MTM elements. (a) SRRs, it can be equivalent to a magnetic dipole, (b) metal wires, it can be equivalent to an electric dipole, (c) slot lines, it can be equivalent to a magnetic dipole, (d) CSRRs, it can be equivalent to an electric dipole, and (e) the empty waveguide operating below the cutoff frequency, the effective permeability is negative for the TM mode, the effective permittivity is negative for the TE mode.