Square slot antennas with modified edges for broadband circular polarization are presented. Slots with only stubs or notches are studied and it is found that the axial ratio (AR) bandwidth is quite sensitive to the perturbations of the stubs and notches. To further enhance the AR bandwidth, slot antennas with combination of stubs and notches are proposed and wide 3-dB AR bandwidth of 15.5% (2.45-2.86 GHz) is obtained. By placing a conducting reflector at the rear of the slot, another modified square slot antenna is designed for practical applications, which achieves a 3-dB AR bandwidth of 6.5% (2.38-2.54 GHz) and peak gain of 8.7 dBic. Agreement between simulated and measured results is satisfactory.
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