Calculation of Antenna Pattern Influence on Radiated Emission Measurement Uncertainty

Size: px

Start display at page:

Download "Calculation of Antenna Pattern Influence on Radiated Emission Measurement Uncertainty"

Emil Arnold
5 years ago
Views:

1 Calculation of Antenna Pattern Influence on Radiated Emission Measurement Uncertainty Alexander Kriz Business Unit RF-Engineering Austrian Researc Centers GmbH - ARC A-444 Seibersdorf, Austria alexander.kriz@arcs.ac.at Abstract In radiated emission measurements an error is introduced by te directive receive antenna. Te Monte Carlo Metod was used to calculate tis error were measured antenna pattern ad been taken into account. Altoug tere are large differences between te classical test set-up and te two improvements antenna tilting and antenna bore sigting te impact to te measurement uncertainty is low. For a test distance of 3 m te measurement uncertainty can be reduced from 5.6 db to 5.43 db wit antenna tilting. Wit a bore sigt antenna tower a reduction to 5.3 db is feasible. Te bias and uncertainty given by CISPR 6-4- are not adequate. Antenna radiation pattern, Electric field measurement, Error analysis, Monte Carlo metods I. INTRODUCTION Te calculation of te measurement uncertainty is an important part of eac EMC compliance test metod. In te past years a CISPR ad-oc group evaluated uncertainty estimations for te most important EMC emission measurement metods. Since 00 a good basis for uncertainty budgets is available in CISPR 6-4- []. Tese budgets are intended as information for te user of te standard and sould elp to calculate teir own estimation. In CISPR 6-4- te metod presented in te Guide to te Expression of Uncertainty in Measurement (GUM) [] was used. All of te uncertainty contributions are so called Type B contributions wic are based on experience, reference data, manufacturer specification, previous measurement data and data provided in calibration certificates. One of te uncertainty contributions for te radiated emission test is called directivity difference. Te reason for tis contribution can be explained by te definition of te measurand and te definition of te antenna factor. Te measurand for radiated emission testing is defined by te maximum electric field strengt between m and 4 m eigt at a certain distance d on an Open Area Test Site. E = V AF () Te electric field is converted to a voltage by te measuring antenna. Tis voltage is measured wit a RF receiver. Te antenna factor of antennas is valid for te main beam direction if directive broadband antennas are used. If te antenna is scanned in eigt te incident angle of te electrical field is not always te main beam direction. So te field strengt is modified by te radiation pattern ( ) E = V AF P () Since te pattern or te angle of incidence is unknown a correction of te field strengt is not feasible or at least impractical. So an uncertainty contribution is introduced if a directive receive antenna is used. Te sign of te directivity error, defined as difference between measured electric field and electric field, is always negative due to te relation ( ) P (3) Te only possibility to reduce te error caused by te directivity is to tilt te receive antenna toward te ground [3][4]. Tere are two metods possible to implement tis: () antenna tiling were te tilting angle is te same for all antenna eigts. () antenna bore sigting were te tilting angle is increased wit te antenna eigt. II. CISPR 6-4- In CISPR 6-4- te directivity error is called directivity difference. Te reason for tis name is a requirement for broadband measuring antennas, called complex antennas, in CISPR 6--4 [5]. It reads: Te main lobe of te radiation pattern of te antenna sall be suc tat te response in te direction of te direct ray and tat in te direction of te ray reflected from te ground do not differ by more tan db. An antenna wic meets tis requirement is used to estimate te directivity error. Tis estimation is sown in Tab. I. In te frequency range from 30 MHz to 00 MHz te error is 0 db for orizontal polarization. Tis is based on te assumption te H-plane pattern is perfect circular. For te upper frequency range and for vertical polarization te error is -.0/ db. Tis means te error is between tese two numbers wit te same probability.

