Patent GB-837564-A (GB)
Title: Improvements in or relating to travelling wave tubes
IPC: H01J23/24
Filed: 1957-04-12
Abstract: 837,564. Travelling wave tubes. WESTERN ELECTRIC CO. Inc. April 12, 1957 [April 26, 1956], No. 12034/57. Class 39(1). In a travelling wave tube the delay line is made up of wires so arranged, preferably horizontally and vertically, in a plurality of contacting layers so as to form a plurality of helical paths in parallel for the wave adjacent helices turning in opposite senses. Each group of wires comprises four layers 101a, 101c; 102e, 102g; 103b, 103d; 104f, 104h the first and third layers being vertical wires and the second and fourth layers horizontal wires, the first and third wires and the second and fourth wires alternating across the structure. In this way, since adjacent groups (of which there may be 10) contact one another, the desired helical paths lie parallel, are built up. The cross section of each helix is preferably square as shown but may alternatively be rectangular, parallelogram shaped, hexagonal or, any other regular geometric figure having an even number of sides'. The delay line structure ensures that the adjacent helices are sufficiently decoupled so that the phase velocity of all parts of the structure is equal to the phase velocity of any component helix in the absence of the other helics. If different signals are applied to different helical paths therefore, each signal is separately amplified (separate input and output connections are provided for each helix and a separate beam passes through each helix). To give further decoupling adjacent helices can if desired have a spacing between them. As shown each wire layer has two wires e.g. 101a, 101c but it may have any number of wires greater than two and adjacent layers can be disposed at an angle other than 90 degrees for example for parallelogram or hexagonal helix cross sections. The tube may be constructed by embedding the ends of the delay line wires in a glass envelope or brazing then to a copper envelope. If the envelope is of copper it may be made of a number of separate washers of a thickness equal to the wire diameter the delay line wires are placed across the apertures in the washers and brazed at their ends to the washers after which the copper washers are stacked up and sealed together. Alternatively the delay line structure may fit within the copper or glass envelope and part of the structure is secured to the envelope to prevent rotation. If the envelope. is of copper the bandwidth is reduced (since the device functions as a hollow pipe loaded guide) with a low frequency cut off at 1 = # but the gain near cut off is higher than for a glass envelope. In another alternative construction the delay line can have dielectric support rods. The input and output are coupled to and from two parallel wires such as 101a, 101c, by two wire transmission lines and coaxial feeders. If the envelope is conductive the outer conductors of the coaxial feeders are connected to the envelope, the elements 101a, 101 care insulated from the envelope. Since the beam is cylindrical a grid of the same general shape as the first delay line mesh is placed between the gun and delay line to intercept electrons which would otherwise strike the delay line wires. Focusing is effected by a solenoid or by two permanent magnets oppositely poled and placed at opposite ends of the tube. A prior art structure comprising a plurality of helices in parallel brazed together at the crossing points is referred to.