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M.J.B. Scanlan

 

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Early Centimetric Ground Radars - A Personal Reminiscence

After the transmitter pulse, the discharge was extinguished and the received signals were coupled to the cavity and hence, by a loop in the cavity, to the mixer, that is, the first stage of the receiver. To aid rapid initiation of the gas discharge, a 'keep-alive' electrode was provided, which ensured that there were always a few electrons available in the discharge gap. Between the TB cell and the magnetron, there was a very similar transmit/receive cell, without a keep-alive, known as an ATR cell. The ATR cell's function was to present such an impedance, combined with that of the quiescent magnetron, that the received signals were directed through the TR cell, rather than back into the magnetron, which would seem a more obvious and natural destination. Both TR and ATR cells could be tuned by means of plungers screwed into the cavity: the TR cell tuned very sharply, the ATR cell so flatly that it was difficult to believe that its presence had much effect,

From the coupling loop in the TR cell, the signal passed via a short length of coaxial cable to the mixer cavity, into which the local oscillator (LO) was also fed (fig. 7). The intermediate frequency (IF) signal at 60 MHz was fed through the base of the cavity via a by-pass capacitor to the first valve of the IF pre-amplifier, whence it passed to the main IF amplifier in the A-scope display. The cable between the two IF amplifiers could be quite short in a 'long waveguide' station (transmitter and receiver both at the foot of the tower) or 70m or more in the more common short waveguide station: this distance was tens of wavelengths at the IF of 60MHz.

There was at this time (early 1944) no microwave test-gear available: one could not measure either the power or the frequency of the magnetron, nor the tuning or sensitivity of the receiver: so, as will be seen later, stations could operate for long periods at well below optimum performance. Every station had its favourite selection of permanent echoes (PEs), and relied on the signal-to-noise ratio (SNR) measured on these as the sole criterion of station well-being: ideally, there would be a range of signal strengths, so that tuning could be done initially on a signal of saturation strength (which at least gave a response when things were well off-tune): later, when tuning was well under-way, the final fine tuning was done with a SNR of say 10:1. Clear and identifiable PEs were a boon, not only for tuning, but also as a check on the range and bearing accuracy of the station.

Where a new magnetron was needed, the complete tuning procedure must be followed for optimum results. After running up the valve, and checking that sonic power was available by the 'hand-over-horn' test, the crucial step was to tune the LO (a klystron) so as to see some echoes. There-after, concentrating on a known PE of moderate size, all the other adjustments were gone through (magnetron insertion into waveguide; waveguide short-circuit position; TR cell signal coupling and tuning; signal coupling to the mixer cavity; LO drive to the mixer cavity; magnet alignment; ATR tuning) and probably reiterated, since some adjustments were interactive. Every hour, the more critical adjustments (LO tuning, TB cell tuning) were checked. The only (and very fallible) criterion of success in this complex tuning procedure was that the station's standard PE was showing its customary signal-to-noise ratio.

The Type 52 stations were designed to watch over the coastal shipping lanes, and initially took no interest in aircraft: as will be related shortly, however, their low cover capability proved useful in another, and quite unforeseen, role. The 60m tower was often sited very near the sea, generally on low (10-30m) cliffs, so that the dish was 70-90 m, above sea level, that is, about as high as the mean height of CR radar arrays. If the reflecting surface were flat enough for a sea-reflection lobe structure to be formed, the first lobe would have been at 0.022o, with nearly 30 lobes in the first degree of elevation, as compared with the first CH lobe at 2.2o(1). However, it is unlikely that any reflecting surface, sea or land, would ever have been smooth enough (that is, smooth to within l/2, or 5cm, over a considerable area) to give the classical lobe structure. For practical purposes, the cover could be regarded as following the surface of the sea as far as the horizon, nearly 40km away, for an antenna height of 80m. This solid cover extended upwards to 3o or 4o, so that the system of 10cm stations was sometimes known as CHEL (Chain Home Extra Low).


 

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Constructed by Dick Barrett

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ęCopyright 2000 - 2002 Dick Barrett

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