GB589603A - Improvements in or relating to directional antennas - Google Patents

Abstract

589,603. Aerials ; wave guides. WESTERN ELECTRIC CO., Inc. July 14, 1944, No. 13547. Convention date, .July 27, 1943. [Class 40 (v)] The directivity of an aerial comprising a transmission channel adapted to radiate or receive waves at points along its length is periodically varied or scanned by cyclically varying the phase velocity of the waves within the channel. In Fig. 1 the transmission channel comprises a rectangular wave guide 1 coupled to the translation device 7 by the probe 11 and co-axial line 10, radiation taking place from the longitudinal slot 13 in a wall parallel to the E component 12 of the H10 waves in the guide. The principal radiation. lobe 23 is at an angle /\‹ to the normal to the slit directed away from the coupling device a secondary lobe being produced at an equal angle on the other side of the normal by energy reflected from the end wall 15 and additional minor lobes between the principal lobe and the slot axis being produced by components of low velocity modes. The angle # depends upon the phase velocity v of transmission in the wave guide and is varied by varying the effective lateral dimension b of the guide. The boundary of the guide opposite to the slot is formed by the cylindrical rotor 14 which is driven by the motor 18 and has a longitudinal slot 16, 27 or flat 26, Fig. 3. The effective lateral, dimension b depends upon the angular position of the rotor slot and thus the radiation lobe sweeps through an angle as the cylinder rotates. In Fig. 6 the top and bottom surfaces of the wave guide 1 and the end adjacent the coupling 8 are extended to form a triangular horn with a wide rectangular aperture 34 normal to the mean direction of the principal radiation lobe. The wide aperture reduces the lobe width in the scanning plane, a further reduction being effected by the provision of flanges 35, 36. Two such systems 38 and 40 for transmitting and receiving can be superposed as shown with the wide apertures in a common plane but with the wave guides directed towards opposite ends of the apertures so that the principal lobes are coincident but the subsidiary and minor lobes are oppositely directed. In this case the horns also serve to prevent interaction between the two systems. When used in a radar system the transmitter lobe may be stationary and the receiver lobe scanned in which case wave guide 1 will be a normal rectangular guide and if the b dimensions for the two guides are different the angles of the triangular horns will be correspondingly different, Fig. 7 (not shown). If the b dimensions for the two guides are equal, the guides can be superposed with the longitudinal slots in a common plane in which case the triangular horns may be omitted and simple flares provided to prevent interaction. Alternatively both lobes may be scanned. In further modifications, Figs. 9 and 11 (not shown), radiation takes place from a series of longitudinally, spaced transverse slots or longitudinally spaced polyrods in one of the b walls of the guide, the areas of the slots being progressively varied to ensure equal energization of the elements. In this case the plane of polarization is parallel to the scanning plane and rotation of the cylinder in the guide varies the space factor of the array. The slot spacing should be less than #a/2 where #a is the free space wavelength and the polyrod spacing should be equal to a greater than #a to prevent the formation of secondary lobes by higher order modes. In the case of the polyrod array, secondary lobes are minimised by giving each element a sharply directive characteristic by tapering the rods and design conditions are given by producing a broadside lobe. The cylindrical rotor may be replaced by a reciprocating member. U.S.A. Specifications 1,562,961, 2,129,669, 2,145,024 and 2,236,393 are referred to.