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A new microstrip antenna using a slot perturbing method is proposed for switchable circular polarisation. The perturbing structure consists of a centred circular slot with an inner conductor and an outer slot in the square patch radiator. By switching the perturbation geometry from controlling the states of two pin diodes, right-hand circular polarisation and left-hand circular polarisation are alternated without a resonant frequency shifting. Simulated and measured results show that the proposed antenna has remarkable circular polarisation performance with symmetrical radiation characteristics at the resonant frequency.