Design of Modern EMC Chambers for Radiated EMC Testing up to 18 GHz

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1 Design of Modern EMC Chambers for Radiated EMC Testing up to 18 GHz Hugo F. Pues Emerson & Cuming Microwave Products N.V. Nijverheidsstraat 7A 2260 Westerlo, Belgium Friedrich W. Tr_autnitz Siemens Matsushita Components GmbH & Co. KG SiemensstraBe Heidenheim, Germany Abslvuct : In the future, EMC measurements above 1 GHz will become mandatory. Currently, most newly realised full-compliance EMC chambers are already specified up to 18 GHz. However, whereas existing standards clearly specify the requirements between 26 MHz and 1 GHz, the standards for the higher frequency range are still in draft form. To construct such chambers, broadband EMC absorbers are required that are effective from 26 MHz to over 18 GHz. The absorber design needs to properly combine both low and high frequency concepts. Measurement results up to 18 GHz are shown for state-ofthe-art anechoic EMC chambers for 3, 5 and 10 m test distance. INTRODUCTION Typically, radiated EMC measurements are to be performed in the frequency range from 26 MHz to 1 GHz. In the future, measurements above 1 GHz will also become mandatory. Currently, most newly realised EMC chambers are already specified up to at least 18 GHz. The anechoic requirements of EMC chambers are different for emission and immunity testing. The emission requirements for alternative test sites such as shielded anechoic chambers are described in EN , ANSI C63.4 and/or CISPR 16. They require that the NSA (normalised site attenuation) in the test volume does not deviate by more than 4 db from the theoretical reference in the frequency range MHz. Two common test distances (3 m and 10 m) are specified. The requirements for immunity testing are described in IEC and/or EN They require that the field uniformity in the test plane is better than 6 db for 75 % of the test points in the frequency range 26 (80) MHz. The preferred test distance is 3 m. For frequencies above 1 GHz, new standards are being developed. Because emission measurements above a groundplane with height scanning of the receive antenna are no longer practical above a few GHz, fully anechoic test environments are being investigated and recommended in new CISPR and CENELEC draft standards and working documents [l]. This can be achieved also in semi-anechoic chambers using a limited amount of floor absorbers as is the case for immunity testing. Whereas the NSA specification may not be extended to frequencies above 1 GHz and be replaced by a NSTL (normalised site transmission loss) specification based on a theoretical freespace model, it is our experience the FU (field uniformity) specification can be extended without much problems to 18 GHz. Currently, the fully-anechoic or free-space environment is not only investigated for emission testing above 1 GHz, but also as an alternative for the standard emission testing above a groundplane between 30 and 1000 MHz [l]. Two common test distances (3 m and 5 m) are specified. DESIGNOFEMCCHAMBERS The basic design of an anechoic chamber for radiated EMC measurements mainly derives from the test distance, the size of the EUT (equipment under test), the frequency range, the types of testing and the specified anechoic performance. Mostly, the volumetric NSA requirement is the most difficult demand and largely determines the chamber design. In general, the difficulty to comply with this NSA specification will increase if the test distance is increased (10 m versus 3 m), the test volume is enlarged (to test larger EUTs) or the lower frequency limit is decreased (30 MHz versus 80 MHz). In order to perform a successful design, the following design steps are essential : High-performance absorbers have to be selected taking into account the specified frequency range and anechoic performance. Computer simulations have to be done in order to optimise the chamber dimensions, the absorber lay-out and the test positions [2]. The installation has to be engineered very carefully in order to meet the requirements (like smoothness of the groundplane) and to not disturb the anechoic performance. The previous steps should lead to a successful NSA design up to 1 GHz. However, to extend the frequency range to above 1 GHz, a lot of high-frequency phenomena have to be considered in addition to the above steps and one should /97/$

