Microstrip circuits, which are widely used in microwave systems such as satellite communication systems and RFID tag systems, commonly adopt patch antennas to radiate power into free-space. However, the patch antennas are known to have very poor radiation efficiencies due to a tight coupling to the metal ground plane. In this paper, we propose the use of an electromagnetic bandgap (EBG) structure to construct a more efficient antenna for microstrip circuits. The EBG structures are generally characterized as a high-impedance plane that prevents the surface-wave propagations. However, we have discovered that by designing a proper antenna feed and selecting an optimum number of cells, an EBG structure could radiate power more efficiently. When the center cells of the EBG structure is excited with a differential feed, a strong E-field is developed at the gaps of all the EBG cells at the resonant frequency. The feed location and the number of unit cells are adjusted for the optimum antenna design. The measured results indicate 7.3% bandwidth at 6.8GHz with 10 dB-mismatch loss when the 12 EBG cells were used. This bandwidth is roughly six times larger compared to the bandwidth of an equivalent patch antenna.