A study of the complete mode spectrum of the dielectric-tube-loaded coaxial cable reveals that only the low-loss slow-wave TM01mode has no lower cutoff frequency. Mode designations are shown to be consistent with those previously adopted for dielectric waveguides. A numerical optimisation procedure is applied to an accurate perturbation analysis of the dominant mode to determine optimum dielectric-tube radial dimensions for minimum loss. Attenuation reductions as high as 75% of the unloaded values are attainable in certain overmoded cases. For the optimum conditions, the cable is well equalised, and the propagation characteristics are relatively insensitive to small variations in the dimensions of the tube.