  Analytical solutions are presented for the electromagnetic radiation by an
arbitrary pulsed source into a homogeneous time-varying background medium. In
the constant-impedance case an explicit radiation formula is obtained for the
synchronous permittivity and permeability described by any positive function of
time. As might be expected, such a medium introduces significant spectral
shifts and spatio-temporal modulation, which are analized here for the linear
and exponential time-variations of the medium parameters. In the
varying-impedance case the solution is obtained for the fourth-order polynomial
time-dependence of the permittivity. In addition to the spectral shifts and
modulation this spatially homogeneous medium scatters the field introducing
causal echoes at the receiver location.
