GB1388230A - Frequency extension of circularly polarized antenna - Google Patents

Abstract

1388230 Aerials BENDIX CORP 19 Dec 1973 [26 Dec 1972] 58737/73 Heading H4A A wide-band microwave aerial assembly comprises a spiral aerial fixed to a surface of a substrate of insulating material, said aerial including a pair of interlaced spiral elements having terminals near the axis of the assembly, a cavity structure with conductive walls and of generally cylindrical configuration supporting the assembly, and positioned at the back of the spiral aerial, and a lower-frequency linearlypolarized aerial extension comprising a ground plane backed dipole element electrically connected to the outside ends of each of the spiral elements. As described, feed conductors 20, 22, Fig. 2, which are normally supplied through a co-axial connector having a shield 24, are respectively connected to the inner ends of spiral tracks 10a, 10b of a spiral aerial 10. The outside ends of spiral tracks 10a, 10b are extended by respective dipole arms 14a, 14b. The radiating elements may be formed by a printed circuit process upon an insulating substrate which is positioned above a ground plane 12. Said ground plane is constituted by the back wall of a conductive cylinderical cavity below the spiral aerial 10, and may also be constituted by the back walls of conductive cavity structures below the dipole extensions 14a, 14b. Said cavities may be lined with microwave absorbing material. The spiral aerial is effective with circularly polarized waves, whilst the dipole extensions permit operation with linearly polarized waves of lower frequency. The overall length of the aerial is #/2, whilst its distance from the ground plane is less than #/4, # being the wavelength at the lowest operating frequency. In one embodiment the insulating substrate for the spiral aerial 10, Fig. 3, is shaped to fit to the underside of a domed radome 25, a spacer 30 made of flexible compressible material be interposed. The dipole extensions 14a, 14b are likewise curved to fit to the radome. A cylindrical conductive housing 12 is positioned below the spiral aerial 10, and has metal plate attachments 26a, 26f below the dipole extensions 14a, 14b constituting ground planes. Microwave absorbing material 28a, 28b is provided between the dipole extensions and the ground plane members. In another embodiment the substrate for the spiral aerial and the dipole extensions is planar, and the cylindrical cavity below the former has extensions below the latter (Fig. 1, not shown). The said dipole extensions are depicted as V-shaped.