In this paper, a single-layer single-element plate antenna with an &OHgr;-shaped slot is presented for broadening impedance bandwidth and suppressing cross-polarization levels. The plate is basically suspended above a ground plane and fed by a coaxial probe. A thin &OHgr;-shaped slot is cut at the middle of the plate. The investigations on impedance and radiation characteristics are experimentally conducted. The measured results show that the achieved impedance bandwidths for VSWR=2:1 reach up to the order of 20% and the improved radiation patterns are stable across the bandwidths. The studies have demonstrated three important points. The first is that the spacing of ~0.1 operating wavelength between the plate and the ground plane is essential to broaden the impedance bandwidth. The second is that the slot effectively compensates for the large inductance arising from the long feeding probe. The last is that the proper selections of the slot shape and the aspect ratio of the rectangular plate are important to depress the high cross-polarization levels. ¿ 2001 American Geophysical Union