Modified cavity model to determine resonant frequency of tunable microstrip ring antenna

In order to compute the resonant frequency of an air-gap tuned annular-ring microstrip antenna without the accuracy being affected from the air gap width, the cavity model is modified by using a new dynamic permittivity and inner/outer radii extension expressions. A close agreement is obtained between the theoretical and the experimental resonant frequency results for fundamental and higher order modes and for different gap widths.