Patent GB-1072540-A (GB)
Title: Waveguide mode coupler
IPC: H01P1/16
Filed: 1965-12-30
Abstract: 1,072,540. Waveguide filters and junctions. COMMUNICATIONS SATELLITE CORPORATION. Dec. 30, 1965 [Jan. 5, 1965], No. 65176/65. Heading H1W. Electromagnetic energy in several different transmission modes is extracted from a circular waveguide connected to a receiving horn 10. The horn receives a communication signal and a beacon signal (e.g. from a satellite) in an adjacent band, the latter signal being arbitrarily polarized. The TE 01  mode component of the beacon signal is extracted by coupler 30 which consists of an annular cavity 31 coupled to the circular guide by a symmetrical array of axial slots e.g. four slots. The cavity 31 forms a twosection filter together with a section 32 of coupled rectangular guide. It is designed to resonate in the lowest possible mode which allows coupling to the desired mode in the circular waveguide e.g. in the TE 011  mode. Waveguide section 32 is coupled to the cavity 31 by means of an axial slot and contains a resonant section formed by a pair of spaced posts. The portion of the horn taper immediately to the left of the cavity 31 forms a rejection filter for TE 01  mode energy by virtue of the fact that it is dimensioned below cut-off for this mode. Beacon-signal energy in the TM 01  mode is extracted by a further two-section filter 51, 52 consisting of a part-annular (270 degrees) cavity 51 and a rectangular waveguide section 52. The dimensions of cavity 51 in the axial direction are sufficiently small to prevent resonance in TE modes. It is dimensioned in the radial and circumferential directions to resonate in the TM 2/310  mode and is coupled to the circular guide by two circular slots near either end of the coupler. The design ensures that the cavity is not excited by the TE 11  modes in the circular guide. In order to prevent slot resonances excited by the TE 11  modes, each slot is provided with a circumferential vane lying in a plane at right-angles to the circular waveguide axis. These vanes short-circuit the wall currents of the TE 11  modes. Rectangular waveguide 52 is coupled to the cavity 51 by a circular slot and contains a resonant section formed by a pair of posts. Any TM 01  mode energy not extracted by the filter is reflected at the section 12 which is tapered to a diameter below cut-off for this mode. The vertical and horizontal components of the TE 11  portions of the beacon signal are extracted by two E-plane junctions 60, 70 containing band-pass cavities tuned by screws. They are coupled to the circumferential H field in the circular guide at points where the E field is zero by means of rectangular irises. In order to prevent iris resonance at frequencies higher than the beacon signal frequencies, the irises are provided with parallel vanes in planes at rightangles to the circular waveguide axis. The vanes extend into the waveguides for a distance equal to one quarter wavelength at the undesired frequency and hence form rejection stubs. The irises may be adjusted by means of tuning screws in the wall of the circular waveguide. Each of the couplers 60, 70 is associated with a band-rejection filter respectively 61, 62 and 71, 72 designed to reflect beacon-signal energy in the TE 11  modes not extracted by the couplers. The associated cavities of each rejection filter are spaced an an odd number of quarter-wavelengths and the couplers 60, 70 are spaced from the nearer rejection cavity by an integral number of half-wavelengths.