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In this letter, an ultrawideband (UWB) microstrip-fed planar monopole antenna with stable radiation pattern is presented. Although compact and simple structure of the planar antennas made them a good candidate in UWB applications, due to unwanted current distribution on the ground section, they suffer from transient distortion and radiation pattern rotation in different frequencies. To alleviate this effect, a metallic staircase plane is placed in the bottom of the substrate. In addition, a mushroom-like electromagnetic band-gap (EBG) structure is embedded on the top layer of the substrate to increase the gain in higher frequencies. Measurement results show 2.2-6 dB gain in broadside direction in 3-10-GHz frequency. A new definition of pattern stability factor (PSFII) is proposed, which is proper for both directional and omnidirectional radiation patterns. Calculated PSFII for the proposed antenna is higher than 95%, which shows a stable radiation pattern. Furthermore, it shows a low distortion manner where the fidelity factor in the main beam direction is more than 95%. In comparison to similar antennas presented in literature, PSFII and gain have been improved noticeably.