Patent GB-889856-A (GB)
Title: Improvements in or relating to waveguide coupling devices
IPC: H01P1/213
Filed: 1960-11-29
Abstract: 889,856. Waveguide band-separation filters. MARIE, G. R. P. Nov. 29, 1960 [Dec. 4, 1959], No. 40987/60. Class 40(8). A coupling device between a circular waveguide and a number of rectangular waveguides comprises dielectric lenses equal in number to the rectangular waveguides arranged to completely surround circumferential slots in the circular guide and designed to direct energy emanating from the slots through intermediate guides into the rectangular guides. As shown, a circular waveguide 8, Fig. 4a, operating in a H on  mode is provided with three circumferential slots 11 communicating with an annular' cavity 9 of square section resonant simultaneously in two orthogonal modes respectively H oqp  and H opq , p and q being even and odd. The cavity is coupled by further slots 10 to an annular waveguide 12 along which energy propagates axially towards an annular opening 15. In the embodiment, the number of output rectangular guides is two and hence the circular guide is completely surrounded by two dielectric lenses 16, 17, Figs. 3, 4a, each having an inner semicircular surface co-incident with the annular opening 15 and centred on the axis 25 of the circular waveguide. The lenses are designed in accordance with optical considerations and are aplanatic, i.e. the image of a plane object perpendicular to the axis of the system lies in a plane perpendicular to this same axis. The points of intersection of the two planes and the axis are called "aplanatic points". In the system shown, rays of energy considered to emanate from point 25 on the axis of the circular guide pass unrefracted into the lenses and suffer refraction at the outer surfaces. The radius of curvature of the outer surfaces is such that, if the extreme rays are produced backwards (broken lines) they meet at the aplanatic points 26, 30 and include an angle of 60 degrees. Thus energy radiating from the cavity 9 with an aperture of 360 degrees is condensed by the lenses into two beams of 60 degree aperture. Intermediate guides 24 have surfaces 27, 29 in the form of elliptical arcs which reflect the energy towards the mouths of rectangular guides 31, 32. The foci of the two ellipses 27, 29 are respectively the points 26, 28 and 30, 33 and the arcs are arranged symmetrically about the minor axes of the ellipses. The other boundaries of the intermediate guides are chosen so that the extreme rays pass along these boundaries. It is possible to replace the elliptical arcs with circular arcs approximating to the elliptical arcs. In choosing a material, e.g. polythene, for the lenses, it is necessary to take into account the thickness of the lenses, since the wavelength in a particular medium of the guided energy is modified by the dimensions of the waveguide and the effective refractive index of the material is modified accordingly. The outputs of the two rectangular guides 31, 32 are combined at the Y-junction 40. In a modified arrangement (Fig. 5a), the inner wall of the resonant cavity is dispensed with and a cavity 49 formed by local increase. in the diameter of the circular guide 38. The cavity has a single annular output slot in which are arranged the lenses 16, 17. In order to prevent generation of unwanted modes in the cavity, circular wires 53 are supported by dielectric slabs 54 along the lines of zero electric intensity of the circular modes. A number of coupling circuits according to the invention may be tuned to different frequencies and arranged at spaced points on a common circular waveguide thus providing a channel-separating or combining circuit (Fig. 6, not shown). The intermediate waveguide may be made from two appropriately grooved metal plates.