Low Noise Tunable Preamplifiers for Microwave Receivers

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.