2 make sure the absorber performance does not drop above 1 GHz. Things to consider are the lay-out of the floor absorbers, the absorber lining of antenna masts, the antenna patterns, the precise measurement set-up, the reflections of dielectric materials (such as wood or plastic). Indeed, whereas a precise computer simulation is most important at low frequencies, at high frequencies the quality of the installation and the practical experience with microwave phenomena become more important. DESIGN OF WIDEBAND EMC ABSORBERS The most common types of absorbers for EMC chambers are resistive pyramidal absorbers, magnetic ferrite tile absorbers and hybrid absorbers (combination type absorbers) [3]. The development of high-performance absorbers can be supported by. numerical analysis techniques. At low frequencies where the absorber dimensions are much smaller than the wavelength, the Method of Homogenization can be used [4,5]. At high frequencies where the absorber dimensions are much larger than the wavelength, ray-tracing techniques can be used [6,7]. Both such low- and high-frequency techniques provide valuable physical insight concerning the optimisation of the absorber performance. This doesn t apply to more rigorous analysis techniques (e.g. [S]) which can be used in the intermediate frequency range. Although the extent of this paper prevents us from covering these topics into detail, we can summarize that the lowfrequency performance is largely determined by the average material properties whereas the high-frequency performance is largely determined by the absorber geometry. However, the fulfillment of both the lowfrequency and high-frequency requirements doesn t guarantee by itself that the absorber will also properly function in the intermediate frequency range. Both types of requirements have to be properly married. Pyramidal resistive absorbers need to be nearly a quarter wavelength long to become effective. This means their thickness should be nearly 2.5 m to be effective from 30 MHz where the wavelength is 10 m. Figure 1 shows the reflectivity of a state-of-the-art 2 m hollow pyramidal absorber which has been optimised for use in 10 m chambers and consists of an outer absorptive layer of 18 mm thickness and an interior support structure. Its performance above 1 GHz is excellent although it has a flat top. However, if the hollow absorber is made using electrically thin resistive foils [9], the absorber performance will deteriorate at GHz frequencies [lo]. Hence, such resistive absorbers are not useful for EMC chambers with an extended frequency range up to 18 GHz or more. Figure 1. Reflectivity of a 2 m hollow pyramidal absorber If there is no space for using large pyramidal absorbers, one can only use hybrid absorbers (which combine a ferrite tile absorber with a small pyramidal one using one or more intermediate matching layers) if the extended frequency range (30 MHz - 18 GHz) is specified. Figure 2 shows the reflectivity of a typical 0.45 m hybrid absorber (developed for use in 3 m chambers) and a 0.75 m hybrid absorber (developed for use in 10 m chambers). The design of such a hybrid absorber is very critical. It has to properly combine a magnetic part which is inherently good at low frequencies and a resistive part which is inherently good at high frequencies. The design requires the use of accurate computer models [l 11. Using such computer models, one can also design matching resistive absorbers to extend the frequency range of existing ferriteonly chambers. EXAMPLES OF WIDEBAND EMC CHAMBERS A 10 m semi-anechoic EMC chamber was recently built with interior shielding dimensions of 22.4 m x 14.3 m x 8.9 m [ 121. The four walls and ceiling were lined with a 2.5 m hollow resistive absorber having a performance similar to that shown in Fig.1. A high-quality groundplane and flush 4 m turntable were installed and much attention was paid to all engineering details. Figure 3 shows the measured NSA deviations at 10 m distance in vertical polarisation in the most critical MHz range in four positions (center, front, left and right) at two heights (1 m and 1.5 m). Because of the excellent high-frequency performance of the absorbers, this chamber can be easily upgraded for frequencies up to well above 18 GHz provided that floor absorbers are used. A 3 m semi-anechoic EMC chamber was also recently built with interior shielding dimensions of 8.7 m x 6.2 m x 5.8 m [ 131. The side walls and ceiling were lined with the hybrid absorbers shown in Figure 2. Figure 4 shows the predicted and measured NSA deviation in horizontal polarisation in 73

3 the center position at 1 m height in the MHz frequency range. B -I -II -II -m -25 ECCOSORB WY-LB <--> ECCOSORB W-38 I I I ly.li M I Finally, both NSTL and FU measurements were performed between 1 and 18 GHz in a semi-anechoic 10 m chamber with inner shielding dimensions of 22.1 m x 12.5 m x 7.5 m [14]. The chamber was mainly lined with HX-80 hollow pyramidal absorbers (see Fig. 1) and had already successfully passed the NSA and FU tests below 1 GHz. To perform the NSTL and FU tests above 1 GHz, floor absorbers were used as shown in Figures 5 and 6. Figure 2. Reflectivity of 0.45 m and 0.75 m hybrid absorbers Figure 5. Floor absorber and antenna positions for NSTL measurements Figure 3. Measured NSA of a 10 m chamber in vertical polarisation (four test positions at two heights) The NSA was measured up to 1 GHz. Between 1 and 18 GHz, measurements were performed using floor absorbers. The chamber met the field uniformity requirements according to both IEC and EN HO- I I 1 I I 1 I I i Figure 4. Predicted and measured NSA of a 3 m chamber in horizontal polarisation (one position at one height) Figure 6. Positions of floor absorber, transmit antenna and test plane for FU measurements The NSTL measurements were performed at 3 m and 5 m distance. Figures 7 and 8 show the results at 5 m which is the more critical distance. As shown in Figure 5, the diameter of the test volume was 4 m. A double-ridged horn antenna was used as the transmit antenna and a logperiodic antenna as the receive antenna. Both antennas were kept at the same height : 1 m and 2 m for horizontal polarisation, 1 m and 1.5 m for vertical polarisation. The FU measurements were performed at 3 m distance over a 1.5 m x 1.5 m test plane. The transmit antenna was a log-periodic antenna which was set at 1.55 m above the floor. The results for both 75 % and 100 % of the 16 test points are shown in Figures 9 and

