GB591053A - Improvements in or relating to electromagnetic radiating or receiving apparatus - Google Patents

Abstract

591,053. Aerials; directive wireless signalling. BOOKER, H. G., WESTCOTT, C. H., and LEES, R. J. March 23, 1944, No. 5448. [Class 40 (v)] A radiating or receiving element comprises a conducting surface 1, Fig. 1, completely bounding a slot 2 which is electrically one or more half-wavelengths long so as to be longitudinally resonant at the operating frequency. The slot which may be fed at .its centre points 4, 5 from a two wire line, has the same properties as a strip dipole of the same dimensions, except that the radiation from the vertical slot is horizontally polarised. A second con. ducting sheet 3 spaced one quarter wavelength behind the slot, acts as a reflector and may be replaced by a hemispherical cavity of radius equal to one quarter of a wavelength. When the slot aerial is used to feed a parabolic reflector, the conducting sheet 1 and the reflecting sheet 3 or its equivalent may be bent along vertical lines on either side of the slot, Figs. 2 and 4 (not shown) so that the convex face is towards the paraboloid. The slotted and reflecting sheets may together form a cavity which is preferably resonant at the operating frequency, energy being fed to or removed from the cavity by a coupling probe. This construction may be incorporated into the skin of an aircraft, the slots being filled with dielectric material. A broadside array may be produced by suitably spacing a number of radiating slots in a conducting sheet, and the slot may be shaped to simulate other forms of aerials, for example a folded dipole. In the arrangement shown in Fig. 5 for feeding a parabolic reflector the radiating or receiving slot 2 is cut in the wall of a conducting cylinder one wavelength high, but of a .diameter of one-quarter of a wavelength which does not permit waveguide propagation along the cylinder. The cylinder 9 is supported by vertical fins 10. on the feeder tube 12, and the feeder cable 17 is coupled to projections at the centre of the slot by an open line quarter-wave transformer 15, 16. A low impedance stub formed of quarter wavelength sections of coaxial line 18 with their inner conductors short-circuited at 19, is connected across the junction of feeder 17 and the line 15, 16, to compensate for the variations in impedance of the slot with frequency. To magnify the effect of the stub it may be made three-quarters of a. wavelength long. Tilting of the beam radiated. by the slot and paraboloid may be produced by raising and lowering the cylinder 9 by means of a supporting arm pivoted at the back of the paraboloid, and a desired radiation pattern may be produced by utilizing two or more radiators and their associated paraboloids. In a modified arrangement for producing beam switching, Fig. 6, a cylinder 22 has a long slot 23 which together with two switching bands 24, 25 constitute two radiating slots. As shown, band 25 short-circuits the lower slot which is therefore inoperative and defines the lower edge of the upper slot which therefore radiates. Simultaneous rotation of the bands 24, 25 causes the feed points of the upper slot to be short-circuited while the lower edge of band 24 defines the upper edge of the lower slot which thus radiates. The arrangement may be extended to include any number of slots and in an alternative method of switching the slots, Fig. 7 (not shown), a plunger is arranged to slide within short cylinders mounted at the feed points, and may be arranged to have an adjustable desired capacity to either side of the slot. Specification 515,684 is'referred to.