Various types of feed structures for microstrip antennas are compared. Broad bandwidth is obtained using aperture coupling with impedance matching. One of advantages in aperture coupling is polarization purity. A modified aperture coupling annular-ring antenna fed by microstrip is presented in this paper. This antenna structure can be considered as the combination of aperture feeding and proximity feeding an annular-ring antenna. It has both advantages in the polarization purity and in reduction of back radiation. From the structure based on the proximity feed annular-ring antenna, an aperture layer is inserted and modified as well as an aperture size to enhance bandwidth.
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