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Following equation (4.5.26b) of chapter 4, the wave impedance η^DNG in a DNG medium is written below:

(5.5.7)

Like the wave impedance in a DPS medium η^DPS, the wave impedance in the DNG medium η^DNG is a positive quantity; showing the outward power flow from the source into a DNG medium. However, the wave impedances of the ENG and MNG media are reactive due to nonpropagating evanescent mode:

(5.5.8)

The inductive/capacitive reactive wave impedances of the ENG and MNG media create the reflecting surfaces. The circuit model of the metamaterials, discussed in section (5.5.3), elaborates on the nature of the RIS. Further details of the artificial RIS surface is discussed in section (20.2) of chapter 20. It is noted that the ENG/MNG medium is realized through the nonpropagating evanescent wave. Such an environment is provided by a rectangular waveguide below the cut‐off region. It is commented in subsection (7.4.1) of chapter 7.

The propagation constants of EM‐waves in the ENG and MNG media are obtained below:

(5.5.9)

Expressions (5.5.9 a,b) show that the ENG and MNG media do not support wave propagation. Figure (5.7) shows that these media are placed in the second and fourth quadrants in the (μ_r, ε_r)‐plane. They only support the decaying evanescent mode.