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Electromagnetic Shielding Pitfalls
Can adding a metal shielding enclosure actually increase radiated emissions? Counter-intuitive as it seems — yes, it can. This article explains why, covering the three weaknesses of any Faraday cage, resonant cavity behavior, and the real mechanism by which shielding suppresses noise.
The Counter-Intuitive Result
Most people would add a metal shielding enclosure to reduce electromagnetic emissions from their electronic equipment — and then expect the radiated emissions to go down. If instead the emissions rise, the immediate reaction might be "no, that's not possible." Yet strange as it may seem, the answer is: yes, it is possible. This result is totally counter-intuitive to what you would expect — and instances like this often lead people to think that EMI/EMC is difficult. Let's look into the reasons why this particular phenomenon occurs.
Typical design engineers often think only about their signals and corresponding circuits — not about noise, that is, the unwanted electromagnetic energy. One key concept that can make EMC easier to understand is that the electrical energy (or signal) in any circuit, when sent from point A to point B, is not travelling in the electrical conductor. Most of this energy actually exists in the space surrounding the conductor.
The capacitance and inductance that govern stored energy are related to the physical space around the conductor, not its resistance. So the energy exists in the space surrounding the conductor — and depending on the frequency of the signal, some of this energy could radiate out or couple to other circuits. This phenomenon is generally described as crosstalk, and it is electromagnetic noise because this energy is going where it is not needed.
The Three Weaknesses of a Faraday Cage
In order to reduce the electromagnetic noise escaping the equipment, a design engineer may attempt to contain this energy by enclosing the equipment in a metal enclosure or shield — sometimes referred to as a Faraday cage. However, not all enclosures are ideal Faraday cages. Every real enclosure has three imperfections:
1
Openings
Openings allow electromagnetic energy to leak out. These take the form of seams (which may not be visible) and slots. The leakage through these openings must be minimized.
2
Conductors Going In or Out
Signal cables, power cables, or ground conductors can pick up energy from inside the enclosure, carry it through the metal walls, and radiate it outside. This is minimized by appropriate application of cable shielding and filtering.
3
Finite Wall Thickness
Depending on the frequency, the thickness of the metal enclosure only provides limited amounts of shielding. The inside surface reflects energy, keeping it inside. Part of this energy is absorbed by the metal as it travels toward the outside — and this absorption is actually the key advantage of using a Faraday cage.
Figure 1: A Faraday Cage or enclosure showing how openings, cable conductors, and metal thickness may lead to energy leaking out.
Noise Propagation — The Enclosure as a Resonant Cavity
The electromagnetic energy contained within the enclosure bounces back and forth between the metal walls. Some escapes by the three limitations described above; some is absorbed by the metal within its thickness. The circuits produce RF energy continuously with clocks and data to perform their control/computing/communication functions.
Initially, the energy level inside the Faraday cage rises — but it cannot rise indefinitely, because that would also increase the energy escaping by each of the three methods. Equilibrium is reached very quickly when the energy produced by the circuit equals the energy escaping the enclosure. This steady state level inside the enclosure is generally much higher than it would be if the enclosure was not present.
Also, some of the energy leaked by the openings or re-radiated by the cables may be directed toward the antenna used for emissions measurement, and possibly more concentrated in some directions. This explains why energy levels may be higher with the enclosure than without it.
We still use the enclosure to suppress noise escaping the EUT. Analyzing this situation helps us realize that reducing noise optimizes the phenomenon that absorbs energy within the materials (metal) of the enclosure. This material absorbs the electromagnetic energy and converts it into heat. Thus, the net energy escaping the enclosure is equal to the energy radiated by the circuit minus the energy absorbed by the Faraday cage.
ⓘ Key takeaway: The real advantage of the Faraday cage is that the enclosure acts as a sink for electromagnetic energy by converting it into heat — not simply by blocking it.
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