Patent GB-734828-A (GB)
Title: Improvements in or relating to electron discharge devices employing travelling waves
IPC: H01J25/38
Filed: 1952-01-25
Abstract: 734,828. Travelling-wave tubes. WESTERN ELECTRIC CO., Inc. Jan. 25, 1952 [Jan. 27 1951], No. 2101/52. Class 39 (1).  In a travelling-wave tube in which the beam is density modulated by an input signal before passing into the wave-transmission circuit, the spacing between the cathode potential minimum and' the nearer end of the wave transmission circuit is less than one radian of the electron beam transit angle, so as to reduce noise. The cathode-grid spacing is ¢-1 mil. and the grid-helix spacing 5=12 mils. In one form, Fig. 2A, collector 14 and helix 12 are biased by concentric sleeves sealed to the end of the envelope with interposed glaze seals, Fig. 1 (not shown), and connections 16, 19. The collector lead is coiled to serve as a choke and spring. The indirectly heated cathode 31 is housed in metal envelope cylinder 32. The input rectangular sectioned wave-guide 53 is connected between the cathode and control grid 39 and the tube extends through the guide #/4 from the lower closed end. The seals across the cavity gap are transparent to microwaves. Collimating grid 40 is connected to helix 12, and a coil 46 focuses the beam. Standing waves are set up in the cathode-control grid space to density modulate the beam, and the modulated beam induces a wave in helix 12 and amplifies that wave. In the modified structure of Fig. 3, metal shells 75 carrying the heater and cathode terminals via glass bead and metal eyelet seals 78, 79 and 61 are sealed together at 76, 62, and control grid 66 is connected via ring 67 to annular terminal 64 spaced from shell 61 by vitreous sealing ring 65. The cathode consists of a sleeve 68 carrying discs 70, 71 and enclosing a heater 69, the emissive coating being on disc 71. The cathode is locked to ceramic spacer 72 by support members (not shown) attached to 68 and extending to slots in 72. Leads 77, 77<SP>1</SP> carry heater shield 80 insulatingly by supports 81 and glass beads 82, 77 being also connected to cathode 68 by strip 85, condenser plate 86 capacitatively coupled to shell 61, and ring 87, and 77<SP>1</SP> being respectively connected to the heater 69 via strips 83. Condenser 86, 61 provides a highfrequency coupling to the external input guide. Ceramic spacer 72 and metal shim 88 maintain the cathode-grid spacing approximately at the thickness of 88. Ring 87 is held in position by ceramic ring 91 biased by spring 92. A ceramic cylinder 22, Fig. 2A, insulates the helix from collector 14. It is stated that keeping collimating grid 95 at approximately helix potential reduces the number of ions in the beam by preventing beam crossovers. The grid 95 comprises a tungsten mesh 111, Fig. 6, welded between discs 112, 113. Helix 12 is welded to disc 112 via matching termination 98. The helix support rods 96 are received in slots in the grid structure which is seated in ceramic cup 94, and the cup 94 together with ring 67 spaces grid 95 from control grid 66. In a modulator, Fig. 9 (not shown), both local and signal waves are applied to a cathode-grid resonator, the local oscillations via a coupling loop and the signal across a capacity in the resonator wall. An inductance is connected in series with the capacity to make the combination resonant at the signal frequency. The helix has no external input. The gain, bandwidth and noise of the devices are discussed mathematically and compared with conventional travelling-wave tubes and high-frequency triodes. The ceramic rods may be glazed to the helix and loss material applied to the central portion of the entire assembly as disclosed in Specification 734,771. Specifications 650,667, 653,153, 668,355, 669,473, 669,475 and 701,034 also are referred to.