Early Centimetric Ground Radars - A Personal Reminiscence
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3 a) The CV76 S-Band strapped magnetron, as used in the Type 277 radar. The device operated at 25kV, 40A and produced radiation at a wavelength of 10cm; and b) diagrams showing construction of the magnetron. |
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By contrast, the Type 52 had one tower, 60 m high, carrying a 2m dish antenna and the receiver/operations room was in a single Nissen hut near the foot of the tower. The total volume and weight of the CH transmitters and receivers must have been 100 times greater than their Type 52 equivalent.
There were in fact two variants of the Type 52. In the more common type, the transmitter and the early stages of the receiver were housed in a 3m cube cabin immediately below the antenna; the received signal at an intermediate frequency (IF) of 60 MHz was fed down the tower in coaxial cable to the main IF amplifier, which was part of the A-scope. In the less common type, the transmitter and receiver were in an extended Nissen hut at the foot of the tower. The transmitter power and the received signal went up and down the tower in waveguide. This was, of course, a much more convenient system, but, as we were able to show later, it paid a considerable price in performance.
Even in the long-waveguide stations, there was turning gear and the rotating joint at the top of the tower, for which maintenance was required. The tower was therefore provided with a small lift that ran from ground level to an open platform some 6m below the antenna: from this, one climbed a vertical 3m ladder into the upper cabin, which in most stations, as already noted, housed the transmitter and the early stages of the receiver. The journey to this cabin was physically undemanding, unlike climbing a OH transmitter tower, but sufficiently tiresome that one took good care to be adequately prepared in every sense before ascending.
The Type 277 Radar
The Type 277 radar, as befitted its naval origins, was powered, not directly from the 50Hz mains, but from a 500 Hz motor generating set, whose output was 180V AC. The pulse recurrence frequency was also 500p.p.s., linked directly to the 500Hz supply: at 500 Hz, of course, the DC power supplies were much more easily smoothed than at 50 Hz.
The transmitter was simple and compact. The 180V AC was transformed to about 6kV, rectified and used to charge a 2 us pulse-forming network. Using a trigger pulse derived from the 500 Hz supply, the network was discharged by a mercury thyratron through a 4:1 step-up pulse transformer to the CV76 magnetron, which was supported between the poles of a powerful permanent magnet. The magnetron output, derived from a coupling loop in one of its cavities, was fed by a probe through the broad face of the waveguide, and launched via an anti-corona sphere on the tip of the probe. Thus, there were three adjustments:
the probe could be moved into or out of the wave-guide (the only degree of freedom available to the magnetron itself), the position of a short-circuiting waveguide plunger could be adjusted, and the permanent magnet could be moved slightly to give optimum alignment between its field and the magnetron axis. Any tools used in this area were of brass, and wristwatches were at considerable risk. The arrangement of magnetron, magnet and waveguide is sketched in figs. 4 and 5.
