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The computing time for a patch antenna with infinite ground plane using image theory is much shorter than that for a finite ground plane. As the latter is the actual practical structure, it would be of advantage to find a CAD formula to account for the impedance perturbation from an infinite ground plane to a small finite ground plane. This paper synthesises such a formula from the ground plane asymptotes aided by the GTD (geometrical theory of diffraction). The GTD has been used to study antenna patterns and backlobes. The paper completes the picture by considering radiation impedance.