An electromagnetic bandgap enhanced active antenna design for microwave-based motion sensing

The design of an active antenna operating at a frequency of 10.5 GHz for microwave-based motion sensing applications is described. The active antenna consists of a feedback-type oscillator circuit where the resonator has been replaced by a microstrip patch antenna. The influence of a uni planar compact photonic bandgap (UC-PBG) structure that is placed around the antenna in order to suppress surface waves on the 0.5 mm FR4 substrate is explored. Its purpose is twofold: the suppression of surface waves can reduce a resonator loss path and thus improve its quality factor resulting in lower phase noise and it can improve the radiation pattern by minimizing backward radiation and surface wave interference. In a first step the UC-PBG structure is adapted for the 0.5 mm FR4 substrate by calculation of the dispersion diagram and experimental verification of the bandgap. Then microstrip patch antennas are designed and characterized. A higher quality factor for the patch antennas with UC-PBG structure can be observed. Moreover, reduced radiation at grazing angles along the UC-PBG is obtained. Finally, two active antenna circuits are built and compared resulting in phase noise reduction of the UC-PBG enhanced active antenna device from -49 dBc/Hz (/wo UC-PBG) to -60 dBc/Hz (/w UC-PBG) at an offset of 10 kHz. We expect the observed effect to be more pronounced for thicker substrates with a higher dielectric constant due to more efficient surface wave excitation.