On the conceptual utility of current flow lines in waveguides

Surface currents in waveguide walls have been very useful in gaining physical insight into the mechanism of waveguide slot coupling from the waveguide modal fields, to the slot radiator and into free space. If the slot interrupts the current flow lines, it will couple to the waveguide field and if the slot is parallel to the current flow lines, the slot will not couple and causes only minor perturbation to the waveguide field. This heuristic concept is extremely useful for the innovation of new or different slot radiators. Examples of the utility of current flow lines in circular waveguide and parallel plate radial transmission lines and how they suggest a class of reflectors with circular caustics are discussed.