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M.J.B. Scanlan

 

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Early Centimetric Ground Radars - A Personal Reminiscence

This affair, and that of the misplaced waveguide gasket at Trimingham, highlighted the difficulty of knowing whether or not a centimetre station was performing as it ought. There was no way for the station staff or, for that matter, the QO party, to check any of the three vital characteristics (transmitter power, receiver sensitivity, aerial gain) which determined the radar range. All they could do was to check the tuning very carefully and establish some performance criterion against a suitable PE. On Type 271, as already mentioned, tuning was an awkward and frustrating process:

moreover, on the west coast, there was not the slightest risk of E-boats, and it is even doubtful if there were minefields. Morale at Great Orme was diminished by sheer boredom, and would have been higher, had a few WAAFs been present; they would probably have swept the cabin every day, and so avoided a thick and revealing layer of dust on the rectifier box! WAAFs were never used on Type 271s, presumably because they were thought not equal to the task of turning the cabin by hand.

A QO party was no better equipped than the station staff to assess the efficiency of a particular station (except for the power meter when it became available). Indeed, the station staff were better acquainted with the local pattern of PEs, the return from which was the only possible criterion of efficiency. Therefore a QO party would spend the first hour or two in carefully measuring a few prominent (but not too large) PEs. This was followed by a careful run-through, and reiteration, of the tuning procedure, noting any improvement. Then, towards the end of the overhaul, we would always change the TR cell and the mixer crystal, the most critical components in the receiver. The TR cell was important in itself, in that signal strength would be lost unless it was on tune and correctly coupled to the outgoing coaxial cable; it was even ruiore important in its role of protecting the crystal from the 40-odd million transmitter pulses per day. The inside of the TR cell cavity was carefully polished, which was said to improve its performance, presumably by improving its Q-factor and so hastening the onset of the gas discharge. (Many years later, it was shown that a TR cell broke down ten or twenty nanoseconds into the transmitter pulse.)

The mixer crystal was an even more problematic device, not really very far removed from the 'cat's whisker' of 1920s domestic radios. In a post-war paper(11), Moxon gave an analysis of 1000 crystals: of these, 20% were not worse than 1.5dB down (compared with the best), 60% not worse than 3dB down, and only 20% worse than 4dB down. He also measured the noise factor of twenty-four crystals: with an IF noise factor of 6dB, the best overall noise factor was 13dB, the worst 16.5dB. It is not known what criterion was used to pass or reject crystals after manufacture, but it might well be thought that the results on the 1000 crystals show either a very good manufacturing process or a reasonably good screening system. Nevertheless, since a loss of 1.5dB in the crystal means a l0% reduction in radar range, only one crystal in five would enable the measurement of range to within l0% of the optimum: at the other end of the spectrum, one crystal in five would yield a range measurement that was 25%

or more down on the optimum. There was no way of measuring crystal performance in the field, by means, for example, of a noise factor measurement, nor were the Moxon statistics available. The QO party did its best in these circumstances to have a reasonable stock of new crystals available (they were never in easy supply) and by keeping them screened (for example, in a tobacco tin) whenever the transmitter was running. (We suspected that the incessant transmitter pulses - well over a billion a month produced a slow degradation of the crystals, even if an occasional rogue pulse did not completely destroy them.)

The QO routine thus comprised two contrasting phases: the three or four days of the overhaul itself - frantically busy and not without anxieties for the whole party, especially perhaps its leader; and the day (or days) between overhauls, when the party would move to the next station, settle in, and perhaps even take a day off.


 

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Constructed by Dick Barrett

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ęCopyright 2000 - 2002 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.