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1、Presentation of the paper at the Workshop: How to use TEM-devices in compliance EM-field testing EMV2000 Dsseldorf, 22.02-24.02.2000 All rights reserved to the authors. TEM Waveguides - Principles, Evaluation Criterions and Examples - Prof. Dr.-Ing. Heyno Garbe, Dipl.-Ing. Jens Peter Krst University

2、 of Hannover Appelstr. 9A D-30167 Hannover, Germany Tel.: +49 511 762 3760 Fax: +49 511 762 3917 e-mail: heyno.garbeieee.org 0Introduction The testing of electromagnetic interference presents, in general, enormous problems due to the very high costs involved in constructing and preparing test equipm

3、ent. Over the past years, investigations have been carried out as to how these very complex field tests could be carried out using simpler, less expensive equipment. Various new test installations have been proposed based upon far field measurements of an antenna. The aim of these installations was

4、to test the emission or interference resistance of a device under far field conditions as realistic as possible. TEM cells and other TEM wave guide constructions have already become part of todays standards. Well-enclosed housings no longer require expensive, shielded rooms. Therefore, a large requi

5、rement has evolved for this type of inexpensive test equipment. Recently, a lot of test facilities have been offered on the market as further possibilities for field measurements. They all claim to fulfil the function of TEM wave guides. The intention of this presentation is to investigate how it co

6、uld be determined whether they fulfil TEM wave conditions. This leads to definitions as presented in IEC 1000-4-3. This standard should appear as EN 61 000-4-3. The general definitions from the standard will be clarified to allow a comparison between the test devices. The proposed limits for field h

7、omogeneity and depolarisation angle are justified. Under these assumption TEM-, GTEM-, G-Strip and MAC-cells are tested. As a result the applicable frequency range for each test facility could be clearly identified. 1Antenna far field = TEM wave propagation As shown in 1 each given electromagnetic f

8、ield can be described as a superposition of well known wave types called field modes: L rrrr L rrrr += += 1010 1010 TMTETEM TMTETEM HHHH EEEE The term Txy stands for There is no component of the x- and y-field in longitudinal direction (=direction of wave propagation). There are only transverse comp

9、onents of x- and y-field. RadioFans.CN 收音机爱 好者资料库 WS: TEM Waveguides - Principles, Evaluation Criterions and Examples - Prof. Dr.-Ing. H. Garbe, 2000 Page 2 of 27 From this we can see that the TEM-wave doesnt have to have a longitudinal component. The ratio of TEM E r and TEM H r is given by = 120 0

10、 0 TEM TEM TEM H E r r for propagation in air. The propagation velocity of the TEM-mode is given by the material constant likewise: 0 00 1 ccTEM= = This is the well known speed of light. Identifying TEM-fields we have to look for the following criterions: 1. Only field components in the transverse p

11、lane 2. Ratio of the absolute value is given by 120 . 3. Propagation velocity is equal to c0. Another important aspect of the TEM mode is that it is independent of the radiator / antenna. After this scientific considerations lets have a look to real applications of field measurements. As proposed by

12、 IEC 1000-4-3 a typical test set-up is shown in figure 1. Fig. 1: Immunity measurement in an ALC (MAZ, Hamburg) Equipment under test Transmitting antenna RadioFans.CN 收音机爱 好者资料库 WS: TEM Waveguides - Principles, Evaluation Criterions and Examples - Prof. Dr.-Ing. H. Garbe, 2000 Page 3 of 27 An antenn

13、a illuminates the EuT for the frequency range from 10 - 1000 MHz. The distance between EuT and transmitting antenna has to be 3 m. It can be show that the radiated field far away from the antenna fulfils the TEM-mode criterions. the rule of thumb for the minimum distance is some what in the order of

14、 the used wavelength. For this case this isnt valid. The wavelength from 10 MHz requires a distance of 30 m. On the other hand a high field strength is needed at the EuT. So a compromise is found and the minimum distance is set to 3 m. Nevertheless TEM-mode conditions is assumed at the EuT. Concludi

