GB750208A - Improvements in or relating to electromagnetic wave devices such as travelling wave tubes - Google Patents

Abstract

750,208. Waveguides. WESTERN ELECTRIC CO., Inc. June 9, 1954 [June 17, 1953; June 17, 1953], No. 16939/54. Class 40 (8). [Also in Groups XXXV, XL (a) and XL (c)] A travelling wave-tube has associated with the helix or other delay line a magneticallybiased element of ferrite material so as to obtain non-reciprocal transmission characteristics. The reflected wave in a travelling wave-tube can thus be eliminated without affecting the forward gain. It is shown that the direction of rotation of the circularly polarized magnetic component depends on the direction of the wave and that the loss varies with the direction of rotation and hence with the direction of the wave. Ferrites are defined as the reaction product (spinel structure) of iron oxide and a small quantity of at least one other bivalent metal such as Ni, Mg, Zn or Mn, e.g. Ni-Zn ferrite. The ferrite is preferably biased to a point near ferromagnetic resonance to give the optimum non-reciprocal effect, e.g. 10-20 oersteds for Ni 0À3 Zn 0À7 Fe 2 O 4 . Since the magnetic vectors of the helix field lie in radial planes a ferrite cylinder surrounding the helix is circumferentially magnetized so that its lines of force cut the magnetic vectors at right angles. Where an axial magnetic focusing field is used it is better to split up the ferrite cylinder into coaxial rings, Fig. 3 (not shown), the gaps between which can be filled with non-magnetic material. The ends of the ferrite structure are tapered to minimize reflection and the helix coupled to the inner conductor of coaxial lines by sections of increased pitch secured to a tapered section of the inner conductors of the coaxial lines. The ferrite rings are preferably permanently magnetized and to obtain a sufficient strength of magnetization it may be necessary to mix the ferrite with a magnetic material of high retentivity or to form a cylinder of laminated layers of ferrite and other permanent magnet material. Alternatively (a) the ferrite cylinder may be provided with a biasing coil; Fig. 4A (not shown). (b) A direct current is passed through the helix or through a straight conductor on the axis of the cylinder, Fig. 4B (not shown). (c) A small gap in a ferrite cylinder is filled by a permanent magnet, Fig. 4C (not shown). (d) The ferrite element is formed as a helix to convert the.axial focusing field into a helical field with a circumferential component, Fig. 4D (not shown). (e) The ferrite element is inserted within the helix. In another embodiment, Fig. 5, ferrite elements 68 are built into soft iron polepieces 66 of annular permanent magnets 69. Adjacent magnets are oppositely poled to give a periodically varying magnetic field for focusing. In another embodiment, Fig. 6, there are two sections of helix 117, 118, whose facing ends of increasing pitch and diameter are connected to transformers 142, 144 comprising metal cylinders - thick and having rectangular irises 4 # # 143, 145 of dimensions - by -. Probes 138 2 4 improve the matching. The transformers are associated with pairs of resistive vanes 148, 149; 154, 155 comprising polystyrene sheets coated with carbon black or carbon sheets and with a ferrite cylinder 150 carried by dielectric rings 151, 152 and with waveguide section 140. The second set of vanes 154, 155 and iris 145 are rotated 45 degrees relative to the first in a clockwise direction and the ferrite cylinder 150 is designed to rotate the wave 45 degrees in the same sense. In operation, transformer 142 converts the helix wave to a linearly polarized wave whose electric vector is parallel to the short side of the iris and perpendicular to vanes 148, 149. The wave is therefore unaffected by either set of vanes but the reflected wave is rotated a further 45 degrees into the plane of vanes 148, 149 and dissipated; any further reflection in a forward direction is dissipated by vanes 154, 155. The magnetic field of the ferrite is provided by beam focusing coil 125, or by a separate solenoid or permanent magnet, or the ferrite may be permanently magnetized. Some turns of helix 117 or 118 can be plated with low loss material. The tube envelope is made of quartz or glass sections of alternately large and small diameters subsequently fused together. In a modification, Fig. 7 (not shown), the second iris may be set at any angle relatively to the first provided the angle of rotation of the wave is greater than 45 degrees, e.g. the irises are parallel and the angle is 180 degrees. The ferrite cylinder is associated with a single pair of helical vanes so that the forward wave has its plane of polarization always normal to the vanes but the reflected wave is rotated into the plane of the vanes for integral multiples of 45 degrees. The ferrite and vane arrangement of Figs. 6 or 7 can be outside the tube envelope. In Figs. 6-8 there may be more than two sections of delay line, a ferrite and vane arrangement being inserted between each pair of sections. In Fig. 8 which shows a cascade amplifier 178 is the ferrite and resistor card assembly similar to those of Figs. 6 and 7, but outside the tube envelope. A band-pass filter 179 is provided in one or both of guides 176, 177. The invention is not limited to travelling wave tubes and the non-reciprocal helix structure can be inserted in any transmission path, e.g. as the inner conductor of a coaxial line. The helices.may be replaced by filter sections, e.g. coupled cavity resonators. The invention may also be applied to backward wave amplifiers or oscillators, in which case the loss is made to predominate in the forward direction. Specification 733,486 is referred to.