Low-loss circular electric mode transmission over long distances imposes special requirements on waveguide structure. In addition to close dimensional tolerances, small random deviations in straightness approximating the 2 or 3 ft beat wavelength must be prevented to avoid unwanted mode conversions. This made it necessary to improve methods for manufacturing solid-walled copper and copper-lined steel waveguide tubing. Helix waveguide with improved mechanical and electrical transmission properties has also been developed. This is made experimentally by winding a coil of fine wire on a mandrel, applying a lossy jacket over it, encapsulating the assembly in a steel tube with a thermosetting resin and finally extracting the mandrel. Techniques for joining sections into long test runs involve accurately threaded butt joints with vacuum and pressure-tight couplings. Facilities for testing such waveguide systems have been installed at the Holmdel Laboratory.