Measurement of the electromagnetic shielding capabilities of materials

Electromagnetic shielding is typically measured in terms of insertion loss, that is, the reduction in the fields coupled between a transmitter and a receiver which results from interposing the shield material. Although the insertion loss concept is simply stated, questions arise when one attempts to interpret specific insertion loss measurements. Insertion loss data depend not only on the inherent shielding effectiveness of the material, but also on the antenna types used for the measurement, the incident field distribution, the sample size, a possible contact impedance between the test material and its mount, and other factors. For a given sample of shield material, varying these factors can lead to a large range of possible measured insertion loss values. Both the above considerations and existing shielding effectiveness measurement systems will be discussed briefly in this paper. The emphasis will be on the potential difficulties in making even relative comparisons of results and on the importance of understanding how the measurement system used affects data.