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M.J.B. Scanlan

 

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Early Centimetric Ground Radars - A Personal Reminiscence

These enemy tactics proved difficult to combat. The best solution would obviously be for a night-fighter to shoot down the Heinkel before it launched its bomb, but this proved to be nearly impossible, given the wide choice of launch points and the slow speed and low height of the bomber. By this time, the best British night-fighter was the Mosquito, equipped with Al Mks. IX and X: how-ever, even if the bombers could be located without aid from the ground, the Mosquito proved to be far too fast to employ the usual tactic of creeping up slowly behind the target until visual contact was achieved. Beaufighters with their obsolete Al radars were brought back from retirement, but achieved little success. Similarly, although radar-directed guns had been successful when the general direction of attack was known, they were difficult to site effectively when the attack might come from a variety of directions.

Thus, although the air-launched V ls were never likely to be a serious threat, given that specially-equipped Heinkels must be used, and given that this method of delivery was intrinsically inaccurate, they were undoubtedly a nuisance and caused quite disproportionate alarm and dismay. Few of the attacking bombers or their bombs could be shot down, and the nuisance seemed likely to persist until every enemy airfield within range had been overrun.

In these circumstances, and for the first time, the centimetre stations came into some prominence. Until the advent of 'Divers' (the code word for Vls), they had suffered somewhat in prestige as compared with the CH and CHL stations, which had larger staffs and purpose-built operation rooms that tracked fast-moving aircraft. The centimetre stations were generally smaller in terms of staff, were housed in Nissen huts and tracked (for the most part) coastal convoys travelling at 10 knots or less. The main enemy was tedium: only the girl on the PPI need be constantly alert, while the A-scope girl and the plotter were involved for only a few minutes every quarter hour, only to find that the convoy had moved along its predictable path by three or four kilometres. However, the CH stations did not see Divers at all, and the CHLs only very late, that is, well after launching: they could never see the launching aircraft. The 73 Wing centimetre stations in Suffolk and Norfolk were the only stations likely ever to 'see' the launching aircraft, and to be able to follow the Diver throughout its path. There were four stations chiefly concerned: Benacre in Suffolk, Hopton, Winterton and Trimingham in Norfolk, and as luck would have it, my QO party dealt with all four. These four stations had the further advantage of being the furthest from Wing HQ in Yorkshire: telephone contact could always be made from our end if required, but the small QO party could be very elusive when phoned from HQ ('Sorry, Sir, he's up the tower').

When Divers first appeared in the North Sea, there was presumably a high-level appraisal of what could be done to improve the effectiveness of Type 52 stations against them. Not much could in fact be done, the Type 52 being very inflexible in its main properties. The pulse repetition frequency was governed by the 500 Hz motor generator set, the pulse width by the pulse-forming network and the peak and mean transmitter powers by the magnetron: the receiver sensitivity was poor in absolute terms, but the best available at the time.

In the event, it was decided that instead of the normal 1 MHz bandwidth, the optional 4 MHz bandwidth would be used, the only modification needed being to improve the bandwidth of the second detector by using four thermionic diodes instead of the standard single diode. This involved some tricky wiring in the already cramped IF amplifier box: more difficult was the greatly increased problem of IF amplifier alignment to give a flat 4MHz bandwidth, especially when, as in all short waveguide stations, the head and main amplifiers were separated by 100m or so of coaxial cable.

This modification, of course, decreased the signal-to-noise ratio (SNR) by four times, but it was supposed that the improved shape of the received pulse would nevertheless give a better PPI response. (The PPI now became the display of choice, because with the antenna rotating continuously at maximum speed, there was very little chance of seeing anything useful on the A-scope). The QO parties carried out the modification and the change of bandwidth as ordered, but I have wondered to this day whether the sacrifice of SNR was really justified.

The QO routine carried on throughout the Diver campaign, and was used as a time for the modifications to the IF bandwidth and the second detector diodes: it was of course even more important than usual that the stations were fully efficient, and that they were back on the air at 1800 hours. We were frantically busy for several months, and it was difficult to avoid the temptation to stay in the operations room until midnight or so to see any


 

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Constructed by Dick Barrett

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ęCopyright 2000 - 2002 Dick Barrett

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