Moment Method Analysis of the Electric Shielding Factor of a Conducting TM Shield at ELF.

This report 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. In order to obtain an accurate and efficient solution the equivalent volume polarization currents representing the shield are expanded in terms of a physical basis expansion corresponding to two plane waves 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 very accurately computed, the method is not capable of computing the extreme near zone electric and magnetic fields of these currents with sufficient accuracy to compute either the electric or the magnetic shielding factor. An alternate method for computing the shielding factors, based upon the use of the volume equivalence theorem to directly compute the total fields in the shield is devised. This alternate method is found to be sufficiently accurate to compute the electric, but not the magnetic shielding factor.