Radio-wave absorption by aluminum and its dependence on the absorption distance

The absorption of radio wave is relevant to electromagnetic interference (EMI) shielding and low observability. The effect of the absorption distance much above the calculated skin depth on the absorption of radio wave (600–2000 MHz) is studied by determining the absorption loss per unit thickness for aluminum sheets of thickness ranging from 0.528 to 2.053 mm. Aluminum is dominantly used for EMI shielding. The absorption loss SE_A increases with increasing thickness, such that when the absorption distance x exceeds 1.3 mm, the increase is slight. The distance 1.3 mm corresponds to ~ 500 times the calculated skin depth for x = 0. Furthermore, when x exceeds 1.3 mm, the linear absorption coefficient α obtained from SE_A/thickness decreases linearly with increasing x. At x below 1.3 mm, α decreases with increasing x in a nonlinear non-exponential manner that is not far from linearity and corresponds to much less decrease for the same x than the exponential case. The non-exponential relationship indicates that the skin effect alone cannot explain the observed relationship. The observed high α at large x, at which the electric field is low, indicates nonlinear dielectric behavior. The factor β that relates α₂/α₁ and x₁/x₂ (where α₁ is the α value at x₁, and α₂ is the α value at x₂) increases with x and levels off at ~ 1.5 when x exceeds 1.3 mm. The effect of x on α is large compared to the effect of the frequency on α.