15、ng this chapter we can say that we need a defined field mode (TEM mode) to ensure reproducible and well defined field measurements. If several test facilities have to be compared the main task is how the test site generates a pure TEM-field. 2Evaluation Criterions The calibration procedure for field

16、 immunity tests is shown in figure 2. Fig. 2: Field calibration according to IEC 1000-4-3 Various criterions which alternative test equipment must fulfil, can be extracted from the IEC 1000- 4-3 standard. They are presented briefly below: Field Homogeneity The IEC 1000-4-3 chap. 6.2 requires the fie

17、ld to be homogeneous over a surface in front of the test object. A surface perpendicular to the direction of the field propagation is selected. This square surface must be at least as large as the irradiated surface of the device under test (DUT), but not more than 1.5m RadioFans.CN 收音机爱 好者资料库 WS: T

18、EM Waveguides - Principles, Evaluation Criterions and Examples - Prof. Dr.-Ing. H. Garbe, 2000 Page 4 of 27 x 1.5m. The DUT is removed and the field strength measured at each of 16 equally distributed points. For at least 12 points, the difference between the minimum and maximum values must not exce

19、ed 6 dB. DUT field propagation 1.5 m 1.5 m min 0.8 m conducting surface Fig. 3:Definition of field homogeneity for irradiation test according to IEC 1000-4-3 The surface in which the field point are located must be as large as the irradiated surface of the DUT. The test points may not be situated on

20、 the conducting surface or under the DUT. A disadvantage of this procedure is that 25% of the measured values are not taken into account in any way. For this reason a new statistical approach was proposed in 4. Assuming a statistical distribution of the measured values the 6 dB criterion can be writ

21、ten as dB sx sx 6 15, 1 15, 1 log20 + + with:mean value = = n i i x n x 1 1 standard deviation() = = n i i xx n s 1 2 1 1 probability()15, 1%75=+kskxxskxp i To obtain the required homogeneity of the field strength at first the mean value and the standard deviation for the n=16 measured field values

22、have to be calculated. If sx sx + + 15, 1 15, 1 log20 is smaller than 6 dB the criterion is fulfilled. Figure 4 shows an example for the field homogeneity of a GTEM cell at 100 MHz. 51015202530 2.5 5 7.5 10 12.5 15 17.5 20logdB r r E E r E V m 0-1-2-3+3+2+1 ( )p E r % r Es + 115, r Es 115, r E Fig.

23、4: Field homogeneity of a GTEM cell WS: TEM Waveguides - Principles, Evaluation Criterions and Examples - Prof. Dr.-Ing. H. Garbe, 2000 Page 5 of 27 Defined field polarisation / TEM wave propagation ” This investigation shall be carried out separately for vertical and horizontal fields .” (IEC 801-

24、3/IEC 1000-4-3 chap. 7) At this point the observed field strengths must be differentiated. The total field strength is often viewed as EEEE gesxyz =+ 222 x y z r E l t Ez Ey Ex Polarisation direction Popagation direction Fig. 5: Definition of the field components On the other hand, the field strengt

25、h component in the polarisation direction is often investigated. For wave guides with dominant higher order modes a consideration of the total field strength is better. Both values are investigated in the following comparison. This requirement should clarify any possible radiation intrusion channels

26、. The dependence of interference resistance on the polarisation direction allows deductions to be made concerning , e.g., intrusive radiation caused by slits or other apertures. By determining the depolarisation between the theoretical and actual polarisation angles, information can also be gained o

27、n the existence of TEM-mode. The existence of a component in the direction of propagation gives information about the presence of a higher mode. The error angle can be deduced from the following equations: y xpol trans E E arctan= and y long long E E arctan= where: Expolis the field strength compone

