A measure of coupling efficiency for antenna penetrations

The electromagnetic penetration into a shielded enclosure through an antenna connected to a receiver inside the shield is discussed. Reciprocity is used to relate the far-zone fields to the short circuit current in the transmission line. Using a Norton equivalent circuit, a measure of coupling efficiency is derived, relating the electric field incident on the antenna to the current incident on the receiver. It is shown that the coupling efficiency can be determined from standard electromagnetic measurements. As a first example, an open-ended coaxial waveguide is considered. As a second example, a transmission line connected to a receiver inside the shield and to a center-fed dipole antenna outside the shield is discussed. Numerical results are presented for the dipole for various receiver bandwidths and transmission line characteristics. The results are given both in the frequency domain and in the time domain for a pulse incident field