Microwaves of a particular frequency strike the metal surface from air. I have absorption and extinction coefficients. How can I calculate total absorbed fraction in metal?
If the thickness of metal film is about 3 times or more larger than the skin depth then the e-m losses in metal become almost constant regardless
of its thickness. In such cases metal can be described by the sheet impedance which the real part and the imaginary part are equal to each other Rs=Xs= (omega*miu0/(2*sigma))^0.5 where
sigma is conductivity of metal, omega is angular frequency, miu0
is permeability of vacuum.
For more details and derivatiion of power losses have a look any book on electromagnetic field theory.
The incident radiation power flux P in watt/cm^2/s will be partly reflected and partly refracted. Apart of the refracted power will be absorbed and the rest will be transmitted. Assuming the the absorbance is roh , then the refracted part will be = P(1- roh) with roh= I rI^2 where r is the reflection coefficient.
The absorbed part Pabs= Prefr( 1- exp - alpha d) with alpha is absorption coefficient and d is the thickness of the absorber.
So, the absorbed power= P(1-roh) (1- exp - alpha d)
Best wishes
Basically the important and exhaustive answers have been given already above.
However there is a kind of tricky situation for a metallic layer which is thin compared to the skin depth at the frequency of interest.This layer ha a certain sheet resistance and there are simple formulae how to calculate the transmission and reflection as a function of the angle of incidence and also as a function of the polarization of the incident wave. However for grazing incidence of such a wave (with vertical polaristion=E-field orthogonal to metal surface) on a thin metallic sheet one can have shielding or not depending on what is behind this layer.In other words ..if there is any significant current (tangential magnetic field= surface current density) flowing in this layer, there is shielding, if not ..then not.We have this case usually in beampipes with dielectric inserts which have a thin resistive layer inside (towards the beam)And the TEM like field of the beam is (locally)very similar to the homogeneous plane wave with grazing incidence and vertical polarisation.
Dear Mr Mistry,
you can see the shielding effect in dependence of the absorption in a cu metal in the file.
Sincerely Yours
F. Gräbner
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