  A Fabry-Perot like interferometer with two microwaveguide adaptors as
reflectors creates a passive dielectric medium with a negative group delay time
due to polarization shift. A rotational strain of the polarization vector by
one of the adaptors is coupled with a drastic negative group velocity. The
adapted rectangular and circular waveguides have the same dispersion. The input
rectangular waveguide mode is linearly polarized, whereas the basic mode of the
adapted circular waveguide is circularly polarized. A 667 wavelengths long
circular waveguide connects the input with the output adaptor. Experiments are
performed in the frequency and in the time domain. We describe, how the helical
polarization change and the spin-flip of the two different circular wave modes
produce the observed negative group velocity.
