A new design of Fabry-Perot cavity antenna (FPCA) has been presented with a unique as well as the most desirable feature of very high-gain maintained over its wide matching bandwidth. About 15.5 dBi average broadside-gain has been achieved over the 19% matching bandwidth. A flat-gain characteristic has been attained using a specially designed conical sidewall. In addition to that, the sidewall will play an important role in reducing the mutual coupling arises due to nearby RF elements. About -16 dB and -19 dB side lobe levels have been obtained satisfactorily in two principle-plane patterns (E- and H-plane) respectively. The cross-polarization purity of the antenna is maintained reasonably below -25 dB over the frequency band. © 2018 IEEE.