A Tunable Transmission Line With Controllable Phase Shifting and Characteristic Impedance

Tunable transmission lines have a high demand in the 5G wireless communication applications. In this brief, a novel transmission line is presented to control both electrical length and characteristic impedance individually, creating flexible functions. For the first function, the proposed transmission line can perform as a tunable phase shifter with 90° phase tuning range and a low insertion loss variation. For the second function, the proposed transmission line can tune the characteristic impedance from 23 <inline-formula> <tex-math notation="LaTeX">${\Omega }$ </tex-math></inline-formula> to 145 <inline-formula> <tex-math notation="LaTeX">${\Omega }$ </tex-math></inline-formula> with a fixed phase delay. The <italic>ABCD</italic>-matrix method is applied to derive the design equations. For the practical implementation, the varactor equivalent parasitic resistances are considered, and the even-odd mode is applied to re-optimize the proposed two tuning functions. To verify the design theory, an experimental prototype is simulated, fabricated, and measured. Both simulation and measurement results match well with the design theory.