Inductively Coupled Non-Invasive Broadband Impedance Measurement Using Optimized Current Probes

The frequency-dependent impedance of electrical connectors, switchgear loops, and related contact structures can indicate degradation at electrical contact interfaces. Conventional contact measurements with a vector network analyzer require dedicated adapters and impedance matching, and the object under test usually has to be removed from the operating system. To simplify field measurements and avoid direct electrical contact, this study employs an established two-probe inductive-coupling/ABCD de-embedding framework in conjunction with a purpose-designed broadband current probe and calibration fixture. Commercial injection and receiving probes were first evaluated to identify practical limitations associated with probe resonance, magnetic-path air gaps, cable clamping, and probe-to-probe coupling. A parametric CST study on ferrite material, winding turns, and core geometry was then used to select a 3W800 NiZn ferrite core with six winding turns and dimensions of 5 mm inner diameter, 10 mm outer diameter, and 5 mm height. The prototype exhibited relatively flat S12/S21 responses from 1 to 400 MHz, with S21 changing from approximately −13.5 dB at 1 MHz to −17.5 dB at 400 MHz and without a pronounced resonance. Quantitative impedance accuracy was validated only over 1–100 MHz: validation against an impedance analyzer using a 108 nH inductor and an 82 pF capacitor yielded mean relative magnitude errors of 4.00% and 4.5%, respectively. Field measurements on a 10 kV switchgear circuit were additionally conducted over 1–30 MHz to demonstrate non-invasive broadband impedance acquisition under practical contact conditions. Accordingly, the 1–400 MHz range in this work refers to probe-transfer characterization rather than experimentally validated impedance accuracy over the full band.