We present a simple model for the input impedance of a rectangular microstrip patch antenna. This model is well suited for computer aided design (CAD). It is based on classical methods: (a) the cavity model determining the frequency and the input resistance at resonance, (b) the dynamic permittivity of a rectangular microstrip patch antenna (to take into account the influence of the fringing field at the edges of the rectangular patch antenna) and (c) the resonant parallelRLCcircuit with an inductive reactance. This model is valid for electrically thick substrates. The theoretical results are in good agreement with experimental data.