28、nt in the transverse direction perpendicular to the polarisation vector Elongis the field strength component in the direction of propagation WS: TEM Waveguides - Principles, Evaluation Criterions and Examples - Prof. Dr.-Ing. H. Garbe, 2000 Page 6 of 27 transverse surface direction of propagation po

29、larisation direction actual field strength trans long Elong Extrans Fig. 6: Definition of depolarisation angle The ideal angle of 0 can never be achieved. Assuming a point source the propagation of the TEM wave is spherical. This introduce an angle of 14. As already shown in 4, a value of 20 for bot

30、h angles is realistic. This leads to the limit that 75% of the measured depolarisation angles have within 20. 3Measuring the field components Normally a high resistive dipole is used to measure the components of the E-field. The radiation pattern of such a dipole is shown in figure 6. WS: TEM Wavegu

31、ides - Principles, Evaluation Criterions and Examples - Prof. Dr.-Ing. H. Garbe, 2000 Page 7 of 27 Dipole with diode to measure rms-values Isotropic radiation pattern Fig. 7: Radiation pattern of a high resistive dipole It can be seen that through the diode the sign of the E-field components is miss

32、ing. This leads to a misinterpretation of the direction of E r as shown in figure 8. y Ez r E x z r E Ez Ex measured vector Ey Ex Fig. 8: Effects of the missing sign To reconstruct the sign the measurement has to be done twice at each point. Figure 8 shows the procedure for a rotation of the coordin

33、ate system of 15. WS: TEM Waveguides - Principles, Evaluation Criterions and Examples - Prof. Dr.-Ing. H. Garbe, 2000 Page 8 of 27 y x x y r E r E measured vector Fig. 8: Reconstruction of the missing sign 4TEM wave guide examples 4.1Measurement set-up The following were investigated: TEM Cell (Craw

34、ford type) GTEM 500 GTEM 1250 G-Strip MAC The positioning of the measurement points is shown in figure 9. Figure 10 displays the measurement equipment for the example of a GTEM-cell. For each field point the components of the electrical field strength are shown. The following nomenclature is used fo

35、r the coordinate system: z coordinate coordinate in the propagation direction or longitudinal direction y coordinate polarisation direction x coordinate direction of transverse plane perpendicular to the direction of polarisation WS: TEM Waveguides - Principles, Evaluation Criterions and Examples -

36、Prof. Dr.-Ing. H. Garbe, 2000 Page 9 of 27 a 3a 3 a 2a bb 6 b 6 b 2 area of field points Septum xz y Fig. 9: Field points in the TEM Cell PC signal generator SMT02 Rohde&Schwarz E-field EMCO 7110 EMCO 7123 amplifier PowerLabs R727LC 50 Septum test volumne GTEM-waveguide broadband termination with di

37、screte resistors and absorbers 50 x z y Millivoltmeter URV55 Rohde&Schwarz Fig. 10:Measurement equipment Nine measurement points in one plane were chosen for each of the three arrangements and this plane was further considered. For the TEM cell, GTEM cell, G-Strip and MAC the frequency range 10 MHz

38、to 1000 MHz was considered. The input voltage (fig. 10) was always set to 10 V. a / mmb / mm TEM490580 GTEM 500450 +/- 40460 +/- 50 GTEM 12501140 +/- 941160 +/- 94 G-Strip350660 WS: TEM Waveguides - Principles, Evaluation Criterions and Examples - Prof. Dr.-Ing. H. Garbe, 2000 Page 10 of 27 MAC22545

39、0 Tab. 1:Dimensions of a and b To judge the different TEM wave guide the results are displayed in the order as written in table 2. FigureValuesolid linedashed lineUnit a)Measurement set-up b)transversal depolarisa- tion angle t s 15, 12 deg c)longitudinal depolari- sation angle l s 15, 12 deg d)Fiel

40、d strength 75% interval sE+ 15, 1sE 15, 1 m V dB e)75% Interval in dB sE sE 16, 1 16, 1 log20 + 6dB Tab. 2:Title of the measurement figures The measurement results can be found in the annex of this paper. 5Conclusion Within this presentation it was is investigated how it could be determined whether

