Low-noise backward-wave amplifiers

The characteristics of backward-wave amplifiers are briefly reviewed and related to general receiver problems which exist in broad-band high-power systems. It is emphasized that a backward-wave amplifier of sufficiently low noise figure offers potential solution to several of these problems and would be more useful in some cases than a conventional low-noise traveling-wave tube. The problems encountered in noise reduction in backward-wave amplifiers are discussed and compared with those for forward-wave amplifiers. Gain, bandwidth, and noise-figure characteristics of experimental S-band tubes are presented. Very low noise figures have been achieved for tuning ranges up to half an octave with minimum programming of the gun potentials. A noise figure of 3.7 db at the center of the tuning range has been attained and was reproducible after several hundred hours of operation. This node figure is well below the usual theoretical minimum and is made possible by the unique geometry of the annular cathode and gun. The results to date indicate that the backward-wave type of amplifier is feasible for receiver applications and may satisfy the increasingly apparent need for rapidly tunable selective preamplification.