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M.J.B. Scanlan

 

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Chain Home Radar - A Personal Reminiscence

Fortunately or not, the perennial shortage of mechanics often meant that only a minimum of tests could be carried out: efforts were then concentrated on those tests and checks which had immediate bearing on the technical health of the station. This was no bad thing, it turned out: a panel thick with dust would carry on quite happily, while attempts to achieve the surgical cleanliness demanded by visiting senior officers did not improve serviceability. Hence the desirability of being far away from Wing H.Q.!

At intervals, nominally every three months, a Quarterly Overhaul party would descend on a station and carry out a complete check of major items: these were teams of two or three engineers, relatively well equipped with test gear, and specializing in receiver or transmitters. These people would carry out a fairly thorough overhaul, extending, for instance, to checking the receiver bandwidths and realigning as required. The transmitter team would check every aspect of the transmitter, including, for instance, the conductivity of the cooling water, which was held in a vast and rather inaccessible tank under the transmitter itself.

Anti-Jamming Provisions

It was clear from the design of a CH station that considerable thought had gone into protecting the system from jamming. Some of these provisions may now be mentioned.

The cathode ray tube itself was specially built to reject transient signals, in favour of target echoes, which would be longer-lasting. This was achieved by using two phosphors in the tube: a blue phosphor excited immediately by the electron beam, and an orange phosphor, excited more slowly by the glow of the blue phosphor. If the tube was observed through an orange filter, transient signals which excited only the blue phosphor were not seen, while the long-lasting echoes were easily visible.

The IFRU (intermediate frequency rejection unit) consisted of a pair of very narrow band rejection filters, which could be tuned separately across the intermediate frequency bandwidth. They were present to reject any accidental or deliberate CW interfering signal.

The AJBO (anti-jamming blackout) circuit was designed to counteract swept frequency jamming. In this very damaging form of jamming, an oscillator is swept in frequency across the pass-band of a receiver, to give a bell-shaped response on the CRT, and completely obliterating any echoes. The AJBO circuit, which worked at video frequency, was designed to discriminate against the slow rise time of the bell-shaped response and blackout the tube: some of the signal time-base would be blacked out and lost, but some would remain, unless the sweep frequency of the jammer was locked to the station PRF (pulse recurrence frequency).

The IJAJ (international jitter, anti-jamming) circuit was designed to introduce a slight random jitter to the PRF of the station, so that jamming pulses triggered by reception of one's own transmitter pulse would not be synchronized with the time-base. Since true echoes would be steady, they could be distinguished from the false pulses from the jammer, which would vary in time from pulse to pulse because of the jitter.

CH stations were also equipped with frequency agility, in that, at least in principle, both the receiver and transmitter could change to any one of four pre-set frequencies in a few seconds under push-button control. In practice, since a need for this facility never became apparent, it was never seriously tested.

Security and Safety

As has been noted above, the Daventry experiment heralded a very large effort towards the implementation of RDF (radio direction finding). The deliberately misleading name was the first of the security measures intended to cover all the developments and their outcome in a cloak of secrecy. Britain was thought to be alone in this field, or, at very least, well ahead of the field. Of course, this was a misapprehension, since as soon as the technology became available, the forecasts (for example, by Marconi himself) of radar were implemented in different ways by almost every advanced industrial nation.

Security, in the sense of preserving secrecy, took various forms. All the documents concerning the system were classified as 'Secret', and elaborate procedures for their destruction in emergency were laid down: for instance, some documents were printed on rice paper, and were to be destroyed in a bucket of acid. Training sessions for operators, and mechanics could be held, but no notes could be taken. A muster of secret documents by an officer from Wing H.Q. was carried out every quarter: this was supposed, impossibly, to account for every page of every document.

†I believe this term should read "Intentional Jitter, Anti-Jamming - Ed.


 

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Constructed by Dick Barrett

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©Copyright 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.