GB929871A - Improvements relating to aerials - Google Patents

Abstract

929,871. Aerials. ELECTRIC & MUSICAL INDUSTRIES Ltd. July 21, 1959 [Aug. 6, 1958], No. 25137/58. Class 40 (7). A radio aerial consists of a slot in the wall of a resonant cavity, the cavity being provided with capacitive loading to tune it to the operating frequency. The aerial may be of the suppressed type embodied in the wall of a structure 1 with the cavity of comparatively small dimensions extending only a little way into the structure. The cavity is in part closed by a plate 2A extending from one side across the mouth and to this plate is attached a channelsectioned baffle 2B whose lower flange is spaced from the bottom of the cavity by dielectric material 2D. The baffle, in effect, folds the cavity about a longitudinal axis and at the same time provides sufficient capacity loading to tune it to the operating frequency. The slot is bounded by the edge 1A of the cavity and the edge 1B of the plate 2A. It extends the full length of the cavity and may be rectilinear or the whole aerial including the slot may be curved. The slot length is made nearly threequarters of the working wavelength and to increase the bandwidth a conductor or rod 3 is placed in the slot. The position of this rod is adjusted so that it taps into the electric field across the slot and provides a transformer with a turns ratio V2/V1 as indicated on Fig. 2, and its length is determined to provide inductive reactance. The aerial feeder consists of a coaxial line 7 whose inner conductor is connected to a conductive film 5 on the inside of the dielectric window of the slot. Embedded in the window itself and parallel to, but spaced from the film 5 is a second film 6 connected to the conductor 3. The films provide series capacitance in the feed to the aerial. An equivalent circuit of the aerial is given (Fig. 3, not shown) with an explanation of how the lumped impedances of the circuit are related to the arrangement of the aerial. Two aerials according to the invention may be formed at diametrically opposite points in the wall of a cylindrical structure or mast as indicated in Fig. 4. These aerials, 13, 14, radiate in phase and are fed by sub-feeders 15 and 16 of equal length from a common feedpoint. Radiation in anti-phase may be achieved by turning one of the aerials upside-down (Fig. 5, not shown).