Author: J.B. Hoag
A metal shield can be used to prevent coupling between two circuits. Capacitive or electrostatic coupling can be prevented by shielding either the primary or secondary, or both, circuits with an enclosing metal container. The shield should be grounded and be made of material of low resistance. Also, metal shields can be used at radio frequencies to prevent magnetic coupling. Here, the eddy currents in the shield have magnetic fields which oppose the original field and more or less completely keep it out of the inside of the shield. The shielding effect is greater at the higher frequencies, is greater for more conductive materials, and also depends upon the thickness of the shielding material. At low frequencies, the eddy currents are so feeble that this method is not satisfactory. The best that can be done in this case is to surround the circuit with a complete shield of soft iron. This will partially divert the magnetic fields.
A shield changes the resonant frequency and the Q of the circuit being shielded. In shielding a coil, the spacing between the sides of the coil and the shield should be equal to at least half the diameter of the coil, and the distance of the shield from the end of the coil should be not less than the diameter of the coil. Copper and aluminum are satisfactory metals for shields.