  A Parity Time-reversal Dual (PTD) symmetric structure constituted by a
Perfectly-Electric-Perfectly magnetic (PEC-PMC) parallel plate waveguide (PPW)
is analyzed. This waveguide supports unimodal transverse electromagnetic (TEM)
edge mode propagation protected against back-scattering from a certain class of
deformations and defects. The TEM solution is found in analytical form by using
three different methods, namely conformal mapping, mode-matching, and
Fourier-transform methods. It is shown through numerical simulations that the
mode propagation is robust with respect to deformations such as 90{\deg} bends
and discontinuity such as transition to free-space. Implementation of the PMC
boundary conditions via both a bed of nails and a mushroom structure is also
successfully investigated.
