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Anechoic Chambers has brought us to the forefront of Shielded and Anechoic Chamber technology.
The following basic information gives our experience in the attenuation of mains frequency magnetic field reduction.
The following is based on full continuity fully welded and gapless joints to all shielded surfaces, augmentation of joints with an overlapping strap may be required.
The behaviour of mains frequency magnetic field is very different to RF and microwave energies and the treatment of reducing the fields is also very different. Aluminium – This medium is common in US Electron Microscopy chambers but not common in Europe, in the Global EMC 50Hz magnetic field cube tests this medium was the lowest in terms of field attenuation. Low Carbon steel – This medium has the lowest commercial cost and is commonly used for 50Hz shielding due to budget constraints, in the Global EMC 50Hz magnetic field cube tests this medium was slightly better than Aluminium in terms of field attenuation. Pure Iron – This medium has the highest permeability compared to the other two shield mediums. Note – There are other exotic ultra-high permeability materials such as Mu-metal, these were not considered as they are not commercially viable. The effect of shielding versus the field strength is not a constant, if the field is low strength then the shield has less effect.
Shielding of cables which emit high magnetic fields can be treated so as to reduce the adverse effects. Flat ferrite tile RF absorbers from Compliance Engineering have excellent absorbing performance, environmental resistance, high strength and good heat resistance. Typical applications of ferrite RF absorbers include RF anechoic chambers for EMC measurements and antenna test facilities. Flat ferrite RF absorbers can be supplied as individual tiles or with the tiles secured to timber panels.
Flat ferrite RF absorbers have good absorption characteristics in the range of 30MHz to 1GHz.
Due to the low profile of flat ferrite absorbers, the volume which the absorbers occupy is significantly reduced compared to pyramidal RF absorber with similar characteristics. Non-flammable characteristics of ferrite absorbers can be utilised for high power radiated immunity test. Easy installationFerrite absorber panels can be screwed directly onto the shielding enclosure to enable easy installation. FU-SE absorber product line is a complete range of high performance pyramidal microwave absorbers.
Based on the principle of having complex magnetic permeability equal (or nearly equal) to complex dielectric constant. Pyramidal Absorbers: The absorber material is cut in a pyramidal or geometrically tapering shape simulating a continuous change in dielectric constant. Flat Absorbers: Layers of homogenous materials with different dielectric constants are stacked so that the final assembly has an increasing impedance (which is stepped instead of continuously varying).
Pyramidal absorber performance is directly related to size - low frequency performance requires larger pyramids. Broadband foam absorbers can be made with "open-cell" materials (typically polyurethane) or "closed-cell" materials (like polystyrene or polyethylene). By matching the impedance of the foam absorber with the ferrite, hybrid absorbers can combine the low frequency advantages of ferrite tile and the high frequency performance of broadband foam absorber. Hybrid absorbers can be made with "open-cell" materials (typically polyurethane) or "closed-cell" materials (like polystyrene or polyethylene). Ferrite tile absorber provides high absorptive performance in the low frequencies from 26MHz to 1GHz. Ferrite tiles can be installed individually using screws or adhesive, but normally, the tiles are mounted on a dielectric spacer before being attached to the walls of the shielded enclosure.
With nearly twice the density of granite, it is conservative to say that cutting ferrite is a bitch.
A safety net for chambers with ferrite tiles on the ceiling is recommended as a low cost safety precaution. Ferrite is a class of ceramic material with a cubic crystalline structure and the chemical formula MOFe2O3 where Fe2O3 is iron oxide and MO refers to a combination of two or more divalent metal oxides. Ferrite tiles are pressed from a powdered precursor and then sintered (fired) in a kiln where they becomes rigid and brittle like other ceramics. The AEP-4 through AEP-48 absorbing materials are based upon a conventional pyramid having a square base and tapering to a point.
The AEP-72 through AEP-96 absorbing materials utilize a unique "twisted 45" geometry with the peak twisted 45 degrees from the base. Broadband wedge absorbers match the anechoic performance of pyramidal absorbers with the exception that, due to the inherent shape of the wedge, they are polarization sensitive. Shielded rooms are notorious for producing multiple reflections and resonances at frequencies from 30 MHz to 1000 MHz.
