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Some problems should be paid attention to in establishing a half - wave anechoic chamber


       In the field of product development, electromagnetic compatibility (EMC) research is becoming more and more important. Many engineering departments wish to have their own EMC testing environment. In EMC testing, the radiation emission measurement of products is particularly important to the test environment and equipment requirements. The environment required for radiation emission is an open field (OATS) or a semi-anechoic chamber (SAC). For other forms of EMC testing, a workbench or a shielded room is sufficient; for the realization of radiation immunity test indicators, a full anechoic chamber is used.

  This article mainly discusses some site design issues for radiated emission testing. An open field is the preferred testing site. However, due to increasing electromagnetic "pollution" and the dependence of open fields on climate, semi-anechoic chambers have become an affordable alternative. In this paper, some introductions are made to the design and construction of SAC for radiation emission testing in combination with civilian EMC testing standards.

  1. Shielded room

  The SAC consists of a shielded room filled with absorbing material. The shielded room isolates the internal chamber from the external electromagnetic environment. The environmental electromagnetic spectrum comes from television signals, radio broadcasts, personal communication devices, and man-made environmental noise. The function of the shielding room is to make the external disturbance intensity in the shielding room significantly lower than the interference field intensity generated by the equipment under test (EUT) itself.

  In the construction of SAC's shielded room, there are two basic construction methods: combined type and welding type. The combined type consists of wall panels and clamps that connect the panels. Wall panels can be plywood or galvanized steel sheets covered on both sides with a thin galvanized layer. The clamp makes the wall panel installation as a whole and ensures the electrical continuity of the wall panel. At the same time, pads and high-frequency absorbing materials are often used to improve shielding effectiveness. Even though most manufacturers apply the same shielding system concept, the performance of each product on the market is not consistent with each other due to the individual characteristics of the equipment.

  The welded structure is a tightly sealed body for radio frequency signals welded by steel plates or copper plates. This is a technique that requires precision. The high-level welding body makes the shielding effect stable and reliable, and the high-performance shielding effect depends on the elimination of weld seam leaks. Of course, the unsatisfactory factor of the welded structure is the higher cost.

  The floor is an important part for EMC testing in SAC. In the radiated emission test, a part of the EUT's transmitted signal is reflected by the floor and measured and received by the receiving antenna, just like the actual situation in the office. Simulate a good floor with conductive continuity and as little surface undulation as possible. We can achieve this effect by building a raised floor. The so-called raised floor is a raised floor made of the same metal material as the walls and ceiling. The cables for measurement and control, the power lines, the mechanics of the turntable are placed under the raised floor. The raised floor generally has a height of 30cm to 60cm according to the mechanical part of the turntable. In order to obtain a complete conductive continuity of the floor, the conductive surface of the turntable is guaranteed to be conductively continuous with the surrounding floor. It is usually realized by the method of equidistant space connection of grounding rings.

  For operational purposes, a perforation of the shielded room is required. Perforations need to be carefully selected and constructed to maintain the integrity of the shielded room. A typical SAC includes several types of basic perforations described below.

  1.1 Access door

  Obviously, at least one door is required. The most common component is a groove contact device, that is, a single-pole double-spring, the door is a single-pole structure, the door frame is a groove structure, and there are double springs in the groove to maintain electrical continuity; or an air-tight gasket, the door and the door frame are pressed through tight to ensure electrical continuity. More popular and inexpensive are revolving doors, which have one or two hinges. The revolving door can be equipped with absorbing material on one or two hinges, but the quiet space after opening the door is very small. To make up for this, sliding doors are also an option, which offer the advantage of being easy to use and affordable.

  1.2 Waveguide window

  For air flow and cooling purposes, airflow below the cutoff frequency enters and exits through honeycomb vents that act as cutoff waveguides with only a small differential pressure change. The operating frequency of most waveguide windows can reach 10GHz. For higher frequencies, such as reaching 40GHz, more advanced designs are required.

  1.3 Power supply

  The power line filter installed in the shielding room is used for power filtering, including turntable, antenna mast, EUT and related equipment in the shielding room. The filter is suitable for AC and DC filtering of high current and high voltage (400V). The referenced standards are MIL-STD-220A for electrical performance assessment and UL1283 for operational safety.

  1.4 lights

  Incandescent lamps can be installed indoors. It is usually installed on the ceiling with high-hat lamps to obtain sufficient lighting and reduce the impact on absorbing materials.

  1.5 Interface board

  The interface board is also a cut-off waveguide, including radio frequency interface for emission measurement, EUT signal interface, filter interface, optical fiber inlet, fire control cable. Fiber optic control cables are used in turntables, antenna masts and CCTV systems. Other perforations include various penetrations, such as refrigeration applications and mechanical systems for air intake and exhaust.

  2. Shielding effectiveness

  The performance of a shielded room is defined by the Shielding Effectiveness (SE). Its meaning is the signal attenuation due to the existence of the shielding room. Currently, the widely used standard for defining SE is NSA65-6 (Table 1). In this standard, the defined attenuation level has exceeded the test requirements of EMC, and it is also sufficient for some other application tests. In the application of EMC, SE is defined on one or some special frequencies. At the commonly used frequency point of 1GHz, the shielding effectiveness of the combined type is 100dB, and the shielding effectiveness of the welded type shielding room can be 120dB.

