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Chain Home Radar - A Personal Reminiscence

The phenomenon involved is exactly analogous to Lloyd's mirror in optics. In purely metric terms (h and l in metres), the first lobe is at 14.3l/ho, the first gap at 28.6,l/ho, and the second lobe at 42.9,l/ho. As applied to CH, l taken to be 10 m, the formulae give lobes at 2.2o and 6.6o with a gap at 4.4o, for the main transmitter array and the 'A' receiving system. For the gapfiller array and the 'B' receiving system, there are lobes at 4.9o and 14.7o and a gap at 9.8o. This pattern of lobes and gaps forms the vertical polar diagram (V.P.D.) as shown in fig. 6, which is reproduced from Neale's fig. 6. Of course, this is a theoretical diagram, and assumes that the reflecting ground in front of the arrays is flat, smooth and highly conducting. If these conditions are not met (as they never are completely), then calibration will be needed, and was in fact always carried out, however good the site might appear to be. Further discussion of the V.P.D. will be deferred until the operating procedure of the station is described. Incidentally, Neale's assertion that the pre-war siting instructions laid down that chosen sites must not 'gravely interfere with grouse-shooting' is, I think, apocryphal: at least two thirds of the 22 East Coast stations, and all the most important stations operationally, were well away from traditional grouse-shooting areas.

The East Coast CH transmitter, type T.3026, has been fully described by its designers(4), and more briefly by Neale(3). The transmitter hall contained two transmitters, each consisting of two large cabinets and a control console fig. 7. The transmitters, although based on a design for a short-wave beam transmitter, needed extensive modification for radar purposes: in particular, in order to generate a well-shaped pulse, the transmitter must be turned on and off quickly at the required times, and the power radiated between pulses must be low when compared with the receiver sensitivity. These were formidable and unfamiliar problems at the time.

Several features of these transmitters are worthy of special note. The RF power was generated in a master oscillator at half the operating frequency, then amplified and doubled in frequency in a driver stage, then amplified to full power by a pair of tetrodes in push-pull. The driver and the two output valves were water-cooled and demountable, continuously evacuated by oil-vapour vacuum pumps, backed by mechanical rotary pumps.


 

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ęCopyright 2000 Dick Barrett

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