Input impedance of a circular microstrip antenna: evolution as function of the thickness

In an earlier study of microstrip antennas excited by a probe-fed, an interesting physical phenomenon had been observed: the presence of a fixed point through which input impedance curves pass, when the thickness was the only parameter varied. We propose an analytical model giving the characteristics of this fixed point and the input impedance of a circular microstrip antenna. This model valid for both electrically thin and thick substrate; is well suited for computer aided-design (C.A.D). The computation compared with experimental data showing good agreement.