Resonant frequencies of rectangular microstrip antennas with flush and spaced dielectric superstrates

This paper presents a predictive model for the resonant frequencies of rectangular microstrip antennas with flush and spaced superstrates. This closed-form model is suitable for CAD and is directly applicable for the integration of microstrip antennas beneath plastic covers or protective dielectric superstrates in portable wireless equipment. The model utilizes conformal mapping and the concept of equivalent capacitance to determine the effective permittivity of a covered microstrip structure. A comparison between calculated and measured resonant frequencies demonstrates that the model provides less than 1% errors for structures with low relative permittivities (ε_r < 3) and slightly higher errors for structures that contain higher permittivity materials. Generally, the model predicts the resonant frequency of a spaced superstrate structure as accurately as possible within the tolerance range of the structure's electrical and physical parameters.