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M.J.B. Scanlan


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Chain Home Radar - A Personal Reminiscence

Assuming a galactic level of, say, 3000 K at 30 MHz and a receiver noise factor of 3dB (=300K), then a signal-to-noise ratio often, say, at the dipoles will be maintained through the cable and the gonio, but the levels of both signal and noise will have been reduced by the losses of, say, 10 dB. The total noise at the receiver will be 600K, and the signal will be one tenth of its level at the dipoles: the signal-to-noise ratio through the receiver is now five, that is, a loss of 10 dB has only reduced the signal-to-noise ratio by 3dB (from ten to five).

The very rapid increase in galactic noise with increasing wavelength, especially as the wave-length exceeds one metre, favours the lower frequencies, when there are severe losses between aerial and receiver, and may have been one reason for choosing 30 MHz, as compared, say, with 60 MHz, the frequency of the Daventry experiment. In any microwave radar system, a loss of 10 dB between the antenna terminals and the receiver input would be catastrophic, and every effort would be made to avoid losses of even a fraction of a dB.

The receiver room of a CH station was also the operations room, where the raw data measured by the CRT operator were converted into the position and height of the target. The raw data consisted of the range, and two gonio readings, one for bearing, the second for height. These were converted into position and height either manually or by an electro-mechanical calculator, the so-called 'fruit machine', which was based on banks of uni-selector switches, as found in Strowger telephone exchanges. These machines were cared for by ex-GPO personnel, usually sergeants in the RAF; these gentlemen were a law unto themselves who looked down on the ordinary RAF mechanic and were in turn treated with great deference by station technical officers, nominally their superiors. Of course, both the manual plotting table and the wiring of the Strowger switches were modified to allow for calibration results.

Besides its main complement of towers, transmitters and receivers, as already described, a full East Coast CH station had its so-called 'buried reserve'. This consisted of a pair of small auxiliary towers, and an auxiliary transmitter and receiver, each buried in a deep pit. The buried reserve transmitter was an MB2, much smaller and more orthodox than the main transmitters (for example, all the valves were sealed off in glass envelopes, and the transmitter was air-cooled); its power out-put was much the same as that of the main transmitters. The receiver was one of the RE series, often one step behind the main receivers in the operations room. The aerials were also smaller and simpler, without the elaborate switching

arrangements of the main arrays. The buried reserve system was intended as a last-ditch stop-gap, in case the transmitter hall or the operations room of the main system was knocked out by bombing. The reserve system was periodically run up and checked as a system, but was rarely used operationally because of the duplication in the main system, and because, as it turned out, the main system towers and buildings were difficult to damage from the air.

The Operation of a CH Station

Every CH station worked at least 23 hours a day, day in, day out: an hour a day was normally allowed for maintenance, but this could only be taken by agreement with the filter room, and would be cancelled if an adjacent station were off the air for any reason, or if heavy air activity were expected. A table (in effect, a black list) was published every month, giving every station's unscheduled time off the air, and good reasons were demanded if this exceeded a few minutes.

If a station was fully up to strength, it would have four watches of operators, each of six WAAFs and led by an NCO, and four watches of mechanics, each of three men, two in the transmitter hall, one in the operations room. Ideally, a WAAF officer, a highly experienced and skilled ex-operator, was present on each watch as a supervisor, and a station technical officer and the senior NCO mechanic would be present all day, and on call throughout the 24 hours. Very often, of course, the station was not at full strength, and would be reduced to a three watch system, in which every-one worked a full 24 hours every three days: such a regime could not be sustained for more than a few weeks. Mechanics were in even shorter supply than operators, and a watch would often be reduced to a single mechanic, who would haunt the transmitter hall, accompanied by an operator for safety. Under such circumstances, the daily maintenance routine became something of a scramble.

As an aid to an understanding of CH operations, consider a station whose 'line of shoot' was due east, that is, on a bearing of 90o. The transmitting arrays faced E, of course, and the receiving dipoles of the 'A' and 'B' systems ran N-S and E-W: the 'C' system dipoles, used only for height-finding, lay N-S. The sensing dipoles lay N-S, to determine whether a target was in front of, or behind, the station, and E-W, to sense whether the target was north or south of the station. The coverage of the station was determined solely by the width of the transmitted beam, about 110o: in our example, this would be from 35o to 145o. The receiving arrays, of course, received signals equally from any direction.


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Constructed by Dick Barrett

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ęCopyright 2000 Dick Barrett

The right of Dick Barrett to be identified as author of this work has been asserted by him in accordance with the Copyright, Designs and Patents Act 1988.