Microwave dielectric-loaded rectangular waveguide resonator for depth evaluation of shallow flaws in metals

This paper presents the design and performances of a partially filled dielectric-loaded waveguide resonator for sensing application, in particular, for evaluating very small flaws on a metal surface. The resonator consists of a rectangular waveguide loaded with a transversally and longitudinally finite dielectric insert that concentrates the electromagnetic field within it and effectively creates a quarter-wavelength resonator. The quarter-wavelength resonator is short-circuited at one end, whereas the other end is formed by tapering the dielectric insert. It is shown that the coupling between the resonator and the feeding waveguide can be widely varied by changing the geometrical and dielectric properties of the insert. Simulations are performed to demonstrate the characteristics of this resonator using a standard K-band (18-26.5 GHz) rectangular waveguide. To demonstrate the potential practical usefulness of such a resonator as a sensor, small perturbations, such as shallow pits or notches (representing damage or flaws), in the surface of the shorting plate of the resonator were used to evaluate the change in the resonator characteristics. These changes can be used for sensing and sizing such surface flaws in nondestructive testing applications (i.e., inspection of aircraft fuselage panels). Measurements are conducted to verify the potential of this sensor for shallow-pit-depth evaluation.