the directions of the wavevectors Inverse Cerenkov Radiation
Figure (5.15c) shows that the charged particle moving with a velocity Vc (Vc > Vp) in a DNG medium with a negative refractive index −|n|. It creates backward radiation with the negative Cerenkov angle,
(5.5.42)
Figure 5.15 Cerenkov radiation in DPS and DNG media.
Figure (5.15d) shows that the wavevectors
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.
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 Zd and kd 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 Zcap 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, Zsurafce(ωres) = Rs(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
