Patent GB-655410-A (GB)
Title: Improvements in and relating to electromagnetic wave propagating structures
Inventor: LINES ALBERT WALTER
IPC: H01P1/207
Filed: 1947-10-06
Abstract: 655,410. Travelling-wave tubes; magnetrons. NATIONAL RESEARCH DEVELOPMENT CORPORATION. Oct. 6, 1947. No. 26794. [Class 39 (i)] [Also in Group XL (b)] An electromagnetic wave propagating structure for propagating electromagnetic waves with a phase velocity substantially less than the free-space velocity comprises a waveguide or coaxial or strip transmission line to which is coupled a plurality of cavity resonators the spacing and/or the natural frequency of which varies in accordance with a repetitive law such that the resonators and the spaces between them may be divided into a plurality of like groups, a group constituting the smallest repeated pattern of the resonators and spaces and comprising at least three resonators and three spaces the resonators of the group being of more than one natural frequency and/or the spaces being of more than one magnitude whereby the structure has a low dispersion over a substantial frequency band. In one form, Fig. 9, the resonators are grouped in threes, each group consisting of two resonators of equal frequency and of one resonator of smaller size and consequently of higher frequency. An electron beam from gun 12 is fired through the rectangular wave-guide to which the resonators are coupled at a velocity approximately equal to that of the wave travelling along the guide, the input wave being fed in through wave-guide section 20 and slot 24 and the amplified wave being taken through slot 25 to wave-guide section 21. If a feed-back path is arranged between the output and input the device functions as an oscillator. It is stated that the frequency of such a device can be adjusted by means of a phase-shifting device in the feed-back path. The electron beam is of wide, thin form; the wave-guide sections 20, 21 are sealed by glass windows 22, 23 and the coupling slots 24, 25 act as matching transformers with the end resonators. The wave propagating structure may be built up of copper stampings which are each silver plated on one side and then bonded together at 800 C. in an atmosphere of nitrogen to form copper-silver eutectic bonds. In modifications the input and output may be coupled to the end resonators, or directly to the waveguide itself, by coupling loops or probes. In other constructions employing resonators grouped in threes, one resonator in each group may be longer instead of shorter than the other two, or all three resonators may be of different lengths and consequently of different frequencies, or the spacing in the groups may be irregular, but the resonators all of the same frequency. Each group may comprise more than three resonators. In all forms, either or both opposite walls of the wave-guide may be formed with the periodic resonator structure. The wave-guide may be circular instead of rectangular, or a coaxial line with the periodic resonator structure formed on the inner or outer conductor may be employed in which case an electron beam of annular cross-section is used. If a wave-guide is used, it may be arcuate or formed as a closed ring, the beam being constrained to follow the desired arcuate or circular path by a magnetic field. In some cases crossed electric and magnetic fields may be used for focusing as in a magnetron. The devices may be operated on fundamental or harmonic frequencies, and the conditions of operation are such that non-dispersive transmission is obtained. The possible modes of operation are discussed in detail and illustrated graphically.