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The design and enhanced bandwidth performance of a monopole antenna with a simple sleeve structure for indoor base station application is presented. Besides a top-loading circular patch shorted to the ground plane through four shorting probes for a low profile, the circular sleeve structure to effectively improve the matching condition of the upper operating frequency edge can be used for bandwidth enhancement. An enhanced impedance bandwidth of about 112% ranging from 750 to 2660 MHz (VSWR ≤ 2) is achieved, and a monopole-like conical radiation pattern is also produced. With this low profile and the widened bandwidth characteristic, it is very suitable for indoor base station applications such as GSM900, DCS1800, PCS 1900, UMTS, IMT2000, Wibro, WiMAX2350, and WLAN (IEEE802.11b).