2 Advanced readers of CISPR 6-4- will recognize te different sign of te values of Tab. and te standard. Tis is because te standard sows te required correction, wic as te inverse sign of te error. TABLE I. DIRECTIVITY DIFFERENCE ERROR OF CISPR 6-4- (30 M VALUES OMITTED) Frequency 30 MHz 00 MHz 00 MHz GHz 3 m 0 m Ver. Ver. db -.0/ db db -.0/ db -.0/ db -.0/ db -.0/ db -.0/ db III. CALCULATION OF INFLUENCE Te propagation of electric waves on Open Area Test Sites is described well in te literature [6]. Te electric field of an omni directional source is Ten te electric field is calculated by E π π η P j d η P j d 0 λ 0 λ jr = e + e e (9) Were R =0 for vertical polarization and R =π for orizontal polarization. If a directive antenna is used Formula 9 is modified in te following way E Dir π η P j d η P j d 0 λ 0 λ jr = P( ) e + P( ) e e (0) Te error due to te directive receive antenna, in db, is π π j d λ η0p E = e (4) Wit te elp of Fig. te pat lengt and incident angles are calculated by TX d ( ) R + = (5) ( ) d = + (6) R + = arctan (7) R + = arctan (8) R d R d Ground Plane RX Figure. Geometry for wave propagation on Open Area Test Site max EDir m 4m E = 0 log () max E m 4m For an EUT te source of radiation is generally not known. It must be somewere witin te EUT. To account for tis a random variable wit an uniform distribution is introduced RX ( ) 0.m }...{ m () Tis means te source of radiation is somewere between te palette (0. m) and te maximum EUT eigt ( m) wit equal probability. If Formula is applied to RX a nonlinear transformation is performed E RX EPattern (3) tat leads to a random variable E Pattern. E Pattern is te probability density function (PDF) of te error. An example for tis nonlinear transformation is sown in Fig.. PDF [m] E PDF Figure. Example nonlinear transformation E Pattern [db] As measures for te random variable E Pattern te mean and te standard deviation are used. Te mean is te bias wic as to be corrected. Te standard deviation is used in te uncertainty calculation.

3 EUT EUT Figure 3. Emission test set-up, antenna parallel to ground plane Anoter nonlinear transformation can be performed wic gives te PDF of te receive eigt. Tis can be useful wen analyzing te incident angles towards te receive antenna. A. Standard mounting In te classical test set-up used by te majority of te test ouses for emission measurements according to CISPR 6--3 [7] and ANSI C63.4 [8] te main beam direction of te receive antenna is parallel to te ground plane, see Fig. 3. B. Antenna tilting One possible way for reduction of te directivity error is to tilt te receive antenna towards te ground plane, see Fig. 4. Te tilting angle is constant over te receive antenna eigt and is calculated by + arctan + arctan = R R (4) Tis means te main beam direction of te receive antenna is placed in te middle between te direct and te reflected ray. CISPR 6--4 suggests tis approac to meet its db pattern requirement. Te transmit eigt as well as te receive eigt are required. Since bot eigts are random variables an optimum tilting angle can not be found. To solve tis issue average values are assumed Figure 4. Emission test set-up, antenna tilting Te modification of a commercial available antenna mast can be done wic a special designed bracket. Care as to be taken tat te automatic polarization canger of te mast is deactivated. Wen canging te polarization te antenna as to be turned inside te mounting bracket. C. Antenna bore sigting Anoter metod for te reduction of te directivity error is antenna bore sigting. Te tilting angle of te receive antenna is increased wit te antenna eigt, see Fig. 5. A special antenna mast is required to implement tis tecnique, see ETS [9] and Sunol [0]. Te mast controller tilts te antenna during eigt scanning to = arctan (7) R If we keep te assumption of an average transmit eigt of te range of te angles are m = (8) 0 3m 45 (9) m = m =.5m (5) wic lead to 3m 0m = 38 = 3.9 (6) EUT Tese angles are used for bot polarizations and in te wole frequency range from 30 MHz to GHz. Using different tilting angles is not practical, since te EMC test as to be stopped several times for canging te angle. Figure 5. Emission test set-up, antenna tilting

