The Important Points for Determining the Requirements of EMC Chambers

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1 1 The Important Points for Determining the Requirements of EMC Chambers Erdem Akpınar, Yakup Erdoğan, ASELSAN Inc. Abstract The establishment of complex EMC test facility requires very big budget, area and special featured building. Due to these reasons, determining the requirements of EMC Chambers is very important. The purpose of this paper is providing general overview of the chamber requirements content and briefly addressing a few aspects of some specification elements. This paper especially evaluates the chambers, compatible for different applications such as military equipment, commercial equipment and military vehicle. Big chambers can be used for vehicle tests and also as 10 meter semi-anechoic chamber provided that the requirements are met. Index Terms EMC Chambers, 10 meter anechoic chamber, military test M I. INTRODUCTION ILITARY and commercial radiated tests can be applied at Open Area Test Site (OATS) provided that the ambient electromagnetic level and reflection are low enough for testing. Due to the fact that providing low enough ambient electromagnetic level is nearly impossible at residential area, electromagnetic shielding is required. The metal walls provide shielding but also cause reflections. The reflected signals are not completely cancelled but can be decreased using electromagnetic absorbers on the walls. If the shielded room also has absorbers on all walls, floor and ceiling the room will be named fully anechoic chamber. Military standard MIL- STD-461F requires only absorption at normal incidence for absorbers. Commercial requirements for radiated emission test area are described in CISPR These are Normalized Site Attenuation (NSA) and Site Voltage Standing Wave Ratio (SVSWR) requirements. Commercial requirements for radiated susceptibility test area is described in IEC or EN This addresses Field Uniformity (FU). Generally EMC chambers does not contain absorbers on the floor and called as semi-anechoic chambers. The purpose of this paper is providing general overview of the semi-anechoic chamber requirements. 30 July This work was supported by ASELSAN Inc. E. Akpınar is, a senior design lead engineer, with ASELSAN Inc. Radar EW and Intelligence Systems Division, Gölbaşı, Ankara. He got his BSc. degree from Istanbul Technical University ( eakpinar@aselsan.com.tr). Y. Erdoğan is, a senior expert engineer, with ASELSAN Inc. Radar EW and Intelligence Systems Division, Gölbaşı, Ankara. He got his BSc. and MSc. degrees from Middle East Technical University ( yerdogan@aselsan.com.tr). II. EMC TEST CHAMBER REQUIREMENTS Test chamber requirements must be determined by the last user in accordance with the purpose of chambers, building limitation, laboratory accreditation requirements and laboratory automation requirements. If it is possible, separate and dedicated building is preferred for big EMC laboratory. Test chamber main requirements are: A. Dimension B. Shielding C. Steel Structure D. Doors E. Floor and Ground Plane F. Filters G. Penetration Panels, Floor Panels and Interfaces H. Earthing I. Lighting J. Fire and Gas Detection K. Honeycombs L. Camera, Mast and Audio Systems M. Absorbers and Performance Requirements N. Testing and Validation O. Control Room, Amplifier Room In this paper, general information can be found for each requirement. A. Chamber Dimensions The main parameters affecting the chamber dimensions are EUT dimensions, absorbers, test antenna and test standard. For military test chamber a general test diagram which shows EUT and associated cables, test table, antenna, test distance, minimum distance between antenna and absorbers, minimum distance between EUT and absorbers must be drawn. In this manner, the required minimum distance between absorbers tips can be found. By adding the absorbers depth and shielding thickness, the required minimum dimensions can be found. However, the power density on the absorber which will be formed in radiated susceptibility test must be calculated and if the power density is over the acceptable level of absorber, the distances must be increased to provide a safe level on absorber. For military test chamber if the EUT is a vehicle, the first method is to fix EUT at one position and use multiple antenna positions. The second method is to place EUT over a turn table and use single antenna position. At first method, the required chamber dimension will be bigger. In contrast, at the

