned to test equipment. The experienced rigger advised Paul that the analyzer was 'only a telly really, ain't it?' and that even an ordinary meter was unnecessary. Paul learnt from this meeting that the best way to do the job is to point the aerial in approximately the same direction as all the others, and then lie through your teeth about the poor reception
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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,

Ten taped ‘T’ junctions and a wailing woman
Suppose you had to connect ten TV sets to an aerial. The tellys are in a ten-storey building, one on each floor directly above each other, so they are about 3 metres apart. You could get a 30 metre length of coax and run it down the building, then cut into it near each telly and splice in a short length of coax to run to that TV set. By ‘splice in’ I mean just that – make a ‘T’ junction by twisting the cables together.

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The other side
Due, I suppose, to the way television has developed, the word 'channel' has ended up with two entirely different meanings. Actually, they aren't entirely different — it would be better if they were — they are related in a way that causes endless confusion. By now you're ahead of me of course. We have ‘channel’ as in ‘8MHz wide slot somewhere between 470 and 860MHz’ and ‘channel’ as in ‘Channel Four’. (Let’s not mention ‘S’ channels or satellite channels or VHF FM channels or DAB channels!). Understandably the public thinks that ‘channel 1’ is the same thing as the button on the remote that has a ‘1’ on it, and is synonymous with BBC-1. By an extension of this logic channel 47 must be what

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