The present invention is directed to a novel foamed polymer material for use as a dielectric in electric transmission lines. The foam dielectric of the present invention is obtained from extrusion of a polymer alloy and a supercritical fluid, especially carbon dioxide. Specifically, the foam polymer dielectric is made by: (1) feeding a polymer alloy into an extruder and heating the polymer alloy, (2) feeding a supercritical fluid into the extruder, (3) mixing the polymer alloy and the supercritical fluid, (4) exiting the polymer alloy and the supercritical fluid from the extruder, and (5) passing the polymer alloy and the supercritical fluid through a crosshead. The foam dielectric of the present invention may be used in coaxial transmission lines. In this embodiment, an inner conductor is surrounded by the foam dielectric, which is further surrounded by a second conductor. Stripline and microstripline transmission lines may also be manufactured using the disclosed foam dielectric.