A new approach to analyze the behavior of a high-gain antenna covered with a frequency selective surface (FSS) superstrate is presented. Using an image theory and effective constitutive parameter retrieval, properties of impedance and a refractive index of the entire cavity structure are investigated. Through the analysis, we show that our antenna inherently operates in the medium whose maximum index of refraction is lower than ‘0.5'. Furthermore, we also demonstrate that the high-gain feature of the Fabry-Perot cavity antenna is not only due to satisfy a conventional cavity resonance condition, but also for a material of an effectively low index of refraction.
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