Absorber performance typically drops off very rapidly as the material becomes less than one quarter wavelength in thickness. The EM series of absorbers is comprised of a pyramidal front material specially loaded to match successive layers of graded dielectric sheets. This graded design also improves the wide angle performance of these products when compared with "conventional" pyramidal materials. Another feature of this series of materials is its ability to be stacked on top of one another. The AEL series absorbers are high performance multilayer, lightweight, flat, laminated absorbers. The Broadband Convoluted absorbers are manufactured from a lightweight urethane foam impregnated with a dielectric material. The convoluted design, when properly loaded, has proven to exhibit exceptional absorption characteristics in the millimeter range, especially at wide angles of incidence. As the wavelengths of the room's operation frequencies become very short, the cell structure of the foam begins to have an effect on the absorber performance. The vent and medium power absorbers are manufactured using special reticulated foam which has a large, open cell structure.
The open cell characteristics of this absorber also increases its ability to dissipate the heat generated by the absorption process. The weatherproof versions of the AEP and AEW series of broadband absorbers offer similar performance to the standard AEP and AEW absorbers, but are manufactured from weather-resistant foam (Type 1) or covered with rubberized cloth (Type 2).
The Type 2 material is manufactured from standard foam absorbers with a manufacturing method similar to our Walkway Series of absorbers and is significantly more robust.

The broadband walkway absorber is manufactured by placing a selfextinguishing polystyrene core in an interlocking pattern over the top of standard AEP type material. When radiated measurements are conducted within a shielded room, it is common to encounter various resonant modes and multimoding depending on the size of the room, the operating wavelength, and the geometrical relationship of the antennas within the room. Ferrite absorbers offer good performance from 30 MHz to 600 MHz, while traditional dielectric foam absorbers deliver performance satisfaction above 500 MHz. The new HyPyr-Loss series of EMC Absorbers incorporates a tuned ferrite absorber with a precision matched urethane pyramidal absorber structure that delivers super-broadband reflectivity performance from 20 MHz to 20 GHz!
AEMI utilizes a proprietary eight-roll impregnation machine that allows for the most uniform impregnation and lossy dielectric distribution in the industry. In the new facility AEMI has installed a custom designed Halogen Tunnel Oven specifically designed to dry the absorbing materials. Many of the specialty absorbers manufactured by AEMI are assembled using multiple layers or materials. AEMI checks 100% of all material manufactured for DJM projects for electrical performance and a sample for mechanical properties.
There are many ways in which to accomplish the placement of anechoic materials on the chamber wall surfaces. The Hybrid Pyramidal Absorber configuration for fully engineered chambers are specifically designed to the customer's specifications utilizing sophisticated computer analysis and ray tracing techniques.
All materials are provided in "kit form" for installation by DJM Electronics at the customer location.
AEMI's hybrid pyramidal absorber is made from homogeneously impregnated polyurethane foam using a proprietary single step impregnation process that locks in both the conductive carbon and fire retardant chemicals.
In the Global EMC 50Hz cube tests this medium was easily the best at shielding 50Hz magnetic fields. Taking the example of a flat plate the energies cannot flow to anywhere, so, the magnetic field will want to re-radiate at the shielding medium edge, thus drastically reducing the shield effect. Taking the example of a sub-station transformer, if enough distance can be afforded then shielding may not be required, although, this is rarely the case! Encapsulation will be required with a choice of permanent shield or with inspection covers. Faraday Cage and Shield Bag consists of a new material: Polyester cloth coated with soft magnetic material by our proprietary process. Materials with this property exhibit an impedance equal to that of "free space" and therefore do not reflect radiated energy at normal incidence.
