  Spatially dispersive (also known as non-local) electromagnetic media are
considered where the parameters defining the permittivity relation vary
periodically. Maxwell's equations give rise to a difference equation
corresponding to the Floquet modes. A complete set of approximate solutions is
calculated which are valid when the inhomogeneity is small. This is applied to
inhomogeneous wire media. A new feature arises when considering spatially
dispersive media, that is the existence of coupled modes.
