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The Chain Home radar system


There were a number of ingenious anti-interference/anti-jamming devices available to the operator. Perhaps the most significant was the use of a double phosphor CRT using a long 'afterglow' material (zinc cadmium sulphide) overlaid by an active blue phosphor (zinc sulphide). The 'flash' of the blue phosphor excited the long afterglow layer, leaving a yellowish image which could be viewed through a yellow filter whilst suppressing the blue flash, the theory being that enemy jamming, interference and the noisy background, being unsynchronized and of a transitory nature, would not build up a long afterglow image and would not be visible through the yellow filter. This system worked very well.

The IFRU, referred to earlier, was a pair of narrow-band notch filters, in series, which could be independently tuned within the i.f. pass-band to reject CW interference.

To counter the effect of 'spoofer' techniques, which enabled jammers to lock-on to the transmitted pulse and return a delayed pulse or sequence of pulses creating false targets, the station p.r.f. was intentionally 'jittered' in a random manner; false targets could then be easily identified as they would appear to jitter on the screen whilst true targets would appear stationary. This device was known as the Intentional Jitter Anti-Jamming Unit (IJAJ). Another device was the Anti-Jamming Black-Out unit (AJBO) which fed a portion of the video signal via a thresholding circuit with appropriate time constant to the grid of the CRT, 'blacking out' un-synchronized interference. A loudspeaker was also provided to assist the operator in identifying the nature of the interference or jamming by listening to the sound of the video signals.

It is worth mentioning at this point that the accuracy of the goniometer method of D/F was earlier thought likely to be inadequate especially in a heavy jamming environment. An alternative scheme, known as the 'Chapman' method (after the inventor, Corporal Chapman) using a system of range cuts to fix the target position, was proposed. One station would display echoes from its own transmitter plus echoes from the transmitter of an adjacent station on a second display. The operator had to swing his goniometer and note the two echoes, one on each CRT, which attained minima at the same goniometer setting to resolve ambiguity. The position of the target giving rise to the two echoes could then be found by range cuts, the goniometer playing no part in the position fixing. To prevent jitter between the two sources of signals, both locked to the grid system, the 'spongy-lock' method of synchronization mentioned earlier was adopted. In the event, the goniometer method of D/F, after careful calibration, was found to give sufficient accuracy for all practical purposes and the Chapman method was abandoned.


Much has been written, and rightly so, of the 'Daventry Experiment' of 1935. Few people will know of an equally important experiment which took place between 1936 and 1937. At the instigation of Henry Tizard, with typical foresight, a series of trials was carried out to establish an operational procedure for interception of hostile aircraft by control from the ground.

It was all very well having the ability to detect and track aircraft from the ground, but how best to use and act on this information? The efficient and successful procedure that was ready for action in 1939 was the direct consequence of a long series of mock interceptions by fighters (Gloster Gauntlets) from the air base at Biggin Hill controlled by R/T from the ground using filtered plots from the South East Coast CH stations. This exercise came to be known as the 'Biggin Hill Experiment'. For details of these historic and vital experiments the interested reader is referred to the biography of Henry Tizard by Ronald Clark.


CH played a vital part towards the end of World War 2 in tracking V2 (A4) rockets launched from occupied Europe towards London and the southern counties. The problem was to detect rockets soon after launch so that the likely point of impact could be estimated and early warning given. Additionally, the point of launch could be pin-pointed to enable Mosquitoes of Bomber Command to make a precision attack on the launching site. A system of simultaneous range cuts from five stations: Bawdsey, Gt. Bromley, High St, Dunkirk and Swingate (Dover) enabled the trajectory to be plotted as the rockets passed through the vertical lobes of each station, with sufficient accuracy to fulfil both these requirements.

The system was quite complex and relied on the integrity of the communication links and the high performance of the CH stations involved. The CH wavelength was most favourable for detection of the rocket, which behaved roughly as a quarter-wave dipole with a very good response and provided detection ranges in excess of one hundred miles. The system was code-named 'Big Ben',


CH was a remarkable achievement when set against the desperation of the times; it seems nothing short of miraculous that following a simple proving experiment at Daventry, the Home Chain was planned, developed, engineered, manufactured and installed just in time to meet the onslaught of the Luftwaffe. There can be no shadow of doubt that without CH, we would not have survived. We should be eternally thankful for men of the calibre of Henry Tizard, Watson-Watt, Arnold Wilkins, E.G. Bowen and many others who had the vision, courage, ability and above all the faith, to forge a system of the magnitude and complexity of the Home Chain in the little time available.

It was, indeed, a close run thing!


(This article is taken from "The GEC Journal of Research", Vol. 3 No.2 1985 pages 73-83 and has been reproduced with the kind permission of the Editor. The copyright of the material remains with the owner.)


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