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In this study, two types of reconfigurable microstrip square ring antennas are proposed. The basic structure of the proposed antennas is an open-loop-type square ring resonator. To increase antenna gain, a square ring slot with narrow width is embedded into the ground plane. The proposed antenna is fed by a T-shaped microstrip through a gap coupling. Depending on the location of the gap in the square ring, the open-loop square ring antenna exhibits different resonant frequencies or polarisations. The state of the gaps in the ring can be adjusted by controlling the bias voltage applied to the Positive-Intrinsic-Negative (PIN) diodes. By altering the connectivity of the vertical and horizontal gaps using the two PIN diodes, the open-loop-type square ring resonator can be used as a frequency switchable antenna. Using a similar method, a polarisation diversity antenna can be realised by independently adjusting the two PIN diodes inserted into diagonally placed gaps. In this latter case, the two polarisations are orthogonal to each other at the same frequency.