Patent GB-813458-A (GB)
Title: Improvements in or relating to magnetrons
Inventor: HOGG HAROLD ALEXANDER CECIL
IPC: H01J25/593
Filed: 1956-03-28
Abstract: 813,458. Magnetron oscillators. M.O. VALVE CO. Ltd. March 28, 1957 [March 28, 1956], No. 9764/56. Class 39(1). In a multi resonator magnetron for generating cm or mm oscillations having an even number N of cavity resonators in an anode block sym. metrically positioned around the axis, the magnetron cathode being located coaxially within the central hole, a sub group of X resonators symmetrically positioned around the axis (X being even and N/X integral and odd) are of larger dimensions, e.g. longer, than those of the remaining resonators, such that one component of the degenerate X/2 mode number doublet in the fundamental mode has a wavelength which is an integral multiple y of the wavelength of mode member N/2 (#mode) the product Xy being made equal to N to make the angular velocities of the X/2 and N/2 modes equal and the output coupling system is designed to extract oscillations of at least the mode (# mode) from the magnetron. Both modes may be extracted, the N/2 mode being of shorter wavelength than the X/2 mode and the X/2 mode extracted energy may be rejected by a filter in the output circuit. By increasing the dimensions of the X resonators relatively to the dimensions of the remaining (N-x) resonators the fundamental wavelengths of all the modes are changed, these changes being increases and relatively large for small values of N and increases or slight decreases and relatively small for larger values of n (n being the mode number). As a result the doublet mode n = X/2 is split, one component having an increased value of # and the other component having an unchanged value of X, the former being the mode of interest. This increase of dimensions of the X resonators is taken to such stage that # (X/2 mode) can be made equal to an integral multiple y of # (N/2 mode). The X/2 component is predominant near the cathode and initiates bunching, while the <SP>N</SP>/ 2  mode is strong near the multiresonator anode surface. The output frequency is about the same as with N resonators of the same size but the cathode can have a larger diameter. The ratio cathode diameter can be made smaller anode diameter e.g. from 0À69 (prior art) to 0À37. In an example N = 24 and X = 8 and the long resonators are more than three times the length of the short ones, the ratio of the wave lengths being y = N/X = 34/8 = 3. If X = 6, y = 5, but X6 and N is not too large. In Figs. 1 and 2 there are 24 rectangular slot resonators, every third one being of increased length (the X group). One of the long resonators is coupled by a narrow slot in a metal plug brazed to the anode block to a waveguide 24, 29 across which a glass window 27 is sealed, a choke formed by metal ring 25 preventing escape of energy. The metal window support 26 and chokemember 25 are brazed to seatings on the stepped surface of 24 while the guide part 29 is brazed to tube 28 similarly brazed to the anode block. Conduits 31 in the anode block carry cooling liquid. The indirectly heated cathode has a recessed portion housing an emissive coating 8 and houses a heater 9 ; the end cathode portions 10, 11 of increased diameter houses inserts 12, 13 of magnetic material to increase the uniformity of the magnetic field. The cathode and heater leads are insulated by glass button seal 17 and insulating spacer 18. The vacuum is completed by glass thimble 15 and metal tube 6. External polepieces are placed adjacent copper end plates of copper anode block 1. In modifications the resonators may be cylinders opening on to the cathode space by slots ; the long and short slots need not be of the same shape and can be of different circumferential width where they open on to the electron orbit space, output can be taken by probes in the long or short slots instead of by the waveguide shown in the Figure. The anode block can have straps at each end.