Self-oscillating evanescent microwave probes for nondestructive evaluations of materials

The design and operation of a self-oscillating evanescent microwave probe (SO-EMP) for very high spatial resolution imaging of material nonuniformities are discussed. Composed of a microstripline resonator in a feedback loop across a 10-dB amplifier with 2.5-GHz bandwidth centered at 1.75 GHz, these oscillator probes are very compact and suitable for high-resolution imaging of materials. A wire tip connected to one end of the resonator enables the microwave probe to interact with a sample located nearby which affects the resonant frequency (f₀) and the quality factor (Q) of the oscillator. Variations in the material properties can be detected by scanning the wire tip over the sample while monitoring f₀ and Q that automatically track the permittivity, permeability, and dissipation in the sample. The SO-EMP outputs versus position of its tip over different high- and low-contrast samples were monitored and pseudo-color maps were generated to image material nonuniformities. Due to the amplifier nonlinearity that comes into play in the oscillation characteristics of this probe and the resonant contribution of the probe-sample interaction, the SO-EMP sensitivity and spatial resolution are improved compared to the other modes of operations we have reported in the past.