Disk microstrip antennas are treated as cavities, from which both internal and external (radiation) field can be determined, based on an efficient improved theory. This theory can predict the impedance characteristics very accurately even for cases where the dominant mode is not strongly excited. The theory is extended to two-port disk microstrip antennas. In particular, if one port is shorted the input resonant impedance at another port on the circumference can be varied over orders of magnitude by simply changing the relative angular positions between the two ports. In case of a single port a similar variation of input impedance can be achieved by moving the feed in the radial direction. All of these provide a means for impedance matching with practically no effect on the pattern. So far as the input impedance matching is concerned the antenna can be represented accurately by a parallel resonant circuit in series with an inductance. The former derives from the resonant mode term and the latter from all non-resonant terms of the field solution in expansion. Experiments have been conducted to verify the theoretical results. Excellent agreement is observed in all cases. (Author)