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Types of absorbers
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Types of absorbers
Electromagnetic absorbers take many forms depending on the purpose and frequency of operation.
- Narrowband absorber (Frequency band of 10% for a reflectivity of less than -20dB)
- Single layer or resonant absorbers
- Single layer or resonant absorbers
- Broadband absorber
- Magnetic Single layer such as ferrite absorber with an advantageous ratio between permittivity and permeability (wave impedance of the material).
- Multilayered or geometric transition absorber.
1. Resonant absorber (Silicone or Polyurethane sheet)
When good reflectivity performances are obtained for a limited frequency band the materials are called narrow band absorbers. They are resonant materials. For a particular thickness and permittivity or permeability the reflected wave is very low.
Quarter wave absorber or resonant absorbers use the cancelling of the reflection at the air/absorber interface with another from the interface absorber/metal.
This resonant condition appears when the difference between these two “paths” is equal to a half guided wave at normal incidence they are in opposite phase (thickness equal to (2n+1) λ/4).
Silicone and polyurethane sheets are a thin flexible resonant absorber available for frequencies within 1.5 to 18 GHz. The typical reflectivity is – 25 dB at the centre frequency and the band width is 10%. These absorbers can line shielded cabinets used for sensitivity testing for narrow band radio equipment.
2. Ferrite absorber
If a combination of dielectric and magnetic materials were to be used in an absorber the velocity of propagation and hence wavelength may be reduced. This can potentially reduce the thickness of an absorber. This is the case for ferrite absorbers.
Ferrites are ceramic materials (hard or soft). For absorber applications two types of magnetically soft material have interesting properties: Nickel-Zinc ferrite (Ni Zn) and Manganese-Zinc ferrite (Mn Zn). But the first one Nickel Zinc ferrite (FE) is more commonly used for EMC chambers.
The thickness of the ferrite tiles varies from 4 to 7 mm (typically 6.7mm). The resonant frequency is approximately 180 MHz and the reflectivity goes down to – 35 dB. Ferrite tiles are resonant absorbers nevertheless if the thickness is optimised reflectivity around the resonant frequency is still interesting for EMC chambers. The reflectivity is -17 dB at 30 MHz and -10 dB at 1 GHz at normal incidence.
To be a broadband absorber the material should have 2 main properties. First the incident wave should not be reflected at the surface (interface) of the material. Any mismatch of impedance should be avoided. The impedance of the material should be close to air impedance (377 Ohms). Furthermore the material should have loss properties. Very few materials in fact comply with both criteria.
3. A multi-layered absorber
An approach toward a gradually tapering resistance profile is to use bulk absorbers. These can be made by loading a structural material like polymer foam with increasing carbon loading in depth thereby decreasing impedance and increasing attenuation.
One of the techniques used to comply with the two criteria is to manufacture multilayer materials. Each layer has a different load. It has different electromagnetic properties and impedances. The first layer (in front of the incident wave) is made of low load.
The impedance of the first layer will be close to the air impedance. In consequence the reflectivity at the interface of the material will be reduced. The load of the next layer will be increased to absorb the electromagnetic energy propagating inside the material. With several layers it is possible to reach the desired reflectivity.
The load can be electric and/or magnetic. Carbon is usually used for the electric load. The figure below shows three-layer absorbers (AT) and five-layer absorbers (AH).
4. Geometric transition absorbers
A second technique to avoid mismatched impedance on the interface of the material is to use a shaped material. This may be a wedge pyramidal convoluted shape or any complex shape in which the section of the material increases in the direction of the wave. The electromagnetic energy propagating inside the material will be transformed into heat.
4.1 Pyramidal absorber (APM)
Pyramidal foam materials (APM) are the most commonly used absorbers. The height of the absorbers determines the reflectivity performances.
The performances of pyramidal absorbers are better than flat absorbers particularly at higher frequencies where any reflections from the surface of the pyramids tend to be channelled down into the absorber. When the height of the pyramids is greater than 10 wave lengths the reflectivity may reach –52 dB.
Such absorbers are mainly used for anechoic chambers used for EMC Radar Cross Section Antenna pattern measurements.
4.2 Truncated pyramidal absorbers (APX)
In some cases truncated pyramidal absorbers (APX) are also used to line EMC anechoic chambers. The height of the absorbers is decreased but the performances are also reduced for frequencies higher than 1 GHz.
4.3Wedge absorber (ADM)
The wedge absorber (ADM) has the same geometry as the pyramidal absorber in one direction and uniform shape in the other. It is used in chamber designs where it is desirable to have the energy guided into a terminating wall. In fact with such absorbers lined on some critical areas it is possible to reduce the non-specular wave energy and to guide the wave in the direction of a terminating wall. By this method it is possible to obtain a double absorption and to reach high reflectivity performance levels.
4.4 Convoluted microwave absorber (APC)
Another broad-band microwave absorber uses a convoluted form (APC). It is especially useful in millimetre bands.
5. Combination of several techniques
To improve the performances of the absorbers some products have been designed which are a combination of several of the techniques described above.
5.1 Hybrid absorbers (HY)
With ferrite tiles it is possible to achieve good reflectivity results for the low frequencies (30 MHz –1 GHz). Also the pyramidal absorbers have excellent reflectivity for frequencies higher than 1 GHz. The idea is to combine both techniques. Nevertheless it is not so simple special loaded pyramidal foam absorbers have to be manufactured. The load of the foam material is reduced to decrease the mismatch of impedance at the interface.
This combination of ferrite and pyramidal foam (HY) is able to cover the frequency bandwidth from 30 MHz up to 100 GHz. These hybrid absorbers are mostly used in EMC anechoic chambers
5.2 Mixed absorbers (MI)Mixed absorbers use the two techniques of geometric gradient and multilayer technique (MI). The frequency band covered by the mixed absorbers is very large: 30 MHz up to 100 GHz. For the highest model it is possible to reach –15 dB at 30 MHz and – 52 dB for a frequency higher than 1 GHz.
MI absorbers are the better absorbers in terms of reflectivity performance criteria; also the frequency bandwidth covers both Radio EMC antenna pattern measurements etc. They are used when the other materials can not reach the required performance levels and when the anechoic chamber has multiple uses.
6. Conclusion
For a given frequency band several choices of absorber models are possible. Nevertheless depending on the final use made of the chamber the best choice depends on cost and technical criteria. With a detailed specification of needs (frequency bandwidth reflectivity level standards to apply) the manufacturer of the chamber is able to define the optimum absorber material.
Bibliography
C.A. Balanis Advanced engineering electromagnetics 1989 1008pp ISBN 0-471-62194-3.
L.H. Hemming Electromagnetic anechoic chambers: A fundamental design and specification guide 2002 240 pp. ISBN 0-471-20810-8.
- Narrowband absorber (Frequency band of 10% for a reflectivity of less than -20dB)
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