 Electromagnetic waveguide

An electromagnetic waveguide confines an electromagnetic wave transversely while allowing propagation along its axis. Conducting or dielectric boundaries select transverse modes. In a uniform guide a mode with transverse eigenvalue $k_{\perp }^2$ has longitudinal wavenumber $k_x^2=k^2-k_{\perp }^2$; below cutoff it is evanescent. At an open aperture, the confined field radiates into an angular spectrum rather than retaining a single guided mode.