Improving antenna arrays characteristics such as power consumption, cost and complexity has gained wide attention in the context of cognitive radio networks. One antenna array configuration that offers great potential is the electronically-steerable parasitic array radiator (ESPAR) antenna. In this paper, we propose a transmitter patch ESPAR antenna system for underlay spectrum sharing in cognitive radio networks. We transmit symbols over switchable weakly-correlated beampatterns of the antenna system so as to maximize the transmitted power to a secondary user receiver, while constraining the interference to the primary users. Our results show the superior power performance of the proposed patch ESPAR antenna system over the dipole ESPAR antennas previously used in the underlay spectrum sharing paradigm. Furthermore, we calculate the bandwidth over which the switchable patterns of the patch ESPAR antenna remains weakly correlated, and it is found to be 131.2 MHz at a center frequency of 4.902 GHz.