  We describe a simple method to experimentally determine the frequency
dependencies of the per-unit-length resistance and conductance of transmission
lines. The experiment is intended as a supplement to the classic measurement of
the transient response of a transmission line to a voltage step or pulse. In
the transient experiment, an ideal (lossless) model of the transmission line is
used to determine the characteristic impedance and signal propagation speed. In
our experiment, the insertion losses of various coaxial cables are measured as
a function of frequency from 1 to 2000 MHz. A full distributed circuit model of
the transmission line that includes both conductor and dielectric losses is
needed to fit the frequency dependence of the measured insertion losses. Our
model assumes physically-sensible frequency dependencies for the
per-unit-length resistance and conductance that are determined by the geometry
of the coaxial transmission lines used in the measurements.
