TV in The Channel Islands 1954
How private enterprise solved the problem of reception beyond normal range.
THE British Broadcasting Corporation decided five years ago that Jersey, Guernsey, Alderney and Sark were beyond the range of even "fringe" TV reception. The challenge was taken up by Broadcast Relay Service, Ltd., the parent company of the Rediffusion group of companies - who serve about 3,000,000 listeners and viewers throughout the world with daily sound or television programmes by wired network direct into their homes, and either broadcast or relay in over 20 foreign languages or dialects.
It was a problem of the greatest magnitude, for the Very High Frequencies used by television are a "straight line" emission. They do not follow the curvature of the earth's surface and therefore cannot be received - except under freak conditions - by any set or station which is out of what we might term the line-of-sight of the transmitter. The Channel Islands, by virtue of their distance from the English transmitting stations and the natural curvature of the earth, were well beyond the ultimate point at which BBC transmissions could be normally received. Seventy miles - cited as the average distance over which television could be received, had hitherto decided who would or would not receive TV transmissions anywhere in the world.
Fortunately, Rediffusion have on their staff a number of peculiarly unconventional engineers. On the principle that physical laws, at any rate, are made to be broken, they set out to find the loophole in this one. And they, succeeded. No one can say who was responsible for the first idea that the freak broadcast could be resolved to normalcy, but it was decided that atmospheric conditions which had hitherto caused solely annoying interference could be disciplined to give service. Tropospheric propagation, which arises where the layer of warm and cool air meets at a height of from one to three miles, is a condition which reflects the Very High Frequency waves used by TV. Rediffusion's technicians endeavoured to "catch" the reflected waves where they come to earth at a spot considerably beyond the "line-of-sight " distance.
The success of the attempts caused these experts to indulge in a rare form of celestial billiards - and their preliminary work was done in so unlikely a place as the files of the old Channel Island newspapers. If TV signals from England were to be caught in their "freak" form against the tropospheric cushion and neatly pocketed in Jersey, it was necessary to make sure that the " cushion " would be a stable one - and only meteorological information could determine the question.
Fortunately, such conditions between England and Jersey were excellent - and the great experiment bore its first fruit on June 2nd, 1953, when all the pageantry of the Coronation was brilliantly received in Jersey on the 110 sets specially installed in the Island by Rediffusion. New and very special aerials, together with other equipment, had been designed, partly by Rediffusion's engineers at the London laboratories and partly by the technicians of the associated company in Jersey, Television Research, Ltd.
Work began on the construction of Rediffusion's Jersey Station - including the setting up of the biggest receiving unit in Britain - and the station was formally opened on June 24th, 1954. The special aerials play a key part in the working of the station, and one of them is set up to pick up the TV programmes radiated from Paris, 200 miles away! What have been the BBC's reactions to this private pioneering in a field where the Corporation has a monopoly? It has been quite objective. There has been no question of reluctance on the part of the Corporation in acknowledging that Rediffusion's "first there" in the Channel Islands has done what might not otherwise have been done for a long time, if at all.
Senior BBC officials have seen for themselves the quality of reception on the Rediffusion screens in Jersey; one of them said "We are extremely interested, particularly in the standard of picture obtained from such weak and variable signals."
Now the Corporation has made concrete acknowledgment of Rediffusion's success by asking the Company for co-operation in the establishment of the BBC's own direct Channel Islands Service. A pilot BBC unit has been set up in Guernsey to pick up all transmissions from Wenvoe and later from the newest transmitter which it is proposed to erect at North Hessary Tor in Devon. At the same time, Rediffusion will monitor these transmissions on Jersey for the BBC. On the result of these cooperative experiments by Rediffusion and BBC engineers will rest the development of the Corporation's service.
Meanwhile, the Rediffusion Group of Companies are not standing still. As soon as the new French TV transmitters - now being built near the French coast are - in operation, Rediffusion will give its Jersey viewers the first-ever alternative programmes. By turning a knob the subscriber will be able to select either the British or the French programmes - and the Commercial Broadcasts as well when they come into being!
The choice will be available every day and is a notable step forward from the present rare and complicated "Eurovision" programmes. Just how good is Jersey's Rediffusion service? A Jersey hotelier who subscribes to the Rediffusion TV service complained that the reception was not as good as he thought it ought to be ... but a visiting holidaymaker from Britain silenced his complaints for ever. "Why," said the visitor, "it is much clearer than we get at home - and we are only 15 miles from the Sutton Coldfield transmitter!"
The first cable laid used for TV signals was 12 gauge unscreened quad in sections with two 22 gauge quads for the sound (four programmes). On test the 22 gauge cable failed completely owing to mechanical damage.
For subsequent demonstration purposes the television signals were fed to Bouley Bay over a two quad (16 gauge) cable unscreened for a distance of 1½ miles. Carrier frequency was 45 Mc/s and four Baird amplifiers were employed with long time constant automatic gain control fitted to these amplifiers. The same method is still used, not the synchronised pulse scheme, and has been found quite satisfactory, and it is understood that the Development Department are now using the same method for some repeaters.
