Correlation of MIL-STD-285 Measurements, Seam Transfer Impedance and EMP Shielding Effectiveness

A model for the penetration of shielded enclosures by electromagnetic fields is developed. The dominant sources are seams, each of which is characterized by its own transfer impedance. This parameter is used to relate shielding effectiveness determined by RFCW MIL-STD-285 measurements to the EMP case. The results of measurements and computer simulations for both RFCW and EMP demonstrate the efficacy of the technique. INTRODUCTION The problem of determining an enclosure's hardness to EMP is of both general and continuing interest. EMP simulators such as the AESOP at the Woodbridge Research Facility at Harry Diamond Laboratories in Virginia, are useful for testing sample shelters and understanding their response characteristics to high intensity EMP fields. This approach is clearly impractical for determining the shielding effectiveness (SE) of each shelter used in a given application. Furthermore, there is the task of verifying that the hardness of units in the field does not deteriorate to unacceptable levels over extended periods of time. The size, cost and location limitations of true EMP simulators make them ill-suited for ongoing evaluation of large numbers of dispersed shelters. MIL-STD-285 [1] testing is an alternative approach to measuring the shielding effectiveness of shelters for certain types of incident EM fields. The equipment used in this set of procedures is relatively inexpensive and portable. Additionally, there is a large body of experience and expertise accumulated in MIL-STD-285 testing that makes it available on a widespread and easily accessible basis. The problem, of course, is that the test fields are not identical. The EMP fields generally arrive as short duration plane waves illuminating the entire shelter and surroundings. Figure 1 illustrates a typical case of EMP incident on a shelter. MIL-STD-285 tests employ more localized CW fields whose wave impedances are determined by the specified antennas and separation. Figure 2 illustrates a typical MIL-STD-285 test setup. The challenge, then, is to understand the nature of electromagnetic penetration into an enclosure for both types of fields [2] and to establish a mapping from MIL-STD285 SE results to EMP SE values. This paper will attempt to establish the MIL-STD-285 to EMP connection by building a single parameter model of field penetration valid in both cases. A numerical * Work performed under subcontract for SOL Telecommunications Services, Inc. HDL Prime Contract No. DAAK21-85-C-0061 C H2294-7/86/000-0423 $ 0 1 .0 0 © 1 9 8 6 IEEE r-i 1