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The circular electric mode in round metallic tubing becomes increasingly more attractive than the dominant mode from the standpoint of minimzing the waveguide size at frequencies above about 10,000 mc for the loss criterion of 0.25 db/100 feet. The circular electric (TE01) mode also makes available a theoretical heat loss of 2 db/mile in waveguides less than 6 inches in diameter at frequencies higher than about 5,500 mc. Increased transmission bandwidth, reduced delay distortion, and reduced waveguide size are factors favoring use of the highest practical frequency of operation. An increased number of freely propagating modes and smaller mechanical tolerances are the associated penalties. Experimental work has been carried out in the 9,000-mc region using the TE01 mode in a pipe about 5 inches in diameter. Transmission of 0.1-Â¿sec pulses has been observed over a distance of 40 miles. Mode conversion and surface roughness of the tubing walls result in observed losses which average about 50 per cent higher than the theoretical values for geometrically perfect, smooth-walled tubing. There is included a brief discussion of several problems unique to transmission in a multimode medium, including pure mode generation, mode filtering, the bend problem, and the effects of mode conversion on transmission loss and signal fidelity.