A single-fed compact circularly polarized quarter-mode substrate integrated waveguide (QMSIW) antenna with improved bandwidth is designed based on a single-layer structure. The antenna consists of four QMSIW elements. Instead of using a complex power divider to excite each element, an inset microstrip line is employed to excite the driven QMSIW element, while the parasitic QMSIW elements are excited by means of gap and direct couplings. Moreover, the phase and magnitude of the electromagnetic fields in each QMSIW element are controlled by gaps and short sections of microstrip line to obtain a circularly polarized radiation. A prototype of the proposed antenna is fabricated and measured, and the measured results are in good agreement with the simulated ones. The measured 10-dB impedance bandwidth is 6.23%, from 5.13 GHz to 5.46 GHz, and the 3-dB axial-ratio bandwidth is 3.62%, from 5.15 GHz to 5.34 GHz. The measured maximum gain of the antenna is 6.46 dBic.
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