@gsuberlandGraham Sutherland / Polynomial I once visited the really interesting but sadly now late Lonsdale twins at their company, Sensor Technology, the company makes transducers for measuring twisting torque on shafts (submarine engine shafts, helicopter winches etc), they work by using a rotary transformer to couple power and signals back from a VNA that's actually spinning on the shaft as it measures S11 on a SAW resonator bonded to the shaft that is subject to deformation as the shaft twists - changing its s-parameters. IIRC the VNA sought a particular value and varied its oscillator until it was obtained and it was that oscillator signal that was coupled back as the measurement through the transformer (transformer was providing power in the other direction). Also it was two SAWs and two VNAs as a bridge to cancel drift - one compresses while the other expands or similar.