2 2 second method the turn table which is compatible to EUT weight and dimension is very expensive. For commercial test chamber, additional to military chamber dimension calculation method, chamber performance requirements are also important. To achieve NSA, FU and SVSWR, chamber dimensions, antenna placing and absorbers are important. To determine these parameters, electromagnetic simulation can be used but the chamber supplier experience is more diagnostic. B. Shielding The shielding effectiveness is very important for EMC chamber. If it is possible, the shielding with better performance must be preferred; because the external electromagnetic field cannot be estimated. There are two common modular methods for shielding type. 1) Pan Type 2) Sandwich Panel Fig. 1. Shielding Types. Pan type is most popular and newer method due to flexibility and modularity. For EMC chamber generally galvanized steel sheets are used and the standard module configuration is bolted every mm from inside/outside with high conductivity mesh gasket inserted and compressed into the gap. Sandwich type consist of 20 mm thick laminated type shield panels. There is particle board between panels. The panels assembly is made without drilling any hole in the shielding and without gaskets, using electro-galvanized steel framing (omega profiles). The fixing of omega profiles must be very tight and this causes wavy appearance of shielding. This technique is not succesfull at the corner and conductive tape can be required to achieve shielding performance. The pan type performance is a little bit better however both of them are enough for EMC testing. The gasket of pan type and the particle board of sandwich type are effected from water and moisture. So, the design of building must be made to keep away water and moisture from the chambers. C. Steel Structure If the chamber dimensions are big, steel structure is necessary to support the shielding and very heavy absorbers. The metallic columns of steel structure must be connected to the building concrete. If it is possible, self supporting steel support frame dedicated to the chamber and connected just from the floor to the main building. Relevant stiffeners and fasteners must be supplied by the chamber supplier. As a precaution to earthquake, separate foundation from main building can be used. The steel structure must support horizontal acceleration of earthquake. D. Doors There are 3 door types used in EMC chambers. One of them is single swing door. This door has small dimensions and used generally for personnel entry. The second type is double swing door. This door has bigger dimensions and used generally for big EUT and vehicle entry. The last type is sliding door. The dimensions and purpose of sliding door are also same with double swing door. The sliding door requires 3 to 4 meters spacing in front of the chamber and the spacing width must be two times the door width. In contrast, the shielding performance of sliding type door is better than double swing door and the maintenance period is longer. For better shielding performance the door must have triple knife system. Since there is height difference between chamber floor and outside, there must be a ramp in front of the door. The ramp must be pneumatic for big door. E. Floor and Ground Plane Floor has 3 important functions. First function is carrying EUT and vehicle. The floor must be designed to be compatible with point load and total load of EUT. If the EUT is a heavy vehicle, the floor must be heavy load area. The whole floor or some part of it can be heavy load area. This heavy load area must be made of concrete floor with immersed steel structure covered by a welded steel ground plane of 8 to 10 mm thickness. The second function of the floor is to constitute ground plane. Ground plane reflects electromagnetic waves and it is important especially for NSA performance. The painting of the ground plane must not affect the NSA performance. The other function of the floor is to be used as raised floor. The electrical and RF cable can be routed under the floor. At the start of the design, all cable must be considered. If required, plastic tubes must be placed inside the concrete for cable routing under heavy load area. F. Filters Electrical power must be filtered at the penetration point to provide low ambient electromagnetic level inside the filter. The filter performance must be measured according to filter measurement standard (MIL-STD-220, CISPR 17 etc). 100 db insertion loss from 10 khz to 40 GHz is preferred. If electromagnetic pulse test is to be applied inside the chamber, EMP protection must be added to the filter. At design phase, it is very important to determine filter location. The location must be easy accessible for especially power supply connection required filters. G. Penetration Panels, Floor Panels and Interfaces At the design phase, all penetration and floor panel location and the interfaces must be determined. To determine the panel location the related standard setup, EUT and its cables and the support equipment must be evaluated. Generally the support

