GB670631A - Improvements in and relating to radio detection and ranging systems - Google Patents

Abstract

670,631. Aerials. BRITISH THOMSONHOUSTON CO., Ltd. (General Electric Co.). Dec. 13, 1948, No. 32177/48. Class 40 (vii). A directive antenna for a radar system operates selectively to provide spiral, conical or linear scanning motions. The antenna comprises a radiating element disposed in front of a reflector, the element being supported for rotation about the axis of the reflector and also about a second axis displaced from but parallel to that of the reflector, the relative speed and direction of rotation about the two axes determining the type of scanning motion. Alternators driven in dependence on the two rotations produce voltages indicative of the angular position of the antenna to control the scanning of a cathode-ray tube. In Fig. 1, a reflector 1 has a central section 2 which is supported to rotate about the axis 29 of the reflector and carries an antenna waveguide 3 inclined an angle to axis 29 and arranged to rotate in bearing 5 about an axis 25 parallel to axis 29. The wave-guide terminates in a dielectric housing 26 and has a metallic plate 27 for reflecting energy from the wave-guide mouth to the reflector. The wave-guide is so inclined that the antenna at its innermost position (shown dotted at 28) lies on axis 29 or is displaced therefrom by a small amount. A housing 4 supports the central section of the reflector and is itself supported on a shaft 9 so as to be rotated by motor 10 driving through a variable speed system (not shown) controlled by lever 12, gear 11 and pinion 8 affixed to shaft 9. A second motor 13 rotates the antenna wave-guide and is connected thereto through a variable speed system (not shown) controlled by lever 14, pinion 15, loose gear 16 journalled on shaft 9, gear 17 and gears 18 and 19. Gears 15-19 function as an epicyclic gear train effective to rotate the antenna wave-guide at a speed determined by the speed and direction of rotation of both housing 14 and gear 15. Energy is conveyed to the antenna, from a stationary wave-guide 20 through rotating half-wave choke 21, rotating section of waveguide 22 formed within shaft 9, curved waveguide 23, rotating half-wave choke 24 and a section of wave-guide within shaft 25. The scanning motion produced depends upon the relative speed and direction of motion of the antenna about axis 29 and shaft 25. For a conical scan the antenna is held at its innermost position 28. If in this position the antenna is displaced a small distance from axis 29 the beam is projected at an angle and as the section 2 rotates the beam describes a cone about axis 29. To produce a conical scan tilted from axis 29 section 2 is held stationary and the antenna rotated about axis 25. To produce a spiral scan section 2 is rotated rapidly and the antenna wave-guide less rapidly. If, for example, section 2 rotates at 2400 r.p.m. and the wave-guide at 100 r.p.m. the beam sweeps out a spiral of approximately twelve turns. The width of the beam and the pitch of the revolutions in the spiral are correlated by choosing the proper gear ratios so that the path of the beam, in returning from its outermost position to the centre, is displaced from the outgoing path. In this way successive spirals interleave in their coverage. A linear or long, thin elliptical scan is produced when the antenna is rotated at twice the speed but in the opposite direction to the section 2, a linear scan resulting if the antenna, at its innermost position, lies on axis 29, and an elliptical scan if there is a small displacement. The direction of scan, vertical, horizontal or at an intermediate position is controlled by the initial angular position of shaft 25 with respect to axis 29. Voltages for indicating the absolute position of the beam are generated by two two-phase alternators 60, 61. The rotor of alternator 60 is driven from shaft 62 by gears 63, 64, the gearing being such that the rotor at all times has the same direction of rotation, speed and phase as section 2. Similarly the rotor of alternator 61 is controlled by gears 66, 67 so as to repeat the rotation of the antenna waveguide. The two phases of the alternators represent the rectangular components of the rotations. Fig. 9 (not shown), illustrates a pulse radar system employed with the antenna and having a cathode-ray tube display, the voltages from each alternator representing the horizontal components of motion being connected in series to the horizontal deflecting plates and the vertical component voltages being similarly connected in series to the vertical plates. The tube spot is caused to trace a pattern which represents at all times the angular position of the antenna. Received echo signals are applied to the tube grid to illuminate the trace. In a modified construction of the antenna, Fig. 5 (not shown), the variable speed systems are replaced by direct-current motors connected to a supply through speed control resistors. A further modification, Fig. 6 (not shown), employs a dipole antenna energized through a coaxial line system, and the driving system comprises a transmission system out of which project two rotating shafts, the relative speed being varied by a single control lever. The housing 4 includes a planetary gear system (not illustrated) which is connected between the two rotating shafts, housing 4 and shaft 25.