The gauge transformation in the theory of waveguide excitation

Various types of equations are known for the study of the excitation of waveguides. These equations are equivalent as far as the computation of the entire electromagnetic field is concerned. Differences regarding the equations are related to details with respect to the partition of the complete field. Such details can be important in connection with the development of the theory of electronic devices. For this reason, an investigation is conducted concerning the structure of the theory of waveguide excitation, taking into account approaches for going from one form of equations to another. The excitation of a cylindrical waveguide by means of an electron beam is considered. It is shown that differences regarding the structure of currently known excitation theories are basically caused by two factors. One factor is related to differences regarding the gauge in the case of the Green's functions, while the second factor involves differences concerning the approach employed in breaking these functions down into simple fractions.