880 IEEE TRANSACTIONS ON ANTENNAS AND PROPAGATION, VOL. 52, NO. 3, MARCH 2004 Multilayer Effects on Cavity-Backed Slot Antennas Marinos N. Vouvakis, Student Member, IEEE, Constantine A. Balanis, Life Fellow, IEEE, Craig R. Birtcher, and Anastasis C. Polycarpou, Member, IEEE Abstract—A hybrid technique of the finite-element method and the method of moments (FEM/ MoM) is used to analyze the effects of multilayered superstrates on cavity-backed slot (CBS) antennas. The spectral-domain approach is used to compute the MoM ad- mittance matrix. An asymptotic extraction technique is applied to improve the computational efficiency of the convolution integrals. Effects on the input impedance, efficiency, gain, and radiation pat- terns are presented for different antennas. The numerical results are validated with measurements and published data. Index Terms—Cavity backed, hybrid method, multilayer, slot antenna, superstrates. I. INTRODUCTION S UPERSTRATE (cover) dielectric or magnetic layers are often used to protect low-profile conformal antennas, such as cavity-backed antennas, from environmental hazards. In other cases, undesirable covers are naturally formed; e.g., ice layers during flights or severe weather conditions. Whether the cover layers are naturally formed or imposed by design to protect the antennas, they may affect the antenna characteris- tics, such as input impedance, efficiency, gain, and radiation patterns. Apart from that, the cover layers provide an additional degree of freedom for the antenna designer to optimize the overall antenna performance. Moreover, studies have shown that multilayered covers can be used to enhance the gain and directivity of patch and printed dipole antennas [1], [2]. For these reasons, it is important to analyze the superstrate effects on perfect electronic conductor (PEC ) mounted cavity–backed antennas. The fundamental superstrate effects on printed antennas were first discussed in [3]–[5]. In these papers, the superstrate effects on current distribution, input impedance, gain, and efficiency were examined in detail for electric current excitations. On the other hand, in cavity-backed slots (CBSs), the excitation is an equivalent magnetic current. The superstrate effects on CBS an- tennas have been discussed in the literature by Lee [6] and Biebl [7]. Lee first used the method of moments (MoM) in the spectral domain to analyze narrow slots residing on homogeneous rect- angular cavities with a dielectric overlay. An extension of that Manuscript received July 5, 2002; revised May 23, 2003. This work was sup- ported by the ONR under Contract N00014-00-1-0222. M. N. Vouvakis was with the Department of Electrical Engineering, Arizona State University, Tempe, AZ 85287-7206 USA. He is now with the Electro- Science Laboratory, The Ohio State University, Columbus, OH 43210 USA. C. A. Balanis, and C. R. Birtcher are with the Department of Electrical En- gineering, Arizona State University, Tempe, AZ 85287-5706 USA (e-mail: bal- anis@asu.edu). A. C. Polycarpou was with the Department of Electrical Engineering, Telecommunications Research Center, Arizona State University, Tempe, AZ 85287-5706 USA. He is now with the Department of Computer Engineering, Intercollege, 1700 Nicosia, Cyprus. Digital Object Identifier 10.1109/TAP.2004.824672 study was the work by Biebl which generalized the method to analyze CBS antennas with multilayer superstrates. Although these methods were rigorous, they were only able to analyze narrow slots residing in canonical-shaped cavities. However, even in Biebl’s study, the formulation was incomplete, since far-field characteristics, such as patterns, gain, and efficiency, were not tractable. Moreover, this method was never applied and tested in multilayer CBS antennas. In this paper, the hybrid finite-element method and method of moments (FEM/MoM) presented in [8], [9] is used instead. The method is able to analyze arbitrarily shaped cavities and aper- tures. Moreover, it is extended to handle multilayer superstrates of dielectric and/or magnetic materials. In the cavity region, the vector FEM is used to compute the electric field, while the spec- tral-domain MoM is used to compute the field in the exterior lay- ered half-space. The method is capable of predicting both near- and far-field characteristics of CBS antennas. An improvement on the fill-in time of the spectral-domain MoM admittance ma- trix is achieved using an asymptotic extraction technique [10], [11] that eventually results in a hybrid spectral/spatial approach for the MoM part. The multilayer effects on two different CBS antennas are presented. Input impedance, efficiency, gain, and far-field amplitude patterns are considered. The results are com- pared with measurements as well as with other existing data. The accuracy and the computational efficiency of the method is demonstrated. II. THEORY A hybrid FEM/MoM approach is used to analyze the radia- tion from cavity-backed antennas. The antenna consists of an arbitrarily shaped cavity that is mounted on an infinite ground plane with multilayered superstrates. Numerically, the problem is separated into two independent domains through the use of the equivalence principle. The FEM with tetrahedral “edge” el- ements is used to solve for the electric-field vector inside the cavity. The exterior multilayered half-space region is efficiently handled by the MoM with linear basis functions of triangular support. Since the hybridization process of the FEM and MoM was included in [9], [12], it will not be repeated here. This paper will focus on the analysis of the exterior multilayered half-space. The MoM admittance matrix of the hybrid FEM/MoM approach is given by (1) where primed and unprimed variables refer to the source and observation points, respectively. is the dyadic Green’s function that gives the magnetic field at the observation 0018-926X/04$20.00 © 2004 IEEE