Interpreting S-Parameter Spectra in Coupled Resonant Systems: The Role of Probing Configurations

The S-parameter $S_{21}$ is widely used to characterize the resonant properties of various systems. However, we demonstrates that the reliability of $S_{21}$ as a true indicator of system resonances depends heavily on the specific probing setup employed. While point-probe or weak probes preserve the integrity of the $S_{21}$ spectrum and accurately reflect system resonances, multi-point and strong probes can introduce significant discrepancies. These discrepancies can manifest as misleading features such as repulsive level anti-crossing or attractive level crossing, even in systems that are fundamentally uncoupled. This highlights the critical importance of selecting appropriate probing techniques to ensure a precise evaluation of system resonances.