GB916989A - Improvements in or relating to microwave devices - Google Patents

Abstract

916,989. High-frequency tubes. BELOIT IRON WORKS. Feb. 24, 1959 [Aug. 8, 1958], No. 6376/59. Class 39 (1). [Also in Group XL (b)] Resonators forming parts of magnetrons, klystrons &c. are made from loops of surfacewave transmission line from which radiation is prevented by loading the inside of the loops with dielectric material. The line may consist of conductive or dielectric material or of a conductive core coated with dielectric material. As shown in Fig. 2, the line 15 is wound as a helix upon a dielectric core 11. The permittivity of the dielectric layer is chosen so that the transit time of wave energy on the inside and outside of the loop are equal. A helix or single loop may be made resonant or anti-resonant by adjusting the length of a complete turn to a multiple of half a wavelength. In the magnetron of Fig. 5, a surface wave progresses between the successive coils 200-205 each of which has a circumference approximating to a whole number of wavelengths whereby it acts as a resonant cavity. The total length of each coil is fractionally greater than a whole number of wavelengths so as to produce the necessary slowing of the wave which progresses around the cathode 207. Some of the coils may be omitted to achieve spatial harmonic operation. The coils are formed from a single line which also forms the anode of the structure. Power is taken off by means of a pick-up loop 220. Tuning may be effected by tapering the dielectric cores of the coils and moving them axially. Alternatively, different gases or liquids may at variable pressure be introduced into porous dielectric cores. The whole configuration may be repeated in a stacked formation using the same conductor throughout. A separate anode may be provided outside the structure. In the reflex klystron structure of Fig. 5, the resonant coil 250 of surface-wave transmission line is of elongated configuration and constitutes the anode of the device. A take-off portion 250a of the line is connected to a source 253 of anode potential, microwave energy being isolated from the source by a reflecting disc 257. The coil may consist of a single loop or several loops, the curved ends of the loops being loaded on the inside with dielectric inserts 275, 276. A cathode 260 and reflector 262 are arranged on either side of a straight portion of the loop. In an alternative Construction, the resonant structure comprises two spaced circular cylindrical coils joined by straight portions of surfacewave conductor. Figs. 9, 10 show an amplifier comprising two axially-spaced single-loop devices 400a, 401a enclosed within an evacuated envelope 404. Rings of dielectric material 410, 411 extend around the insides of the loops to prevent radiation. Input and output lines 400b, 401b extend tangentially from the loops. The input loop 400a is provided with a thermoemissive surface 415 and an internal heating wire 419, whilst the output loop 401a is maintained at a positive potential. The operation of the amplifier depends upon bunching of the electrons and a critical spacing between anode and cathode. In a modification, the anode and cathode loops are made from a continuous spiral line, the two loops being isolated from each other for D.C. by means of a spaced pair of coupling discs. In a further modification a third loop electrode forming a control grid is introduced between the anode and the cathode. Specification 916,990 is referred to.