Patent GB-795863-A (GB)
Title: Improvements in or relating to travelling wave tubes
IPC: H01J23/52
Filed: 1955-10-28
Abstract: 795,863. Travelling wave tubes. WESTERN ELECTRIC CO., Inc. Oct. 28, 1955 [Oct. 29, 1954; Oct. 29, 1954], No. 30905/55. Class 39(1). [Also in Group XL(b)] A travelling wave tube comprises two concentric helices coupled over a coupling region, wherein the axial phase velocities of the two helices are different at the ends of the coupling region and vary progressively from the ends to zero in the intermediate region. In one form F (b) the tube has inner helix 14 of uniform or varying pitch and outer helices 19, 23 wound oppositely to the inner helix. The phase velocity of the outer helices decreases from a value greater than that of the inner helix to a value less than that of the inner helix or vice versa by varying the pitch of outer helices 19, 23. The helices are terminated at their free ends in reflectionless impedances 15, 16, 20, 24. The inner conductors of the coaxial line feeders are continued as the outer helices 19, 23. The tube envelope is of glass or quartz. In a transverse field travelling wave tube, Fig. 2A, the inner helix is of uniform pitch and the outer helix, wound oppositely to the inner helix, has uniform pitch except at the ends of the coupling regions, where it has increased pitch. In the intermediate portions of the helices the phase velocities of the helices are equal and the coupled helix system carries a T.E. mode ; slight inequalities can be tolerated but result in reduced coupling. The so called out-of-phase mode is used, i.e., corresponding points on the two helices are # out of phase. Alternatively the outer helix has uniform pitch in the coupling regions and the pitch of the inner helix is decreased at the ends, or the pitch of both helices can be varied simultaneously. A quartz cylinder 40 rests against the envelope and supports the ribbon outer helix and the ribbon inner helix is wound around rod 38 of quartz. The beam is annular and flows through the space between the two helices which are at the same D.C. potential. For high output forms the phase velocity can be reduced at the output end of the intermediate portion of the helices to keep in step with the reduced velocity of the beam. The inner helix may have the same length as the outer helix or may be shorter as shown and the coupling regions may be several beat wavelengths long. The ends of the outer helix can be connected to the inner conductors of coaxial feeders, the outer conductors of the coaxial feeders being connected to a shield. Accelerating electrode 51 has three arcuate openings for the annular beam, the openings being aligned with the cylindrical potential surface of purely transverse field, and it is stated that electrons passing along this potential surface produce no noise in the delay line. A longitudinal focusing magnetic field can be prov.ded. The in-phase or out.of-phase modes are excited by supplying the input to the helix of faster or slower phase velocity respectively. The separation between the two helices can be varied as well as the pitch to shorten the energy transfer regions. The characteristic impedance of the helices can be uniform and equal to one another over the coupling region ; to keep the characteristic impedance of the outer helix constant, it can be surrounded by a conductive cylinder of uniform diameter, or flared. Loss material is provided. The internal helix may be excited by the feeder, e.g., by connecting it to the inner conductors of coaxial lines, or the feeder may be coupled to the inner helix at one end and to the outer helix at the other end. The pitch of the outer helix in the coupling regions may decrease at the junction with the intermediate section of the helices. In a modification, Fig. 3 (not shown), which is a longitudinal field type travelling wave tube, the inner helix is of uniform pitch and the pitch of the outer helix decreases as shown to give a phase velocity less than that of the inner helix or the pitch of both helices can be varied in energy exchange regions. The inner conductors of the external coaxial line feeders can be connected to the outer or to the inner helix the outer coaxial line conductors terminating in shields 6, 9, 70. The beam can be annular and projected in the space between the two helices or a solid cylindrical beam is projected along the axis. The outer helix is supported by the external surface of the tube envelope while the inner helix is wound on a rod as in Fig. 2A. Reflectionless terminations for the inner helix are formed by lossy material sprayed on to its supporting rod. The in-phase wave is excited, i.e., corresponding points on the two helices are in phase. The accelerating potential can be applied to a stannic oxide coating on the inner surface of the envelope, or to the helices. The phase velocities of the two helices are equal in the intermediate region so that the energy is divided equally between them, and magnetic focusing can be used. In general the longitudinal field travelling wave tubes preferably use the faster in phase mode and the transverse field tubes the slower out-of-phase mode. There are four normal modes-two forward (in-phase and out-of-phase) and two backward, the backward wave modes being undesired. Hypercoupling i.e., transfer of energy between modes, is made as small as possible. Specifications 761,279 and 795,862 [Group XL(b)] are referred to.