Locally Strong-Coupled Microwave Resonator Using PEMC Boundary for Distant Sensing Applications

This research study reports on a new coupling structure to demonstrate a constant coupling strength for longer distances in planar microwave sensors. It is shown that there is a positive correlation between the gap size in a pair of gap-coupled transmission lines (GCTLs) and the effective distance of reader and sensing resonators in a planar microwave sensor. On the other hand, the coupling strength in a pair of GCTLs is negatively correlated to the gap size between the two lines, where the larger gap sizes result in weaker coupling strength or vice versa. A technique is introduced to make the coupling strength partially independent of the gap size in GCTLs. The technique enables a sensor designer to increase the gap size, and subsequently, the distance between the reader and sensing resonators of a sensor. Strong electromagnetic coupling is locally demonstrated between a pair of GCTLs using a new artificial perfect electromagnetic conductor (PEMC) boundary, which does not allow electromagnetic power reflect toward the source over larger gap sizes in GCTLs. The concept is experimentally verified with a gap size up to 10 mm between GCTLs with a constantly measured insertion loss of 0.9 dB. Moreover, the structure is employed to design distant microwave resonator for humidity sensing applications. The design has demonstrated the ability of sensing, while the sensing resonator is placed at 35-mm distance from the readout circuitry. The structure is reported to increase the sensing distance with a ratio of about 12 (36:3) compared to the conventional microwave sensors.