Patent GB-688610-A (GB)
Title: Improvements in or relating to antenna systems
IPC: H01Q3/32
Filed: 1947-01-09
Abstract: 688,610. Aerials. WESTERN ELECTRIC CO., Inc. Jan. 9, 1947 [Jan. 17, 1946], No. 777/47. Class 40 (vii). In a directive aerial system wherein the primary aerial element is moved repeatedly through the focus of a reflector associated therewith, thus providing a scanning action, the primary element comprises an aperture formed in an element comprising a cylindrical wave-guide having a helical slot cut therein and an outer member coaxial therewith having a linear slot cut therein, the inner member rotating continuously. A wave-guide element 17, Fig. 1, comprising two coaxial members 32 and 33, Fig. 4, extends along the latus rectum of a parabolic reflector 20, and is coupled to the transmitting or receiving apparatus 15. The inner member 32 has a helical slot 34 taking one turn upon the cylinder and having an axial length equal to an integral number, e.g. 7, of half-wavelengths in the guide cut therein. The outer member 33 has a linear slot 35 having a length at least equal to the axial length of slot 34. The member 32 is rotated by the motor 24, which is coupled thereto through the shaft 25 and gears 27, so that the aperture formed by the intersection of the slots 34 and 35 travels from left to right through the focus of the reflector 20. The polarization of the electric vector is as shown in Fig. 9 and the energy radiated through the aperture is polarized vertically. It may also be shown that the energy escaping through the spacing between the members 32 and 33 is also polarized vertically and consequently cannot be distinguished from the wanted radiated energy. Accordingly wave-traps 36 are arranged as shown to prevent undesired energy from escaping through the space between members 32 and 33. In the arrangement shown in Fig. 13, the axial length of the turn is made less than before so that a nearly square aperture is obtained, and the electric vector is polarized as shown in Fig. 14. In this arrangement the signal radiated from the aperture is horizontally polarized while the unwanted radiation via the spacing between the coaxial members is vertically polarized. A large number of closelyspaced plates 41 are arranged in front of the linear slot and terminated in side-walls 42. The spacing between the plates 41 is less than a halfwavelength while that between the side-walls 42 is greater than a half-wavelength, so that only the desired horizontally polarized signal is radiated. Phase-shifting and matching arrangements. In order to maintain the electrical lengths between the aperture and the unit 15 and between the aperture and the termination 30 of the guide 17 constant, phase-shifting arrange. ments 21 and 22 are provided. These arrangements comprise members 43, 44, 45, 46 and 47, as described in Specification 575,577, and preferably modified, for wide-band operation, as described in Specification 677,842. Members 43 and 47 are rotated through gears 26 and 28, respectively, member 43 producing a phasechange of 360 degrees for every 180 degrees of rotation and member 47 producing a phasechange of 360 degrees for every 360 degrees of rotation, and the gear ratios are accordingly so arranged that members 43 and 47 rotate at 1¥ and 3¢ times the rate of rotation of member 32, respectively.