  We demonstrate theoretically and experimentally that a periodic three-way
microstrip coupled waveguide exhibits a stationary inflection point (SIP). The
SIP is a third order exceptional point of degeneracy (EPD) where three
eigenmodes of the guiding system coalesce to form a stationary frozen mode with
zero group velocity. Here the frozen mode is shown in a reciprocal waveguide;
therefore three coupled waveguides (called three-way waveguide) are required,
supporting three modes in each direction. We illustrate the occurrence of the
frozen mode regime by observing the dispersion diagram and by using the concept
of coalescence parameter that is a measure of the separation of the three
coalescing eigenvectors (polarization states). Results based on full-wave
simulations and measurements demonstrate the coalescence of three modes and how
this coalescence is not perfect due to the presence of losses in the microstrip
structure at 2 GHz. The coalescence parameter is used to determine how close
the three-way system is to the ideal frozen mode condition.
