A miniature controlled-impedance transmission line consists of a flexible cable having side-by-side conductors transmitting high frequency signals. The cable is preferably in the form of a pair of conductors, each surrounded by respective inner and outer dielectric layers of different compositions. The inner and outer dielectric layers are applied to each conductor independently of the other conductor, after which the respective outer dielectric layers of the two conductors are bonded together in side-by-side relationship without altering the inner dielectric layers. The result is a conductor pair having minimum cross-section for high-density applications and uniform capacitance which is also stable in that it will not change with subsequent bending or handling. Preferably, the conductors, with their inner and outer dielectrics, are helically twisted together prior to bonding so that the bonding forms a permanently twisted pair having not only uniform and stable capacitance but also uniform and stable lay length with resultant uniform electrical delay characteristics of both conductors.