Low frequency waves on transmission lines of composite section

Three types of transmission lines are analyzed, and it is found that the principal wave is not transverse electromagnetic in transmission lines for which the medium between the conducting boundaries is other than a homogeneous isotropic dielectric. Calculations of characteristic impedance and velocity of propagation on the basis of electrostatic and magnetostatic field distributions are accurate in the low-frequency limit. The cut-off frequency for the first higher order mode lies between those that would be obtained if the transmission lines were entirely filled with one dielectric material or the other.