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A wideband quasi-Yagi antenna utilising a microstrip-to-coplanar stripline balun is presented. The antenna is designed to be compact, directional and wideband so that it can be used to form a circular array for a microwave imaging system. The feeding structure of the antenna includes an ultra-wideband balun that is formed using a pair of microstrip to slotline transitions. The simulated and measured results of the proposed antenna indicate more than 70% fractional bandwidth with more than 10 dB return loss, more than 18 dB front-to-back ratio and around 4 dBi gain.