GB936456A - Waveguide to strip transmission line directional coupler - Google Patents

Abstract

936,456. Directional couplers. SPERRY RAND CORPORATION. April 1, 1959 [May 14, 1958], No. 11079/59. Class 40 (8). A directional coupler comprises a hollow waveguide coupled to a strip transmission line either by slots in a ground plane conductor of the strip line which co-operate with a slot in the waveguide wall, or by slots in the waveguide wall arranged adjacent a dielectric layer of the strip line, the slots being arranged at an acute angle, preferably 45 degrees, to the longitudinal axis of the waveguide and being spaced from each other a round-trip distance of 180 electrical degrees at the design frequency to provide cancellation of backward-coupled waves. It is stated that the slots are not spaced at quarter wavelength intervals as is usual because the velocities of wave propagation in the primary and secondary lines are unequal. In the construction shown in Fig. 1, slots 6, 7, 8 in the copper ground plane conductor 9 of the strip line 1 co-operate with a slot 18 in a narrow wall 5 of the waveguide 2, the copper strip conductor 11 being spaced from the ground plane conductor 9 by a dielectric layer 10 and from the copper ground plane conductor. 13 by a dielectric layer 12. If desired, the ground plane conductor 13 and the dielectric layer 12 can be omitted. The strip line 1 is connected at one end to a coaxial line 17 and at the other end to a termination 22 and brass plates 14, 15. The termination 22 absorbs any backward energy directed along the strip 11 and comprises, Fig. 5, a narrow band of evaporated metal film 21, having a resistivity matching the impedance of the strip line, centrally located on a heat-resistant electrical insulator, e.g. a ceramic or glass wafer. Two bands of solder film 23, 24 are placed on each side of the metal film and a thin coating of quartz may be provided to seal the film. The strip conductor 11 is connected to one soldered end 23, 24 of the termination 22 and the other end of the termination is clamped between the brass plates 14, 15. In the construction shown in Fig. 6 the slots 6 &c. are arranged in a narrow wall 50 of the waveguide 20 adjacent to the dielectric layer 10 of the strip line 19, the ground plane conductor 9 being omitted. As in Fig. 1 the other dielectric layer 12 and ground plane conductor 13 can be omitted. TE 10 mode propagation is referred to. The backward wave from the slot 6 can pass to an energy absorbing termination or to a coaxial line. The slots are arranged with their coupling coefficients obtained from the binomial or the Tschebychev expansion. The array of slots shown in Fig. 8 is derived from the binomial expansion and is suitable for an L-band coupler providing maximum narrow wall coupling. It is stated that all types of hollow waveguides can be used including circular waveguides. A mathematical treatment is provided. Reference has been directed by the Comptroller to Specification 820,212.