Leakage of Electromagnetic Interference along Stationary Conductors Passing through Conducting Walls

A mechanism by which electromagnetic energy at quasistatic frequencies may leak out of seemingly shielded compartments via conductors such as external shields of cables, pipes, shafts, bolts, etc., protruding from the interior of the compartment is investigated both theoretically and experimentally. The mechanism involved may be illustrated by considering a conductor, such as a shaft passing through a hole in a conducting wall, and contacting the wall at a finite number of contact regions. These regions represent a short circuit to the TEM mode, however higher modes may pass along the guide whose walls may be represented by the actual regions of contact and the inner and outer coaxial conductors. Attenuated transmission may thus occur at frequencies below the cutoff frequency of the higher modes. As verification of the above hypothesis an impedance relationship, representing the shunt impedance that the contact regions offer at below cutoff frequencies (in particular f = 15 kc), was developed and checked experimentally for continuous pressure and soldered contacts. The experimental data show relatively close comparison with the theoretical model.