EMC Question of the Week: April 15, 2019

The Schelkunoff Decomposition breaks the overall shielding effectiveness value into three components: Reflection Loss (R), Absorption Loss (A) and a multiple reflections correction term (M). The Absorption Loss (A) expressed in dB represents the ratio of

1. the incident power to the absorbed power
2. the absorbed power to the incident power
3. the power not reflected to the transmitted power
4. none of the above

Answer

The correct answer is "d." In fact, the value of the Absorption Loss (A) tells us nothing about the relative amount of power that is absorbed by the shielding material. The general formula for the Schelkunoff Decomposition is:
$SE(dB)=20\log \frac{{\eta }_0}{4{\eta }_s}+20\log e^{\raisebox{1ex}{$t$}\!\left/ \!\raisebox{-1ex}{$\delta $}\right.}+20\log \left|1-e^{-2\raisebox{1ex}{$t$}\!\left/ \!\raisebox{-1ex}{$\delta $}\right.}\right|=R\left(\text{dB}\right)+A\left(\text{dB}\right)+M\left(\text{dB}\right)$.

The term in the middle is basically expressing the ratio of the field strength at the one face of the material to the field strength at the other face for a wave passing through the material in one direction one time. This effectively represents an absorbed power, but it cannot be expressed in terms of the incident power or transmitted power in any straightforward or intuitive manner. The Schelkunoff Decomposition is an interesting and convenient way to point out that shielding materials both reflect and absorb power, but it cannot be used to quantify relative amounts of reflected and absorbed power.

A more useful decomposition for quantifying how well a material absorbs power as opposed to reflecting power is the mismatch decomposition,

$\text{SE(dB)}=L_D(dB)+L_M(dB)$

where L_D is the dissipation loss and L_M is the mismatch loss. L_M is a measure of the actual reflection loss,

$L_M=-10{\log }_{10}\left(1-P_R\right)$

where P_R is the normalized reflected power. It can be expressed in terms of the material properties as,

$L_M=10{\log }_{10}\left(\frac{{\left|{\eta }_0+{\eta }_{in}\right|}^2}{4\mathrm{Re}\left({\eta }_0\right)\mathrm{Re}\left({\eta }_{in}\right)}\right)$, where ${\eta }_{in}={\eta }_0\left(1+{\Gamma }_{in}\right)/\left(1-{\Gamma }_{in}\right)$ .

L_D is a measure of the actual power absorbed, $L_D=-10{\log }_{10}\left(\frac{P_T}{1-P_R}\right)$

where P_T is the normalized transmitted power reaching the load. 

The shielding effectiveness calculator on the Clemson Vehicular Electronics Laboratory website, calculates the plane wave shielding effectiveness of materials. It can be used to calculate and plot both the Schelkunoff and mismatch decomposition terms.

Have a comment or question regarding this solution? We'd like to hear from you. Email us at This email address is being protected from spambots. You need JavaScript enabled to view it..