3 3 equipment are at the outside of the chamber, so the penetration panel location must be near to the support equipment. At the same time, the EMC laboratory personnel and support equipment must be at the same place to observe the EUT condition. Penetration panels shall contain water, air and fiber optical interface for special EUT requirements. H. Earthing The chamber shielding and all filter must be earthed at a single point near the filter location. The grounding impedance (earth-electrode resistance) must be guaranteed to be less than 0.5 Ohm. I. Lighting The required illumination must be determined at design phase. The number of lamps must be determined by analysis and an analysis report must be requested from the chamber supplier. This requirement is not mandatory in proposal stage. Absorber colors affect the illumination so some companies use white cap on absorber for more illumination. The emission levels of lamps are also important for EMC chambers. Halogen lamp emission is not a problem but the lifespan is short. Metal halide lamps have emission over the required ambient level. So, metal halide lamps must be switched off for testing. But the full illumination is very late after switching on. In spite of low efficiency and short lifespan halogen lamps must be preferred. J. Fire and Gas Detection Every anechoic chamber must have fire detection. Traditional smoke detectors can be used in chamber but EMI filter must be used for copper cable penetration. They can be susceptible to electromagnetic field and placing the detector between the absorbers is not easy. For better performance another technique, which is called air sampling system, can be used. The system consists of nonmetallic air sampling tubes inside the camber and smoke analyzer is placed outside. The system can be used also for exhaust gas analyzing. Some of the gases can be poisonous and the gases must be detected to save personnel. The important points are the gases types which are desired to be detected and the gases locations. K. Honeycombs For the ventilation of the chamber, honeycombs have to be installed. The performance of these elements should not affect the characteristics of the chamber. In order to provide efficient ventilation in the chamber and to be able to guarantee the possibility of reproducing the same temperature conditions for each test, the dimensions and the number of the honeycombs must be decided. For this decision the EUT power consumption, the EUT location, allowable air flow rate and the target temperature must be clear. However, the cutoff frequency of the honeycombs has to be compatible to required shielding effectiveness. To easily connect honeycombs to air duct, they must be delivered with a connection flange. Electrical isolation of the air ducts and honeycombs with flexible interface is useful for blocking the ground loops. L. Camera, Mast, Turntable and Audio Systems For observing the EUT while it is being tested a video camera must be used. For EMC chamber, camera emission level, susceptibility level, must be compatible to the test standard. It must be battery powered, shielded and its interface must be fiber optical. Commercial radiated emission test requires the movement of receiving antenna from 1 to 4 meter. Special antenna mast is used for this purpose. The weight capacity must be compatible to antenna. It has to turn the antenna to both polarizations and must be controlled with fiber optical interface. Commercial radiated emission test also requires 360 degrees measurement around the EUT. Thus, EUT has to be turned 360 degrees by a turntable. The dimension and weight capacity must be compatible to EUT. It must be controlled with fiber optical interface. To communicate with a test person inside the chamber an audio system is required. The system has to be filtered to make no interference. M. Absorbers and Performance Requirements The most important parts of the chamber are absorbers. The performance of the chamber and the important portion of the price are directly related with the absorbers. Ferrite and pyramidal absorbers are the two types of absorbers which are commonly used in EMC test chambers. Ferrite absorbers performance is effective up to 1 GHz. Pyramidal absorbers are effective generally from a few hundred MHz up to a few 10 GHz. By using ferrite and pyramidal absorber, frequency range can be increased. Hybrid absorbers also have impedance matching to provide better performance. Ferrite tile is a magnetic absorber that has the best absorption where the H-field is maximum, i.e. close to the shielding surface of the room. The tile exact performance is obtained from a combination of metal oxides pulverised, pressed, sintered and ground. By controlling the intrinsic parameters μ and ε, together with the dimensions, the ferrite is tuned to a specific frequency. Ferrite absorbers generally have a form of square tile. Pyramidal absorber which is used for EMC Chambers can be separated in group as: Foam (Carbon based) o Polyurethane o Polystyrene o Polyethylene Thin Film Polyurethane foam absorber is most common type of the foam absorbers. Performance is good and enough for EMC Chambers. They are flammable and they emit gases that harmless to human. Fire retardant models can be found. If the absorber depth is big, the absorber can bend in the future due to it s weight. Another problem of this type absorber is the carbon dust due to the open cells. Painting of the absorber is a solution of carbon dust. After 6-8 years the absorber will get tough and will be easily broken. At some models white cap can be used to provide better illumination. Polystyrene and polyethylene absorbers are closed cell absorbers. Performance is better due to the carbon distribution

4 4 is equal at everywhere. They are nonflammable and emit gases that relatively harmless to human. At some models, white cap can be used. The lifespan is longer than polyurethane absorber (over 20 years). Thin film absorber doesn t contain carbon. The performance is enough for EMC chamber. Non combustible model can be found. Due to the light color better illumination can be provided. The lifespan is longer than polyurethane absorber (over 20 years). Military standard s absorber requirement starts 80 MHz, so only pyramidal absorber is enough to meet the requirement. But for low uncertainty of measurement hybrid absorber can be used. Commercial standard requirement starts from 30 MHz so hybrid absorber must be used. However, some companies can success NSA requirement with only pyramidal absorbers. But these absorbers must be very long for working at 30 MHz. N. Testing and Validation Shielding effectiveness test must be applied to every shielded room to see the performance. At some points of the rooms shielding is not enough due to incorrect installation, missing mesh, insufficient tightness etc. For SE test can be applied according to EN To evaluate the SE rapidly Leakage Test can be applied. Leakage test at a frequency of MHz with a modulation of 1 khz FM is done on joints, penetrations panels, and accessible honeycombs around the chamber. If there is a problem, leakage has been verified and repaired. To see the real performance of the room test with antenna is required. These measurements can be separated in 3 group according to the frequency range as magnetic, electric and plane wave. With the compatible antenna pair the SE performance is measured. Due to long measurement time and test price the measurement points and frequency must be limited. The most important points such as filter, honeycombs, door etc must be selected as test points frequencies for per points are enough to see the SE performance. For military chambers, there is no requirement for chamber testing related with absorber performance. The manufacturer's certification of their RF absorber material (basic material only, not installed) is acceptable For commercial chambers, there are 3 tests related with absorber performance. These are: NSA Test according to CISPR FU Test according to EN SVSWR Test according to CISPR NSA performance is measured by using two antenna spaced 10 and 3 meters. There must be no absorber on the floor. One of the antennas is at test volume and the other is at measurement point. The antenna to antenna isolation is measured in the frequency range of 80 MHz to 1 GHz. By subtracting antenna factors the NSA value can be found. The receiving antenna must scan 1 to 4 meters. The measurement must be applied for different transmitting antenna position and height in the test volume. See Figure 2. Fig. 2. NSA Test Points FU test shows the test volume whether is compatible for radiated susceptibility test or not. The frequency range of the test is 26 MHz to 18 GHz and requires floor absorbers. The floor absorber must be hybrid absorber. The test distance is 3 meter and the electric fields generated by transmitting antenna are measured at 16 points in a vertical plane. The electric field deflection must be maximum +6 db at %75 of the all points. See Figure 3. Fig. 3. FU Test Points SVSWR test shows the test volume whether is compatible for radiated emission test in the frequency range of 1 to 18 GHz. The floor absorber must be only pyramidal absorber. The test distance is 3 meter and the electric fields generated by transmitting antenna are measured at 6 points in a line. The difference must be <6 db between first point and the other five points. These measurements must be repeated at different line in the test volume. See Figure 4. Fig. 4. SVSWR Test Points

