How to estimate the coupling Q factor from two-port 5-parameters

This paper presents a sequential procedure to estimate the coupling quality factor (kQ) for general reciprocal two-port systems. Starting from the well-known impedance matrix, we focus on its non-diagonal component as a transfer function. It is normalized by the equivalent scalar resistance (ESR). The ESR is regarded as an extension of one-port effective series resistance to two-port schemes. This normalization finally leads to a versatile formula for kQ. The sequence also extends to the reactance compensation by two-port simultaneous resonance, and the optimum load resistance that maximizes power transfer efficiency.