4 IO Frequency [GHz] Figure 7. Normalised Site Transmission Loss results in horizontal polarisation at 5 m distance ( 5 test positions at two heights ) F 1 El s 0 P.m jj J I I I I Frequency [GHz] 18 Figure 8. Normalised Site Transmission Loss results in vertical polarisation at 5 m distance ( 5 test points at two heights ) 75

5 6 I I.I I I nr I I I I I I I I I I I I 1 I I I I I Frequency [GHz] Figure 9. Field Uniformity results in horizontal polarisation for 75 % and 100 % of test points 6 1 o! I I I I I I I I I I I IO 10 II Frequency [GHz] Figure 10. Field Uniformity results in vertical polarisation for 75 % and 100 % of test points 76

6 CONCLUSION Although the existing EMC standards only specify the anechoic performance of test chambers up to 1 GHz, it appears necessary that new chamber projects are specified up to 18 GHz. This can only be achieved by using highquality broadband absorbers which are specified over the full frequency range. If hybrid absorbers are used, a twostep approach may be possible (1st step : up to 1 GHz ; 2nd step : up to 18 GHz) if the second step is taken into account from the initial design. It has appeared from our investigations that the standard FU test technique can be extended from 1 GHz to 18 GHz whereas this is not the case for the standard NSA technique. However, the NSTL technique can be used from 30 MHz to 18 GHz in both fully anechoic chambers and semi-anechoic ones with a limited number of removable floor absorbers. REFERENCES [ 1 ] CENELEC SC2 1 OA/WG , Concept EMC Standard Anechoic Chambers : Part X : Emission Measurements in Fully Anechoic Chambers, November [2] J. Dauwen, M. Van Craenendonck and H. Pues, Design of Anechoic Chambers for Radiated EMC Testing by Computer Model&g, Proceedings of the 12th International Wroclaw Symposium on EMC, June 1994, pp [3] H. Pues, Specific EMC Test Methods: Electromagnetic Wave Absorbers, Proceedings of Intertronic 95, Paris, June 1995, Part EMC, p [4] E. Kuester and C. Holloway, A Low-Frequency Model for Wedge Pyramid Absorber Arrays - I : Theory, IEEE Transactions on EMC, vol. EMC-36, pp , November [5] C. Holloway and E. Kuester, A Low-Frequency Model for Wedge or Pyramid Absorber Arrays - II : Computed and Measured Results, IEEE Transactions on EMC, vol. EMC-36, pp , November [6] B.T. Dewitt, Analysis and Measurement of Electromagnetic Scattering by Pyramidal and Wedge Absorbers, Ph.D. Dissertation, The Ohio State University, [7] B.T. Dewitt and W.D. Burnside, Electromagnetic Scattering by Pyramidal and Wedge Absorber, IEEE Transactions on AP, vol. AP-36, pp , July [8] N. Marly, B. Baekelandt, D. De Zutter and H.F. Pues, Integral Equation Modeling of the Scattering and Absorption of Multilayered Doubly-Periodic Lossy Structures I, IEEE Transactions on AP, vol. AP-43, pp , November [9] A. Enders, Advancements in Anechoic Chambers, ITEM 1996, pp and [lo] H. Pues, Reflectivity of Foil Absorbers, Emerson & Cuming Microwave Products, Internal Report, February [l l] T. Ellam, An Update on the Design and Synthesis of Compact Absorber for EMC Chamber Applications, Proceedings of the 1994 IEEE International Symposium on EMC, Chicago, August 1994, pp [12] W. Mtillner, H. Pues, P. Rost and F.W. Trautnitz, Auslegung und Errichtung einer Absorberhalle fur 10 m and 3 m Messentfernung, Proceedings of EMV 94, Karlsmhe, February 1994, pp [13] V. La Fragola, H. Pues and F.W. Trautnitz, Errichtung eines Normenkonformen Messplatzes mit 3 m Messstrecke, Vergleich der Simulation mit den Messergebnissen, Proceedings of EMV 96, Karlsruhe, February 1996, pp [ 141 W. Miilhrer, Transmission Loss and Field Untformity Measurements at ABB Industrial Systems AB, Vaster& Sweden, Austrian Research Centre Seibersdorf, Report No. EMV-H23/96, November

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):