  We demonstrate the existence of a special degeneracy condition, called
degenerate band edge (DBE), between two Bloch modes in periodically loaded
circular all-metallic waveguides at microwave frequencies. The DBE condition
has been associated with a dramatic reduction in group velocity and with some
unique resonance properties, but it has not been shown in hollow waveguide
structures yet. Hence, we show here its existence in two periodic waveguide
examples. The unit cell of the first case is composed of a circular waveguide
loaded with two inner cylinders with elliptical irises with misaligned angles.
The second structure is composed of loading the waveguide with elliptical
rings. The demonstration of DBE in those waveguide is explained through a
simple multi transmission line approach where the conditions to obtain DBE are
clarified, and suggests that the DBE can occur in several other analogous
periodic waveguide. These structures can be potentially used to investigate
unconventional gain schemes in traveling wave tubes or other kinds of
distributed amplifiers and novel pulse compressors.
