# Radiation from a Cavity-Backed Circular Aperture Array Antenna Enclosed by an FSS Radome
## 1 Introduction  
A radome, a portmanteau of radar and dome, is a structural and weatherproof enclosure, and thus it is used to protect microwave antennas. The radome usually comprises dielectric layers that minimally affect electromagnetic signals to be transmitted or received by the antenna. Due to the effects of the radome on the electromagnetic signals, a thorough analysis of the radome is needed. To understand electromagnetic properties of the radomes, there have been extensive studies on various radome configurations [1; 2; 3; 4; 5; 6; 7; 8; 9; 10]. The radiation from a circular aperture surrounded by a hemisphere radome was predicted based on the dyadic Green's function technique and physical optics (PO) method [2]. The Von [PERSON] radome, one of the renowned radome structures, has been analyzed using the method of moments (MoM) [3], the coupled surface integral equation [4], and the aperture integration-surface integration (AI-SI) [5]. Furthermore, the radiation characteristic of hemisphere, tangent-ogive, and cone-shaped radomes with multiple sources has been investigated based on the multilevel fast multipole algorithm (MLFMA) [6]. The iterative physical optics-boundary integral-finite element method (IPO-BI-FEM) was used to analyze the sandwich tangent-ogive radome [7]. These studies focused on the radome consisting of dielectrics; however, the dielectric radome is generally responsible for the increase in radar cross section (RCS) of aircraft due to its broadband transmission characteristic. Instead, a frequency selective surface (FSS) radome, whichis a curved FSS within a multi-layer radome, is employed to reduce RCS over a wide frequency range because it exhibits bandpass or bandstop characteristics. Previous studies have investigated electromagnetic characteristics of single- and multi-layer FSS radomes based on the pole residue matching (PRM) [8] and the ray tracing technique [9; 10]. The FSS are one kind of the metasurfaces showing a bandpass or a bandstop characteristic. These characteristics are not found in natural materials. Metasurfaces, metamaterials, graphene, and plasmonics are gaining increasing popularity among researchers due to their ability to adjust permittivity or permeability purposely and manipulate electromagnetic waves passing through the materials. Due to numerous potential applications, a variety of studies have been conducted for sensing technology, plasmonics, graphene, and photonics [11; 12; 13; 14; 15; 16]. There have also been studies on analyzing the properties of metamaterials and metasurfaces [17; 18]. However, the investigation of electromagnetic properties for the complex structures which consist of a radome with the metasurfaces such as FSS and an aperture array antenna has not been presented. This is because it is hard to use the full-wave methods to analyze them, for a given memory usage and time, when an object is electrically large in size and complicated. Therefore, hybrid techniques combining the full-wave methods (such as MoM, FEM, finite-difference time-domain (FDTD), and the mode matching method) and the asymptotic methods (such as the ray tracing technique and geometric optics (GO)) are required, but related studies on radiation properties from array antennas enclosed by an FSS radome seem to be lacking. Therefore, it is of great significance to investigate electromagnetic properties of the aperture array antennas with the FSS radome using a hybrid technique combining the mode matching method for modeling aperture fields and the ray tracing technique for analyzing the radome. In this paper, we analyze the radiation from a cavity-backed circular aperture array antenna enclosed by an FSS radome using the hybrid analysis method combining the mode matching method, the ray tracing technique, and Huygens's principle. Towards this purpose, in particular, three different ways are carried out step by step. Firstly, electromagnetic characteristics of the cavity-backed circular aperture array antenna is predicted based on the mode matching method [19].