Electromagnetic momentum associated with waveguide modes

Theoretical expressions, originally derived by Stratton, are considered in relation to waveguide modes, and the relationship between momentum transfer and mechanical force is discussed. It is suggested that this electromagnetic equivalent of the conservation of momentum is a useful theoretical tool, and examples of its application are described. The first example shows that the conservative principle leads to an upper limit for the magnitude of the reflection coefficient, when two waveguides, each supporting an H mode, are coupled together. The second example illustrates the use of the principle in deriving the series reactance in the equivalent T network, for an inductive post in a rectangular waveguide.