Читать книгу Introduction To Modern Planar Transmission Lines - Anand K. Verma - Страница 251

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.