See through metallic sensor implemented using proximity resonant coupling.

Metal detection plays an important role in applications ranging from security to industrial quality control. This work presents a novel ground-less circular patch resonator which behaves as an open stub, engineered for high-sensitivity detection of metallic objects. The key innovation lies in utilizing electric field coupling between the stub's top surface and nearby metallic targets. The detection is achieved by monitoring changes in the transmission coefficient. The proposed design offers two key advancements by extending detection range up to 8 cm and selective sensitivity exclusively to metallic materials by remaining unresponsive to dielectric objects. Experimental results show that as the distance between the metallic object and the sensor decreases, the resonance coupling becomes stronger, demonstrated by a resonance frequency shift and dip of approximately -10 dB in the transmission coefficient. The sensor is also capable of "see-through" metal detection behind dielectric barriers, and its selective sensitivity to metallic materials helps minimize false responses from non-metallic objects. This approach allows for the detection of cracks in metallic objects and applications involving ground penetration radar (GPR) systems.