Читать книгу Introduction To Modern Planar Transmission Lines - Anand K. Verma - Страница 249

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].