The frequency of a signal that can propagate without being attenuated through an aperture in the EMI shield is called the cut-off frequency. The corresponding wavelength is called the EMI shield wavelength.
When the dimension of an opening in an EMI shield is equal to one-half the wavelength of interference, then the shield radiates EMI and becomes useless.
Depending on the operating frequencies and power levels, the dimension of the aperture should be smaller than 50and not greater than20.
In the electronics industry, the EMI shielding technique is used to protect signals from interference caused by external electromagnetic waves and to prevent the generated signal from causing interference to neighboring equipment. The oldest EMI shielding technique is the Faraday cage, where a metallic screen surrounds the sensitive circuit, absorbing radiated and conducted EMI. Most metallic screens come with an opening, an aperture, and slots. These openings cause detrimental effects if they disobey the relationship between the EMI shield wavelength.
Apertures in EMI Shields
In EMI shields, openings are incorporated for ventilation, mechanical supports, optical displays, and connectors. The size of the opening plays a significant role in increasing the effectiveness of the shielding.
At higher frequencies, the size of the seams or apertures in the EMI shield affects its performance. If the wavelength of interest is shorter than the aperture size, high-frequency EMI may escape through the openings and cause EMI leakage. If the size of the aperture is smaller than the wavelength of the signal, then it is considered an optimum design. For the long-term reliability and maximum performance of EMI shields, it is important to optimize the size of EMI shield openings by considering the EMI shield wavelength.
Recommendations for Apertures in EMI Shields
Apertures are essential in EMI shields, and there can be more than one aperture in the EMI shield. However, it is advisable to keep the small openings distributed on all sides of the EMI shield, rather than aligning them on one side. This can help maintain shielding effectiveness by preventing the radiation of EMI in different directions.
Depending on operating frequencies and power levels, the dimension of the aperture should be smaller than 50. In cases where the design crosses the 50 dimension, the maximum upper limit is20. If openings cannot be avoided, then conductive gaskets, screens, or paints should be used along with the aperture to limit its dimension to the range given above. It is important to use circumferential metallic shields for the wires, cables, and connectors entering the EMI shield to ensure firm physical bonding. In case the wires or cables used are unshielded, filters are recommended at the entry point.
EMI Shield Wavelength
The size of the aperture can result in the attenuation of signals in EMI shields. The frequency of a signal that can propagate without being attenuated through an aperture in the EMI shield is called cut-off frequency, and the corresponding wavelength can be referred to as the EMI shield wavelength. The signals of wavelengths lesser than the EMI shield wavelength pass freely through the aperture, whereas signals with wavelengths greater than the EMI shield wavelength get attenuated.
How Does the Size of the Aperture Impact Shielding Effectiveness?
Normally, at high frequencies, the openings in EMI shields act as slot antennas. To avoid this slot antenna effect in EMI shield openings, a dimension for the aperture is calculated, above which it serves as a slot antenna. The calculated dimension of aperture is suitable for increasing the shielding effectiveness and can be given by the following equation, where is the wavelength of interference and L is the longest dimension of the aperture.
Shielding Effectiveness, SE(dB)=
When the length of the aperture increases, the shield becomes more susceptible to EMI leakages and shielding effectiveness is reduced.
From the above equation, we can come to the following conclusion: when the dimension of the opening in the EMI shield is equal to one-half the wavelength of interference, then the shield radiates EMI and becomes useless. The ideal dimension of an aperture is 120 of the EMI shield wavelength, as it gives 20dB shielding effectiveness.