41、a new proposed test site is equal to the old Open Area Test Site (OATS) or anechoic chamber (ALC). The general definitions from the standard will be clarified to allow a comparison between the test devices. This leads to the condition that a pure TEM wave must be generated on such a new test site. A

42、 new statistical approach characterising the field homogeneity was shown. The criterion depolarisation angle determines the dominant present of the TEM mode. Under these assumptions TEM-, GTEM-, G-Strip and MAC-cells were tested. As a result the applicable frequency range for each test facility coul

43、d be clearly identified. References: 1 M. Koch: Analytische Feldberechnung in TEM-Zellen PhD-Thesis University of Hannover, 1998, Shaker Verlag 2 H. Garbe, M. Koch Normgerechtes Testen mit einer G-Strip? Research Report, Hannover, 1996 WS: TEM Waveguides - Principles, Evaluation Criterions and Examp

44、les - Prof. Dr.-Ing. H. Garbe, 2000 Page 11 of 27 3 H. Garbe Beurteilungskriterien fr TEM-, GTEM-Zellen und andere Wellenleiter EMV97, Dresden 1997, MESAGO, Stuttgart 4 H. Garbe, M. Koch, H. Haase Specification of Alternative Test Sites with Respect to Given EMC Field Standards EMC97 Zrich, Feb. 199

45、7 5 J. P. Krst, M. Koch, H. Garbe Vergleich der Feldhomogenitt verschiedener TEM-Wellenleiter pp. 291-298, EMV98, Dsseldorf, Feb. 1997, VDE-Verlag 6 P. Wilson On Correlating TEM Cell and OATS Emission Measurements IEEE Trans. on EMC, Vol. EMC-37, pp. 1-16, Feb. 1995 7 C. Groh, J. P. Krst, M. Koch, H

46、. Garbe TEM Waveguides for EMC Measurements IEEE Trans. on EMC, Vol. EMC-41, pp. , Nov. 1999 WS: TEM Waveguides - Principles, Evaluation Criterions and Examples - Prof. Dr.-Ing. H. Garbe, 2000 Page 12 of 27 Annex ATEM cell (Crawford type) Site view top view TEM-Zelle 100 100 150150 580 150150 150 15

47、0 980 290 490 Fig. a:Measurement set-up WS: TEM Waveguides - Principles, Evaluation Criterions and Examples - Prof. Dr.-Ing. H. Garbe, 2000 Page 13 of 27 01002003004005006007008009001000 -30 -20 -10 0 10 20 30 40 50 60 f /MHz t /deg tem-Zelle, transversaler Fehlerwinkel Fig. b:Transversal depolarisa

48、tion angle 01002003004005006007008009001000 -30 -20 -10 0 10 20 30 40 50 60 f /MHz l /deg tem-Zelle, longitudinaler Fehlerwinkel Fig. c:Longitudinal depolarisation angle WS: TEM Waveguides - Principles, Evaluation Criterions and Examples - Prof. Dr.-Ing. H. Garbe, 2000 Page 14 of 27 0100200300400500

49、6007008009001000 120 125 130 135 140 145 150 155 160 f/MHz |E|/dB V/m tem-Zelle, Feldstaerke, 75% Intervall Fig. 11d: Field strength 75% interval 01002003004005006007008009001000 0 1 2 3 4 5 6 7 8 9 10 f /MHz Schwankungsbreite, 75% /dB tem-Zelle, Homogenitaet der Feldstaerke Fig. e:75% Interval in dB WS: TEM Waveguides - Principles, Evaluation Criterions and Examples - Prof. Dr.-Ing. H. Garbe, 2000 Page 15 of 27 BGTEM 500 Fig. a:Measurement set-up 01002003004005006007008009001000 -30 -20 -10 0 10 20 30 40 50 60 f /MHz t /deg gtem-Zelle