1,031,502. Travelling wave tubes. ENGLISH ELECTRIC VALVE CO. Ltd. Feb. 27, 1963 [June 29, 1962], No. 25077/62. Heading H1D. A travelling wave tube comprises a band-pass slow wave structure which has different portions with different cut-off frequencies, the arrangement being such that the cut-off frequencies of one portion lies outside the bandpass of the remaining portion. A backward wave tube has a series of disc-like transverse metal partitions forming the delay line, each having an aperture for the passage of the beam, and each having a coupling aperture, the dimension of the coupling aperture decreasing from the input end to the output end of the structure, the lower frequency cut-off of the structure near the input being thus above that near the output. The variation of cut-off frequency may be gradual, as shown, or effected in a single step. In a forward travelling wave tube operating in the fundamental mode, the beam apertures themselves act as coupling apertures and gradually decrease in diameter from the gun end of the structure to the collector.
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