Measurement of differential-mode and common-mode scattering parameters of symmetric coupled-line discontinuity structure

The coupled-line thru-reflection-line (TRL) calibration technique is developed to measure the differential-mode (DM) and common-mode (CM) scattering parameters of the symmetric coupled-line discontinuity structure. Under DM and CM excitation conditions, a four-port symmetric coupled-line network can be treated as equivalent DM and CM two-port half-circuits. The developed coupled-line TRL calibrators can also be equivalent to DM and CM half-calibrators, and then applied to measure the DM and CM scattering parameters of the symmetric coupled-line discontinuity structure. To validate the effectiveness of the developed measurement method, the DM and CM characteristics of a coupled-line guided-wave structure, the periodically non-uniform coupled microstrip-line structure, are measured over the frequency range of 1-8 GHz. The measured results are shown in reasonable agreement with the simulated results. It demonstrates that the developed coupled-line TRL technique is an effective approach to characterise the symmetric coupled-line discontinuity structure.