Why are coaxial lines more efficient at microwave frequencies?

Coaxial lines are more efficient at microwave frequencies primarily because the electromagnetic fields are completely confined within the conductors. This characteristic minimizes the loss of signal energy and reduces the potential for interference from external sources. The design of coaxial cables, which comprises an inner conductor surrounded by a dielectric and an outer conductor, ensures that the fields remain tightly bound within the structure, leading to lower radiation losses compared to other types of transmission lines. This confinement improves the performance of coaxial cables at higher frequencies, thus making them suitable for microwave applications.

While other aspects like weight, resistance, and installation ease may have their importance in different contexts, the key feature that makes coaxial lines particularly efficient at microwave frequencies is the complete confinement of the electromagnetic fields by the conductors.