Input impedance, resonant frequency, bandwidth and radiation characteristics of microstrip circular patch antennas coaxially shorted by a cylindrical post are investigated. The analytical results for the dominant and higher modes are compared with experimental data. The radiation conductance, the fringing field and the effect of feed position, i.e. a coaxial probe, are included by the mode expansion and matching methods. The dielectric effect of the substrate is also taken into account for the computation of the radiation characteristics. The results show that the electrical performance parameters of a centrally shorted circular microstrip antenna are sufficiently adjustable by a modification of the radius of the shorted post. The characteristics indicate that the proposed radiation element is a useful antenna with higher gain, wider bandwidth and improved impedance behaviour.