Medium noise periodically focused, permanent magnet traveling-wave tubes

Although periodic-permanent-magnet-focused traveling-wave tubes using magnetically shielded convergent-flow electron guns are available with a wide range of gain and power-output characteristics, the noise figure of such tubes is usually of the order of 25 db or higher. In some applications, this noise figure is a limiting factor. This paper describes traveling-wave tubes of the periodic-permanent-magnet-focused type having octave coverage at L, S, and C bands and noise figures of the order of 12 to 16 db. This reduction of noise figure is achieved through the use of a multi-electrode parallel-flow electron gun in which both the noise-reducing electrodes and the cathodes are immersed in a uniform magnetic field. The remainder of the tube, which is less critical for noise, operates in a conventional periodic-permanent-magnet focusing structure. A typical tube at S-band has a noise figure of the order of 13 db, a small-signal gain of 40 db, and a power output of 20 milliwatts. The same general order of performance has been achieved at both L and C bands. Design features which permit tube operation under severe environmental conditions are also discussed, and some typical environmental performance data are presented.