Image reproduced with kind permission of Ordnance Survey
Ordnance Survey of Northern Ireland.
Bearley is a former BT/PO radio station located in Warwickshire, located on the site of the former RAF Snitterfield approximately 5km due north of Stratford Upon Avon. It was a receiver station, being paired primarily with Leafield transmitting station.
Bearley in the 1950's
The site had been used as an HF radio receiving station since the early 1950s and was part of the General Post Office External Telecommunications Executive. In those early days, the accomodation consisted of Nissen huts as seen in the picture above, and the receivers were Marconi GF552 units, which used valves. However, the accomodation was unsuited to the expansion required to meet the demand of the rapidly rising volume of international traffic.
A new station was therefore built at a cost of £0.5 million (equivalent to £6.5 million today) opening in 1967 and which was ‘state of the art’ with new transistorised telephony receivers, the Plessey PVR800. Each RX occupied a full 19 inch rack standing 6ft tall. There were no visible controls except 6 banks of thumbwheel switches to set the channel frequencies. Everything else was managed from a Central Control Unit, CCU.
PVR800 Receivers at Bearley
The PVR800 itself was a quadruple-conversion superheterodyne receiver which could be set to any one of 6 preset frequencies or by fully synthesised tuning it could be set to any of the 200,000 available channels in 125Hz increments. The receiver could search for and identify transmissions automatically and once the correct one had been found, it could lock to it and follow variations in the transmitted frequency as long as it stayed within internationally agreed limits.
The receivers used independent sideband which allowed up to 4 voice channels on a single frequency. This was accomplished by splitting each sideband into 2 segments. The audio was also inverted which made it impossible to listen to in an ordinary RX.
The telegraphy receivers were modified Marconi HR11s which again occupied a whole 6ft rack. The heavy bit was the huge PSU at the bottom which provided the HT and heater supplies for the many valves. The HR11s were also controlled from the CCU.
Marconi HR11 receivers at Bearley
The HR11s looked more like conventional receivers with 2 RF units (double diversity) and a mixer unit each having a large slow motion drive. Converting them to remote control entailed fixing a box containing motors over the space where the knobs for the slow motion used to be. When the operator changed channels there was a loud whirring as the motors moved the dial to one end of travel and then back to the pre-set frequency. Sometimes the automatic system did not quite get things right and manual adjustment was required via a small knob on the motor box.
The HR11 local oscillators had been replaced by frequency synthesisers which allowed pre-set channels to be programmed in. The PVR800 receivers and HR11 synthesisers were locked to a 100Khz standard buried 35ft in the ground for temperature stability. At this depth, the temperature stays within about 0.5° of 10°C all year round without any artificial control. There were three identical standards at the bottom of the wells, at ground level were racks of distribution amplifiers to distribute the standard frequency around the site.
The receivers were located at one end the building and the multiplexing and decoding equipment at the other with the CCU in the centre.
The Cenral Control Unit (CCU) at Bearley
The telegraphy side was mostly multiplexed teleprinters using time division multiplex, TDMX. It was possible to get up to 48 channels on a single frequency. There were some TDMX decoders coupled to printers for testing and it was amusing to scan through the channels and watch the printer slow down to a point where each character was taking a few seconds to print.
All the decoded signals were sent down the line to various terminal buildings in London. There were a few racks of line equipment which were often visited by colleagues from PO telephones.
The new station aroused a lot of interest and people from many parts of the world came to visit. There was particular interest in the PVR800 and most visitors seemed to gravitate to the test RX in the maintenance room as there were many problems in the early days. The first models used some germanium transistors which were not particularly reliable and were soon replaced by silicon types.