Distance No Object 

15 May 2023 tbs.pm/76921

When you sit and watch a swimming match from Morecambe or a race for the Hunt Cup at Cheltenham; when you enjoy a variety programme from Blackpool or see army tanks being built in Leeds, do you ever wonder just how the picture gets from the camera to your set? The answer is one of the minor miracles of TV, the micro-wave relay, the working of which is explained in this article

 

 

TV Mirror cover

From TV Mirror for 8 January 1955

IN an outside broadcast on “steam radio” the link between the event and the transmitter is an ordinary Post Office telephone line.

There are times when the TV engineers wish their job was as easy. But, except over very short distances, GPO telephone lines are just not suitable for carrying a TV picture.

The main TV transmitters — Alexandra Palace, Sutton Coldfield, Holme Moss and the rest — are certainly linked by a cable, along which the majority of your studio programmes travel every day. But this is a special type of cable designed for the job. It is very expensive — nearly £1 [£30 in today’s money, allowing for inflation – Ed] per foot [30.5cm] — it takes a long time to make, and as yet very little of it has been laid in Britain.

If a TV outside broadcast happens to come from a place reasonably close to this main cable, it is possible for the GPO engineers to feed the picture along a line into the cable and so on to the transmitters.

But in nine cases out of ten the OB point is too far from the cable and so the BBC engineers have to find an alternative method of carrying their pictures across the country.

What they do is to send the picture by wireless, from a mobile transmitter van at the OB to a receiver at the nearest TV transmitter. To do this they make use of very short waves — micro-waves as they are called — which can be made to travel in a straight line like the beam of a torch and which need very little power to transmit.

That, too, may sound simple enough, but in fact there is a big snag. These micro-waves have one peculiar property. While the medium waves used in Sound broadcasting — the Home, Light and Third for example — will arrive safely at your home whatever obstacles may be in the way, micro-waves can only be received satisfactorily if the transmitter and receiver are in a direct “sight line.” That is, you must be able to see the receiving aerial from the transmitter van — providing your eye can reach that far!

The trouble here is that so many of the cities and towns from which OB programmes are taken are built in valleys or with high hills between them and the transmitters. So the chances of a TV broadcast coming from a spot which has an uninterrupted sight line to the transmitter are pretty remote.

To find out just how the problem is tackled I went to see Alec Hill of the BBC. His official title is Senior Television Engineer in Charge Radio Links, Midlands and North. In fact Alec is the man who plans and supervises these “radio links” for the greater part of the country.

You may not be surprised to know that the principal piece of equipment in Alec’s office is a large contour map of the British Isles, showing the heights of all the mountains and hills that he is likely to encounter.

“When a request comes in for a radio link,” he told me, “we first of all draw a section through the surface of the earth along the line between the OB site and the nearest TV transmitter.

“That shows us what to expect. We can see the sort of country there is in the path of the micro-waves and the heights of the hills. Remember, it’s not the distance that worries us — distance is no object on this job! — but the ‘sight line.’ And you’d be surprised at the number of times one apparently insignificant little hill can ruin the whole scheme.”

But the maps and drawings in the office at Castle Bromwich are only the start of the job. Maps don’t show woods on the top of hills, and the extra feet of tall trees may make all the difference. Nor do they show such man-made obstacles as slag-heaps. Once a possible path has been planned, a physical test must be made to make sure all is in order before the broadcast.

“Nowadays we’ve got quite a lot of paths properly mapped and recorded,” said Alec Hill, pointing to the chart covered with coloured pins joined by cords. “In two years we’ve used and recorded more than 100 paths, and some of them, of course, are used over and over again — like the one from Blackpool to Holme Moss where we use the famous Tower for our transmitting aerial.

