MoM ANALYSIS OF A CIRCULARLY POLARIZED CONICAL BEAM SPHERICAL SLOT ARRAY ANTENNA Chuwong PHONGCHAROENPANICH ! , Monai KRAIRIKSH ! , and Jun-ichi TAKADA " ! Faculty of Engineering and Research Center for Communications and Information Technology, King Mongkut’s Institute of Technology Ladkrabang, Bangkok 10520 Thailand Phone: (662) 3269967 Ext.3342 Fax: (662) 3269086 E-mail: kpchuwon@kmitl.ac.th " International Cooperation Center for Science and Technology, Tokyo Institute of Technology, Tokyo, 152-8550, Japan, Email: takada@icc.titech.ac.jp 1. Introduction A circularly polarized conical beam spherical slot array antenna is proposed for the application of the low bit rate or low gain over temperature ratio (G/T) land mobile satellite communication subscriber and broadcasting service in addition to the wireless Local Area Network base station [1], [2]. The advantage of this antenna type is that the structure is simple and suitable for mass production i.e., a ring of perpendicular slot pairs cut on an outer surface of the concentric conducting spherical cavity enclosed by the conducting conical surface. The feeding structure is also simple, a linear electric probe excited at the center of the inner surface of the cavity to generate axially symmetrical mode, and it is integrated with the power divider. From the preliminary results of external radiation characteristic investigations [2], the radiation pattern appears to be the conical beam i.e., the elevation pattern possesses the null in broadside direction and the azimuthal pattern is omnidirectional. Additionally, the elevational beam direction toward the geostationary satellite is relatively low which is suitable for applying to the land mobile subscriber unit located far from the equator. However, to transfer the power from a transmitter to the antenna or the antenna to a receiver efficiently, impedance characteristics of the antenna should be estimated in order that appropriate matching is accomplished. Therefore, phenomena of the field inside the cavity in conjunction with outside the cavity must be taken into account. Method of moments plays an important role in solving unknown current of integral equations. The system of integral equations is formulated by enforcing the boundary condition that the tangential magnetic fields inside and outside the cavity are continuous through the slot aperture and the source model is considered at bottom of the feed probe. Using the aids of the dyadic Green's function in conjunction with the method of moments, the impedance and radiation characteristics are realized. Experimental results verified the theoretical ones. 2. Antenna Structure The structure of a circularly polarized conical beam spherical slot array antenna consists of a number of perpendicular slot pairs cut on an outer surface of a concentric conducting spherical cavity enclosed by the conducting conical surface as shown in Fig.1. The slots in a pair are excited with orthogonal phases to provide the circularly polarized radiation. These slots are arranged as a ring along an azimuthal circumference of the spherical surface at the positions where the adjacent pairs are in phase to form a conical beam. Figure 1(a) shows the perspective view of the circularly polarized conical beam spherical slot array antenna. The slot length and width are l s and ε s , respectively. Each slot in a pair is separated, along an elevation plane d θ , at the distance so that the phase quadrature is obtained [3]. The azimuthal spacing between each slot pair is denoted by s φ . One of the slots in a pair is oriented at 45° counterclockwise with the horizontal line of the spherical surface whereas the perpendicular counterpart is oriented at an angle of 135°. The cross-section view of the antenna is shown in Fig.1 (b). The inner and outer radii of the concentric conducting spherical cavity are R a and R b , respectively, and this cavity is enclosed by the conducting conical surface at an angle θ c . The excitation probe is located at the center of the inner surface of the cavity (R a ≤ R ≤ R b , θ=0°,φ=0°). The center of each slot pair is located at an angle θ s . Each slot in a pair is offset from the position in opposite direction so that phase quadrature between these slots is obtained. R a R b θ θ θ c θ θ θ s z Slot Pair Probe z x y γ γ γ γ2 1 φ φ φ β β β ij θ θ θ α α α 1 1 1 α α α 2 2 2 R o Rs2 Rs1 z' x' y' s φ d θ R a R b θ θ θ c z M -M J (a) (b) (c) Fig 1 A circularly polarized conical beam spherical slot array antenna (a) perspective view (b) cross-section view (c) Equivalent model PROCEEDINGS OF ISAP2000, FUKUOKA, JAPAN