For monopole antenna mounted on a finite ground plane, the direction of its peak-radiation has an angle elevated off the horizontal plane. From the analysis based on spherical vector wavefunctions (SVWF), it is shown that the elevated angle arises from higher order transverse magnetic $(TM)$ modes, which are mainly excited by the ground plane. By inserting resonance structures, i.e., slots, in the ground plane, a dipole-like ($TM_{{01}}$ mode dominated) radiation pattern can be achieved with a maximum radiation along the ground plane. Accordingly, a new monopole-based Yagi-Uda antenna is proposed with the main beam along the ground plane. It has an improved gain compared with the traditional one without inserting resonant structures on the ground. A sector of a circular Yagi-Uda array operates at 3 GHz is then simulated and measured for the verification of the method.