Abstract: A length of strip transmission line uses two symmetrically spaced center conductors between two groundplanes. These conductive strips produce an even-mode electric field between the two groundplanes when excited in-phase and an odd-mode electric field when excited in anti-phase relationship. For the latter case, the phase velocity of the odd-mode is significantly affected by the electric field in the gap region between the conducting strips. By varying the relative dielectric constant of a material located in the gap region, e.g., by means of a voltage-controllable dielectric such as barium-titanate compositions, the phase velocity and, hence, the phase shift of an RF signal propagating through the strip transmission medium can be controlled.