Open beam waveguides: theory.
The propagation of an electromagnetic wave through an open beam waveguide is theoretically investigated in the case where the guiding elements are not inserted in an absorbing screen. Numerical results are derived for the dielectric-frame beam waveguide. The low loss per cell of such a waveguide and the existence of a quasi-repetitive field configuration are explained by the presence of an internal caustic surface, supported by the frames, bounding the axial portion of the beam. The finiteness of the transverse width of the frames and the absence of any outer screen suggest the formation of an external caustic surface, which is closer to the internal one the narrower the frames are. Knowledge of the features of the caustic due to each single frame is of help in the design of the low-loss beam waveguides with prescribed characteristics.