Input impedance of annular ring microstrip antenna using circuit theory approach

Input impedance and bandwidth of an annular ring microstrip antenna have been determined by modeling the antenna as a section of radial line loaded with wall admittances. The effect of mutual coupling between the radiating apertures has been taken into account. The theoretically calculated values of input impedance for TM12-mode are compared with measurements. The agreement is good. Higher order modes have been found to be present in the vicinity of TM12-mode. The reactance due to these modes can be utilized to increase the voltage standing-wave ratio (VSWR) bandwidth of the antenna.