4 a) E-plane 30 MHz e) E-plane 50 MHz i) E-plane 500 MHz b) H-plane 30 MHz f) H-plane 50 MHz j) H-plane 500 MHz c) E-plane 90 MHz g) E-plane 00 MHz k) E-plane 000 MHz d) H-plane 90 MHz ) H-plane 00 MHz l) H-plane 000 MHz Figure 6. Radiation Pattern of Scwarzbeck VULB 960

5 IV. ANTENNA PATTERN All te simulations in tis paper are performed wit a VULB 960 from Scwarzbeck []. Te radiation pattern from 30 MHz to GHz can be seen in Fig. 6. Below 50 MHz te E-plane pattern is nearly ideal. Above tis frequency te antenna becomes directive and exceeds a gain of 7 dbi above 00 MHz. It can be seen clearly tat te H-plane pattern below 50 MHz is not perfect circular. Tis broadband receive antenna is typical for EMC emission measurements. Te construction is similar to oter popular antennas like ETS Lindgren 34C, Scaffner BiLog CBL 6C or Sunol JB Series. V. RESULTS Te PDF of te error E Pattern is calculated using te Monte Carlo Metod. Tis numerical metod can be used in uncertainty calculation and is described in a supplement of te GUM []. A. Statistical measure of E Pattern Fig. 7 sows te mean and te standard deviation tat are calculated from te PDF of E Pattern. As expected te error is smaller for a test distance of 0 m. Te reason for tis is te smaller angles of incidence at tis distance. Furter a reduction of te error is seen wen tilting or bore sigting is used. a) For a test distance of 3 m te bias can be up to -.7 db. Tis is muc larger tan te bias suggested by CISPR 6--4 of -0.5 db. Also te standard deviation of. db exceeds te value 0.9 db given by te standard. It is important to mention tat te given uncertainty is valid after te correction of te bias. Interesting is tat te largest error does not occur at te frequency wit te narrowest beam widt. Tis fact can be explained wit te probability distribution of te receive eigt. At a frequency of GHz te receive antenna is placed between m and.5 m were te angles and are small wit a probability of 88%. At 00 MHz te maximum field strengt is found above.5 m wit a probability of 57%. Large errors can only occur if te two angles are large. b) B. Impact on measurement uncertainty Wen comparing te results of capter V.A. it is very important to keep te combined standard uncertainty in mind. In te combined standard uncertainty all uncertainty contributions of te measurement are united. Fig. 8 sows te dependency on te antenna pattern error. Reduction of te antenna error, by using tilting or bore sigting, will ave a strong impact if te gradient of te curve is ig. Since te calculated standard deviations are below. db te pattern error as a low impact. It is not a contribution wic dominates te measurement uncertainty. Classic; Classic; Mean [db] sigting; sigting; Ver Mean [db] sigting; sigting; Ver c) d) Standard Deviation [db] Classical; sigting; sigting; Ver Standard Deviation [db] Classic; sigting; sigting; Ver Figure 7. Statistical measure of E Pattern a) Mean 3 m b) Mean 0 m c) Standard Deviation 3 m d) Standard Deviation 0 m

