A novel aperture-coupled, asymmetrical C-shaped slot, square microstrip antenna is proposed for circular polarisation (CP). A narrow and asymmetrical C-shaped slot, microstrip antenna is fed at the centre using an aperture coupling to obtain a CP operation. The compactness of the antenna is easily obtained by inserting a C-shaped slot. Wide CP radiation is achieved simply by making the C-shaped slot asymmetrical. With this antenna, the measured 3dB AR bandwidth is around 3.3 and the 10dB return loss bandwidth achieved is 16.0. The overall antenna size is 0.48λo×0. 48λo×0.092λo at 2.4GHz. The proposed slot microstrip patch technology is useful to design compact, broadband, circularly polarised antennas and arrays.