Abstract: A dielectric substrate for RF, microwave, or millimeter wave devices, circuits, or surfaces includes a propagating region for transmitting or reflecting an electromagnetic field, and one or more material-filled vias located within the propagating region. The application of an external electric or magnetic field to the material-filled vias may be used to tune the electric permittivity or the magnetic permeability of the fill material and hence control the effective electric permittivity or the effective magnetic permeability of the dielectric substrate within the propagating region. A dimension of the material-filled vias may be less than half of a wavelength of the propagating electromagnetic field. The fill material may include liquid crystals, a ferroelectric crystal composite, a ferromagnetic crystal composite, organic semiconductors, and/or electro-optic or magneto-optic polymers.