A cavity model is used to analyse an aperture-fed nearly square circularly
polarised patch antenna. The form of the aperture is that of a symmetric cr
oss-slot that couples the excitation between a single microstrip feed line
and the patch antenna. Using equivalent magnetic current sources at the slo
ts. the modal electric and magnetic fields under the patch are obtained, an
d hence analytical expressions for the patch admittances at the aperture ar
e derived and used to obtain an equivalent circuit of the circular polarise
d antenna. Good agreement is obtained between the circuit modelling and pra
ctical results.