Variable ZC transmission line and its application to a tunable impedance transformer

This paper presents a new type of a transmission line with the variable characteristic impedance which is electronically tuned by RF switches or varactors. Furthermore, a tunable impedance transformer, which can be used to enhance the impedance matching between an antenna and RF front-end, using this variable Z_C transmission line is fabricated and measured. By means of inserting a third metal between signal and ground metal, we can adjust the equivalent capacitance or inductance per unit length, which makes the novel transmission line have different characteristic impedances. The impedance transformer can be electronically varied from 10 to 65 Ω in a 50 Ω environment at the center frequency of 900 MHz.