A simple rectangular microstrip antenna with triangular slotted ground plane has been studied both theoretically and experimentally to improve shortcomings like low gain (5 - 6 dBi), narrow bandwidth (3% - 4%), and poor copolarization (CP) to cross-polarization (XP) isolation, i.e. polarization purity (typically 10 - 12 dB), of conventional rectangular microstrip patch antennas. By placing two pairs of triangular shaped slots on the ground plane just below the nonradiating edges of the patch, high gain (around 9 dBi) and more than 22 dB polarization purity over a wide elevation angle has been achieved. The proposed antenna covers almost the full X band of frequency from 9.55 GHz to 11.43 GHz while resonating at 10.35GHz (i.e. impedance bandwidth of 17%), so the proposed antenna offers improved gain, impedance bandwidth, and polarization purity imultaneously as compared to conventional rectangular microstrip antennas. Parametric studies have been documented to achieve the optimum defect dimension. The specialty of the proposed antenna is that the gain and radiation pattern is quite stable in the entire operating frequency band, along with its attractive gain, impedance bandwidth, and polarization performance. The simulated and measured results show close resemblance to each other. The proposed geometry is quite simple and easy to fabricate and therefore may be ideal for applications where high gain, stable radiation characteristics, and wide impedance bandwidth along with high copolarization to cross-polarization isolation over wide elevation angles are required.