Development of Separated Turnstile Antenna for Space Applications Elham Sharfir Moghaddaml and Shervin Amirf 1 lIran Telecom Research Center(ITRC) End of North Karegar Street, PO Box 14155-3961, Tehran, Iran Tel:+98 21 88630360; Fax: +98 21 88027762; E-mail: esharifi@itrc.ac.ir 2Iranian Research Organization for Science and Technology (IROST) No.71, Forsat Street, Tehran, Iran Tel:lFax: +98 21 66281015; E-mail: amiri@irost.org Abstract Dual-band operation and circular polarization are the main characteristics of the separated turnstile (ST) antenna that make it suitable for space applications. In this paper, we describe the design and optimization of a separated turnstile antenna to obtain a saddle-shaped pattern in the UHF band and an omnidirectional pattern in the VHF band, with simultaneous impedance matching in both bands. The antenna model and the effects of the body of the satellite and the solar panels were analyzed using the Moment Method. The effects of the physical parameters of the antenna on the radiation pattern, such as the length of the monopoles and their mounting angle, were investigated. A radio-frequency distribution unit - which included a diplexer, phase shifters, and a power divider/combiner for both bands - was also designed, analyzed, and measured. The test results agreed with the analysis results. The axial ratio and polarization efficiency of this antenna were calculated for incident waves with different polarizations. The antenna was fabricated and its radiation parameters were measured. The test results were compared with the analysis results to confirm the correctness of the design. Keywords: Satellite communication; satellite antennas; turnstile antenna; separated turnstile antenna; circular polarization 1. Introduction T he separated turnstile antenna has many applications in small low-Earth-orbit (LEO) satellites. Small LEO satellites usually operate in the UHF and VHF bands, and provide services such as data and short-message communications in store-and-forward mode. The turnstile, separated turnstile, helical, quadrifilar, and conical spiral antennas are appropriate antennas that are used in small LEO satellites, work in the UHF/VHF bands, and have cir- cular polarization [ 1]. The separated turnstile (ST) antenna consists of four monopoles that are driven in phase quadrature, and that are mounted symmetrically on the bottom plate of the satellite. Sim- plicity, low cost, and deployment after launch of the satellite are reasons for using this antenna in satellite applications. A tangential separated turnstile antenna was analyzed and designed in [2]. This antenna was a shorted quadrifilar helix that had circular polarization and the proper pattern for a satellite appli- cation. For the first time in [3], a separated turnstile antenna on a large cylindrical satellite was analyzed in the VHF band using computer programs. Four sizes of large cylindrical satellite were modeled during this study, and the effects of the inclination of the monopole on the polar radiation patterns in the constant- (P plane was investigated in this paper. In [4], a crossed-dipole turnstile with single or double bays was designed to operate as a circularly polarized antenna for LEO satellites. This antenna operated in the 84 IS SSN 1045-9243/2008/$25 @2008 IEEE UHF band, and had an omnidirectional pattern. An integral-equa- tion method, employing Galerkin's technique, was applied to ana- lyze the antenna. The effects of changing the height of the crossed dipole above the square plate on the radiation pattern and on the other antenna characteristics were investigated in the article. The frequency response of the antenna's input impedance and the VSWR with respect to a 50-ohm feed were presented with the antenna in free space and above a square plate. In [51, a new single- fed crossed-V dipole antenna for circular polarization was designed and measured. The input impedance of the V dipoles and the cir- cular-polarization condition were derived in the paper. A separated turnstile antenna on a cylindrical satellite was analyzed in [6]. hIn that article, the effects of the antenna's geomet- rical parameters on the radiation pattern were studied. In [7], a separated turnstile was designed as a dual-band antenna for a LEO satellite. The input impedance of the antenna was calculated by use of the Moment Method, and the VSWR as a function of frequency was calculated. In [8], a tangential separated turnstile antenna was designed in the UHF band by means of a genetic algorithm. The definition of a suitable cost function for obtaining an arbitrary pat- tern was one of the main features of the article. In the present work, a separated turnstile antenna that can work in the UHF and VHF bands was designed, analyzed, and manufactured. This separated turnstile antenna was intended to have a saddle-shaped pattern in the UTHF band for data and mes- IEEE Antennas and Propagation Magazine, Vol. 50, No. 4, August 2008