Introduction To Modern Planar Transmission Lines. Anand K. Verma

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Figure 5.15 Cerenkov radiation in DPS and DNG media.

Figure (5.15d) shows that the wavevectors and Poynting vectors , for both the upper and lower radiations, are anti‐parallel that causes the backward radiation. Thus, a DNG medium supports the reverse, i.e. inverse Cerenkov radiation. The metamaterial medium could be continuously varied from the DPS to DNG forming the forward end‐fire to broadside to backward end‐fire antenna [J.26, J.27, B.6, B.7]. The inverse Cerenkov radiation has been experimentally confirmed in the microwave frequency range [J.28]. The realization of such a microstrip antenna is illustrated in the subsection (22.4.1) of chapter 22.

      5.5.8 Metamaterial Perfect Absorber (MPA)

      The absorbing materials are needed to absorb undesired reflected EM‐waves and interfering signals. The perfect absorbing materials absorb 100% of incident RF power without any reflection, scattering, and transmission. The frequency‐dependent absorbed power A(ω), i.e. attenuation, of an absorber, in absence of scattering and diffraction, is expressed as

      (5.5.43)

where R(ω) is the reflectivity (reflectance) related to the reflection coefficient (S₁₁) and T(ω) is the transmissivity (transmittance) of the absorbing sheet [B.14]. The attenuation A(ω) is also called absorptivity (absorptance). For 100% absorption, both R(ω) and T(ω) must be zero. To get R(ω) = 0, the absorbing sheet must be matched to the free space impedance η₀ = 377 Ω over a frequency band and for getting T(ω) = 0 the absorbed power must be dissipated in the absorbing sheet. The transmissivity can be made zero even by placing a conducting sheet behind the absorbing sheet. However, it may result in multiple reflections degrading R(ω) [J.29].

      Salisbury Absorber

The classical Salisbury absorber, shown in Fig (5.16a), is a narrow band resonant type absorber. The absorbing resistive screen has resistive 377Ω/sq impedance so that it is matched to free space. A metallic sheet (electric‐wall) placed behind the absorbing screen at a distance λ₀/4 creates the magnetic‐wall, i.e. the high impedance surface (HIS) at the plane of the resistive screen. The equivalent transmission line model of free space shows the absorbing screen as a 377Ω load followed by an open‐circuited termination corresponding to the magnetic‐wall. The concept of magnetic‐wall is discussed in the subsection (7.2.2) of chapter 7. At the magnetic‐wall, the total tangential components of the incident and reflected waves provide a high‐intensity electric field, i.e. . However, the total tangential component of the magnetic field is zero, i.e. . The total electric field induces a current on the resistive screen placed at the magnetic‐wall. The absorbed incident RF power is dissipated. Salisbury absorbing sheet is narrowband, thick, and bulky. By placing several absorbing resistive screens above the conducting plane, over a distance λ₀, a broadband absorber, known as Jaumann absorber, is obtained [B.15]. Again, it is a thick and bulky absorber. The metasurface provides an alternate arrangement for creating the high impedance surface to develop a thin and compact absorber. The realization of the metasurface is discussed in section (22.5) of chapter 22.

Figure 5.16 Composite surface absorber.

      Metasurface Absorber

      Figure (5.16b) shows the composition of the metasurface‐based absorber [J.30, J.31]. A thin dielectric sheet d < < λ₀ is backed by a conducting surface. An inductive surface is created at the dielectric surface. The capacitive grid of lines or patches is constructed on the dielectric surface such that the surface resonance creates the metasurface, i.e. the high impedance surface at the plane of the air‐dielectric interface. Like a Salisbury absorber, again 377 Ω/sq resistive screen is placed at the interface to get the impedance matching with free space. The absorbed RF power is dissipated as heat.

Figure (5.16b) also shows the equivalent transmission line model. The total surface impedance that creates a metasurface is

      (5.5.44)

      where Z_d and k_d are characteristic impedance and wavevector of the conductor backed dielectric sheet. At a certain resonance frequency, the denominator of the above expression is zero giving the needed value of the surface impedance Z_cap of the capacitive grid:

      (5.5.45)

      The surface parallel resonance creates the high impedance metasurface at which the resistive screen is located. In this case, Z_surafce(ω_res) = R_s(Screen) = 377 Ω is obtained to achieve matching condition. The absorbed RF power is dissipated in the resistive screen to get nearly perfect absorber. By using multilayer metasurface, the broadband thin absorber has been developed.

      DNG Slab Absorber

The lossy DNG slab can also be used as a perfect absorber, in place of the metasurface and resistive screen combination. The DNG slab absorber is developed around two configurations shown in Fig (5.17). Figure (5.17a) shows the lossy DNG slab with a conductor backing, and Fig (5.17b) shows it without any conductor backing. The working principle of a lossy DNG slab absorber is completely different. However, it is not based on the negative refraction property and backward wave nature of the DNG medium.

      The characteristic impedance of any magneto‐dielectric slab, DPS or DNG, is computed as . The impedance matching with free space could be achieved μ_r(ω) = ε_r(ω). Such natural DPS materials are not available. However, μ_r(ω) and ε_r(ω) of a DNG slab are engineered using two different structures. So both μ_r(ω) and ε_r(ω) can be tuned independently to obtain their equalization at the same frequency. Thus, impedance matching could be realized in a DNG slab. If a DNG slab is sufficiently lossy, then absorbed RF power could be dissipated in the DNG slab of appropriate thickness.

Figure

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