  Table 1 NSA 65-6 Performance Requirements for Shielding Effectiveness

 

  Before installing the absorbing material, the SE of the shielded room should be tested to confirm that the shielded room meets the specified shielding level. Similar to NSA65-6, the current standards for shielding effectiveness testing include MIL-STD-285 and IEEE299-1997. Academically, IEEE299-1997 is considered to be a more detailed and wider version after MIL-STD-285 written in 1956. It not only describes the test plan, but also has strict test locations (doors, panel seams and other perforations). Since it is difficult to guarantee SE near the perforation, extra attention should be paid to the integrity of the shield near the perforation.

  3. Electromagnetic absorbing material

  The electromagnetic absorbing material is installed on the wall and ceiling of the shielding room to reduce the electromagnetic reflection of the surface. Electromagnetic radiation is absorbed by the absorbing material when it is incident, and part of the electromagnetic energy is converted into heat energy. Of course, some lingering reflections exist and can interfere with testing.

  Table 2 Common broadband EMC absorbing materials 

       In SAC, there are currently two widely used broadband electromagnetic absorbing materials, which are distinguished according to their working mechanism: ferrite which absorbs magnetic field radiation and carbon-added foam which absorbs electric field radiation. A hybrid material is composed of both materials. Of course, there are some special designs, but they are not widely used. Table 2 shows the design of some typical absorbing materials and their characteristics. Most of the foam-type absorbing materials are made into a cone shape, while the hybrid type is made into a wedge shape. Ferrite patches are generally installed on a non-conductive wall (usually plywood), so that the high-frequency performance of the patch can be improved.

  The design of broadband EMC absorbing materials is a complex process, which requires trade-offs and coordination between low-frequency and high-frequency performance, size and engineering cost. Usually, manufacturers often use the trial and error method when designing absorbing materials, and work through the process of design, trial and error. To speed up the design process and make it economical, many manufacturers use computer-aided design. With computer-aided design, the manufacture and measurement of absorbing materials are ignored, only advanced design is needed, and computer-based optimization design is required. If accurate models are used and a large number of absorbing material parameters are determined, high-quality absorbing materials can be produced either by extensive trial-and-error design methods or by computer-aided design.

  When most manufacturers explain the performance of absorbing materials, they only consider the case of vertical incidence. This is an optimized figure, only with absorbers that perform well for direct normal incidence. However, it is more important to consider oblique shooting in SAC than vertical. It is related to the attenuation of the wave incident on the surface of the shielded room. Most absorbers perform well at normal incidence. But it is more important to consider the oblique case than the vertical in SAC. As the incident angle increases, the performance of the absorbing material decreases obviously. Therefore, this is an important factor when designing an anechoic chamber.

  In SAC, the performance of the absorber is not only determined by the basic design performance of the absorber. The quality of the installation of the absorber also plays a large role. Ferrites in particular, regardless of hybrid design or not, can suffer from performance degradation due to improper installation. Due to the size limitation of a single ferrite full patch, there is a small air gap between two adjacent patches. These small air gaps act like reluctance, reducing the continuity of magnetic energy between the patches, thus reducing the absorption effect. With careful installation, individual air gaps can be less than a few tenths of a millimeter wide. A large air gap will result in a considerable reduction of incident small attenuation, thus permitting larger reflections from certain parts of the shielded room wall. In the design of absorbing materials and anechoic chambers, the so-called air gap effect must be considered, because air gaps are often encountered in actual installations. Even a small air gap will degrade the performance of the ferrite patch, making the actual level lower than the theoretical level.

  The measurement of absorbers is an important part of confirming their performance. Due to the strict requirements on the low-frequency performance of SAC, the performance of the absorbing material should be confirmed to the lower limit of 30MHz. From 150MHz to 30MHz or lower can be measured with coaxial waveguide. In the high frequency band, other types of waveguides (100MHz and above) and free space methods (above 800MHz) can be used for testing.

  4. Anechoic chamber design technology

  In order to construct a SAC that meets site attenuation requirements, the difference between the measured normalized site attenuation value and the ideal value for an ideal open field (according to standard ANSI C63.4-1992) should be less than 4dB. This indicator faces many challenges, especially in the low frequency band, the electric field size of the absorbing material is small, and the electromagnetic performance is poor. Therefore, before the construction of the anechoic chamber, digital simulation is needed to confirm and optimize the design of the anechoic chamber. Fabricators may choose to try and design, but this is time consuming and costly. Numerical simulation, combined with corrections to measured performance data of as-built anechoic chambers, is an effective design tool for designers of today's anechoic chambers.

  In the middle and high ranges of the working frequency range, the electromagnetic waves incident on the absorbing material can be considered as plane waves. In this case, using the method of ray tracing to simulate the performance of the anechoic chamber, a credible estimation of the performance of the anechoic chamber will be obtained. For low frequencies, however, the plane wave assumption is no longer valid.

