Quadrifilar conical helical antenna with travelling- wave current distribution H.Nakano N.lkeda zyxwvutsrqp J .Yamauchi zyxwvutsrqpo Indexing terms: Helical antenna, Qwdrifilar helix, Wideband antenna zyxwvutsrq Abstract: zyxwvutsrqp The radiation characteristics of a quadrifilar conical helix antenna are analysed. Special attention is paid to the generation of a circularly polarised wave over a hemispherical region above the antenna. The present antenna has wideband characteristics with respect to the radiation pattern and the input impedance by virtue of the radiation mechanism characterised by a travelling-wave current. The effects of the cone angle and the pitch angle on the radiation field are also investigated. 1 Introduction In satellite communications and navigation systems, use of circular polarisation is attractive since it elimi- nates the need for control of the antenna orientation about the bore sight. A circularly polarised (CP) wave can be obtained using a planar antenna, such as a spi- ral or a microstrip patch. When installation space is limited, a rod type CP antenna, such as a helix, is pref- erable. Data on resonant helices in [l-31 are particu- larly useful in some applications where the operating frequency band is not wide. Recent communication systems often require a wide CP radiation pattern over a wide frequency band. A bifilar conical helix antenna (Bi-CoHA) [4], character- ised by a travelling-wave current along the arm, is an antenna satisfying this requirement. However, it has the disadvantage that the radiation intensity varies with the azimuth angle owing to the two-arm configuration, i.e. the circular symmetry of the radiation pattern with respect to the conical axis is degraded. To obtain a more symmetrical pattern, one must use a multiple-arm configuration, for example, a quadrifilar configuration This work numerically investigates the characteristics of a quadrifilar conical helix antenna (Q-CoHA), and then realises a Q-CoHA radiating a CP wave over the hemispherical region above the antenna while main- taining a broadband input-impedance response. Special attention is paid to the effects of the cone angle and the [5, 61. zyxwvutsr 0 IEE, 1997 ZEE Proceedings online no. 19970977 Paper first received 2nd September and in revised form 5th November 1996 The authors are zyxwvutsrqp with the College of Engineering, Hosei University, Koganei, Tokyo 184, Japan pitch angle on the 3-dB axial ratio angle coverage (3- dB ARAC). Note that when the cone angle of a Q- CoHA zyxwvu (200 in Fig. 1) is zyxw O", the antenna corresponds to a conventional quadrifilar helix wound in a uniform cylinder [5, 61. Also note that the present Q-CoHA is different from helices characterised by a standing-wave current distribution [l-31. zyxw 2 X j 2Lf j 1 ....... .*,! 0 Y ........ - a.* ... zyx i X 3rd Fig. 1 antenna Configuration and co-ordinate system of quadrifilar conical helix zy 2 Antenna configuration and numerical technique Fig. 1 shows the configuration and co-ordinate system of a Q-CoHA. The configuration parameters are desig- nated as follows: 20, is the cone angle, 2Lf is the feed wire length, and z is the pitch angle. The antenna arm, whose wire radius is p, is defined as Y = roexp(aq5w) where yo (= LjsinO,) is the generating line length, a (= sin 0, tan z) is the winding constant, and #w is the winding angle. The arm length L from the feed point 0 to the arm end Lend (open end) is given by L = "du2 + sin2 do[exp(u$,) - 11 + ~f To obtain an axial beam, the four arms are excited with a phase relationship of (OO, -go", 180", 90'). The U 53 IEE Proc.-Microw. Antennas Propag., Vol. 144, No. 1. February 1997