5 5 O. Control Room, Amplifier Rooms For EMC chambers shielded control room and amplifier room can be used. However, there is no requirement in the standards for shielded control and amplifier rooms. If EUT has copper interface and support equipment outside penetrate the chamber. Identically, the amplifiers can generate high level emission and this emission can interfere to any antenna in the same building and/or to the EMC chambers. Using shielded room is a solution for both problems. III. CONCLUSIONS EMC chambers can have very different requirements according to the standards, EUT, building, test facility. All the requirements affect the chamber performance and price. So the real requirement must be decided at design phase with general aspect of all requirements. Towards that goal this paper addressed a general overview of the chamber specification content and briefly discussed a few aspects of some of the specification elements. While an in-depth study of the chamber specification and other areas of the chamber procurement process were beyond the scope of this paper, the information contained in this paper should be useful to those who might be actively involved in a new chamber project. of the chamber, then penetration panels must be used. If there are high level electromagnetic signals outside of the chamber that signals can penetrate inside the chamber through the EUT interface cables. By placing the support equipment in a shielded control room the high level ambient signals cannot REFERENCES [1] CISPR , Specification for Radio Disturbance and Immunity Specification For Radio Disturbance and Immunity Measuring Apparatus And Methods, IEC Standard, 2007 [2] IEC , Testing and Measurement Techniques Radiated, Radio-Frequency, Electromagnetic Field Immunity Test, IEC Standard, 2006 [3] EN , Anechoic Chambers Part:1 Shield Attenuation Measurement, European Standard, March 1996 [4] MIL-STD-285, Attenuation Measurements for Enclosure, Electromagnetic Shielding, for Electronic Purposes. Method of, Military Standard, June. 25, 1956 [5] MIL-STD-461F, Requirements for The Control of Electromagnetic Interference Characteristics of Subsystems and Equipment, August. 20, 1999 [6] Hugo F. Pues, Design of Modern EMC Chambers for Radiated EMC Testing up to 18 GHz, presented at IEEE Electromagnetic Compatibility Symposium, Austin, Aug , 1997 [7] Manny Barron, Specifjling and Procuring a 10 Meter Semi-Anechoic EMC Chamber, presented at IEEE Electromagnetic Compatibility Symposium, Washington DC, Aug , 2000 REFERENCES [1] CISPR , Specification for Radio Disturbance and Immunity Specification For Radio Disturbance and Immunity Measuring Apparatus And Methods, IEC Standard, 2007 [2] IEC , Testing and Measurement Techniques Radiated, Radio-Frequency, Electromagnetic Field Immunity Test, IEC Standard, 2006 [3] EN , Anechoic Chambers Part:1 Shield Attenuation Measurement, European Standard, March 1996 [4] MIL-STD-285, Attenuation Measurements for Enclosure, Electromagnetic Shielding, for Electronic Purposes. Method of, Military Standard, June. 25, 1956 [5] MIL-STD-461F, Requirements for The Control of Electromagnetic Interference Characteristics of Subsystems and Equipment, August. 20, 1999 [6] Hugo F. Pues, Design of Modern EMC Chambers for Radiated EMC Testing up to 18 GHz, presented at IEEE Electromagnetic Compatibility Symposium, Austin, Aug , 1997 [7] Manny Barron, Specifjling and Procuring a 10 Meter Semi-Anechoic EMC Chamber, presented at IEEE Electromagnetic Compatibility Symposium, Washington DC, Aug , 2000

Semi Anechoic Chamber (SAC)

1 of 9 Semi Anechoic Chamber (SAC) Approximate Dimensions of 3m Semi Anechoic Chamber (SAC) Length: 10m Width: 9m Height: 9m Frequency range of Semi Anechoic Chamber: 9 KHz to 40 GHz Emission test (EMI):