Low noise C-band traveling-wave tube

A developmental C-band traveling-wave tube with gain of 20b and noise figure in the 4.5 db range is described. The tube is characterized by operation with a hollow electron beam and employs permanent magnet focusing in which a peaked magnetic field is produced at the cathode. In order to improve the "d" factor, a ratio of beam diameter to helix diameter of the order of 0.75 is employed. In addition to improving the noise performance, the peaking of the magnetic field at the cathode aids in achieving this close beam-helix coupling. The tube is matched to waveguide with a return loss of better than 20 db over a 10% bandwidth. Some additional features of interest from the viewpoint of system applications are presented.