Patent GB-768563-A (GB)
Title: Improvements in or relating to travelling wave magnetron oscillators
IPC: H01J25/587
Filed: 1954-03-25
Abstract: 768,563. Magnetron oscillators. M-O VALVE CO., Ltd., ROBERTSHAW, R. G., and WILLSHAW, W. E. March 23, 1955 [March 25, 1954], No. 8790/54. Class 39(1). A travelling wave magnetron oscillator of the multi-cavity type comprises a metal plate anode having a central electron orbit hole of which the diameter is not greater than <#>/4 preferably not greater than <#>/10 and the axial length is not greater than <3#>/10 preferably not greater than <#>/10 and an even plurality not greater than six of holes forming cavity resonators surrounding the central hole and opening into it by slots, a cathode mounted in the central hole, the arrangement being such that output power can be radiated directly through the envelope, i.e. with no output connection sealed through the envelope the device being designed to oscillate on a space harmonic of the # mode the desired mode corresponding to k=<Np>/2 for an unsymmetrical resonator arrangement or h=N(f+¢) for a symmetrical resonator arrangement. For 2 resonators operation is on the second space harmonic while for 4 or 6 resonators the first space harmonic is used. It ia shown that the phase difference between adjacent resonators can be represented by the expression 2#<n>/N, where N = number of resonators. For symmetrically spaced resonators, h+n=pN where k represents the number of waves of the desired spatial harmonic mode around the anode, and p is O, 1, 2 or any other integer representing the order of the space harmonic and for the # mode n=<N>/2 so that h=N(p+¢). For an unsymmetrical spaced arrangement of resonators k can have additional values given by k=<PN>/2 for the # mode. The unsymmetrical arrangement of resonators must be such that there are N resonators each centred on one of 2K resonator points ' spaced equally around the circumference and <N>/2 pairs of resonators, then there must be an even number (or zero) of resonator points between resonators of a pair, and an even number (not zero) of resonator , points between adjacent resonators of adjacent pairs, there being the same number of resonator points between each pair of resonators an example being given. The magnetron is especially suited for low power continuous wave operation but may be designed for pulse operation ; the ratio of the cathode and anode diameters being 0À3 and 0À6 respectively. The angle subtended by the communicating slots between the resonators and central hole can be chosen to favour spatial harmonic operation in the desired mode or to prevent excitation of an undesired space harmonic. The anode 5, Figs. 1, 2, is a rectangular copper plate with four sector shaped cavity resonators 10 between rectangular anode segments 11 and a central interaction space 9. A T-shaped support 8 carries choke cup 6 on which anode 5 is mounted, there members being secured together by screws 7. The indirectly heated cathode 12 is carried by two mica plates 13 supported from anode 5 and has end shields 18 secured bv bent over tags passing through holes in 13; end shields 18 are coated with titanium oxide to increase their heat emissivity. The mica plates 13 are secured to anode 5 by two pairs of Cu-Ni pins 15, nickel eyelets 14 the heads of which space the mica sheets from the anode and nickel retaining sleeves 16 are spot welded to pins 15. The cathode and heater leads 20, 22 have single turn loops to provide resilience and allow for thermal expansion ; the leads pass through ceramic bushings 21, 23 at the base of choke cup 6. In manufacture, pins 15 are fitted accurately in accurate holes in anode 5, being staked and clenched thereto using a circular tool. Anode plate 5 is mounted in a jig with pins 15 in position, with a locating member fitting in hole 9 and having an extension the same diameter as the cathode, a mica sheet 13 carrying 14, 18 is then fitted over this extension, the tags of 18 bent over to grip the mica plate and nickel eyelets 14 pressed on to the mica. The mica sheet is withdrawn from pins 15, the anode plate reversed in the jig and the above procedure repeated for the other mica sheet 13. The locating member is now withdrawn from hole 9 and replaced by the cathode one end of which is inserted through the mica sheet fitted to the anode plate and the second mica sheet is fitted in position. The mica sheets are now permanently fixed by spot welding nickel sleeves 16 to pins 15. The anode-cathode assembly is secured to choke cup 6 and T- support 8 and the cathode and heater connections to leads 20, 22 completed. The high frequency power generated is radiated directly through the walls of the envelope and the device may be inserted in a circular waveguide with resonator 10 furthest from the choke cup 6 magnetically coupled to the guide to generate an H10 mode therein. The magnetron of Figs. 1 and 2 operates on a first space harmonic of the # mode. A wire loop carried on a lead wire 4 is completed by getter 24 so that the latter can be flashed by eddy current heating, and a coating of magnesium oxide is provided on the inner surface of glass base 2 to increase the insulation between leads 4. In a modification the resonator system can be rotated through 45 degrees and a probe extending perpendicular to the flame of the anode plate is attached to, that anode segment which is furthest from choke cup 6. In another modification, Fig. 3, the anode plate has two resonators 27, the slots 28 being parallel sided or having 45 degree corners where they join the resonators. The magnetron may be inserted in and magnetically coupled to a waveguide with the anode segments parallel to the guide axis, the device operating on a second space harmonic of the # mode. In a modification of Fig. 3, the resonator system can be rotated through 90 degrees the waveguide being coaxial with the axis of the envelope. The T-shaped member 8 can be replaced by a hollow member flanged to enable it to be screwed to the choke cup and open or perforated at its lower end to permit pumping. A mica sheet between 5 and 6 touche envelope 1 for spacing and is cut away where 5 and 6 are screwed together ; leads 20, 22 pass through the mica sheet. A second mica sheet can support the free end of the anode plate from the envelope. In a further form, Figs. 4, 5 (not shown) the device of Fig. 3 is inserted in a circular sectioned waveguide with the anode segments at right angles to the guide axis, the resonator remote from the choke cup being magnetically coupled to the guide, the degree of coupling depending on the thickness of the anode plate and the distance of the resonator from the end of the anode plate this part of the plate may in fact function as an output coupling loop. In another arrangement, Fig. 6, the magnetron of Fig. 3 is inserted at the junction of a circular sectioned waveguide 34 and a rectangular sectioned waveguide 35, 36 with tuning plunger 38 which varies the frequency of the magnetron and load 40 which may be a mixer in a radio or.radar receiver. Choke cup members 6, 33 prevent escape of energy along guide 34. The magnetron resonators are coupled to the rectangular guide 35, 36. In further modifications, Figs. 7, 8 (not shown) the anode segments are tapered, or made thicker at the electron orbit region. If the anode is made by hobbing the undesirable rounding of the edges of slots linking the resonators with the central hole is minimised by inclining the slots to the radial, the inclination commencing a short distance away from the mouths of the slots. A prior arc multi-cavity magnetron operated on a space harmonic of the # mode is referred to.