  For the low-frequency range, there are two methods for the performance model of the anechoic chamber: one is the simulation of the ray-tracing technique in the high-frequency case, and the other is the Maxwell equation in the three-dimensional case for the shielded room equipped with absorbing materials solution.

  In the case of ray tracing, due to the low-frequency properties of the absorbing material and the size of the anechoic chamber, multiple reflections must be considered. Since the test data of absorbing materials in the low frequency band is more difficult to measure at any angle than the vertical case, digital simulation data is often used. It should be noted that this simulated absorber performance data is closely related to the normal incidence measurement data to avoid systematic errors in the anechoic chamber simulation. In the multi-level ray tracing model, the performance simulation of the measured 10m anechoic chamber is better than that of the 3m anechoic chamber, because the electric space in the 10m anechoic chamber is large enough, the volume of the absorbing material used is large, and the low-section performance is good.

  Since solving Maxwell's equations in 3D is an intensive computational task, the finite element method or finite difference method is usually used. These methods are to divide the space to be calculated into discrete units so as to use Maxwell's equations for calculation. For the case of low frequency band, the absorbing material is approximated as a low frequency thin layer, which can reduce the difficulty of calculation. However, the accuracy of this algorithm largely depends on the use of absorbing material models, tests of absorbing material performance and a large amount of data. In theory, this method is more accurate and reliable than the ray tracing method. However, compared with multi-level ray technology, the installation of absorbing materials and the limitations of anechoic chamber measurement lead to uncertainties in the implementation process, and also limit the accuracy of the actual design.

  5. Construction of the laboratory

  In the above sections, we have introduced several main issues, including SAC design, shielding effectiveness, absorbing materials and anechoic chamber model. This section focuses on the overall implementation of these aspects.

  The multi-level reflection ray tracing method has the advantage of being convenient for calculation. Using this technique, designers can select optimal designs from numerous design drafts. An experienced design engineer can analyze and collate the data to assure the performance of the anechoic chamber, regardless of the inherent limitations of modeling techniques.

  When building an EMC test laboratory, a considerable amount of space is required to accommodate the anechoic chamber and related equipment. Typical design dimensions are shown in Table 3. In addition to the data given in Table 3, we also have to consider fire protection facilities, raised floors, steel structures that reinforce the shielded room to be able to load the quality of the absorbing material and ensure its integrity.

  Table 3 Half-wave anechoic chamber parameters

  After the construction of SAC and related equipment, its performance should be verified to prove that it is feasible to replace the ideal OATS with SAC. In private EMC facilities, SAC performance testing follows the alternative field method described in standard ANSI C63.4-1992, CISPR22 or related standards. These test procedures demonstrate the performance of the anechoic chamber by comparing the field attenuation of the anechoic chamber with the OATS. Site attenuation is measured in a stationary area around the EUT on a turntable according to the theory described in the standard for an alternate site. The frequency range of this test procedure is determined according to the requirements of radiated emission for EUT testing. After the initial verification is established, the operation of the SAC shall be established on an annual verification basis.

  The performance of SAC depends on many factors, one of which is the installation of the absorbing material. Special attention should be paid to the air gap effect of ferrite patches, especially in doors and other perforated places, where the absorbing material is discontinuous. Care should also be taken with the arrangement of doors, interface panels and windows. Be careful not to cause performance problems at discontinuities in the absorbing material, and to avoid parasitic reflections and emissions caused by untreated reflective materials. In addition, the floor must be very flat and electrical continuity must be ensured around the turntable. When validating an anechoic chamber, the antenna factor plays a critical role. In addition, over time, the wave-absorbing material, especially the wedge foam, will tilt, which has little impact on performance, but has some negative effects.

  An important issue is quality control when selecting absorbers or manufacturers of darkrooms. Since the performance of the absorbing material is the most important factor in the electromagnetic performance of SAC, attention should be paid to whether the manufacturer can guarantee that the performance of each batch of absorbing materials produced in the factory is consistent. It is best to have a quality control program to ensure that the electromagnetic properties of each batch of absorbing materials are strictly tested in the low frequency range. Moreover, the performance of the anechoic chamber is related to the installation quality of the absorbing material, so it is necessary to arrange experienced personnel to control the quality during the installation.

  In general, EMC test equipment is not just SAC. According to the needs of the budget and experiments, shielded control rooms and laboratories can be added, as well as full anechoic chambers and pre-test anechoic chambers for measuring immunity. At a minimum, there must be sufficient space to accommodate test equipment and test personnel.

  6. Summary

  This article touches on the general situation in constructing SAC, but it cannot fully relate to all problems in constructing SAC. Some important issues such as fire safety and structural integrity require further study. In conclusion, building a SAC is not a simple task, and there are a large number of factors that affect the electromagnetic performance and functionality of a SAC. Especially for fully adaptive anechoic chambers, for a test distance of 3m or 10m, quality control, design capabilities and existing work performance all play an important role in selecting an anechoic chamber manufacturer. Moreover, the successful operation of EMC equipment is related to the use of test ancillary equipment (turntables, antenna masts, antennas, cables) and measuring instruments. At the same time, the experience of the test personnel is also important.

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