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EMI / RFI Shielding Considerations
In general, the following is true for shielding of unwanted radiation emissions for Electromagnetic compatibility (EMC):
- It is desired in most cases to reflect and / or absorb Electromagnetic Interference (EMI) radiation.
- Absorption improves with increasing EMI, Radio Frequency Interference (RFI) frequency, and / or increased barrier thickness, increased conductivity (lower material or coating resistance), and / or increased barrier permeability.
- Reflection improves with increasing conductivity and / or decreasing permeability.
- E-Field Reflection increases with decrease in frequency and / or decrease in distance between the noise source and shield barrier.
- H-Field Reflection increases with increase in frequency and / or increase in distance between the noise source and shield barrier.
- Plane Wave Reflection will increase with a decrease in frequency.
- Apertures, or “openings” in the shielded environment should be no larger than 1/20Th the wavelength of the highest noise frequency for Commercial applications and 1/50th the wavelength of the highest noise frequency for Military (mil-spec) applications.
- Seams in conductive-shielded enclosures should a 5 to 1 overlap to gap spacing ratio.
Terms
- Absorption - Electromagnetic energy loss or dissipation through a transmission medium.
- Absorption Loss - Ratio of energy entering a medium to that absorbed by the medium.
- Conductivity - Unit cube conductance of a material or reciprocal of the volume resistivity in ohms per centimeter.
- E-Field - High impedance or electric component of an electromagnetic wave. E- Field induces a charge on a shield and is measured in volts per meter.
- H-Field - Low impedance or magnetic component of an electromagnetic wave. H-Field induces current in a shield.
- Permeability - The ability of a material to serve as a path for magnetic lines of force based on air as = to 1. Permeability is also defined as magnetic induction B in gauss divided by the magnetizing force H in oersteds.
- Plane Wave - Electromagnetic wave in which all points normal to the direction of propagation or in phase.
- Reflection - Redirection of electromagnetic energy.
- Reflection Loss - Ratio of energy incident at an air-metal boundary of a shield to that reflected. Measured in dB.
Appropriate Shielding Materials Properties
Order of Conductivity
| Material |
Conductivity |
Permeability @100Khz |
Absorption L @1Mhz |
Reflection L @1Mhz |
| Group 1
|
| Silver
| 1.08
| 1
| 3.46
| +0.3
|
| Copper
| 1.00
| 1
| 3.33
| 0.0
|
| Gold
| 0.70
| 1
| 2.79
| -1.6
|
| Chromium
| 0.66
| 1
| 2.71
| -1.8
|
| Aluminum
| 0.61
| 1
| 2.60
| -2.2
|
| Brass
| 0.47
| 1
| 2.29
| -3.3
|
| Magnesium
| 0.37
| 1
| 2.03
| -4.3
|
| Tungsten
| 0.31
| 1
| 1.86
| -5.1
|
| Zinc
| 0.30
| 1
| 1.83
| -5.2
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| Cadmium
| 0.23
| 1
| 1.60
| -6.4
|
| Nickel
| 0.22
| 1
| 1.56
| -6.6
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| Phosphor-Bronze
| 0.22
| 1
| 1.56
| -6.6
|
| Tin
| 0.15
| 1
| 1.29
| -8.2
|
| Beryllium
| 0.10
| 1
| 1.05
| -10.0
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| Lead
| 0.08
| 1
| 0.94
| -11.0
|
| Monel
| 0.041
| 1
| 0.68
| -13.9
|
| Manganese
| 0.040
| 1
| 0.67
| -14.0
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| Titanium
| 0.039
| 1
| 0.66
| -14.1
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| Group 2
|
| Silicon Iron
| 0.23
| 10
| 1.60
| -6.4
|
| Permalloy
| 0.21
| 50
| 1.53
| -6.8
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| Hypernik
| 0.21
| 95
| 1.53
| -6.8
|
| Mu-Metal
| 0.20
| 400
| 1.49
| -7.0
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| Supermalloy
| 0.20
| 2000
| 1.49
| -7.0
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| Iron (purified)
| 0.17
| 100
| 1.37
| -7.7
|
| Iron
| 0.17
| 10
| 1.37
| -7.7
|
| Steel
| 0.10
| 5
| 1.05
| -10.0
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Total Shield Effectiveness
Total Shield Effectiveness can be reduced to the following:
Total Shielding = Reflection + Absorption + Re-Reflection
Loss Model, Solid Conductive Barrier:
Aperture size vs. highest noise frequency:
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