Patent GB-772002-A (GB)
Title: Improvements in or relating to travelling wave tubes
IPC: H01J25/40
Filed: 1954-11-16
Abstract: 772,002. Travelling wave tubes. WESTERN ELECTRIC CO., Inc. Nov. 16, 1954 [Nov. 18, 1953], No. 33150/54. Class 39(1). [Also in Group XL(c)] A travelling wave tube comprises a first delay line carrying a spatial harmonic backward wave, i.e. a wave of negative phase velocity, for modulating the beam, and a second separate delay line in which a forward wave is induced by the modulated beam, the output being taken from the downstream end of the second, delay line. The backward wave circuit can be a bifilar helix, Fig. 1 (not shown) whose axis is parallel to but displaced from the tube axis, a lecher line 44, 45, Fig. 2, having cylinders 46 connected alternately to 44, 45, a waveguide 64, Fig. 3A, loaded with slats 65 extending diametrically thereacross successive slats being rotated relatively to one another and carrying cylinders 66 for passage of the beam or a slotted cylinder, Fig. 4 (not shown) the slots being cut in an interleaved pattern. The forward wave circuit is preferably a helix 50 or 72. Both ends of the backward wave circuit and the upstream end of the forward wave circuit have reflectionless terminations of colloidal graphite coated on the tube envelope the thickness of the coating being tapered, or alternatively the backward wave circuit terminations can be two pairs of dielectric rods 48, 49 or 67, 68 having a coating of loss material of varying thickness. The downstream end of the forward wave helix is of gradually increasing pitch for impedance matching. The backward wave used is a spatial harmonic, the forward wave usually a fundamental wave. In Fig. 1 (not shown) the two helices of the bifilar helix can be at different D.C. potentials to give electrostatic focusing or are at the same potential if magnetic focusing is used. Frequency modulation is achieved by connecting the modulation source between the cathode and backward wave circuit and amplitude modulation by modulating the voltage on helix 50 by source 52 or by modulating the beam current. Fig. 2 can use periodic electrostatic focusing or conventional magnetic focusing. The beam velocities in the forward and backward wave circuits can be the same or different. To improve the uniformity of the output power level with changes in frequency the forward wave circuit can have a frequency response complementary to the frequency response of the backward wave circuit, so the output power level does not change with frequency or the frequency responses of the backward and forward wave circuits can be chosen to give an output of any other desired frequency characteristic. In the mixer of Fig. 3A an external signal is applied at 70 and the desired mixed frequency selected by operating the forward wave helix in a dispersive region ; the output can include a filter to remove fundamental and undesired modulation products. The coating of loss material 72 can be dispensed with if the forward wave helix is given non- reciprocal characteristics by insertion of a ferrite element as described in Specification 750,208. In another embodiment, Fig. 4 (not shown) an external input is applied to the downstream end of the backward wave circuit and the slot length decreases gradually towards the downstream end of the backward wave circuit for impedance matching. In all forms, an additional electron source can be provided between the forward and backward wave circuits to increase the power output. The beam path can be non-straight, e.g. circular when used in magnetron type travelling wave tubes. Pulsed operation can be achieved by placing deflecting means between the backward and forward wave circuits, e.g. in Fig. 1 (not shown) the output will be frequency modulated pulses. Specification 772,001 also is referred to.