The variation of attenuation with weather was too great at 45 Mc/s, and one cable was replaced with 18 gauge aluminium quad and this is still working. The signals were fed at the carrier frequency at 45 Mc/s for a period of 12 months. Carrier frequency was then changed to 9.72 Mc/s and this frequency was used for the Coronation demonstrations.
Vision signals were fed on one-pair of the screened cable and the mains supply for four repeaters working at 45 Mc/s phantomed over the quad. These repeaters operated at a 30db gain. Each pair of each cable carries an A.F. programme at 45 volts, one programme being TV sound.
Vision signals are now fed at 9.72 Mc/s with only one three valve repeater with the mains supply phantomed. With regard to the tilted wire aerial arrays, these were in two sets, one for Sutton Coldfield Station and one for Alexandra Palace Station, and automatic diversity reception was employed. Interference was so strong, however, from continental transmitters that the scheme of automatic diversity reception proved unsatisfactory and the switching was then done manually. The present installation comprises three sets of tilted wire arrays beamed on Alexandra Palace, Wenvoe and Paris. Diversity was originally at 45 Mc/s with a converter to change the 61.5 to 45 Mc/s.
For feeding the signals to St. Helier, repeaters were designed and used similar to the scheme employed at Thanet where the carrier frequency is now 9.72 Mc/s. The repeaters for the seven mile link were designed with negative feedback in the output stages and not only gave a bandwidth of at least 7 Mc/s but enabled the output stage to function at its optimum load and be independent of quite wide variations of the 12 gauge unscreened cable characteristics. This cable is buried in ground which is literally lying in water at certain times of the year and at others in completely dry earth. The variations in the magnitude of characteristics impedance are from 53 to 90 ohms with a considerable variation in the phase angle.
At the aerial there are two A154 low-noise amplifiers in cascade for each signal followed by an RG109 frequency changer. A power output stage made in Jersey design provides 16 volts into the link. There are seven repeaters, the last of which is in Colomberie Close.
The link is built up as follows:-
To first repeater - 2,000 yds underground 12 gauge aluminium.
To second repeater - 1,600 yds underground 12 gauge aluminium.
To third repeater - 600 yds 12 gauge unscreened joined to 900 yds of 18 gauge screened.
To fourth repeater - 1,200 yds of 12 gauge unscreened.
To fifth repeater - 1,200 yds. of 12 gauge unscreened.
To sixth repeater - 1,760 yds of 18 gauge screened.
To seventh repeater - 1,760 yds of 18 gauge screened.
This link was used throughout for the Coronation demonstration. The penultimate 1,760 yds was laid in a sewer, and the last 1,760 yds partly in a brook and partly under the street in a duct. Since the Coronation, mechanical faults have developed on 1,200 yds of the first 12 gauge section, and since Rediffusion was not allowed to dig up the road it has been replaced with a catenary supported overhead 18 gauge copper squad mounted on the Electricity Company's poles.
The overhead cable allows for feeders to other parts of the island also for subscribers' connections en route to Colomberie Close. The Carrier scheme had been abandoned. A 22 s.w.g. quad will be added to the squad for country areas, and the four A.F. programmes amplified in the subscribers' units. For distribution in St. Helier it was eventually decided to use unscreened squad for the drop-ins, and also for sections of the network, while for other parts copper screened quad would be used.
The feeder repeaters are TV31s - a 3 valve small repeater in a housing giving 4 watts output. These cost £36 including power pack, housing and sound by-pass units.
For unscreened drop-ins the signal has to be 100 millivolts, and for screened drop-ins 20-40 millivolts at the subscribers unit. All the terminal units have only one I.F. valve. In St. Helier the sound distribution is at audio.
A four programme unscreened network was in existence so a squad was added to the quad. At the ends of spurs lengths of up to 200 yds of existing unscreened squad were used for the TV feed. Chokes were not put in for the non-TV subscribers, who just had the "D" programme drop-in disconnected. By using this method there was a considerable saving in cost of blocking chokes.
All the inserts were designed and made locally. The terminal units are made by Felgate Radio modified to the Jersey design on 9.7 Mc/s, 100 of these with 12in. tubes were used for the Coronation, while 14 inch and 17inch diameter tubes also from Felgate will be used in the future.
Outside St. Helier, all the cabling is to be run on the Electricity Company's poles using an amplified audio system. The network consists of one 22 s.w.g. quad and one squad. As feeders of up to 12 miles length are needed, audio is amplified, in the subscribers' unit.
It is anticipated that there will be three Rediffusion programmes and two TV sound programmes. Four of these will be at audio, and one either phantomed or using a 50 Kc/s carrier. The terminal units for this scheme are now being designed, and a high gain vision repeater has been designed for pole mounting with A.G.C.
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