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Reflectivity

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Reflectivity To ensure that the phenomenon will be reproducible a perfect reflective support is chosen for absorbers (usually a metallic support so the transmission coefficient is null). Most of the time reflectivity is given in normal incidence for a plane wave excitation (far field). In this case where we are in the far field of the absorber and the field takes the structure of a plane wave reflectivity is defined by Equation X or in dB (Equation X).
where S is the power density (W/m²)

4.1 Single Layer material Reflection coefficient (R) for single layer absorber material backed by metal is given by:
Where r is the reflection coefficient from the interface between air and absorber material. Φ is the electrical thickness of the absorber. For perpendicular polarization:
For parallel polarization:
The electrical length of absorber material is dependent on material thickness d permittivity ε permeability μ and angle of incidence at air/absorber interface θ. Angle of refraction


4.2 Multilayer material Wave impedance Z₁ can be calculated by a recurrence formula given by
K_n is the wave impedance of layer n given by
U_n is the propagation constant of layer m given by

Where
And
The last layer is a perfect conductor because like a single layer absorber this one is metal backed then
The reflection coefficient of the layered media in parallel polarisation is given by
And in perpendicular polarisation by
Where K0 is the wave impedance of the incidence region for an angle of incidence θ