The paper presents the performances of microstrip patch antennas excited by the aperture-coupling feed that is composed of a T-shaped microstrip feed line and an annular-ring slot. Because the annular-ring slot is designed at a full-wavelength resonant mode, a broad impedance bandwidth can be obtained by combing the resonant modes of the coupling slot and radiating patch; moreover, a low cross polarization is also observed, especially around the direction with maximum gain. For reducing the considerable back radiation induced by the resonant coupling aperture, additional slots are introduced and embedded along the perimeter of the annular-ring slot. Experimental results show that the reformed coupling slot can improve the front-to-back ratio of the aperture-coupled microstrip antenna by more than 5 dB. Simulation analyses are also performed to support the measured results.
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