APPLICATION OF COHERENT MEASUREMENT METHODOLOGY TO SHIELDING EFFECTIVENESS MEASUREMENTS

Shielded enclosures are used in a variety of safety, TEMPEST, and measurement applications. The most critical performance criterion of these enclosures is shielding effectivess (SE) where SE is defined as 20 times the logarithm of the ratio of the electric fields inside and outside the enclosure or 20 times the logarithm of the ratio of the magnetic fields. SE of enclosures has traditionally been measured by teither MIL-STD-285 or NSA 65-6. Both of these standards are over 25 years old and have poor repeatability, as great as 40 dB. Recently, the IEEE has promulgated IEEE-STD-299-1991 which addresses and corrects some of the deficiencies of MIL-STD-285 and NSA 65-6, but the new standard is also subject to poor repeatability. None of these standards attempt to measure SE as a functio of wave impedance where wave impedance is deifned as the raio of the magnitude of the electric to magnetic field. Wave impedance is particularly important to the Navy as topside enclosures are exposed to high-power HF communications whive have widely varying magnetic and electric field components. The proposed Phase I effort is a multi-step process of which the final product will consist of the theory, statistics, and software necessary to measure the shielding effectiveness of electromagetic barriers with respect to frequency and wave impedance using established coherent-measurement techniques.