Progress in low-noise microwave tube design

Low-noise microwave amplifier tubes are used primarily in microway relay and radar receivers. A commercial S-band low-noise travelling-wave tube is used as an example to illustrate the basic components, their philosophy and design. Particular stress is laid on the tube processing and the design of the cathode-first-anode region of the electron gun. The wave theories for travelling-wave-tube noise figures are reviewed, and the significance of the older theory in the light of recent results is discussed. While the generalized noise theory of longitudinal-wave devices is well in hand, theories for the region of multi-velocity flow are still in the early stages. Noise figures of other types of longitudinal-wave tubes should theoretically be as low as those of travelling-wave tubes, but the latter have enjoyed greater development. Empirical results from the work on travelling-wave tubes are stated, although they are not fully understood. Finally, suggestions are made as to promising areas for noise-figure improvement.