GB1364941A - Aerials - Google Patents

Abstract

1364941 Aerials DEFENCE SECRETARY OF STATE FOR 5 Jan 1973 [5 Jan 1972] 393/72 Heading H4A An aerial which transmits or receives circularly polarized waves comprises a dipole mounted substantially within the aperture of a slot aerial, a cavity adjacent the dipole and the slot aerial and coupled thereto, means for coupling the slot aerial to the dipole asymmetrically relative to a line of symmetry parallel to the shorter side of the slot aerial, and feed means connected to the dipole. As described, a rectangular fibreglass substrate 1 has a conductive layer 2 on one side, from which portions are removed to provide a rectangular slot 3 within which is disposed a folded dipole 4 having an outline 5 and comprising a feed gap 6 in one limb. The slot and the folded dipole are symmetrically disposed about centre line 7 of the substrate 1. On the reverse side of the substrate 1 a rectangular conductor 8 and a conductive strip 9 are provided. One end of the strip 9 overlays the gap 6 and is connected to the folded dipole 4 near said gap by a lead 10 passing through the substrate 1. The strip 9 is shaped to lie under rather more than half of the folded dipole 4, and at its other end is provided with a matching section comprising a series arm 11 and a parallel arm 12. The conductor 8 provides capacitive coupling between the slot and the folded dipole and is positioned with its shorter edge parallel to the strip 9 so that it overlays part of the slot 3. Its centre A is a little more than half the length of its shorter side from the line of symmetry 7. The inner conductor 16 of a coaxial feed cable passes insulatedly through the ungapped arm of the folded dipole and through the substrate, and is connected to the end of the extension 11 of the strip 9, the outer conductor of said cable being connected to said arm of the dipole. A conductive cover 13 fits over the layer 2 and is connected thereto around its outer edge 14. The coaxial feed cable passes through the centre of the cover 13. and is also connected thereto. The aerial may be flush-mounted in an aircraft, with the dipole vertically orientated in normal flight. The aerial may be arranged so that the orthogonally related linearly polarized radiations from the dipole and the slot are also in phase quadrature, whereby circularly polarized radiation may be obtained. In another embodiment (Fig. 4, not shown) a mirror image of the conductor 8 is provided on the other side of the line of symmetry 7, but a portion of each conductor is separated from the remainder by an insulative gap, which is bridged by a diode. One of said diodes is forwardly biased, whereby the associated capacitive coupling conductor becomes effective, and the other diode is backwardly biased, so that the associated capacitive coupling conductor remains ineffective. In this way, the aerial may radiate circularly polarized waves of either hand.