IEEE Demonstration -
Penetration of an EM field Into and Through a Shielding Barrier Material

A demonstration illustrating the penetration of an EM field into and through a shielding barrier material was presented in Seattle at the 1999 IEEE International Symposium on EMC. The demonstration illustrated that the value of the E and H fields emanating from a shielding barrier can be predicted by knowing the value of the impedance of the shielding material and the value of the H field prior to the placement of a shield.

This picture shows the set-up for measuring the H field radiating from an electric dipole antenna located inside a shielded box with the face of the box open.

Figures 1, 2 and 3 illustrate the measured test values of the E and H fields before and after the placement of an aluminized mylar sheild material on the box, where the shield material possessed an impedance of 2.4 ohms. As shown in Figures 1 and 2, the attenuation of the H field by the shield was negligible where the predicted attenuation of the shield to the H field was 0.02% or a loss of .002dB.

Figure 1: H Field Test Data with Face of Shielded Box Open

Figure 2: H Field Test Data with Shield Attached to Face of Box

Figure 3 illustrates the difference in the measured E field before and after the shield material was attached to the box.

Figure 3: E Field Test Data

Table 1 illustrates the level of the predicted and measured E field with the shield in place. The predicted E field was performed by multiplying the value of the H field with the impedance of the barrier (i.e., by 2.4). In using dB the E field is obtained by adding 20 log2.4 to the value of the H field in dB uA (i.e., adding 7dB to the value of the H field in dB.)

Table 1: Measured and Predicted E field Values (dB uV/m)

Frequency 100 kHz 500 kHz 1 MHz Measured 63 69 69 Predicted 66 72 73