Evaluation of the effectiveness of an electromagnetic shielding enclosure at low frequencies

Abstract

Abstract We construct an electromagnetic shielding enclosure with dimensions of 95 cm × 95 cm × 95 cm. It is made from aluminum and copper which are high electrical conductors and nonmagnetic materials, so the enclosure does not produce additional static magnetic field. The enclosure is composed of a cubic frame (25-mm aluminium rails) and 6 removable panels. Each panel consists of a copper mesh (wire diameter of 0.25 mm and 0.58 mm spacing) sandwiched in a double layered aluminium frame, two aluminium handles, and 4 aluminium hooks. The electromagnetic shielding effectiveness of the enclosure is evaluated using the standard method at low frequencies, 0.4–25 MHz. A function generator and a circular flat coil with a diameter of 5 cm are used to transmit sinusoidal electromagnetic waves directed to the enclosure. The electromagnetic waves transmitted into the enclosure are measured by a spectrum analyser and another similar circular flat coil. The measurements show that when the frequency is increased from 0.4–2.5 MHz, the shielding effectiveness slightly increases from 42.2–48.6 dB. For the frequency between 3–25 MHz, the shielding effectiveness remains at around 49.6 dB with a standard deviation of 0.6 dB. Such shielding enclosure is potentially improving the accuracy of many investigating techniques which use electromagnetic waves with low frequencies including nuclear quadrupole resonance and magnetic induction tomography.