GB1338753A - Multimode antenna - Google Patents

Abstract

1338753 Aerials INTERNATIONAL STANDARD ELECTRIC CORP 6 Jan 1972 [12 Jan 1971] 590/72 Heading H4A An omnidirectional aerial which can operate simultaneously with vertically polarized waves and with circularly polarized waves has a vertical axis along which are disposed a disc 17 and, below said disc, four radial arms 1, 2, 3, 4, said arms being angularly spaced at 90 degrees intervals. A circular coaxial member 5 provides physical and electrical connection means for the arms, portions of which are bent downwards at an angle of less than 90 degrees. Each pair of oppositely disposed arms constitutes a dipole, and when said dipoles are respectively fed with orthogonally phased energies they operate as a turnstile aerial, from which circularly polarized waves are emitted. The disc 17 and the arms 1, 2, 3, 4 together constitute a discone aerial, which when fed with energy emits vertically polarized waves. As described, the disc 17 is supported on a cylinder 9, inside which there rises a coaxial feed-line 18 to a power divider or adder 19. Coaxial lines 20, 21 extend from said divider, their centre conductors 22, 23 being taken through holes in the disc 17 to respective feed points on oppositely disposed arms 1 and 3, and their outer conductors being connected to said disc. The coaxial line 18 has a characteristic impedance of 50 ohms, and each of the coaxial lines 20, 21 has a characteristic impedance of 100 ohms and is a quarter-wave in length at the mean frequency of operation of the discone aerial. The turnstile aerial is fed from the output ports of a quadrature hybrid 12 by coaxial lines 6, 7 each having a first portion 8 extending upwardly towards adjacent arms 1, 4, respectively, substantially parallel to the cylinder 9, and a second portion 10 coupled to, and extending substantially at right angles to the portion 8, within said arms. The centre conductors 11 of the portions 10 are electrically connected to the circular member 5. To prevent distortion of the radiation pattern owing to excitation of the cylinder 9, R.F. chokes are provided under each arm by means of radial bars which extend from the said cylinder to the outer conductors of the coaxial lines 6, 7, in the cases of the arms 1, 4, or to vertical bars simulating said outer conductors, in the cases of the arms 2, 3. The radial bars are disposed a quarter-wave below the arms at the mean frequency of operation of the discone aerial. In another embodiment (Figs. 4, 5, not shown) the power divider or adder is disposed below the aerial and three-quarter wavelength coaxial lines extend upwardly therefrom to the disc. Coaxial feed lines for the turnstile aerial enter at the extremities of two adjacent arms, and extend therethrough to the coaxial circular member.