GB853600A - Improvements in antenna systems - Google Patents

Abstract

853,600. Aerials. ALFORD, A. July 21, 1958, No. 23330/58. Class 40(7). In order to increase the bandwidth of a directive aerial system comprising a primary element, or a plurality of primary elements, associated with a reflector the primary element or elements are in the form of flared slots fed at their narrowest end with the direction of flare arranged either towards or away from the reflector. Figs 1 and 3 show the construction of an embodiment utilizing four slot elements mounted in front of a plane reflector consisting of rods 31 extending between bars 32. The slots, of which three 5, 6, and 5<SP>1</SP> are indicated in Fig. 3 are formed between the central portions 1, 1<SP>1</SP>, and caps 2, 3, 2<SP>1</SP>, 3<SP>1</SP>, of aerial members that may be circular, ellipsoidal or of other regular or irregular shape. These members are connected centrally by electrically neutral supporting stems 4 to the main structure. A coaxial feed line 27, 28 feeds the system through a coupling arrangement 23, 24 from which four sub-feeders extend one to each slot. The sub-feeders extend through tubes (e.g. 19, 20) as far as the stems 4 and thence along the inside of the stems and the central parts of the aerial members to be connected across the narrowest ends of the slots. The slots are narrowest at the ends closest to the reflector and widest at the point furthest therefrom. If the circumference of the aerial member, measured round the slot is of the order of a quarter of the working wavelength or less the primary element radiation pattern has, it is stated, been found to be approximately omnidirectional whilst circumferences of the order of a half wavelength give a directional pattern having a maximum in the direction extending from the feed point through the widest part of the slots. In dimensioning the system described above for wide-band operation it is arranged that the directivity of the slots themselves is utilized at the upper end of the band whilst at the lower frequency end of the band the slots become omnidirectional and the reflector is spaced to give the required directivity. Dimensions, and an explanation of operation, are given in the Specification for an aerial system capable of operation over the band 300 - 1050 megacylces. In another embodiment (Fig. 6) two primary element slots flared towards a parabolic reflector 40 are shown, at lower frequencies the slots are omnidirectional and the reflector spacing is such that direct and reflected radiation add. At higher frequencies the major part of the radiation is reflected as the slots become directive and there is little loss due to cancellation of the reflected by the direct signal.