EFFICIENT ANALYSIS OF DICHROIC PLATES FOR LARGE REFLECTOR ANTENNAS Maurizio Bozzi and Luca Perregrini Department of Electronics, University of Pavia – Via Ferrata 1, Pavia, Italy E–mail: bozzi@ele.unipv.it, perregrini@ele.unipv.it ABSTRACT This paper presents a novel method for the analysis of dichroic plates, used for combining signals at different frequencies in the beam–waveguide feeding system of large reflector antennas for deep–space applications. This method applies to the analysis of either single or multiple metal screens perforated periodically with holes. The holes perforating the metal screen may have an arbitrary cross–sections and, possibly, include steps. This method, named the MoM/BI–RME method, is based on the solution of an integral equation by the Method of Moments (MoM) using entire–domain basis functions, efficiently obtained numerically by the Boundary Integral–Resonant Mode Expansion (BI–RME) method. The MoM/BI–RME method was used for the analysis and design of many dichroic plates for radio– astronomy. Some of these examples are reported in the paper. INTRODUCTION Large reflector antennas for radio–astronomy applications usually operate in more than one frequency band (typically the S, X, and Ka bands). The feeding system of such antennas frequently consists of a beam waveguide, where the combination of signals at different frequencies can be performed by dichroic plates [1]. In the basic configuration, dichroic plates (also called FSSs, Frequency Selective Surfaces) consist of a thick metal screen perforated periodically with apertures (Fig. 1a) [2]. The geometry of the apertures and the thickness of the metal screen determine the frequency bands where the dichroic plate is transparent and the ones where it is a perfect mirror. More complicated configurations are sometimes used, in order to achieve better performance: multi–grid structures (Fig. 1b) and metal plates perforated periodically with stepped–holes (Fig. 1c) have been proposed [3]. Recently, we developed an efficient method for the analysis of dichroic plates [4]. This method, named the MoM/BI– RME method, is based on the infinite array approximation and permits the analysis of dichroic plates with arbitrarily shaped apertures. The peculiarity of this method consists in the use of the Boundary Integral–Resonant Mode Expansion (BI–RME) method for the numerical calculation of entire–domain basis functions, needed in the analysis by the Method of Moments (MoM). The MoM/BI–RME method was implemented in fast and flexible computed codes. One of them applies to the analysis of single–grid dichroic plates, perforated with arbitrary apertures and illuminated by a uniform plane wave incident at an arbitrary angle [4]. This code permits the wideband analysis of dichroic plates in few seconds on a standard PC. This code was used for the design of a S–/X–band dichroic mirror to be operated in the deep space antenna of the European Space Agency (ESA) in Perth, Australia [2]. This antenna is required for future deep space missions by ESA, such as Rosetta and Mars Express. Another code was implemented for the analysis of multi–grid dichroic plates [5]. By using the segmentation technique and the infinite array approximation, the structure reduces to a number of step discontinuities between a metallic waveguide and a waveguide with periodic boundary conditions, where the field is expressed as a combination of Floquet modes. Each discontinuity is analyzed by the MoM/BI–RME method. Finally, a code was implemented for the analysis of metal plates perforated with stepped holes [6]. In this case, the structure reduces to the cascade of two types of discontinuities: the discontinuity between two metallic waveguides with different cross–sections and the one between a metallic waveguide and a waveguide with periodic boundary conditions. The basic theory of the MoM/BI–RME method is presented in this paper, along with its application to the analysis of single– and multi–grid dichroic plates, and metal plates perforated with stepped apertures. Some application examples are also addressed, both in the microwave and in the mm–wave range. metal hole incident plane wave transmitted plane wave reflected plane wave metal holes incident plane wave transmitted plane wave reflected plane wave air spacing metal stepped hole incident plane wave transmitted plane wave reflected plane wave (a) (b) (c) Fig. 1 – Configurations of dichroic plates: (a) single–grid; (b) double–grid; (c) plate perforated with stepped holes