Patent GB-817585-A (GB)
Title: Improvements in or relating to microwave duplexing systems
IPC: H01J17/04
Filed: 1957-02-13
Abstract: 817,585. Combined radio receiving and transmitting systems. MONOGRAM PRECISION INDUSTRIES Inc. Feb. 13, 1957 [May 21, 1956], No. 4892/57. Class 40 (5). [Also in Group XL (b)] A microwave duplexer comprises a circulator, branches of which are respectively adapted for connection in order of the direction of circulation to a transmitter, an antenna, and an attenuator, including a gyromagnetic element in a waveguide section surrounded by a polarizing coil, connected to a receiver. In the embodiment described a circulator 1, Fig. 1, comprises two hybrid units 6 and 6<SP>1</SP> with flanged connections 9 and 11 connected to a pulsed transmitter and antenna respectively. Waveguide section 5 is connected through a 90-degree elbow 13 to a Faraday rotator 17, which terminates in a flange 19 connected to the receiver. Waveguide 21 terminates in a dissipative resistance element 23. A gyrator magnetized by permanent magnets 27 is introduced into waveguide 3 to retard by 90 degrees the phase of waves propagated from left to right, and advance by 90 degrees the phase of waves propagated in the opposite direction. A polystyrene compensator 28, Fig. 2 (not shown), so loads waveguide 5 as to advance by 90 degrees the phase of waves propagated through it. A further 90 degrees phase shift occurs as between a wave transmitted directly from an input A through waveguide 3 to the transmitter, and a wave crossing into waveguide 5. A 90-degree phase shift in the same direction also occurs in transferring through coupler 6<SP>1</SP> back to the other straightthrough path. Thus where a wave enters at A through waveguides 3 and 5 in parallel, a 180- degree rotation takes place through gyrator 27, and through the two couplers, the waves reinforcing the output to the antenna, but a wave at branch D is 180 degrees out of phase and the two waves therefore cancel in branch D. Received signals in arm B are advanced equally in waveguides 3 and 5, and reinforce in arm C, while the waves in arm A are 180 degrees out of phase and cancel. Attenuator 17 reflects the major portions of any wave received into branch D where it is absorbed. The attenuator comprises two Faraday polarization rotators connected in cascade. The fibre-glass body 31, Fig. 4, very thinly silver plated, has axially within it two ferrite pencils 35, 351 supported in polystyrene foam 37, the ends being closed by flanges 39, 39<SP>1</SP>. A polarization analyser, midway between pencils 35, 35<SP>1</SP>, transmits waves polarized 90 degrees from those transmitted by the input and output guides, which comprises a short section of rectangular waveguides at right angles to the input and output guides, or a septum 40 half a wavelength long and extending diametrically across the guide. A winding 43 of several sections in parallel surrounds the tube, a damping rectifier, to prevent oscillation, being connected across it. An absorber 41 of carbon coated sheet covers the output of the guide. When no current flows in winding 43, the Faraday elements being unmagnetized, a wave from the input guide flows unchanged to septum 40 is reflected back into guide 13 and absorbed in arm D. Any unreflected portion is rotated 90 degrees from the input wave and is absorbed in attenuator 41. When winding 43 is excited by a pulse from the transmitter, each Faraday element rotates the plane of polarization of transmitted waves by 90 degrees. Thus the waves pass septum 40 without reflection or absorption. In passing the second section of the attenuator they are rotated through a further 90 degrees and are accepted by an output waveguide oriented similarly to guide 13. A further embodiment, Figs. 5 and 6 (not shown), employs a unidirectional transmission line, in which Faraday elements with permanent magnets give a 45-degree rotation of polarization, so that waves entering arm A pass into arm B. A wave entering arm B emerges polarized at 90 degrees from waves propagated by arm A. An arm C receives waves polarized at 90 degrees from those of arm A, an absorption or reflection attenuator being used to give additional isolation.