SHIELDING EFFECTIVENESS

Transcription

1 SHIELDING Electronic devices are commonly packaged in a conducting enclosure (shield) in order to (1) prevent the electronic devices inside the shield from radiating emissions efficiently and/or (2) prevent the electromagnetic fields external to the device from coupling efficiently to the electronics inside the shield. (1) (2) The effectiveness of the shield in preventing externally-directed radiation or internally-directed radiation is a function of the shield material and thickness, along with the enclosure geometry. Ideally, the shield would be a completely enclosed structure. However, the need for power and communication conductors to penetrate the enclosure, along with the need for effective ventilation, will compromise the effectiveness of the shield. The shielding effectiveness of an electromagnetic shield is typically defined as the ratio of a field magnitude (electric or magnetic) without the shield in place to the field magnitude with the shield in place. This definition of the shielding effectiveness is equivalent to that of insertion loss in microwave circuits where the insertion loss of a given component is typically defined as the ratio of the signal obtained without the component in the circuit to the signal obtained with the component in the circuit.

2 SHIELDING EFFECTIVENESS In order to make a straightforward comparison of the shielding capabilities of one material to another, the simple geometry of a planar metallic shield of thickness t in air is considered, as shown below. The shielding effectiveness of a given shield is actually a function of the distance from the incident wave source (near-field sources and far-field sources). The source is initially assumed to be a far-field source such that the incident wave can be approximated by a normally-incident uniform plane wave. As the incident wave encounters interface #1at z = 0, a portion of the wave is reflected away from the interface, while the remainder of the wave is transmitted into the metal, and is attenuated as the wave travels through the metal. A portion of forward wave in the metal is reflected from interface #2 at z = t producing a reverse wave, while the remainder of the wave is transmitted into the air region (z > t). The reflection/transmission process at the two interfaces produces, in theory, an infinite number of reflected, forward, reverse and transmitted wave components.

3 The electric field shielding effectiveness (SE E) and the magnetic field shielding effectiveness (SE ) in db of the planar shield are defined by M For far-field sources, SE E = SE M since the ratio of the electric field to the magnetic field for a uniform plane wave is constant (equal to the wave impedance of the medium). For near-field sources, in general, SE E SE M given the rapid variation of the near fields in the vicinity of the source. Thus, the electric and magnetic shielding effectiveness terms are different and vary as a function of distance from the source. The shielding effectiveness of the planar shield is governed by three distinct mechanisms involving the interaction of the incident wave with the air/conductor interfaces and the conducting medium of the shield. These mechanisms are: (1) Reflection loss A portion of the incident wave is reflected from interface #1. The amplitude of the reflected wave fields are equal to those of incident wave fields multiplied by the reflection coefficient for waves moving from air into the conductor (Ã ). a-c

4 (2) Absorption loss All of the forward and reverse waves propagating within the conducting shield are significantly attenuated according to the attenuation constant for the conducting shield. This attenuation of the wave corresponds to the loss of wave energy in the form of heat. The complexvalued propagation constant (ã) within the conducting shield is given by where á is the attenuation constant and â is the phase constant for the shield material. The amplitudes of the waves internally reflected from interface #1 and interface #2 are proportional to the reflection coefficient for waves moving from the conductor into air (Ã ) given by c-a For good conductors, the attenuation constant can be approximated by the inverse of the skin depth (ä). The thickness of the shield relative to the skin depth (which is a function of frequency) dictates how significantly the wave is attenuated as it propagates through the shield.

5 (3) Multiple reflections A portion of each of the forward waves within the planar shield is transmitted into the air region (z > t). The transmitted fields used in the SE calculations are the vector sum of the fields associated with these forward waves. Likewise, a portion of each of the reverse waves within the planar shield is transmitted into the air region (z < 0). The reverse waves transmitted out of the planar shield represent additional losses which enhance the shielding effectiveness value. Both of these transmitted waves are proportional to the transmission coefficient for waves moving from the conductor to air (T ). c-a The significance of the multiple reflections is related to the thickness of the planar shield relative to the skin depth. If the shield is several skin depths thick, there is significant attenuation as the initial wave progresses across the shield, making the effect from multiple reflections negligible. Conversely, the effect of multiple reflections can be significant for shields that are only fractions of a skin depth (low frequencies).

