GB906614A - Aerial system with slot radiators - Google Patents

Abstract

906,614. Aerials; radio frequency couplings. HOLLANDSE SIGNAALAPPARATEN N.V. March 24, 1960 [March 25, 1959], No. 10481/60. Class 40 (7). A microwave aerial system for radiating and/or for receiving energy reflected by an object illuminated by one or both of two beams diverging from each other by a small angle, the direction of the beams being variable by variation of the frequency of the radiation, comprises a number of radiating slots in two wave-guides. All the slots are arranged along a single line, the slots in one wave-guide being interspersed along this line alternately with those in the other. The distance along the line between two successive slots in either guide is less than the free space wavelength and is substantially less than the distance along the guide, and the phase difference between the radiation from two successive slots in one guide differs from the phase difference between that from corresponding slots in the other. Two waveguides A and B, Figs. 4 and 5, of rectangular cross-section are formed into a double start helix with slots a and b in each guide arranged with their centres along a line parallel to the axis of the helix, the distance along the line between slots being less than a free space wavelength by reason of the radial disposition of the larger dimensions of the guides. The slots in each guide are arranged so that the angles between them and the longitudinal axis of the guide increase with distance from the fed end of the guide so as to obtain equality of radiation from the slots. This, together with the close spacing of the slots, results in each guide radiating a broad beam which varies in direction with variation of radio-frequency due to a corresponding variation of the phase difference of the radiation from successive slots. The guides are so arranged that the phase difference between the energy radiated from two successive slots in one guide differs from the corresponding difference in the other guide, so that the two beams diverge slightly. This relative difference may be created by positioning diaphragms in the guides between the slots, those in guide A being parallel to the longer walls of the guide 501 to cause capacitive reactance, whilst those in guide B are parallel to the shorter walls of the guide 502 to cause inductive reactance. This arrangement is stated to result in the relative phase difference between corresponding successive slots in the two guides remaining sensibly constant when the frequency of radiation is varied. The relative phase difference may be provided by differences of lengths of the two guides between successive corresponding slots, or by the employment of reactive diaphragms between slots in only one of the guides. The wave-guides may be constructed in other configurations, for example from partcircular and straight sections, Fig. 7, such a construction being particularly suitable for use with a reflector such as a parabolic cylinder. An aerial for radiating a single beam whose direction may be varied by variation of the frequency of radiation and which is stated to be known, consists of a rectangular waveguide formed into a helix, with the minor dimension radial, a row of slots 1, 3, 5, 7, Figs. 1 and 2, in the major wall of the guide being arranged along a line parallel to the axis 16 of the helix. Grooves such as 2, 4, 6, which are approximately a quarter of a wavelength deep, are arranged between the slots to prevent undesirable coupling therebetween due to the component of the electric vector along the common line of the slots. Such grooves may also be used in the embodiments described with reference to Figs. 4 and 5. A waveguide hybrid for coupling a pulsed radar transmitter and a receiver to receive both the sum and difference of the energies reflected from a target illuminated by both radiated beams consists of two sections of waveguide A<SP>1</SP> and B<SP>1</SP>, Fig. 8, with inductive diaphragms 803 in A<SP>1</SP> and capacitive diaphragms 802 in B<SP>1</SP>, the phase-shift produced by each set of these being 45 degrees. The two sections are coupled by an aperture 802, the two waveguides of the aerial system described being connected at A<SP>1</SP> and B<SP>1</SP>, respectively. The transmitter is connected at Z, the " sum " receiver also at Z via a T-R cell, and the " difference " receiver at V. The aperture 801 divides a transmitted burst of energy equally between the two guides, advancing the phase of that passing into guide A<SP>1</SP> by 90 degrees, so that after passing the diaphragms 802 and 803 equal amplitude inphase radiation is fed to the two guides of the aerial. Energies received by the aerial fed to A<SP>1</SP> and B<SP>1</SP> are suitably shifted in phase by the diaphragms and apertures to produce at Z their sum and at V their difference.