Shielding attenuation

The shielding attenuation is a dimensionless measurement that quantifies the effectiveness of shielding . While the shielding is a technical measure, the shielding attenuation is a measure of the quality of a shield in terms of electromagnetic compatibility . The screen takes over the function - z. B. on the principle of the Faraday cage  - to protect a spatial area that it surrounds against an external electric field , its effectiveness is recorded with the shielding attenuation. The shielding attenuation also describes the protective effect against the magnetic field and the electromagnetic field . In the case of shielded cables, shielding attenuation is also often specified, but the usual, technically unambiguous measured variable for recording the shielding effect of a cable is transfer impedance or outdated coupling resistance.

MIL-STD 285, the military defense equipment standard VG 95370, part 15, or the US standard NSA 65–6 define common measurement methods for shielding attenuation. The IEEE-STD 299 replaced the MIL-STD 285 in 1997.

definition

The shielding attenuation is defined as the ratio of the power density S ₁ at a point in space before a screen is introduced to the power density S ₂ at the same point in space after a screen has been introduced. The power densities have the unit W / m². The shielding attenuation is dimensionless and is usually expressed in logarithmic units in decibels . In decibels it follows for the shielding effectiveness ( se for shielding effectiveness ):

${\displaystyle {\frac {se_{p}}{\mathrm {dB} }}=10\cdot \log _{10}{\frac {S_{1}}{S_{2}}}}$

Transferred to the electric field component E , the formula is:

${\displaystyle {\frac {se_{e}}{\mathrm {dB} }}=20\cdot \log _{10}{\frac {E_{1}}{E_{2}}}}$

Transferred to the magnetic field component H , the formula reads:

${\displaystyle {\frac {se_{h}}{\mathrm {dB} }}=20\cdot \log _{10}{\frac {H_{1}}{H_{2}}}}$

It is assumed that the Poynting vector S , with the field sizes E and H for transverse electromagnetic fields via the equation

${\displaystyle {\vec {S}}={\vec {E}}\times {\vec {H}}}$

are related to each other. This restriction is laid down in MIL-STD 285 with reference to the wave impedance of the electromagnetic field.

The shielding attenuation determined according to the above equations consists of the contributions of the reflection of an electromagnetic wave , the absorption of the wave within a shielding material and the multiple reflection within the material. Assuming a plane wave, the size with index 1 would be the incident field size, the size with index 2 would be the field size transmitted through the screen . Their ratio gives the shielding attenuation, which is represented in the equations in logarithmic measure. In all three of the above equations, the index 1 relates to the field without and the index 2 to the field with shielding at the same point in space.

Location dependency

The sizes with index 2 are within the shielded area. Depending on the type of screen, there may be considerable deviations from the last-mentioned equation for the sizes within the screen, because then the influence of the screen, e.g. B. by its geometry or by apertures in the shield cover, a transverse electromagnetic field may not necessarily be assumed in all cases. The shielding attenuation is therefore highly location-dependent and can vary considerably within a shielding device. It is therefore essential to determine the location at which the screening attenuation is to be determined within the screening. Usual methods define the center point of a screen facility as the reference point. However, as expected, the attenuation is greatest there, so that this choice of location is often not suitable for detecting a weak point in the shield. Measurement regulations also stipulate distances between one meter and thirty centimeters from the screen wall for the measurement location.

Effectiveness of a shield and influence on the shielding attenuation

In order to explain the mechanisms that lead to the degradation of the shielding attenuation, the effect of a shield is briefly explained. More detailed descriptions of the modes of action can be found under the keyword shielding (electrical engineering) . The shielding attenuation quantifies the effect of a shield against electrical, magnetic or electromagnetic fields. The modes of action of shields against such fields are briefly summarized below.

Shielding of a low-frequency electric field

In relation to electrostatic fields and low-frequency electric fields, an electrically conductive screen works according to the Faraday cage principle.

Shielding of a low-frequency magnetic field

A screen acts against low-frequency magnetic fields due to the high permeability of the screen material, which means that the magnetic flux density is concentrated in the screen material. A low-impedance drain wire or coaxial cable shield creates a magnetic shield because the current induced in the low-impedance wire counteracts the exciting magnetic field with its magnetic field. This mechanism only works with a cable shield or drain wire attached on both sides, because no current can flow when the shield is attached on one side.

Shielding an electromagnetic field

Compared to high-frequency electromagnetic fields, an electrically conductive screen acts against the electrical field component based on the Faraday cage principle. Compared to the high-frequency magnetic field component, a screen acts on the basis of equalizing currents which in turn generate an opposing field that compensates for the external incident magnetic field component (see Lenz's rule ). So that a compensating current can flow, z. B. Cable shields can be connected at both ends. With increasing frequency, the shielding attenuation increases due to the skin effect in the shielding material, which keeps the field penetrating into the shielding wall away from the inside of the shield. Shields can also be made of lossy materials that convert the electromagnetic field energy into heat.

Degradation mechanisms

The shielding attenuation is degraded by openings (leaks) in the shield. This also includes narrow gaps where the maximum expansion (the gap length) is the dominant effect for the degradation of the shielding attenuation. The actual field penetration at an aperture for the ideally conductive screen is proportional to the surface current that is established at the location of the aperture due to the application of the field.

Furthermore, a shielding effect is impaired by the reduced electrical conductivity of a shielding material or, with respect to low-frequency magnetic fields, by a low permeability of the shielding material.

Compared to high-frequency electromagnetic fields, a screen is generally considerably weakened if no equalizing current can flow, e.g. B. with one-sided shielding on a shielded cable. A shield then only has a capacitive effect, while the decisive unwanted inductive coupling of the magnetic field component acts directly on the shielded assemblies or conductors.

The shielding effect is also degraded if there is insufficient absorption in a lossy material.

Measurement method

Well-known standards for measuring methods for determining the shielding attenuation are the MIL-STD 285, which has been repealed since 1997 but is still frequently used, the military defense equipment standard VG 95370, part 15, or the US standard NSA 65-6. The MIL-STD 285 has been replaced by the IEEE-STD-299 regulation.

literature

• H. Kaden: Eddy currents and shielding in communications engineering . 2., completely rework. Edition. Springer Verlag, 2006, ISBN 3-540-32569-7 (first edition: 1959).

Individual evidence

1. ↑ Military Standard MIL-STD-285 ( Memento from July 14, 2007 in the Internet Archive )
2. ↑ Troubleshooting in shielded rooms ( Memento from August 25, 2014 in the Internet Archive ) (PDF; 417 kB)