Very Low-Noise Traveling-Wave Amplifier

Recent improvements in design and techniques are described 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 4.8 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.