GB706958A - Improvements in or relating to travelling wave tubes - Google Patents

Abstract

706,958. Travelling-wave tubes. STANDARD TELEPHONES & CABLES, Ltd. March 14, 1952 [April 19, 1951], No. 6640/52. Class 39(1) A compact unit-construction travelling-wave tube comprises a sealed envelope which is provided at one end with a terminal base for the electrodes of an electron gun and at the other end with input and output terminals for the radio frequency energy. In one arrangement, Fig. 2, the tube comprises a cylindrical housing 1 of non-magnetic material containing an electron gun 2 and a radio frequency transmission line coupling unit 3. A magnetic unit 4 comprising an electromagnetic coil or alternatively a permanent magnet is mounted with one end abutting a shoulder 5 on the housing. The units 2 and 3 have alignment plates 6 and 7 which rest against abutments 8 and 9 on the inner surface of the housing. The electron gun comprises a cathode 11 and focusing electrode 12 supported on insulating rods 13 from an anode cylinder 14 which together with a shaped anode 15 is brazed to the plate 6 which consequently acts as a heat radiator for the anode. The contact pins 22 are mounted in a base 21 attached by means such as screws 25 to posts 24 supported on a cup-shaped diaphragm 16 which surrounds a plurality of electrode-lead seals consisting of ceramic beads 17 and 18 and a tubular part 19 preferably of nickeliron alloy, and an exhaust tube 23. The diaphragm 16 is secured by tie rods 28 to the plate 6. During assembly the nut 29 is retracted slightly so that the plate 6 may position itself against the abutment 8. The outer rim of the diaphragm 16 is then brazed to the housing I. The radio frequency coupling unit 3 comprises an annular diaphragm 30 secured to the plate 7 and surrounding a coaxial line input connection 31, comprising conductors 34 and 35 sealed by a ceramic bead 36, an output connection 32 comprising conductors 37 and 38 sealed by a bead 39, and a collector electrode 33 supported on the plate 7 by nickel-iron tubes 56 and 57 connected by a ceramic sleeve 58 of matching expansion coefficient. The helical radio frequency transmission line 40, which may alternatively comprise a plurality of annular discs or plates, is supported within a quartz tube 41 which is coated for approximately half its length with lossy material 41a and has its ends supported in the inner conductor sleeves 42 and 43 of matching transformers 44 and 45 having outer conductor sleeves 48 and 52 connected respectively to extensions of the outer conductors 34 and 37 of the output and input connections 32, 31. The helix 40 is connected at its ends to sleeves 50 and 53 connected respectively to conductive plates 49 and 54 and capacitively coupled to the sleeves 42 and 43 to which are connected the inner conductors 35 and 38 of the output and input connections 32 and 31. The plate 54 and cylinder 52 are secured to the plate 7 while plate 49 is provided with alignment pins 60 which engage openings 59 in the plate 6. The entire radio frequency coupling unit 3 including the matching transformers 44 and 45 and the helix 40 is assembled in a jig and then placed with the plate 7 in position against the abutment 9, whereupon the diaphragm 30 is brazed to the housing 1. The plates 6 and 7 are preferably of magnetic material to concentrate the magnetic flux along the axis of the tube and thus ensure a pencil-like electron beam throughout the length of the helix 40. In an alternative arrangement, Fig. 4, the helix 64<SP>1</SP> is supported in a quartz tube 81 which is held in V-shaped notches in two clamps 71 and 72 carried by an extension 46 of the outer conductor 34 of the input connection 31. The inner conductors 35 and 38 of the input and output connections are connected to matching transformers or transition sections 62 and 70 each having a plurality of turns, the pitch of which increases from that of the helix to infinity. The extensions of the outer conductors 34 and 37 of the input and output connections terminate in the neighbourhood of the points at which the inner conductors are connected to portions 67 and 69 which are connected to the ends of the helix. In this arrangement, the plate 7a has a flange 85 which abuts a recess 84 in the housing la and is brazed thereto so that the plate 7a terminates the housing. The plate 6a rests against an annular abutment 89 on a ring 87 of magnetic material which is brazed to the housing 1a and to a cylindrical extension 90 which forms an abutment for the magnetic unit 4. The electron gun is secured in position by brazing a diaphragm to the extension 90, as described in connection with Fig. 2. In a further modification the ring 87 is made integral with the plate 6a so that the electron gun including the housing 90 may be completely assembled on the plate 6a which is then brazed to the housing la.