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This study describes a microstrip-fed planar monopole antenna incorporating a resonant structure in the form of a ladder-shape, which is etched on its back-plane. The radiation element of the antenna comprises a simple rectangular-shaped patch. The feature that makes the antenna different from a traditional monopole antenna is the resonant structure's configuration. The resonant structure has an effect of transforming the monopole antenna into an ultra-wideband (UWB) antenna. The prototype antenna's performance was verified through simulations and measurements. The antenna's dimensions are 22 × 22 mm2 and its measured impedance bandwidth (S11 <; -10 dB) stretches across approximately more than 17 GHz, between 2.7 and 20 GHz. The proposed antenna<;s performance extends significantly beyond the UWB frequency band as defined by the Federal Communications Commission, and its omnidirectional E- and H-plane radiation patterns are stable across its bandwidth. The results presented show that the proposed technique can be applied directly to the design of planar monopole antennas. The antenna exhibits UWB behaviour with near omnidirectional characteristics and radiation efficiency >80%. Additional features that make this antenna attractive and viable for modern UWB system applications are its simple configuration, low profile, compactness and low fabrication cost.