GB664150A - Improvements in and relating to slot aerials - Google Patents

Abstract

664,150. Aerials. BRITISH BROADCASTING CORPORATION, BROWNLESS, S. F., and MONTEATH, G. D. Jan. 9, 1950 [Jan. 10, 1949], No. 677/49. Class 40 (vii). Relates to means for increasing the bandwidth of a slot aerial comprising a conducting surface having formed therein an open or closed-ended slot. The invention is described as a modification of a known form of folded slot aerial. Fig. 2, as disclosed in Specification .591.053 in which the central conductor 16 forms in conjunction with the longitudinal edges of the slot 14 two open-circuited sections of transmission line when viewed from the feeder terminals 17 and 18. When the length of the central conductor is #/2 at the resonant frequency, the two sections of transmission line provide a reactance having a positive reactance/frequency slope, Fig. 1(b) (not shown), which tends to cancel the negative reactance/ frequency slope of the slot, Fig. 1(a) (not shown), and in accordance with the present invention the compensating positive reactance slope is increased by terminating one or each end of the central conductor by a reactance which may comprise: (a) an even number of A/4 transmission-line sections in cascade, the end section being open-circuited, or an odd number of #/4 sections in cascade, the end section being short-circuited; (b) a parallel combination of open-circuited and shortcircuited sections, e.g. connecting an end of the central conductor to a #/4 stub at a point intermediate between the short-circuited end and the open-circuited end; (c) parall eltuned circuits, or filter networks of lumped elements or combinations of lumped and line elements. In one embodiment, Fig. 3, the central conductor 16 comprises a #/2 rod terminated by short-circuited coaxial line stubs 19 and 20 of approximate length #/4, the exact length of the stubs or rod being chosen to make the input impedance of the aerial purely resistive over a frequency band whose width may be varied by varying the diameter of the rod or the characteristic impedances of the stubs. The outer conductors of the stubs are bonded to the plate 15 and the aerial is fed by a coaxial line 21 whose outer conductor is also bonded to the plate. In a modification, Fig. 4 (not shown), the upper half of the rod 16 and the stub 19 are omitted thereby doubling the slope of the correcting reactance. The rod and stub need not be in the plane of the sheet. This form may be further modified by omitting stub 20 and extending the lower end of rod 16 by #/4 and connecting the end of the extension to the sheet 15. The input impedance of the aerial may be varied by displacing the rod 16 towards one or other of the longitudinal edges of the slot, by adjusting the length of the rod 16 or stub 20, or by connecting a variable capacitor across the end of the stub adjacent the rod, any resultant reactance being balanced out by a variable capacitor across terminals 17 and 18. The coaxial feeder 21 may be replaced by an unbalanced feeder comprising a single conductor connected to terminal 17 arranged parallel to and adjacent sheet 15 which provides the return path of the feeder. Fig. 5 shows an application of the invention to an open-ended slot aerial formed by bending the sheet 15 to form a cavity with two edges of the sheet spaced from one another to form a slot in a wall of the cavity. A #/4 rod 16 is terminated by a short-circuited #/4 stub 19 comprising a metal plate 22 parallel to the back of the cavity and spaced therefrom by insulating material 23, the lower end of plate 22 being shorted to the back of the cavity.