an impedance matching circuit includes a conductor line having an input port and an output port , a ground conductor , a tunable dielectric material positioned between a first section of the conductor line and the ground conductor , a non - tunable dielectric material positioned between a second section of the conductor line and the ground conductor , and means for applying a dc voltage between the conductor line and the ground conductor . the impedance matching circuit may alternatively include a first planar ground conductor , a second planar ground conductor , a strip conductor having an input port and an output port , and positioned between the first and second planar ground conductors to define first and second gaps , the first gap being positioned between the strip conductor and the first planar ground conductor and the second gap being positioned between the strip conductor and the second planar ground conductor . a non - tunable dielectric material supports the first and second planar ground conductors and the strip conductor in the same plane . a connection is provided for applying a dc voltage between the strip conductor and the first and second planar ground conductors . a plurality of tunable dielectric layer sections are positioned between the strip conductor and the first and second planar ground conductors so as to bridge the gaps between the said first and second planar ground conductors and the strip conductor at a plurality of locations , leaving non - bridged sections in between , defining a plurality of alternating bridged and non - bridged co - planar waveguide sections .