Coupling of apertures in enclosures to external cabling structures

In addition to the direct radiation from enclosure apertures, an indirect contribution also exists due to the coupling to external cables. Based on an inductive coupling model, a relation for the external coupling voltage is derived and combined with a resonant dipole-antenna model for worst case estimations of the radiation level. An expression for the direct-to-indirect radiation-amplitude ratio is deduced. The model is well confirmed by a numerical example for a rectangular box with a slit. The results clearly indicate that in the low-to-intermediate frequency range the indirect cable radiation exceeds by far the direct radiation from the aperture.