Very low-noise traveling-wave tube

This paper describes recent improvements in design and techniques which have lowered the noise figure of a developmental traveling-wave amplifier from 9 db to 6 db. Some selected tubes have had noise figures as low as 4.7 db. The improved tube is now commercially available as the RCA-6861 low-noise traveling wave tube. A general explanation of beam noise is presented and the presently employed noise-reducing schemes and theories are discussed in relation to the 6861. The specific factors contributing to the very low noise figures are evaluated, including low helix loss, low QC with high circuit impedance, maximum beam diameter with minimum intercepted current, a flexible low-noise gun, and, most important, a smooth and highly emissive dense oxide cathode operating at about 600°C. Application considerations are also discussed, including phase sensitivity, life, saturation effects (modulation and harmonic generation), allowable voltage variations, and typical noise figure, gain, and match over the 2700-to-3500 Mc frequency range of the tube.