A "'closed waveguide "'is an electromagnetic waveguide ( a ) that is tubular, usually with a circular or rectangular cross section, ( b ) that has electrically conducting walls, ( c ) that may be hollow or filled with a dielectric material, ( d ) that can support a large number of discrete propagating modes, though only a few may be practical, ( e ) in which each discrete mode defines the propagation constant for that mode, ( f ) in which the field at any point is describable in terms of the supported modes, ( g ) in which there is no radiation field, and ( h ) in which discontinuities and bends may cause mode conversion but not radiation.
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