Patent GB-1280841-A (GB)
Title: Feed system
IPC: H01P5/02
Filed: 1969-08-14
Abstract: 1280841 Aerials; radio direction-finding RCA CORPORATION 14 Aug 1969 [21 Aug 1968] 40621/69 Headings H4A and H4D [Also in Division H1] In a monopulse multimode feed system a first plurality of waveguide modes is provided by first means coupled to a sum mode port of a mode coupler 16, Fig. 1, and a second plurality of waveguide modes is provided by second means coupled to a plurality of difference mode ports of said coupler, the modes of said second plurality being circularly polarized. Third means, including a single aperture radiating horn 18, is coupled to another port of said coupler. As described, the mode coupler 16, Fig. 2 (an end view), has a large conically shaped port 25 at the centre of a square waveguide section and includes eight symmetrically located waveguide ports 26 to 33 in corner pairs forming four circularly polarized exciting sub-apertures, with each port being orthogonally polarized with respect to its corner paired port. The centre port 25 increases in diameter from its end 34 near the receiver equipment to its end 35 near the horn 18. There is a step between the larger end of the port 25 and the square waveguide section for exciting modes higher than the fundamental. Propagation of unwanted modes is prevented by an inner ring 37 in the port 25 and by tuning stubs 40 in the ports 26 to 33. The horn 18 has an increased flare angle at its end and is provided with internal fins along its four sides, and square waveguide sections inside its corners, to improve bandwidth (Fig. 7, not shown). A mode filter 17, in series between the horn 18 and the mode coupler 16, has a square cross-section, a half cosine function profile along the axis of propagation, and symmetrically located ridges in its two main planes (Figs. 5, 6, not shown). The minimum width of the square aperture at the larger end of the mode filter, and the length of said mode filter are, respectively, 2# and 5#, where #=wavelength corresponding to the geometrical mean of the limits of the operating frequency band. Operation in tracking mode.-The total step aperture 36 of the mode coupler 16 is considered as divided into four sub-apertures as shown in Fig. 4, each sub-aperture having a respective horizontally polarized port and a respective vertically polarized port. The ports are coupled in associated pairs 26, 27; 28, 29; 30, 31; 32, 33 to the inputs of respective short slot hybrids 44 to 47. With right-hand circularly ploarized signals, output ports 1, 3, 5, 7 of the hybrids 44 to 47 are coupled to terminations 55 to 58. Output ports 2, 4 of the hybrids 44, 45 are coupled through a waveguide section 48 to the input ports of a hybrid 50, and output ports 6, 8 of the hybrids 46, 47 are coupled through a waveguide section 49 to the input ports of a hybrid 53. Magic tees 52, 53 are each coupled to each hybrid 52, 53 and respectively provide elevation and azimuth difference mode signals at their output ports 61, 62. For left-hand circularly polarized signals, the ports 2, 4, 6, 8 of the hybrids 44 to 47 are terminated, and the ports 1, 3, 5, 7 are used, in conjunction with waveguide sections 59, 60. Also, the roles of the output ports 61, 62 are reversed. Operation with sum mode signals.-These signals are coupled between one port of a threeport polarizer assembly 12, Fig. 1, and the smaller end of the conical port of the mode coupler 16. The assembly 12 includes an orthogonal coupler 71 (Fig. 8, not shown) constructed of square waveguide sections, in which transmitted signals in the TE 10  mode are combined with received signals in the TE 01  mode. A fixed phase shifter 73, in the form of a square waveguide section which is internally ridged and which is provided with two sets of corner irises, converts the orthogonally polarized transmitted and received waves into right- and left-hand circularly polarized waves (Figs. 9, 10, not shown). A rotatable phase shifter 75, which is similar to the fixed phase shifter 73, is provided at each end with a rotary joint 74, 76 and is geared to an electric motor (Fig. 11, not shown). Circularly polarized waves from the fixed phase shifter 73  are accepted by the rotatable assembly and are transformed back to linear polarized waves at the output of the rotary joint 76. The attitude of the output polarizations is dependent upon the orientation of the phase shifter 75, the orthogonal relationship of the transmitted and received waves remaining at all attitudes. By removing the second phase shifter 75 and the rotary joints 74, 76, operation with circularly polarized signals at the horn is obtained. Reference is made to Specification 1,280,842.