On the remarkable accuracy of the quasi-static TEM approximation for distributed network models

When a two-conductor waveguide contains a homogeneous dielectric, its low-frequency mode is formed by multiplying the static transverse electromagnetic (TEM) fields by the phase factor e(j(omegat-kz)), with the wave number given by k(omega) = rootepsilonmu, and the voltage-to-current ratio given by the characteristic impedance rootL/C. When the dielectric is nonhomogeneous, however the low-frequency mode cannot be TEM. Yet the measured behavior of this mode has been quite accurately modeled using the "quasi-TEM assumption," that the mode follows the distributed network model with wave number and characteristic impedance taking their static values. Herein we explain the remarkable accuracy and ubiquity of the TEM approximation.