Sensorless Coupling Estimation via the Input Reflection Coefficient of a Chireix Combiner for Adaptive Wearable Wireless Power Transfer

Adaptive wireless power transfer (WPT) systems for wearable and biomedical applications require continuous knowledge of the magnetic coupling coefficient to maintain stable power delivery under motion-induced variations. Conventional approaches rely on auxiliary sensors or dedicated RF measurement stages, which increase system complexity and limit integration in compact platforms. This paper introduces a sensorless coupling estimation method based on the input reflection phase of a Chireix outphasing combiner operating in in-phase mode (φ=0°). The reflection phase ∠Γin exhibits a numerically monotonic dependence on the coupling coefficient over the wearable-relevant range k∈[0.05,0.70], arising from a structural decoupling between coupling-dependent conductance and fixed susceptive compensation in the combiner input admittance. The method enables coupling estimation using only intrinsic RF observables at the transmitter port, without interrupting power transfer or requiring additional sensing hardware, via a two-step protocol combining sub-millisecond sensing with coupling-aware outphasing control. Validation combines analytical modeling, numerical evaluation, Monte Carlo analysis under component tolerances, and circuit-level S-parameter simulation at 13.56 MHz, demonstrating robust monotonic behavior under component variations, phase noise, and resonant detuning. The framework reinterprets the Chireix combiner as a dual-function structure enabling simultaneous power combining and coupling observability, providing a low-overhead solution for adaptive wearable WPT systems.