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M.J.B. Scanlan

 

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Early Centimetric Ground Radars - A Personal Reminiscence

 

The magnetron power was delivered via a rotating joint to the antenna feed horn, which illuminated the 2 m circular dish antenna. There was no method of power measurement available, and the custom was to place one's hand over the feed horn to ensure that the magnetron was delivering at least some power. This crude attempt at power measurement was also a very early example of microwave cooking! As has been noted elsewhere(5), the mean power of a radar magnetron, even such an early example as the CV76, was closely comparable to that of the magnetron in a modern microwave oven. The CV76 was equipped with a heater for the cathode, but the heater was switched off as soon as the magnetron reached full power: thereafter, the cathode was heated only by electron bombardment.

The received signals ('echoes') were collected by the antenna and passed, via the feed horn and rotating joint, to the vertical waveguide run leading to the magnetron. However, they were inter-cepted about 30cm above the magnetron by a TR (transmit/receive) cell clamped to the narrow wall of the waveguide, to which it was coupled by a slot in the wall. The TR cell (fig. 6) ingeniously combined a toroidal resonant cavity and a gas discharge device. During the transmitter pulse, the gas discharge across the centre of the cavity effectively threw an open-circuit across the waveguide wall slot, and confined the power to the waveguide, save for the small fraction needed to sustain the discharge.


 

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ęCopyright 2000 - 2002 Dick Barrett

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