GB735578A - Slotted cylindrical antenna - Google Patents

Abstract

735,578. Aerials; aerial coupling arrangements. ALFORD, A. Sept. 2, 1952, No. 22057/52. Classes 40 (5) and 40 (7). An aerial comprising a conductive cylinder 1, Fig. 8, with a longitudinal slot 2 is coupled to a coaxial feeder by connecting the outer conductor 17 of the feeder to the cylinder by a flared matching section 18, and extending the inner conductor 19 into the cylinder along its axis and connecting it by a branch 20 to one side of the slot at a point 21 a distance S=5#/4 from the end plate 22, adjustable shorting bars 23, 24 being set at a distance L=# from the point 21. In a modification, Fig. 9 (not shown), a collinear array of the slots is effected by using a single slotted cylinder of length 9#/4, the slot being divided into two separate slots by a central shorting bar and each slot being energized at its centre by a branch from the centre conductor. The radiation pattern of the aerial in a plane perpendicular to the axis of the cylinder is substantially circular and according to a further feature of the invention the pattern is made directional, e.g. substantially cardioid, Figs. 5, 5a and 5b (not shown), by attaching radiating " wing " elements 7, 8, Fig. 1, to the cylinder on either side of the slot, the width of the wings being not less than the diameter of the cylinder. The shape of the radiation pattern is controlled by the size and shape of the wings and by the forward angle between them and this angle may be varied within the range 130-226 degrees without affecting the impedance characteristics of the aerial. When the angle is less than 130 degrees, the loading effect becomes appreciable and can be compensated by increasing the capacity per unit length between the edges of the slot and decreasing the cross-sectional area of the cylinder; minor adjustments of slot length may also be necessary and may be effected by movable shorting bars. The cylinder may be formed of metal sheet or netting and its cross section, which may be other than circular, is usually within the limits 0.01-0.02#<SP>2</SP>, a preferred value being 0.014#<SP>2</SP> for a slot 2# long and 0.01# wide; the slot may be sealed with dielectric material. The wings may be formed of sheet metal or spaced wires, Fig. 5d, and they may be rectangular or have curved or serrated edges; they may be curved or bent back as shown in cross section in Fig. 3a and Fig. 7 to form a second slotted cylinder 9 whose slot is coincident with that of the first cylinder or the edges 55, 57, Fig. 5c may be folded back, the space enclosed by each fold acting as an auxiliary slot which radiates backwards in opposite phase to the radiation from the main slot thereby neutralizing the minor back lobe, Fig. 5 (not shown), which is produced by plane wings. Back radiation may also be reduced by a plane rear reflection, Fig. 10 (not shown). As shown in Fig. 1, the wings are formed by a bent slotted sheet mounted on the cylinder 1 with its slot coincident with the cylinder slot but alternatively the wings may comprise separate sheets, each sheet being attached to the cylinder along the edge of the slot or along a line parallel to but displaced from the slot. When the wings are directed backwards they may be attached to a supporting mast 10 as illustrated in Fig. 2 (not shown) and in cross section in Fig. 3. Directivity in the plane perpendicular to the cylinder axis may be increased by employing a broadside array of the units backed by a plane reflector, Fig. 10 (not shown), the units being protected by a dielectric housing, Fig. 11 (not shown), mounted on the reflector and heating elements disposed parallel to the cylinder axes being embedded in the dielectric to effect de-icing. It is shown with reference to Figs. 1a ... 1c (not shown) that the directivity in the plane containing the cylinder axis is better than that of a conventional pyramidal horn of comparable aperture.