6 An exact solution for the shielding effectiveness (SE E = SE M = SE) can be obtained for the case of a far-field source assuming normal incidence. The general form of the fields associated with the separate wave components are shown below. Interface #1 Interface #2 z = 0 z = t Applying the boundary conditions (continuous tangential electric and magnetic fields) at interface # 1 (z = 0) gives Applying the boundary conditions at interface # 2 (z = t) gives

7 Given the incident field amplitude, the preceding four equations can be solved for the four unknowns (the reflected, forward, reverse and transmitted amplitudes). The resulting ratio of the incident field to the transmitted field is The shielding effectiveness of the planar shield is then The three terms in the equation above can be identified separately as the contributions to the shielding effectiveness from reflection loss, multiple reflections and absorption loss. The shielding effectiveness in db can then be written as db db db where R, M and A represent the contributions to the shielding effectiveness in db due to reflection loss, multiple reflections and absorption loss, respectively.

8 The separate terms in the shielding effectiveness expression can be simplified for typical shields made from good conductors (ó ùå), for which the following approximations are valid. This gives

9 Inserting these approximations into the SE component equations gives The terms above represent the far-field shielding effectiveness contributions for a good conductor. Example Determine the shielding effectiveness in db for a 20 mil thick sheet 7 of copper (ó = S/m) at 1 MHz due to (a.) reflection loss from the surface of the copper sheet (b.) multiple reflections within the copper sheet (c.) absorption loss within the copper and (d.) all three shielding mechanisms (the total SE of the copper sheet).

10 (a.) (b.) (c.) (d.)

11 NEAR-FIELD SHIELDING EFFECTIVENESS The determination of the near-field shielding effectiveness is a much more difficult problem than the far-field case due to the complexity of the near fields. However, we may approximate the near-field shielding effectiveness by replacing the far-field wave impedance (ç o) in the far-field shielding effectiveness equation by an equivalent near-field wave impedance. The near-field wave impedance is defined using the same equation as that used for the far-field wave impedance (the ratio of electric field to magnetic field). The near-field source can be classified as an electric field source or a magnetic field source as to which field component is dominant in the near-field. An electric field source can be represented as a superposition of Hertzian dipoles (elemental electric sources) while a magnetic field source can be represented as a superposition of small loops (elemental magnetic sources). The wave impedances of these sources are where r represents the distance from the source (see Figure 10.10, p. 738). Note that the electric field is dominant in the near field of a Hertzian dipole while the magnetic field is dominant in the near field of the small loop. The wave impedances of both sources approach ç o in the far-field. Inserting the respective wave impedance into the far-field shielding effectiveness terms yields the near-field shielding effectiveness contributions. The reflection loss and multiple reflection terms are functions of the wave impedance (type of source) while the absorption loss term is not.

12 Electric sources Magnetic sources Note that the electric field source generates a high-impedance field in the near-field, while the magnetic field source generates a lowimpedance field in the near-field. The near-fields of these sources have the following variation.

13 The wave impedances very close to the electric field or magnetic field source (assuming â r 1) can be approximated by o The magnitude of the wave impedances very close to the source in terms of wavelength are By identifying the type of near-field source in a given application, one can translate the shielding results for far-field sources to the case of the nearfield source. In addition to simple dipole and loop geometries, examples of electric field and magnetic field sources include such things as spark gaps and brush contacts (electric field sources) and transformers (magnetic field sources).

14 Example Determine the shielding effectiveness in db for a 20 mil thick sheet 7 of copper (ó = S/m) at 1 MHz given (a.) an electric source at a distance of 1 m from the shield (b.) a magnetic source at a distance of 1 m from the shield. (a.) Note that the multiple reflection loss and absorption loss for this nearfield electric source are the same as that found for the previous farfield shielding example.

15 (b.)