Patent GB-1265226-A (GB)
IPC: H01Q25/04
Filed: 1969-12-15
Abstract: 1,265,226. Radio direction-finding; aerials. J.P. POITEVIN. 15 Dec., 1969 [13 Dec., 1968], No. 61090/69. Headings H4A and H4D. [Also in Division H1] A microwave system for separating TM 01  waves at beacon frequency from other signals in a satellite tracking system also comprises means for transmitting communications signals to the satellite, and for receiving communication signals from it. As described, a communications satellite 52, Fig. 1, transmits circularly polarized waves, carrying beacon and communication signals, which are picked up by a conical horn 51 connected to a circular wave-guide 1. If the axis of the horn is aligned with the satellite, so that the planes of the horn aperture and of the incoming wave-front coincide, only TE 11  mode waves are propagated inside the waveguide 1. If there is mis-alignment, TM 01  mode waves are also propagated inside the waveguide 1, and these are extracted by means of a mode coupler 50, which is fed via two diametrically opposite rectangular slots 10 in said waveguide. The slots are inclined so that their longer sides are perpendicular to the directions of the magnetic field in their vicinities arising from the said TE 11  mode waves, from which they are consequently substantially decoupled. The mode coupler further comprises rectangular waveguide portions 11, 21, Fig. 5, extending perpendicularly from the circular waveguide 1 and having their longer sides parallel to the longer sides of the said slots. The portions 11, 21 support TE 10  mode waves and form cavity resonators at beacon frequency, being tuned by tuning screws 12, 22. The input arms 301, 302 of a magic-T 30 are fed from the portions 11, 21 via respective T-members 15, 25, balancing phase-shifting attenuators 13, 23, and anglemembers 16, 26. Adjusting screws 14, 24 can move absorbent dielectric strips inside the phase-shifting attenuators 13, 23 to enable balancing. The TE 10  waves produced by the TM 01  waves in the circular waveguide 1 owing to any misalignment of the aerial then combine in arm 303 of the magic-T 30, and produce tracking signals in a coaxial cable 30. Any TE 10  waves produced by the TE 11  waves in the circular waveguide 1 combine in arm 304 of the magic-T 30, and are absorbed in a matching termination. As is shown in Fig. 1, the circular waveguide 1 is continued to a circular-to-rectilinear polarization converter 53 from which received signals are obtained, and into which orthogonally polarized signals for transmission may be fed. Received signals at beacon frequency are compared with tracking signals from the mode coupler in an amplitude and phase comparator 60, which supplies a signal showing the amplitude ratio and a signal showing the phase difference, which are applied to a co-ordinates converter 61. Azimuth and elevation signals generated by the converter 61 are applied respectively to motors 62, 63, which align the horn 51. In a second embodiment, the inclined slots as shown in Fig. 6 are replaced by crossed slots whose longitudinal and transverse components are respectively parallel to, and perpendicular to the axis of the circular waveguide 1 (Fig. 7, not shown). The crossed slots are so dimensioned that they are equivalent in performance to said inclined slots. In a third embodiment, each slot as shown in Fig. 6 is replaced by a pair of parallel slots spaced apart by a quarter of a wavelength in the waveguides at the beacon frequency (Fig. 8, not shown).