The thickness of the barrier is greater than 2Π δ meters. The equation for “B” is a correction factor when Z w is less the Z b (as is the case at 100kHz using the low impedance source).
As illustrated in Figure 1 and Table 1, there is not a loss of shielding under these conditions. The equation for “R” yields a reflective loss when Z w < Z b.
#SCHELKUNOFF ELECTROMAGNETIC WAVES SKIN#
This phenomenon is classified as “skin effect” where the average depth of the current flowing on the surface of an infinitely thick (greater than an extinction depth or 2Π δ meters) is one skin depth where δ is one skin depth. The H field in turn creates a back EMF (voltage) which forces the current flowing in the barrier to flow close to the incident surface of the barrier. This current generates an H field at right angles to the direction of the current. When an EM wave is impinged on a shielding barrier, current (surface current density -J s) is coupled to the barrier. The term “absorption loss” implies a power loss (or an I 2 R loss as defined in our high school physics classes). No loss in the H field was detected during the test. Figures 1 and 2 compare the results obtained using the “SE” equations with the E field attenuation obtained by Broaddus and Kunkel. The reflective loss “R” is derived from transmission lines as obtained using the equations associated with “wave theory.” It is assumed that the reflection coefficient (R) using the wave theory equation on a transmission line is identical to that of when a radiated wave is reflected from a shielded barrier, where the loss is equally the same for the E and H fields. The test results obtained by Broaddus and Kunkel (and shielding effectiveness analysis) are based on the following test conditions: 1) the test barrier has a resistance of 1.4 ohms (impedance of 2.0 ohms) 2) the EM wave sources are a high impedance (electric dipole) antenna and a low impedance (magnetic dipole) antenna 20 centimeters from the barrier and 3) the frequency range is between 100 kHz and 10 MHz.Ĭomparing the results of the analysis with the test results (as illustrated in Figures 1 and 2 and Table 1) yields significant insight into the meaning and value of the equations. Table 1 illustrates the analysis results using the shielding effectiveness equations (as given in the Sidebar “Shielding Effectiveness Equations”) on the test conditions used by Broaddus and Kunkel.
These equations are:Ī (absorption loss) = 20 log e -t/δ= 8.686 t/δ Most of the literature dealing with the shielding theory of electromagnetic (EM) waves defines the level of attenuation of the wave through a barrier by the use of shielding effectiveness equations. An analysis of circuit theory versus wave theory is performed, and the results conclude that wave theory does not represent the actual phenomena associated with a conducted wave on a transmission line, and that wave theory is only a theory.
The contribution to shielding theory by Schelkunoff is also evaluated. The analysis consists of deriving the values of R and A, and evaluating and comparing the values of R, A and B with test results obtained by Al Broaddus and George Kunkel in their paper entitled “Shielding Effectiveness Tests of Aluminizing Mylar.” The analysis and test data presented clearly demonstrate that the equations have been misinterpreted by Schelkunoff and others, and that there is no reflected loss inside a shielding barrier. In this article, we analyze the shielding effectiveness equations (SE = R + A + B) as defined by Ott, Schnelkunoff, White, and Frederick.
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