GB611248A - Improvements in and relating to directive antenna systems - Google Patents

Abstract

611,248. Directive radio,systems. SPERRY GYROSCOPE CO., Inc. March 29, 1946, No. 9894. Convention date, March 31, 1945. [Class 40 (vii)] A directive aerial system for producing an asymmetrical directivity pattern comprises a parabolic reflector with a main aerial positioned at the focal point and a parisitic aerial for modifying the directivity pattern positioned at a point displaced from the axis of the reflector and lying between the focal point and the reflector. The invention is described as applied to modifying the vertical directivity of the horizontal beam of an airborne radar system so that there is appreciable energy transmission at relatively large angles below the axis of the beam, Fig. 5, thus enabling objects on the ground to be detected throughout a greater range of distances. According to a further feature of the invention, the parasitic aerial is automatically rendered ineffective when the beam is scanned conically or spirally for D.F. purposes so that a symmetrical beam is obtained during such scanning. As shown in Fig. 1, a symmetrical pencil beam is produced by mounting two horizontal dipoles 23 and 24, spaced #/2 apart, at the focus of a paraboloidal reflector 22 of focal length 4# and the beam may be scanned through a limited angle in azimuth by oscillating the aerial assembly about a normally vertical " nod " axis by means of the motor 31 and the eccentric and crank connection 30, 33, 34. For spiral scan, the " nod " axis is simultaneously rotated about the transverse horizontal " spin " axis of the system by means of the motor 26 and for conical scan the motor l 31 is stopped with the reflector axis at a small angle to the " spin " axis. The dipoles 23 and 24 are energized by a rectangular wave-guide 44 projecting through the reflector which is coupled through the rotary coupling 40, coaxial line 37 and rotary coupling 38 to the waveguide 36 connected to the transmitter 35 and receiver 39. According to the invention, a horizontal parasitic dipole 25, Fig. 3, is mounted half-way between the focus and the reflector at a distance A below the axis 21 of the reflector. This dipole is mounted on the end of a vertical tubular member 55, Fig. 6, which has a helical slot 56 engaging a pin 57 fixed in an outer tubular member 58 which is attached to the underside of the wave-guide 44. The member 55 is normally forced upwards by the helical spring 62 bearing against the flange 61 and in this position the dipole 25 is parallel with the dipoles 23 and 24 so that it is effective to modify the vertical directivity as shown in Fig. 5 as is required during azimuthal scanning. During conical or spiral scanning, the member 55 is forced radially outwards by centrifugal action and is thereby rotated through 90 degrees by the action of the pin 57 and helical slot 56 so that the dipole 25 is now in the position 25<1>, Fig. 3, parallel to the axis of the reflector and in this position it has no effect on the directivity and a symmetrical beam is obtained. Instead of rotating the reflector and the radiating element during conical or spiral scan, the radiating element only may be rotated and the radiating element may be a single dipole or a wave-guide. mouth or horn instead of the dipole pair shown.