A novel design concept of enhancing the impedance bandwidth of a single-layer shorted microstrip patch antenna (MPA) is proposed under simultaneous radiation of the one- and three-quarter wavelength resonant modes (TM _0,1/2 and TM _0,3/2 modes). Initially, a conventional MPA with the shorting wall is studied to demonstrate that none of the redundant modes can be excited between the dual-resonant modes. Then, a pair of shorting pins are appropriately installed underneath the patch to progressively turn up the resonant frequency of TM _0,1/2 mode without significant influence on that of TM _0,3/2 mode. After that, a V-shaped slot is etched out on the patch to dramatically reduce the resonant frequency of TM _0,3/2 mode with little effect on that of TM _0,1/2 mode. With the use of these arrangements, the dual-desired radiative resonant modes are gradually moved in proximity to each other, thus widening the impedance bandwidth under emergence of dual resonant poles. In order to validate the principle and design method, the proposed antenna is designed, fabricated, and measured. The measured results demonstrate that the impedance bandwidth of the proposed shorted patch antenna has been extended to about 11.8%, and it is about 1.82 times of 6.5% of the traditional shorted MPA. Meanwhile, the fabricated antenna exhibits other attractive performances including stable radiation pattern, same polarization, and a low-profile property with the height of 0.042 free-space wavelength.