 

A camera with a long lens

This is a typical television camera used for OBs. It is fitted with a lens which can give a close-up view of events happening some distance away

 

Nothing is impossible

“But in the early days it meant a lot of preparatory work and our mobile units were working at all times and in all weathers to get suitable links worked out for future programmes. Sometimes we could kill two birds with one stone and, while on one job, try out another path that might possibly be wanted in the future. We’re getting quite expert at anticipating the requirements of the TV planners!”

To cover these radio links, each of the OB camera units has a micro-wave unit attached to it. There are two such units at the moment in Hill’s area – one shared by the Midlands and the North, the other by Scotland and the North. Soon a third will be added.

Each unit consists of seven vehicles, one of which carries a 90ft. [27.4m] telescopic mast on a “fire escape” and one of which tows a diesel generator in a trailer. Each is fully equipped to work two main “hops,” with a short starting link of a mile or so between the cameras and the highest convenient transmitting point. The unit also carries radio telephone equipment so that the engineers at both ends can keep in touch with each other during setting up and rehearsals.

“As a rule,” said Alec, “these two ‘hops’ cover most eventualities. For example, a TV broadcast from Blackpool will first travel on the starting link to an aerial half way up the Tower. The first ‘hop’ takes the picture to Horwich, about 18 miles [29km] from Manchester, where it is received and re-transmitted to the end of the link at Holme Moss.

“Recently we’ve added several miles to these ‘hops’ by putting aerials 600ft. [183m] up the TV masts at Holme Moss and Sutton Coldfield. However, if a more complicated route has to be followed for some reason, we ^an borrow part of another unit and add a third or even a fourth ‘hop.’

“Nothing is impossible. So far as I remember only one OB has been turned down on technical grounds. But it may not be worth while broadcasting from some places. Personnel and equipment may have to be tied up for too long and the cost will become prohibitive. The engineers will do anything that is asked of them, but the programme staff must decide whether the time, trouble and expense is justified by the programme.”

Each OB is scheduled to take three days — the first for travelling and rigging, the second for testing, and the third for rehearsals and the broadcast. But in practice this just doesn’t work out.

“The boys often have to work a whole week on end with no time off for maintenance of equipment,” Alec told me. “Sometimes, when there are three or four OBs from the same place on successive days, things ease off a bit. But don’t forget — if the equipment does break down, there’s no duplicate we can switch over to. It’s a risk we have to take, but in fact breakdowns are extremely rare — only one in two years.”

When I left Alec Hill I travelled north to watch one of the units at work on an outside broadcast from Harrogate. And I saw how the micro-wave signal is beamed from the transmitter van with its telescopic aerial to the receiver at Holme Moss.

 

Men stand around a dish

Micro-wave transmitters are also used as permanent links between some BBC transmitters and the main network. This wintry scene was taken at the opening of the Aberdeen transmitter, when engineers in a blinding snowstorm struggled on top of the 1,400-foot-high [247m] Cairn o’ Mount to maintain the link between Aberdeen and Kirk o’ Shotts

 

All in a day’s work

Both aerials look rather like enormous electric bowl fires, each 4ft. [1.2m] across and mounted on tripods. They must be focussed exactly on each other — an error of only 2½° over a distance of perhaps 60 to 80 miles [96 to 129km] may mean that the signal is lost altogether.

The micro-wave links, by the way, are used only for the TV picture. The sound part of the programme is carried on GPO telephone lines, exactly as for a Sound broadcast, and joins up again with the picture at the transmitter station.

Only one breakdown in two years — only one broadcast turned down for technical reasons. That seems to sum up very neatly the work that the radio links are doing every day to bring y^our programmes into the home. But to the micro-wave boys it’s all in the day’s work, and their only aim is to make each job they do a little better than the last one.

As Alec Hill said to me, just as I was leaving — “the whole thing’s quite new and we’re learning all the time.”

 

You Say

2 responses to this article

Ray Wilson 15 May 2023 at 1:37 pm

Another great article

Thanks…

Phillip Hallinan 18 May 2023 at 9:15 am

I did this job for HTV and used to provide 4 hop links regularly between north and south wales.

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