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Fig. 5, which shows the plots made by an American military radar of the Japanese aircraft approaching Pearl Harbour. These plots were never used even to give a warning, because there was no effective system of assessing them and reporting the results to the defence.

One thing which Watson-Watt always claimed to have 'invented' was the highly successful use of WAAF's (Women's Auxiliary Air Force) as radar operators. The only entertaining moment in the whole dreary proceedings of the Royal Commission on Awards to Inventors in 1951 was when Lord Justice Cohen, who I thought was asleep, suddenly interrupted a very long speech by Watson-Watt about WAAF's to ask if he was claiming to have 'invented women'. Watson-Watt claims to have replied, 'My Lord, that was before my time'; maybe he did, but if so it was lost in the laughter.

The Home Chain (CH)

A second air exercise was staged in 1937 using three of the original five radar stations and was sufficiently successful for the future of the coastal chain of radar stations (Home Chain-CH) to be firmly assured; in August 1937 an order was placed for twenty more stations. Construction went ahead at a great pace and by the outbreak of war there were about 19 stations on the east coast and six on the south, giving radar coverage from the north of Scotland to Portsmouth (Fig. 6). By the end of the war there were some 50 CH stations giving coverage more or less all round Great Britain.

A typical CH station7 on the east coast was equipped with four 110 m steel towers (Fig. 7) which supported the main transmitting array, a vertical stack of eight half-wave dipoles. The transmitter used water-cooled, continuously evacuated demountable tetrodes to radiate about 350 kW at one of four spot frequencies in the range 20-30 MHz; the pulse width was 20 us and the pulse recurrence frequency was 25 or 12.5 Hz. The receiver was equipped with either three or four wooden towers (73 m high) for different spot frequencies; each tower supported a pair of crossed dipoles (with remotely switched reflectors) at a height of 65 m; there were also auxiliary dipoles at 30 m and 15 m which were used for height finding and filling gaps in the vertical polar diagram of the main system (Fig. 8). At the receiver, direction-finding was accomplished by comparing the signals from the crossed dipoles in a goniometer and the 1800 ambiguity in the direction of the target (the 'sense') was resolved by using the switched reflectors. Height-finding was accomplished by measuring the ratio of the signals from the dipoles at 65 m and 30 m with a goniometer and feeding this ratio, together with range, into an electro-mechanical computer which calculated the target's height. The receiver itself had low-noise pentodes (EF8s) in the input stage, a triode-hexode mixer and an IF frequency of 2 MHz with optional bandwidths of 500, 200 and 50 kHz. The receiver output was displayed on a cathode-ray tube whose timebase could be calibrated with marker pips. The


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Updated 18/12/00

Constructed by Dick Barrett

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ęCopyright 2000 Dick Barrett

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