STUDY ON TECHNICAL FEASIBILITY OF AN OPEN ...

2014-36-0176

STUDY ON TECHNICAL FEASIBILITY OF AN OPEN AREA TEST SITE IN BRAZIL FOR THE UHF BAND (300 MHz to 1 GHz)

Kenedy Marconi G. Santos1,2, Marcelo Bender Perotoni3, Marcela Silva Novo1, José Osvaldo Saldanha Paulino4, Décio R. M. Faria5, Caio Luminatti Andrade6, Thayane Almeida Alves2 Federal University of Bahia1, Federal Institute of Bahia2, Federal University of ABC3, Federal University of Minas Gerais4, 6 Federal University of Itajuba, SENAI – Cimatec

Copyright © 2014 SAE International

Abstract This paper will present a set of measurements and simulations in order to experimentally investigate the technical viability of an open area test site for the UHF band, in the frequency range of 300 MHz to 1 GHz. The experimental and computational investigation analysis discloses the possibility of an emission tests in a reduced size open area of tests, through interventions in the setup of the test proposed by international standard CISPR 25. Keywords— Antennas, CISPR 25, Computational Analysis, International Standard and Open Area Test Sites.

Introduction A few years ago, the cars were essentially provided with mechanical arrangements, used to perform many types of control, for example, the pressure carburetor. Recently with the advancement and lower costs of vehicular embedded systems the use of electro-electronic command systems, controller of pollutant emissions, board computer, vehicular self-diagnostic and reporting capability are more present, however the need for the electromagnetic compatibility (EMC) is necessary and more present [1][2]. Nowadays, the car is smarter and comfortable, but as the consequence of this technological progress, there is a great increase of electromagnetic signals emitted by vehicles. Beside to design vehicles to their desired functional performance, the cars must also meet legal requirements in virtually all countries of the world before it can be sold [3] [4]. Design an electronic product to perform a new and exciting function is a waste of effort if it cannot be placed on the market. Recently, EMC techniques and methodology have become an integral part of the process involved in the design [3] [4]. In electric vehicles the Electromagnetic interference (EMI) in the power supply system can affect the safety of the vehicle and passengers [3]. The international standard, CISPR 25, is designed to protect Page 1 of 9

on-board receiver from the disturbances produced by conducted and emitted vehicular radiation produced by any electronic or electrical component installed in the vehicle [5][6]. Therefore, new products should always prove that they comply with those regulations prior being regularly commercialized. In order to investigate the technical viability of an Open Area Test Site for UHF band, it is necessary to acquire enough knowledge about the electromagnetic field levels in the site. The site used in our tests is a farm located near the city of Capelinha, state of Minas Gerais, Brazil (Lat. 17.8651°S Long. 42.4313° W). The Open Area Test Sites (OATS) provide low-cost and accurate measurements [8]. Nevertheless, before using an OATS for measurements involving ultra-high frequency signals the intrinsic behavior of the site must be carefully scrutinized.

Problem Formulation The OATS are used in radiated electromagnetic interference (EMI) measurements, and the antenna calibration in these sites is performed through antenna-factor (AF) measurements. Therefore, site calibration is essential before conducting any measurement or test. The calibration procedures of the normalized site attenuation (NSA) and the procedure to measure the ambient radio frequency in a test site are presented in CISPR 16 [8]. The ambient radio frequency levels at a test site shall be sufficiently low compared to the levels of measurements to be performed. In our tests, the ambient radio frequencies were 6 dB or more below the measurement levels to be performed [6] [8]. For perfect results, an ambient level 20 dB below the emission level measured is recommended [8]. The NSA has become the international standard parameter to electromagnetic emission measurements and acceptability of an open area test site [8]. It is defined as the relation of the power input in a matched, balanced, lossless, tuned half wave dipole used as a transmitter and the output of a correspondingly matched,

balanced, lossless, tuned half wave dipole receiver antenna, for a specified polarization, separation, and heights above a flat reflecting electromagnetical surface [8].

