Huygens' principle and field equivalence relations for cylindrical enclosing surfaces
Frequency-domain Huygens' principle and equivalence relations for an infinitely long cylindrical surface enclosing the field sources or scatterers are addressed. Assuming the harmonic dependence of the fields along the cylinder axis, we derive three equivalent versions of line-integral representations for the fields in free space at any point outside the enclosing cylinder. In one of the forms, similarly to the Helmholtz-Kirchhoff scalar diffraction theory, the integrals contain a longitudinal field component and its normal derivative. Another presented form contains longitudinal and normal field components, similarly to the three-dimensional boundary integral representations by Stratton and Chu. This form eliminates the need to calculate the field derivatives. Furthermore, we present a two-dimensional version of the three-dimensional Schelkunoff-Franz representation, which contains only tangential components of the fields, complies with the field equivalence theorem and can be regarded as a rigorous formulation of Huygens' principle for a cylindrical enclosing surface. All three forms of the line-equivalence relation are exact and describe the fields through equivalent field distributions on a line enclosing the cross section of the source region. A physical interpretation of Huygens' principle in terms of conical waves emanated by virtual linear sources on the cylindrical surface enclosing the true sources is given. Specialization of the relations to the intermediate and far-field zones are presented. The derived expressions are applicable for studies of scattering and radiation in a very general class of structures that are infinite and periodic along one direction.