Losses in dielectrically loaded coaxial cables

Loss reduction in coaxial cables brought about by dielectric linings has been experimentally observed, but various theoretical considerations fail to explain this feature. To clarify this situation, the paper gives consideration to the general conditions for obtaining lower losses by means of a thin concentric dielectric shell placed anywhere between the two conductors. The analytical criteria obtained agree well with numerical results derived directly from the exact Maxwellian field problem, but are much easier to apply and give a more unified insight into the significance of the various parameters concerned. The analytical method is particularly useful in the range of special interest, namely for thin linings. For electrically thick linings, the exact results may be understood from coupled-mode considerations. While not explaining the experimental observations, the present analytical theory admits to reduced attenuation in other, more favourable circumstances discussed in the paper. This is in accordance with exact numerical computations, but restricts the use of quasistatic transmission-line theories to frequencies lower than previously assumed.