Power transport theorem (PTT) governing the transport process of the
power-flow passing through waveguide is derived. The input power as the
source term in PTT is found to be the one for sustaining a stationary
power transport, and the corresponding input power operator (IPO) is
formulated. The travelling-wave condition satisfied by the
travelling-wave modes one-directionally propagating along the waveguide
is introduced as a counterpart of the famous Sommerfeld’s radiation
condition satisfied by the radiative fields distributing in far zone.
Employing the travelling-wave condition and some other necessary
conditions, the dependent currents in IPO are effectively eliminated.
Under PTT framework, the recently developed decoupling mode theory (DMT)
for wave-port-fed antennas is further generalized to waveguides. The
PTT-based DMT (PTT-DMT) focuses on constructing a set of
energy-decoupled modes (DMs) for any pre-selected objective waveguide by
orthogonalizing the IPO with only independent current, and any two
different DMs don’t have net energy exchange in an integral period. The
PTT-DMT is an effective alternative for classical eigen-mode theory. The
alternative realizes an effective unification for waveguide-oriented
modal analysis theory and antenna-oriented modal analysis theory, such
that the theories can be easily integrated into a single theory for
whole waveguide-antenna combined system in the future.