Far field pattern for rectangular PIFA antenna from the cavity model

A conventional patch antenna (PA) with its rectangular radiating sheet shorted to a ground plane along the zero potential line forms a rectangular planar inverted-F antenna (PIFA). The PIFA therefore can be considered as an effective compact antenna system with a patch area of less than one-half of usual square patch antenna. It is modelled as a resonant cavity with magnetic walls between the radiating plate and the ground plane. The far field expressions are derived using potential theory with the assumption of magnetic currents flowing around the cavity boundaries. An improved model for the far fields is described which accounts for the PIFA side slots. These affect both the impedance and the pattern. The model agrees well with experiments and gives insight into the behaviour of the impedance and pattern of the general PIFA.