Tiles are thin (approximately 0.26 inches), but can be heavy (roughly 10 lbs per square foot). By steadily increasing the impedance from that of "free space" at the incident surface of the absorber to a high impedance, lossy material at the rear surface, energy is progressively absorbed and attenuated through ohmic loss as it propagates through the absorber. Although the flat-faced design makes this absorber flexible, lightweight and inexpensive, performance is limited to 20dB because of the higher front-face reflections. Compared to foam absorbers that cover this frequency range, ferrite tile offers the best price for performance in a small, compact package. DJM Electronics can provide tiles pre-mounted on a dielectric backer board in various sizes and configurations.
Straight cuts can be achieved using polycrystalline diamond blades and adequate cooling to prevent stress cracking and chipping. The different metal oxides create ferrites that exhibit different magnetic and absorptive properties.
The mechanical and electromagnetic properties of the ferrite are heavily affected by the mixture of the different ferrites and the sintering process itself.
This unique feature increases wide angle performance, reduces the mismatch at the base transition, and provides additional mechanical support to the peak. This design is used to direct energy down a given path so that it may be efficiently absorbed by large, high performance materials which compose the chamber's termination.
In situations where ferrite absorber cannot be used or is cost-prohibitive, specially designed foam absorbing material can be utilized to increase the repeatability of tests performed within a shielded room. Due to the extremely long wavelength at 30 MHz, conventional pyramidal absorbing materials would normally need to be extremely thick. After the energy has entered the material through the pyramidal front, it is progressively passed on to the higher and higher dielectrically loaded sheets which enhance the energy dissipation characteristics of the absorber. This allows the engineer to provide a temporary wall of absorptive material for the elimination of hot spots within a shielded facility.
These products are made from open cell polyurethane foam layers which are specially impregnated and laminated with flexible adhesives and latex.
The convoluted material has properties especially desirable for applications requiring high reflectivity performance in conjunction with low forward scattering characteristics for use in RCS chambers in the millimeter wave region.
By proper selection of the carbon loading, reflectivities on the order of 45 to 50 dB have been attained consistently at normal incidence over the 18 to 40 GHz frequency range. By properly selecting the carbon loading, reflectivities on the order of 50 to 55 dB have been attained consistently at normal incidence at 95 GHz.
This material is designed to provide about -35dB forward scatter loss at those angles from 18-40 GHz. This allows air to move freely through the material, making these products ideal for HVAC vents or outdoor applications where passage of air is necessary.
The choice of weatherizing method depends on the severity of the environment and the life expectancy requirement of the absorber. These are constructed from a reticulated foam, which allows water to flow freely through the material and not become trapped in the product. A standard pyramidal foam absorber is mated with a non-absorbing, low loss front wedge producing a rectangular shape. This structure is topped with a higher density polystyrene laminate that provides excellent load bearing and wear characteristics that exceed most competitive products. In order to reduce this effect, we have developed the Absorbing Wall Module, which can be easily moved and repositioned within the cavity to absorb interfering signals on a selective basis. The combination of these individual materials can yield unpredictable results if the selected materials have mismatched impedance characteristics.
During a recent EMC chamber project an independent consultant was hired to survey all absorber manufacturers to determine which manufacturer produced the most consistent materials. The new dryer has three independently controlled heat zones as well as a variable speed conveyer system. Each of the necessary components goes through the manufacturing process outlined above before being joined together with special adhesives.

It is common to treat the corners of an anechoic chamber with little or no material to simplify installation.
All materials are then manufactured in strict accordance with mathematical formulas generated from this analysis and design. Materials for corners and wall intersections will be pre-cut at the AEMI factory in order to simplify installation and maintain the integrity of the absorber layout. As a result, AEMI's absorber does not display the unsightly migration of fire retardant chemicals to the surface of the absorber when exposed to damp and highly humid weather.
These absorbers have been extensively tested and is certified to meet the requirements of NRL Fire Test Standard 8093, Parts 1, 2, and 3. The samples must self-extinguish following a 30-second exposure to flame from a Bunsen burner. AEMI standard manufacturing processes and quality assurance practices include sample inspections to the above referenced standards. It is always better to get as close to total encapsulation as possible, thus allowing the energies to flow around the Faraday cage.
