Electromagnetic waveguide
= 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.