Aperture Leakage Cutoff

Formula

Description

An aperture or slot in a shielded enclosure acts as a waveguide with a cutoff frequency determined by its longest dimension. Below the cutoff frequency, electromagnetic waves are attenuated exponentially and cannot efficiently pass through the opening. Above the cutoff frequency, the slot becomes an effective antenna that radiates or admits energy. This is why long narrow slots (like those along enclosure seams) are more problematic than round holes of the same area. The critical dimension is the longest linear extent of the opening.

Variables

• f_c — Cutoff frequency above which the slot leaks efficiently (Hz)
• slot_length — Longest dimension of the aperture (m)

Practical Notes

A 15 cm slot (typical enclosure seam) has a cutoff of 1 GHz, meaning it radiates efficiently at Wi-Fi frequencies (2.4/5 GHz). To mitigate, use overlapping seams, EMI gaskets, or conductive tape. Multiple small holes have much less leakage than one large hole of the same total area. The rule of thumb is to keep slot lengths below λ/20 of the highest frequency of concern.