Measurements and analysis of transient electromagnetic shielding effectiveness for nested shield configurations

Electromagnetic (EM) shielding studies on totally closed box shields, one inside the other, were carried out earlier for steady magnetic fields in the frequency range from 50 Hz to 150 kHz and for plane waves from 1 MHz to 1 GHz. The calculated and the experimental values of the shielding effectiveness for a few selected materials like copper, tin-plated iron, Mu-metal and their composites in single and nested configurations, for wall thicknesses in the range of 50 /spl mu/m to 250 /spl mu/m were compared. In order to explain the discrepancies in the calculated and experimental results, need was felt to simplify the experiments and improve the accuracy and sensitivity of the measurements. The present study is a sequel to the one above but differs essentially in two aspects, viz.; (a) the shielding effectiveness is measured for the broadband (DC to 100 MHz) transient, intense (E-field in the range of 50 to 100 kV/m) and fast (rise time less than 10 nano seconds) EM fields generated in a simulation facility, and (b) the closed nested box shields are configured as open ended structures, to overcome the experimental complexities of the former. The transient fields comprising E-field, D-Dot field and B-Dot field, were characterised by indigenously developed sensor probes. A high sensitivity of 750 mV/kV/m has been achieved for the electric field sensor. The open-ended shield structures are made from 1.5 mm. thick aluminium sheets supported at the four corners of the sheets by copper rods of 6 mm. diameter. The corresponding dimensions for the shields are 250/spl times/200/spl times/75 mm. and 200/spl times/150/spl times/50 mm respectively. The paper discusses the results of the measurements thus performed on shielding effectiveness and sufficiently elaborates the design and development of E, D-Dot and B-Dot field probes. Also, discussed are the characteristics of the transient intense EM fields generated in the simulation facility.