New developments in multiconductor slow-wave transmission lines of circular cross-section

Equispaced parallel wires located around the circumference of a circle are used to form a cylindrical grid structure operating as a waveguide at 3 GHz. It is shown that a number of distinctive slow-wave modes can propagate along such a guide and among them one finds the Goubau wave, a variety of TEM configurations and the dipole mode. Launching filters enable each mode in turn to be identified and preserved for individual measurements. At the same time, a theoretical investigation is made applying superposition of separate circularly symmetrical surface waves assumed to be associated with each conductor as a single-wire transmission line. Calculations from the theory show good agreement with experiment. It is found that dipole-mode propagation along an 8-wire guide of the kind considered here, has an attenuation 40% below that of the corresponding 2-wire line and provides for a close approach to uniform energy-density over the cross-section of the structure. Moreover, the field outside the surface of the guide is much smaller than in the case of the 8-wire Goubau line.