On the modal resonant properties of microstrip antennas

The chosen rectangular and fractal microstrip patch antennas above an infinite ground plane are analyzed by the Theory of characteristic modes. The resonant frequencies and radiation Q are evaluated. An method by G. Vandenbosch for rigorous evaluation of the radiation Q is employed for modal currents on a Rao-Wilton-Glisson (RWG) mesh. It is found that the resonant frequency of a rectangular patch antenna with a dominant mode presents quite complicated behaviour including having a minimum at a specific height. Similarly, as predicted from the simple wire model, the radiation Q exhibits a minimum too. It is observed that the presence of out-of-phase currents flowing along the patch antenna leads to a significant increase of the Q-factor.