The problem of an aperture-coupled patch antenna is studied using the spectral domain technique. The general case of a slot inclined to both the feedline and the patch is considered. The equivalent circuit for the structure is obtained and the effects of various parameters, such as slot dimensions and inclination on the resonant frequency and on the input impedance, are studied. Comparison between theory and measurement shows good agreement. The analysis could be useful in designing antennas for dual frequency operation or for circular polarisation where inclined slots could be used.