A transmission structure having high propagation velocity and a low effective dielectric loss. The structure comprises a dielectric, a first reference conductor disposed below the dielectric, a signal conductor disposed above the dielectric, and a second reference conductor disposed over the signal conductor. The second reference conductor has a recess portion facing the signal conductor, the recess portion defining a gap between the second reference conductor and the signal conductor. The gap may be filled with air which has a relative dielectric constant approximately equal to one (1). Because of the physical and dielectric constant characteristics of the gap, the structure concentrates an electric field in the gap resulting in an effective dielectric constant approximately (approaching) one (1) and an effective dielectric loss approximately equal to zero (0). Thus, the structure exhibits a propagation velocity approximately equal to the speed of light.