Robust non-contact microwave crack detection via deterministic stand-off resonance modeling

Non-contact microwave inspection using open-ended waveguides is strongly affected by probe–surface stand-off variations, which distort the reflection response and can obscure defect signatures. This work demonstrates that the probe–surface separation can be modeled through a virtual short-circuit interpretation that produces a predictable ladder of stand-off resonances governed primarily by the probe–surface separation. Experiments were conducted in the Ka-band (26.5–40 GHz) using an open-ended WR-28 waveguide inspecting F114 steel samples containing a surface-breaking crack over stand-off distances from 4 to 45 mm. A time-domain gating workflow was introduced to isolate the dominant round-trip response and suppress competing spectral features arising under specific stand-off conditions. The TD-gated analysis removes resonance ambiguity and yields stable resonance frequency estimates typically within ± 3 % of the stand-off resonance model across most stand-off distances. The resulting crack-induced resonance features remain clearly detectable, with signal-to-noise ratios reaching up to 60 dB. These results demonstrate that stand-off resonance features can be reliably detected and uniquely assigned across a wide stand-off range, enabling robust non-contact microwave inspection without requiring tightly controlled probe–surface separation.