Shielding

If a system radiates too much and fails the FCC specification, metallic shielding can be used to reduce the magnitude of the radiation. Most of the notebooks and desktop computers employ some kind of metallic shielding reduce the radiation.

To understand the mechanism of shielding refer to the figure below.



Figure - An electromagnetic wave gets, reflected, attenuated and then re-reflected before coming out of the shielding material

An electromagnetic wave is generated from an electronic product start traveling as a free space wave after some distance. After traversing a certain distance substantially greater than its wavelength its impedance is same as the free space impedance or 377 Ohm. This wave hits the surface of the metal. The surface of the metal has typically low impedance. We can think of this situation similar to the transmission line case where a signal traveling in high impedance hits a low impedance boundary and gets reflected. In case of the electromagnetic wave the magnitude of this reflection depends upon the frequency of the incident wave, the permeability of the shield material and the conductivity of the shield material relative to copper. After reflection, a part of the signal enters the shield material. As it propagates the shield material, a part of it gets absorbed. The magnitude of the absorbed signal depends upon the thickness of the shield material. The signal gets substantially reduced in magnitude by the time it reaches the right side of the shield boundary. At the right boundary, it faces a low impedance to high impedance boundary and is again reflected. Multiple reflection may take place with the amplitude of the wave decreasing significantly at each reflection. In many cases, magnitude of the second reflected wave and the multiple reflection can be ignored. The wave finally emerges out of the metal shield.

Shield effectiveness is the term used to denote the amount of the loss in dB due to the shield. If R is the shield effectiveness in dB due to the reflection, A is the shield effectiveness in dB due to the absorption and B is the correction term due to multiple reflections, then the total shield effectiveness is given by

SE = R + A + B

Where,

R = shield effectiveness due to reflection, dB
A = shield effectiveness due to absorption, dB
B = Correction term due to re-reflections , dB


This shield effectiveness term does not take into account the other factors such as leakages due to the opening in the shield enclosure. In general, these factors may reduce effectiveness of the shield enclosures.

Reflection

As soon as a wave radiating from a PCB hits a metallic shield, a part of it is reflected. The wave traveling in air can be assumed to have an impedance of 377 Ohms. It hits the conductor acting as a shield. The impedance of the shield conductor is low. As a result there is a reflection, similar to the reflection when a incoming signal on a transmission line encounters a low impedance.

If the wave is at a distance of few wavelengths from the source, it can be approximated by as a plane wave an the formula for the reflection term can be given by,



Example Find the shield effectiveness due to reflection from a sheet made up of copper at a frequency of 100 MHz. Assume that the sheet enclosure is far from the source of radiation, so that plane wave propagation can be assumed.

Solution

The sheet effectiveness is given by,




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