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Early Centimetric Ground Radars - A Personal Reminiscence

So, back to radar Type 277. There was only one job the QO party could perform which was outside the scope of the station personnel. The QO party had a signal generator covering the IF frequency of 60 MHz, and an important part of the QO was to check, and if necessary, to re-align the IF amplifiers, which, as already noted, were generally split, with a head amplifier of three stages at the top of the tower, and a main amplifier of five or six stages as part of the A-scope. The separation was generally 90-100m, that is, nearly thirty wave-lengths. Even over a bandwidth of 1 MHz, which was the standard, this meant that any reactance at one end of the IF cable showed as a rapid variation of reactance at the other. To complicate the issue, the signal generator, made by Marconi-Ekco (a fore-runner of Marconi Instruments) covered frequencies up to 60MHz in one band, and over 60 MHz in another. 60 MHz could be achieved in either band, but not necessarily with the same power in both. IF alignment was therefore a tedious and frustrating process, one which would of course have been almost trivial with a swept signal generator.

After some months, we acquired, hot from the Telecommunications Research Establishment (TRE) laboratory a microwave power meter. This device consisted of a section of waveguide, perhaps a metre long, which was inserted in the main waveguide run: another length was always required to make up the length (~3m) of a standard waveguide section. A small fraction of the power in the main guide was coupled into an auxiliary guide, and this coupled power was used to light up a filament. This filament could be viewed, and compared with another, powered by a variable DC source. The DC power required for equal brightness was used as a measure of the microwave power in the main waveguide. This was hardly a precision instrument, but it was incomparably better than the 'hand-over-horn' guess-work that preceded it.

Use of this power meter enabled us to do some undoubtedly valuable work. First, we measured the transmitter power at the top and bottom of the waveguide run on a 'long waveguide' station. This showed that the waveguide loss in one transit was 3dB - a long waveguide station must therefore be 6dB less effective (allowing for transits either way) than a short waveguide arrangement. Such a loss implies a short-fall in range of some 40%.

In a second series of measurements, we were able to eliminate a long-standing problem at Trimingham. For a considerable time, perhaps even from initial installation, there had been complaints of intermittent arcing in the rotating joint. Measurements above and below the joint showed a loss of 3dB, far above the expected level. The joint was dismantled, and a misplaced gasket, which reduced the waveguide aperture by a factor of three, was found. This obstacle caused a severe mismatch, of course, and was solely responsible for the arcing and the loss of power. The station staff were delighted at the obviously improved performance and, on this occasion at least, were not dismissive of the efforts of the QO party.

In April or May of 1944, 73 Wing, which had previously covered the east coast from the Scottish border to Suffolk, took over the corresponding extent of the west coast. There were three centimetre stations in the new responsibility, and two of us were dispatched to carry out QOs on them. We were of equal rank and seniority, and, rather than squabble, we agreed that we would toss for the doubtful privilege of being in charge. I won and was therefore in charge for the first station (Cregneish, on the Isle of Man, Type 271) and the third (South Stack, on Anglesey, a Type 277, but mounted on a low gantry, rather than a tower). Nothing remarkable happened at either station, except that while we were at South Stack, the allies landed in Europe. However, the second visitation to Great Orme, near Llandudno, also a Type 271, was more eventful. We found that the station was about 20dB off tune, and apparently had always been so. There had been, of course, no objective way of assessing performance, but once a station had achieved a certain standard, it should never fall far below that standard. At Gt. Orme, the standard had been set at an appallingly low level: in a very short time, and without any special equipment, we improved its performance by a staggering margin. PEs appeared as if by magic: in particular, the Liver building at Liverpool now gave a strong echo, which had never been seen by the station staff. My colleague, whose turn it was, wrote a scathing report, with which I fully agreed. In retrospect, one can sympathize with the station staff: evidently, no one had ever shown them how the station should perform.

Unfortunately, we had left ourselves open to counter-attack. On the floor of every Type 271, there was a small metal box containing a copper oxide rectifier, used to top up the emergency 50V battery: there was a space on the QO pro forma for comment on this device, to which no one ever paid any attention. We had written 'Inspected and cleaned', (the universal euphemism for doing nothing, which satisfied the RAE mania for cleanliness). The station staff, in reply to the damaging report, picked up this point and complained to Wing HQ; it was all too apparent that neither we, nor they, had touched the rectifier box for weeks, or even months. This was another example of hubris: again it con-cerned a Type 271 station, and again nemesis took the burly form of Wing Commander Scott-Taggart. My colleague, as author of the QO report, was summoned to see the great man and explain the discrepancy, albeit on an almost trivial point, between the QO report and the undeniable fact. It must have been an uncomfortable interview, but the W/Cdr was lenient, and took no further action: probably he was as anxious as anyone not to aggravate the incident.


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Constructed by Dick Barrett

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ęCopyright 2000 - 2002 Dick Barrett

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