Low noise transverse wave electrostatically focused B.W.A. for U.H.F.

It has often been pointed out in the literature that transverse wave electron beam amplifiers can make use of noise reduction methods not applicable to longitudinal space charge wave tubes. In the middle 1950´s electrostatically focused traveling wave tubes based on this principle were built by Adler. Such tubes have the additional advantage of low weight and of potentially low cost. With a recently renewed interest in voltage-tunable amplifiers, a simple structure has been proposed which consists of a stack of plates and small coils to form a bandpass filter propagating a transverse backward wave. Two different d.c. potentials are applied to form a series of lenses for beam focusing; with the particular structure this requires no additional elements. Theory shows that a cascade backward-wave amplifier built around this structure is possible with gains of 20 to 30 db, tunable over an octave, instantaneous bandwidths of a few megacycles and noise figures of the order of 3 db or less. Experimental tubes have shown an octave tunable bandwidth with voltage swings of less than 500 V; gains of 20 db are possible with tube lengths of approximately 5 inches. Noise figures of about 5 db were obtained by combining a gun originally designed for a low noise transverse wave TWT with a backward wave structure designed to operate with similar circuit voltages.