Moment method analysis of the electric shielding factor of a conducting TM shield at ELF

This paper presents an integral equation and method of moments (MM) solution to the problem of TM transmission by a metallic conducting shield at extremely low frequencies (ELF). To obtain an accurate and efficient solution, the equivalent volume polarization currents representing the shield are expanded in terms of physical basis functions, corresponding to two planewaves propagating normal to the surface of the shield. ELF approximations are used to obtain closed form expressions for certain crucial elements in the MM matrix equation where extreme accuracy between the relative magnitudes of self and mutual impedance terms are required. Numerical data will illustrate that despite the fact that the equivalent polarization currents are being computed very accurately, the method is not capable of computing the extreme near zone electric fields of these currents with sufficient accuracy to compute the electric shielding factor. An alternate method, based upon the use of the volume equivalence theorem to directly compute the total electric field in the shield is devised, and is found to accurately compute the electric shielding factor