An Effective Method to Extract Properties of Slow-Wave Structures for TWT Applications

An effective numerical method for accurate extraction of the properties of slow-wave structures (SWSs) with the full-wave simulator is proposed in this article. With the short-open-load (SOL) calibration technique, the ABCD matrix of the core SWS can be extracted by calibrating out the effect of error boxes at its two terminals under approximate excitation in full-wave modeling. Then, the per-unit-length properties including the complex transmission coefficient and the complex characteristic impedance can be calculated with the ABCD matrix of the core SWS. The coupling effect among unit cells is reduced by introducing more periods of core SWS in modeling. This method allows us to determine all four per-unit-length parameters of the SWS with good accuracy. After a detailed description is executed on its calculation process, two verification structures (microstrip meander line and planar helix with straight edge connections) are studied to verify the effectiveness of the proposed method. The calculated propagation constant turns out to be consistent with those from the eigenvalue method. Furthermore, the calculated characteristic impedance of the SWS provides a powerful ability in the design of an SWS circuit terminated by other circuits or loads. Both structures are fabricated and their measured S-parameters are confirmed with good impedance matching properties as predicted.