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Hail and Farewell
The BBC opens its new London transmitting station
The BBC's new TV transmitting station at the Crystal Palace site, in South London in 1956, replaced the original transmitting station at Alexandra Palace with which the BBC service opened in 1936. Alexandra Palace was the first high-definition TV station in the world. Little was known in 1936 about the range of a high-power transmitter working on a frequency of 45 Mc/s, and it was thought that the range for satisfactory reception would not exceed thirty miles. The site of the first station was therefore chosen so that the maximum number of viewers would be included within that range.
In fact, the range of thirty miles was considerably exceeded. Moreover, technical developments made it possible to use transmitters of considerably higher power and a range of at least fifty miles became possible. This factor, and the density of population in the area to be covered, indicated that the maximum number of people would be covered by moving the station to a site south of the Thames. The site at Crystal Palace was chosen after consideration of a number of alternatives.
The station building was underground in order to comply with the requirements of the London County Council (who owned the Crystal Palace grounds) that public access to this area should not be impeded. The building was planned so that it would be possible to extend it in the future to house further equipment which might be needed, for example, for transmissions in Bands IV and V or for the development of colour television.
The Crystal Palace was believed to be one of the most efficient TV stations in the world. In planning the actual transmitting installation, great emphasis was placed on reliability. The vision transmitter consisted of two identical units each producing a peak-white power of 15 kW, and the sound transmitter of two identical units each producing an unmodulated carrier power of 4 kW.
Each transmitter had its own programme-input equipment for vision or sound as the case may be, and the inclusion of common lines in the chain from the programme input to the aerial was avoided wherever possible. Where this was not possible, the equipment was duplicated and provision made for an immediate change-over if a fault should develop. This arrangement permitted an important saving in staff costs because if a fault should develop in a particular piece of equipment, the service was not interrupted. Immediate attention to faults was, therefore, not vital and this enabled the staff on watch to be reduced to two men.
The transmitting aerials were carried on a self-supporting tower which is tapered to a height of 440 feet above ground level, and then continued as a parallel-sided structure for a further 250 feet. The aerial system consisted of eight tiers of dipoles mounted partly on the support tower and partly on the parallel-sided portion above it. The tapered section of the uncompleted tower is shown in the photograph above.
The aerial system was divided electrically into two halves, one vision and one sound transmitter feeding power to one half of the system, while the duplicate vision and sound transmitters fed power to the other half via a separate feeder and combining unit.
Power-supply equipment had already been installed to take account of possible future requirements. It had sufficient capacity to handle the load imposed by two 50 kW vision transmitters and two 12 kW sound transmitters. Provision had been made for additional transmitters of comparable power for a second programme channel later.
Adapted from an article in: "The Television Annual for 1957".
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