GB807806A - Improvements in or relating to electromagnetic wave guides - Google Patents

Abstract

807,806. Waveguides. SPERRY RAND CORPORATION. July 12, 1956 [July 12, 1955 (2); July 14, 1955], No. 21583/56. Class 40 (8). A waveguide comprises a channel member 10 having a pair of inner parallel planar surfaces spaced substantially less than a half wavelength apart at the lowest frequency of the transmission band and a connecting surface 12 extending between and perpendicular to the parallel surfaces, a conductive fin 20 being centrally positioned between the parallel surfaces and dimensioned to project from the surface 12 a distance less than the width of the parallel surfaces but not less than a quarter wavelength at the lower frequency of the transmission band. The mode of propagation in such a guide may be understood by considering the guide to be derived by bisection of a " sandwich " type strip line (Fig. 2) supporting a second order mode of propagation. Cut-off for this mode occurs when the width of the fin 20 is equal to a free space quarter wavelength and cut-off for the next higher mode occurs when the width of the fin is three quarters of a free space wavelength. The guide thus has a bandwidth one and a half times that of a conventional rectangular guide. A coaxial terminal 50, Fig. 7, may be coupled to the guide by extending the central conductor 34 through an end wall 30 to form a probe. The end of the fin 20 lies under the probe and is terminated short of the end wall 30. Alternatively the probe may extend vertically from a carriage adapted to slide horizontally along the edges of the parallel walls of the guide (Fig. 5, not shown). The guide may be terminated by a slotted resistive card 44, Fig. 6, arranged obliquely in the guide. Figs. 9, 10, 13 shown respectively a plan, elevation and cross-section of a transistion between a rectangular waveguide 110 and a guide 120 according to the invention Since the rectangular guide has a broad dimension greater than a half wavelength, the broad walls of 112, 114 must be tapered to the width of the connecting wall 126 of the guide 120. In order that the rectangular guide 110 shall be above cut-off during the transition it is provided with a tapered ridge 132 which is joined to the fin 128 of the guide 120. The transition is completed by gradually widening the narrow walls 116, 118 of the rectangular guide 110 to the width of the parallel walls 122, 124 of the guide 120 and at the same time by opening the top wall of the guide 120 by means of a V-shaped slot 134. The transition may be designed (Figs. 14, 15, not shown) so that the narrowing of the broad walls and the broadening of the narrow walls of the rectangular guide occur together and not in sequence.