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Chain Home Radar - A Personal Reminiscence


At first sight, the CH radar would not promise to be a very effective system. The PRF was very low at 25 p.p.s, giving a non-ambiguous range of about 6000 km: under 'anaprop' (anomalous propagation) conditions, even this PRF proved too high, and returns from one pulse appeared on the next time-base. This phenomenon, aided by modern data processing, is the basis of over-the-horizon radars: in CH, the nuisance was overcome by halving the PRF to 12.5 p.p.s. As a result of the low PRF, the mean power was also low (200W at 25 p.p.s., only 100W at 12.5 p.p.s). At 30MHz, the galactic noise level is very high, but as explained above, this was not as serious as it might appear, given the high losses between dipoles and receiver. The aerial gains were also low: 8 or 9 dB for the main transmitter array, perhaps 3 or 4 dB for the receiving arrays. However, the saving grace of the system was that it was a floodlit system, which meant that 25 pulses were received from every target within the coverage in every second, save for the brief intervals when the genie passed the bearing of a target: the mean strike rate was therefore at least 103 per minute. A rotating beam radar, on the other hand, would put ten to twenty pulses on the target in every revolution, which would take 10 - 15 seconds: the mean strike rate was at most about 100 per minute, moreover, returns from successive revolutions would be completely uncorrelated. One concludes that CH was designed not so much by theory as by a series of careful step-by-step experiments.

Whether by awe-inspiring foresight, or merely by serendipity, the CH radar system, for all its shortcomings, proved a most valuable factor in the air war over southern England in 1940. The battle was fought as dictated by the enemy, an assault by massed aircraft flying at considerable height in daylight. These were ideal conditions for a defence guided by CH, which gave early warning of high flying aircraft, together with a sufficiently accurate indication of position and height for the defence to be deployed in good time and in commensurate strength. Another great strength of the system was the fact that all the stations in a given sector reported to one filter room, which therefore had an overview of the complete picture: it did not matter too much if a single station was out of action for any reason. If the enemy, having monitored the radar transmissions and measured the heights of the towers (the towers at Dover were plainly visible from across the channel) had appreciated the lack of low cover and attacked much nearer sea level, or had expended more effort on jamming the stations instead of trying to destroy them by bombing, the outcome might have been very different.

It is perhaps worth recalling that the commanders on both sides were veterans of the air battles of 1914-18, where superior height was generally decisive. The British were therefore content with the cover given by CH; the enemy, for his part, even if he appreciated that CH could not give low cover, evidently decided that superior height was too valuable an asset to be sacrificed, even if this meant early detection. They probably underestimated, not so much an individual CH station, but the integrated and centralized defence system in which detection, plotting and height-finding by CH was only the first, if vitally important, step.


This account relies heavily and gratefully on Bruce Neale's 1985 paper on CH and on Bill Baker's 'History of the Marconi Company': I count ito privilege to have known and worked with both these authors. The CH people I knew in 1943 also contributed greatly, by a process almost of osmosis, to my knowledge: alas, they are too numerous to mention, and I have completely lost touch with them.

If it is true, as has been alleged, that all old radar people suffer from nostalgia, it is above all true about CH people. Just as a steam train enthusiast despises diesels, so CH people tend to look down on later radars, however efficient or effective. If this paper helps to foster and preserve that nostalgia, it will have served its purpose.


1 NEALE, B.T., 'The Deventry Experiment', Marconi Radar Systems News and Views, Radar History Special Edition, 1991.

2 BAKER, W.J., 'A History of the Marconi Company', Methuen and Co., 1970.

3 NEALE, B.T., 'CH - The First Operational Radar', GEC Journal of Research, 3, 2, p. 73-83, 1985.

4 Dodds, J.M and Ludlow, J.H., 'The C.H. radiolocation transmitters', Journal of the IRE, 93, IIIA (Radiolocation) 6, p. 1007-1015, 1946.

5 JENKINS, 1W., 'The development of CR-type receivers for fixed and mobile working', Journal of the IEE 93. IIIA (Radiolocation) 6, p. 1123-1129,1946.

This article has been reproduced from the "GEC Review", vol 8, no.3, 1993 with the kind permission of the Editor.


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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.