Low Noise Tunable Preamplifiers for Microwave Receivers

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Abstract
An investigation of noise reduction in backwardwave amplifiers has yielded two significant results. 1) This type of amplifier has been demonstrated to be capable of very-low noise figures and therefore constitutes an entirely new class of microwave receiver tubes. Tube noise figures less than 6 db for a 25 per cent tuning range and less than 4.5 db for a 10 per cent tuning range of the amplifier's narrow pass band have been attained. 2) Tube noise figures of well under 4 db have been measured, which are the lowest recorded to date on any type of microwave tube. This performance results from a special low-noise gun which was developed for use with hollow electron beams and which features a new type of beam launching mechanism. The experimental S-band tubes and "Christmas-tree gun" are described. Detailed noise performance of the tubes is presented, as well as other data relating to the operation of the tube as a receiver component. On the basis of these experiments, it is concluded that still lower noise figures are possible using the basic concepts of this new gun, not only for backward-wave amplifiers, but also for other types of microwave tubes.

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