A new formula has derived for calculation of coupling impedance of accelerator components in experiments with the wire method. The formula requires the measurement of both transmission coefficient S21and the reflection coefficient S11, which is usually not zero. For most cases where the coupling impedance of the device is small and matching is good in experiments, the new formula would have minor difference from the formula derived by Vaccaro.
Wang, J. G., & Zhang, S. Y. (2001). Measurement of coupling impedance of accelerator devices with the wire-method. Nuclear Instruments and Methods in Physics Research, Section A: Accelerators, Spectrometers, Detectors and Associated Equipment, 459(3), 381–389. https://doi.org/10.1016/S0168-9002(00)01024-X