Electromagnetic Wave Interaction with Apertures
An aperture refers to an opening in a conductive surface, such as a ventilation hole, a seam where metal pieces join, or a mounting hole for electronic controls. When an electromagnetic wave reflects off a surface containing an aperture, some of the field can penetrate the opening. Even small apertures, like a window in a building wall, allow maximum light intensity in the resulting beam.
The reflected wave induces surface currents on the conductive surface, and this current flow pattern is influenced by the presence of the aperture. On the conducting surface, the electric field is zero except in the area of the opening. The challenging question arises: What is the electric field on the other side of the aperture?
Complexity of Aperture Field Calculation
Calculating the field that penetrates through an aperture is highly complex. Even in an idealized scenario with a thin conducting surface, zero resistivity, infinite area, and a round hole, the emerging wave is no longer a plane wave, and surface currents exist on the far side. Problems become more intricate for apertures of different shapes, such as rectangles, due to fine structural details, especially near corners.
In practical applications, energy entering through an aperture is usually within a conductive enclosure with associated electronics. Determining field strength inside a real enclosure is often unsolvable, making practical assessment through specific circuit impact crucial. The worst-case calculation (WCC) approach assumes certain conditions, including perpendicular arrival of external waves to every surface, alignment of the electric field with the aperture’s maximum dimension, and a thin, perfect conducting surface. The idea is to ensure that the field energy entering through the aperture is less than or equal to the WCC level, allowing for a manageable assessment of field strength.