Waveguide as a Communication Medium

The circular electric wave in round metallic tubing has an attenuation coefficient which decreases as the frequency of operation is increased. A corollary to this behavior is the fact that any preselected attenuation coefficient can in theory be obtained in any predetermined diameter of pipe through the choice of a suitably high carrier frequency. The attenuation which is characteristic of microwave radio repeater links, about 2 db/mile, is in theory attainable in a copper pipe of about 2″ diameter using a carrier frequency near 50,000 mc. Scale-model transmission experiments, conducted at 9,000 mc, showed average transmission losses about 50 per cent above the theoretical value. These extra losses were due to (1) roughness of the copper surface and (2) transfer of power from the low-loss mode to other modes which can also propagate in the pipe. The latter effect may have serious consequences on signal fidelity because power will transfer (at successive waveguide imperfections) from the signal mode to unused modes and, after a time delay, back to the signal mode. This effect has been studied experimentally and theoretically, and it is concluded that (1) either mode filters must be inserted periodically to absorb the power in the unused modes of propagation, or (2) the medium itself must be modified to continuously provide large attenuations for the unused modes of propagation. The latter approach is attractive in that it also provides a solution to the problem of bending this form of low-loss guide.