An improved design of microstrip patch antennas using photonic bandgap structure

Microstrip antennas inherently have a high input impedance at resonant frequency. This may not be convenient for some applications. For active integrated antennas, moreover, the radiated power of microstrip antennas needs to be very low at harmonic frequencies. The main goals of this study are impedance matching and harmonic suppression of microstrip antennas. In order to meet these requirements, two one-dimensional (1-D) photonic bandgap (PBG) structures, namely, the defected ground structure (DGS) and the compact microstrip resonant cell (CMRC), are applied to the feed line of microstrip antennas. The characteristic impedance of the microstrip line is controlled by the additional effective inductance of the PBG structure. Without any matching circuits, microstrip antennas can be easily fed by a simple 50 Ω microstrip line with a PBG structure at the operating frequency. Additionally, the second harmonic of the proposed antennas is properly suppressed compared to a conventional antenna. Measured results indicate that the two PBG structures are quite effective for harmonic suppression. Therefore, the proposed antennas are suitable for active integrated antennas.