An alternative EM shielding effectiveness measurement method for buildings

摘要

Here we present an efficient method for evaluating the EM shielding effectiveness of a commercial building to determine approximately the amount of shielding available before beginning a “hardening” program. A traditional approach is to use a transmitter/antenna matched to an antenna/receiver. In that standard approach the setup is calibrated in an open area outside, with a set separation distance. Then the same setup is used, but with the antenna/receiver moved into the building - with shielding effectiveness determined by the reduction in the received field, as provided by the building structure. However, done properly, this approach can be expensive, time consuming, and may present some logistical problems. To address these problems, we present a much simpler alternative approach. For this method only the internal antenna/receiver part of the setup is used. The external source is replaced by using existing external RF sources, such as AM and FM radio transmission, and signals from cellular phone towers. This approach also accounts for the fact that at lower frequencies (∼1 MHz) the shielding effectiveness differs for electric and magnetic fields. After discussing this approach, we then present a test in which this method and the traditional approach are used on a building, and the results compared. All approaches have strengths and weaknesses - this method produces reasonable results with greatly reduced effort. It is especially useful in that there is no need to procure the right to illuminate the building and to gain access to all required transmitter sites all around the building.