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This paper presents some dominant wave transmission characteristics of multimode round waveguide lines in the 4-kmc range of frequencies. The use of such waveguide lines offers the advantages of lower transmission losses than obtainable with single-mode rectangular waveguide, and relative ease of making good joints. Possible mode conversion effects, including dominant mode elliptical polarization, have been examined and found to be innocuous. As a result, cross-polarized dominant waves can be used to provide two reasonably independent signalling channels at the same frequency in one pipe. The experimental results obtained with a straight line 2.812-inch inside diameter and length of 150 feet are given.