A finite-size superstrate based on identical patch arrays for broadband resonant cavity antennas

This paper presents the characteristics of broadband radiation of a resonant cavity antenna (RCA) over the X-band (8∼12 GHz). The study focuses on an effective antenna aperture in obtaining the flat ripple directivity with broad bandwidth. The investigation is performed on a finite-size metamaterial superstrate. The RCA structure is composed of a two-layer metamaterial superstrate and an aperture-coupled patch antenna source. The metamaterial configuration is formed by stacking two identical patch arrays with a small air-gap separation of 0.133λ at 10 GHz. Its property enables a transmission resonance and this leads to ability to enhance the primary field excitation. In the RCA synthesis, the radiation of the broadband RCAs is studied for the superstrate with different array sizes of the unit cell. The simulation shows a decrease of the broadside radiation at the middle of the frequency bandwidth. In the study, the effective antenna aperture with a flat ripple is obtained for the cell array size of 5x5. This corresponds to 1.25λ by 1.25λ for a broadband radiation bandwidth and high directivity with the array period of 0.25λ. The radiation results are simulated and obtained in the observation of the non-uniform surface current distribution on the patch array. By using an electric line source as a primary radiator, the electromagnetic (EM) field distribution is illustrated to rely on the nature of the primary antenna source which provides a spherical wave-front radiation.