  Work-energy principle (WEP) governing the work-energy transformation process
of Yagi-Uda array antennas is derived. Driving power as the source to sustain a
steady work-energy transformation is introduced. Employing WEP and driving
power, the essential difference between the working mechanisms of scattering
objects and Yagi-Uda array antennas is revealed. The difference exposes that
the conventional characteristic mode theory (CMT) for scattering objects cannot
be directly applied to Yagi-Uda array antennas. Under WEP framework, this paper
proposes a generalized CMT for Yagi-Uda antennas. By orthogonalizing driving
power operator (DPO), the WEP-based CMT can construct a set of energy-decoupled
characteristic modes (CMs) for an objective Yagi-Uda antenna, and then can
provide an effective modal analysis for the Yagi-Uda antenna. In addition, a
uniform interpretation for the physical meaning of the characteristic values /
modal significances (MSs) of metallic, material, and metal-material composite
Yagi-Uda antennas is also obtained by employing the WEP-based modal
decomposition and the field-current interaction expression of driving power.
