Transmission in Multimode Waveguide with Random Imperfections

Long distance waveguide transmission via the TE 0 i mode in circular waveguide is an attractive goal because the theoretical TE 01 heat loss decreases monotonically as the operating frequency increases. As is well known, operating frequencies far above the TE 0 i cutoff frequency are required to realize sufficiently low heat loss and delay distortion, so that the guide must operate far into the multimode region. Thus, considering a typical case, a 2-inch I.D. perfect copper circular guide operating at 55 kmc will have a theoretical TEoi heat loss of 1.54 d b / mile; but this guide will propagate 223 additional modes, which we call spurious modes, at this frequency. 1 The TEoi transmission loss will approach the theoretical TE 0 i heat loss in a copper waveguide only if the waveguide is a geometrically perfect right-circular cylinder over its entire length. Any departure from this ideal geometry will couple the TE 0 i mode to some of the spurious modes 2 to 7 the net effect of this coupling will be to increase the TEoi transmission loss above the theoretical heat, loss, and to cause the TEoi transmission loss to vary with frequency. 1,8 Two types of geometric imperfections are of interest: (a) Intentional deformations introduced in the guide for various reasons, e.g., to go around corners,3,6'7'9 to taper from a small guide to a larger one,1" etc. Mode conversion effects control the design of such devices, but we will not discuss them further. (b) Random geometric imperfections arising during the manufacturing or the laying of the guide; these imperfect ions will increase the TEoi loss and cause it to vary randomly with frequency.