Patent GB-869201-A (GB)
Title: Improvements in or relating to travelling wave tubes
Inventor: MINAKOVIC BORIVOJE
IPC: H01J23/24
Filed: 1958-01-16
Abstract: 869,201. Travelling wave tubes. STANDARD TELEPHONES & CABLES Ltd. Jan. 16, 1958, No. 1525/58. Class 39(1). Travelling wave apparatus comprises a main helix interacting with the electron beam an intermediate and an outer helix the intermediate and outer helices being contrawound to form a coupler; the intermediate helix has a larger diameter and shorter length than the main helix and is within the tube envelope, the outer helix is outside the envelope. The intermediate helix may have the same sense pitch as the main helix when connected thereto, Fig. 1 A, 3B, 4 (Fig. 4 not shown) or contrawound when there is no direct connection, Fig. 5A, 6A. If it is so connected it may be of the same wire as and continuous with the main helix 1, the end turns of the latter being pulled out at 11 to give a smooth impedance transition. The object of the invention is to extend the benefits of coupled helix feeding to tubes in which the helix 1 is carried by a set of three ceramic rods 2 instead of by the tube envelope and consequently in which the coupling would be insufficient without the presence of a third helix 6. The R.F. shielding tube 8 connected to choke sleeve 3 may extend to the junction point of the helices 1, 6, to prevent interaction between the electron beam and helix 6: choke sleeves 3 carry rods 2. The intermediate helix 6 is preferably wound round parts of the ceramic rods 2 which are ground down slightly to allow room for the helix between the rods and envelope. The helix 6 may be wound to surround helix 1 to reduce the length of the non-useful part of the electron beam, Figs. 3B, 4, 5A, 6A (Fig. 4 not shown). The outer helix 9 has a cylindrical shielding tube surrounding it and connected to the outer conductor of a coaxial line feeder, the outer helix being continuous with the inner conductor of the feeder. The three helices 1, 6, 9 are all of Mo wire. In Fig. 3B the inner and intermediate helices are connected by looping the wire around one of the support rods 2 and 12 so that the helices 1, 6 are wound in the same sense while the outer helix 9 is wound in the opposite sense. In Fig. 4 the connection between helices 1, 6 causes them to be wound in opposite senses. In this case 1, 6 have different phase velocities to avoid coupling between them. In Fig. 5A the helices 1, 6 are as is preferred not directly connected, the intermediate helix being wound in the different sense to the inner and outer helices which are wound in the same sense. In all cases the D.C. connections to the helices are preferably made at points where the R.F. power flow is a minimum. The outer helix may be coupled to the coaxial line feeder by a tapered transition section of helix (as the outer helix and surrounding shield function as a 'microstrip' transmission line). In a practical embodiment Fig. 6A, of which only the output end is shown (the input end being similar) the outer shield is of non-magnetic material and is in three parts 17, 18, 19, and also acts as a choke with inner sleeve 3. Low loss dielectric material 22 fills the space between the inner and outer conductors of the coaxial output feeder and also forms a cylindrical lining tube 22 to align outer helix 20; the dielectric cylinder 22 is held in position by annular members 20. The collector electrode 14 is, a hollow frustoconical structure and seats in a tapered metal bush 15 having a cylindrical casing 16. The shield 19 is carried in the end plate 25 of a periodic permanent magnet focusing assembly 26 which consists of annular permanent magnets 27 and high permeability spacers 28, having short flux concentrating cylinders 29 integral therewith. One magnet and cylinder 29 is recessed for the coaxial output feeder. The magnet assembly also supports the magnetic shield for the electron gun. A theoretical analysis and dimensions of examples are given. In particular in theory neither the innermost or outermost helices need be terminated if the length, diameter and pitch of the helices are such that a complete transfer of power takes place between them, but in practice some termination may be necessary, e.g. a lossy termination for the free end of the outermost helix. The phase velocities of all the helices are the same when uncoupled and the coupling between 1 and 6 is the same as the coupling between 6 and 9. If input power is applied to the innermost (main) helix in addition to and independently of the input helix coupling, a complete transfer of power between inner and outer helices 1, 9 takes place if the length of the helix coupler is #/2, where #b is the 'spatial beat wavelength', 'spatial beating' being it is stated caused by the interference of the three forward waves on each helix separately. The invention may be applied to a backward wave oscillator or amplifier.