Investigation of improved magnetic materials for PPM-focused low-noise traveling-wave tubes

Low-noise traveling-wave tubes focused by a combination of a uniform field in the gun region and periodic field (PPM field) over the remainder of the tube usually have a noise figure from 3 to 5 dB higher than those of tubes focused by a uniform magnetic field (PM field). This paper describes how this disadvantage of higher noise has been largely overcome as a result of improvements in magnetic materials and PPM-focusing circuit design. The electrostatic lens effects were reduced by reducing the transverse magnetic field with more uniform magnets and by reducing the period for improved transmission. The magnetic lens effect in the drift region was minimized by extending the length of the uniform gun field so that at least 3 dB of gain occurred in this region. The final tube showed improved electrical and environmental results. A maximum noise figure of 7.9 dB was achieved from 6.4 to 12.0 Gc/s. Noise figures of 6.7 to 6.9 dB were obtained over the 9.2 to 10.2 Gc/s band. The final 5.3 lb low-noise amplifier offers a substantial weight and volume reduction for most low-noise applications and demonstrated compliance with MIL-E-5400F Class 2 equipment specifications.