6 Combined Standard Uncertainty [db] ,0 0, 0 Uncertainty contribution Directivity Difference [db] Figure 8. Impact of te antenna directivity contribution to te combined standard uncertainty. Te calculated combined standard uncertainty is sown in Tab. II. For a test distance of 0 m te error is in te same range for all tree metods. Te improvement from 5.4 db to 5.0 db is not significant. Te differences are larger for a test distance of 3 m. Te measurement uncertainty of 5.6 db can be reduced to 5.43 db wit tilting and to 5.3 db wit bore sigting. TABLE II. Metod Classic Tilting sigt Pol. Ver. Ver. Ver. DIRECTIVITY ERROR FOR ANTENNA BORE SIGHTING R [m] Probability db criterion [%] Combined standard uncertainty [db] 30 MHz 00 MHz MHz GHz C. Probability of compliance wit db criterion Wit te formulas given in capter 3 it is possible to calculate te probability distribution function of te receive eigt as well. Tis information can be used to calculate te incident angles and. Using ( ) ( ) P C = 0log (0) db P te PDF for te db criterion is calculated. Tab. II. sows also te lowest probability tat te db requirement is met. Especially in te classical test set-up in vertical polarization te probability tat te requirement is met is only %. Wit none of te presented metods it is possibly to comply wit te limit in vertical polarization. Since te measurement uncertainty is acceptable and te antenna pattern is typical te criterion is questionable. VI. CONCLUSION Wit antenna tilting and antenna bore sigting te directivity error can be reduced. It was sown tat improvement of te measurement uncertainty is minimal. Only 0.3 db can be gained at a test distance of 3 m using a bore sigt antenna tower. It must be analyzed if te improvement by a fraction of a decibel in te combined standard uncertainty justifies te investment of suc a mast. Tese masts consist of more dielectric material so te coupling wit te measuring antenna may be iger and must be taken into consideration. Te Monte Carlo Metod is very effective to solve tis problem. It is te only tecnique to take measured data into consideration. Furter work will be te calculation of te uncertainty contribution for different EUT sizes. Wit te size of te EUT te incident angles to te receive antenna will cange significantly. So te results will be different for floor standing and table top EUTs. REFERENCES [] CISPR 6-4- Ed : Specification for radio disturbance and immunity measuring apparatus and metods Part 4-: Uncertainties, statistics and limit modelling Uncertainty in EMC measurements, 003- [] BIPM, IEC, IFCC, ISO, IUPAC, IUPAP, OIML: Guide to te Expression of Uncertainty in Measurement (GUM), 993 [3] Z. Cen, M. Foegelle, T. Harrington: Analysis of Log Periodic Dipole Array Antennas for Site Validation and Radiated Emissions Testing, 999 IEEE Int. Symposium on EMC, Volume, Page 68-63,999 [4] H. Garn, W. Müllner, M. Bucmayr: Antennas, Field Sensors and Cambers for Radiated EMI Testing Correct Application and Calibration, Worksop Notes, Worksop No., EMC Asia 997, November 5 t 997, Singapore [5] CISPR 6--4: Specification for radio disturbance and immunity measuring apparatus and metods - Part -4: Radio disturbance and immunity measuring apparatus Ancillary equipment - Radiated disturbances, IEC, Second Edition, [6] A. A. Smit, R. F. German and J. B. Pate: Calculation of Site Attenuation From Antenna Factors, IEEE Transaction on Electromagnetic Compatibility, Vol. EMC-4, No. 3, August 98 [7] CISPR 6--3: Specification for radio disturbance and immunity measuring apparatus and metods Part -3: Metods of measurement of disturbances and immunity Radiated disturbance measurements, IEC, Second Edition, [8] ANSI C63.4: American National Standard for Metods of Measurement of Radio- Noise Emissions from Low-Voltage Electrical and Electronic Equipment in te Range of 9 khz to 40 GHz, IEEE, Revision 003, 30 January 004 [9] ETS EMC Test Systems: Test Site Hardware, Antenna Tower Model 070-, 070-, 07, Information leaflet, 999, [0] Sunol Sciences: EMC -Sigt Antenna Positioning Tower Model TLT, Information leaflet, [] Scwarzbeck Messelektronik: VULB 960 TRILOG Broad Band Antenna, [] ISO: Guide to te expression of uncertainty in measurement (GUM) Supplement : Numerical metods for te propagation of distributions, International Organization for Standardization, 004

On the relation between radiated and conducted RF emission tests

On the relation between radiated and conducted RF emission tests Presented at te 3 t International Zuric Symposium on Electromagnetic Compatibility, February 999. On te relation between radiated and conducted RF emission tests S. B. Worm Pilips Researc Eindoven, te