When the field strength becomes very high the shielding can go into saturation and becomes less effective. The shielded should run past a sensitive area by at least 3m but could be as much as 6m so as to negate the re-radiation effect.
Variety of thickness gives the chamber designer the opportunity to choose grades appropriate for specific frequencies and incidence angles. Pyramidal absorber is relatively inexpensive, but is confined to higher frequencies because of this size trade-off. Tiles come in sizes as large as 200mm x 200mm, and as small as 60mm x 60mm, but 100mm x 100mm (approximately 4" x 4") tiles are the industry standard.
Each tile is permanently mounted on a dielectric composed of medium density particle board using a high strength polyurethane adhesive. It is through this gradual change in dissipation that high loss at low frequencies can be achieved.
This mobility also gives the engineer the freedom to alter the test environment with movable walls of material whenever conditions and requirements change. Consequently, they are flexible and wrappable and may cut and bonded with contact adhesive for a strong, permanent bond. The geometry in conjunction with the proper loading provides a material with excellent attenuation properties. It is necessary that the foam be "windowless" - the cell structure must be homogenous and open throughout the entire front face of the absorber or the reflections from the irregular geometry will limit the amount of energy that will propagate into the absorbing medium. The material has a second layer which is necessary to provide the low frequency performance of -40 dB at 28 GHz. These absorbers air dry quickly and are best used in an environment where thunderstorms and brief showers may occur spontaneously.
This permits increased efficiency in absorbing characteristics and also allows the user to shield the test environment from personnel or equipment behind the wall modules. However, with proper impedance matching, these two materials can be joined to create a hybrid absorber structure that benefits from the inherent performance characteristics of each of the individual materials.
AEMI's materials were shown to have the most consistent distribution throughout the absorber and were subsequently awarded the project.
However, this is inadvisable due to the possibility of reflections originating from these "bald spots." To eliminate this possibility, AEMI incorporates a "picture frame design" for corner treatment. During the manufacturing process, sample lots are tested regularly for 100% compliance with the specific dielectric loading formulas derived for the customer's application. The absorber materials will then be mounted to the ferrite tile panels with a contact adhesive specially formulated to insure proper attachment. The guarantee period will begin on the date of final acceptance of materials and does not cover any damages due to customer misuse, modifications, or neglect. Another benefit of AEMI's unique impregnation process is the uniform distribution of the conductive materials into the foam, leading to uniformly consistent electrical performance.
No part of this material may be reproduced, translated, transmitted, framed or stored in a retrieval system for public or private use without the written permission of the publisher. This material prevents RF interference from cellular phone, wireless LANs, Bluetooth devices and can suppress outgoing RF by as much as 40 dB for R&D testing. In addition to speeding installation, the dielectric separates the ferrite from the shield wall by at least 12mm thereby maximizing the higher frequency performance of the ferrite.
DJM Electronics has the facilities and equipment necessary to satisfy most cutting requirements. Maintaining correct dimensional tolerances as well as the prevention of cracking and warping related to this shrinkage are fundamental challenges of the manufacturing process.
Resonance of the shielded cavity can be altered at will through the proper placement of this series of absorbers.
The absorber should be backed by a reflective surface, and can be supplied with a conductive backing if required.
These materials can withstand UV and very harsh weather conditions and have a life span of 1 to 5 years depending on the local environmental conditions. However, the module can also be ordered without the metal backing for customers who prefer not to have any additional reflecting surfaces inside the enclosure. It is with this special impregnator that AEMI is able to achieve the desired distribution of the lossy materials within the foam matrix. Once the material is placed on the conveyer it takes from 17 minutes to 42 minutes to dry the product.
The "picture frame" design treats the perimeter of the chamber surface junctions with specially designed 3" to 4" thick flat absorber material that extends the full length of the pyramidal absorber along the wall surface.
Ferrite tile absorber is made of the "soft" variety (usually zinc, nickel, manganese or copper oxides).
The drying time varies with the size and weight of the individual materials being processed. AEMI's "picture frame" approach allows for the edges of the chamber to be shadowed and is the most efficient method of "loading" the edges to eliminate corner reflection interactions.

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