Ambient emissions Measurements The electromagnetic field distribution is influenced by channel frequency, distance from the source, wave polarization, size of the antennas, land contours, buildings, tower height, earth’s propagation constant, time schedule, seasons, and radiated power [9]. The ambient emissions measurements were performed in the far-field region. The chosen site was a parking lot of a farm located near the city of Capelinha, state Minas Gerais, Brazil, characterized by an area of weak signals. The Figures 1, 2 and 3 shows the site.

A set of measurements was performed so that the fields along the cardinal directions North, South, East and West could be characterized along the angular variations of 0◦, 90◦, 180◦ and 270◦, in both vertical and horizontal polarizations. The Figure 4 shows the setup used to address the ambient emissions Measurements. The results of measurements in horizontal polarization are presented in Figures 5, 6, 7 and 8. The vertical polarization measurements showed lower amplitudes and are not presented in this paper.

Figure 4 - Setup used to measurements ambient emissions

The Table 1 shows the limits for radiated disturbances for some services in the UHF band [6]. Figure 1 - Farm located near the city of Capelinha, state Minas Gerais, Brazil.

Figure 2 - Parking lot of the farm located (Lat. 17.8651°S Long. 42.4313°W).

Figure 3 - Parking lot of the farm located (Lat. 17.8651°S Long. 42.4313°W).

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Table 1 – Limits for Radiated Disturbances - CISPR 25

Figure 5 - Ambient electromagnetic noise around the proposed OATS – 0º horizontal polarizations.

Figure 8 - Ambient electromagnetic noise around the proposed OATS - 180◦ horizontal polarizations

In all measurements, presented in Figures 5, 6, 7 and 8, the ambient noises were 6 dB below the limit line. For perfect results, an ambient level 20 dB below the emission level measured is recommended [8].

Simulation Scenarios

Figure 6 - Ambient electromagnetic noise around the proposed OATS – 90º horizontal polarizations

A metallic ellipse with larger and smaller radii of, respectively, 6 and 5.19 meters was modeled as a ground plane, presented in Figure 12. Its material was considered of being made of perfect metal (lossless). Two similar dipole antennas, resonating at the frequency of 678.7 MHz were positioned; one set as a transmitter and the other as a receiver. Its lengths were 0.197m and radius equals to 1 mm. They were positioned to generate a horizontal polarization.

Figure 7 - Ambient electromagnetic noise around the proposed OATS – 180º horizontal polarizations Figure 9 - Dipole antenna, the port spotted in red can be seen.

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Figure 10 - Computed return loss dipole antenna

In this simulation, the transmitter antenna stands at a height of 2 meters from the ground plane, whereas the receiver is at the heights of 1 and 4 meters, one at a time Fig. 11 shows the top view of the ellipse and antennas position.

Figure 13 - The mesh used on the antenna.

The simulation took almost two hours, with an 8 GBytes laptop, Intel I-7 with 2.6 GHz processor. It required 1.3 GBytes of RAM memory (maximum required value).

A. Results and Discussion

Figure 11. View of the open site measurement area. The ground plane has an ellipse form, and the antennas are located at the points “a” and “b” in the figure.

Since the simulation is comprised by perfect electric conductors, the I-Solver (Integral Equation) of CST Microwave Studio was used. It is based on the MLFMM-MoM method, which basically turns the matrix generated by the Method of Moments (MoM), originally dense (strong coupling among the elements), into sparse, by means of the Multi-Level Fast Multipole Method (MLFMM). The main motivation to use a variant of the MoM is that only the metallic surface areas are meshed, leaving a large open space volumes unmeshed. It helps save large memory amounts in comparison to volumetric meshes (such as Finite Element Method FEM or Finite Difference Time Domain FDTD) [11]. Figures 11 and 12 shows the mesh seen on the ground plane and on the antenna.

Two scenarios were evaluated, having the model set according to Fig.11: 1. 2.

Receiver antenna at a height of 1 meter from the ground plane; Receiver antenna at a height of 4 meters from the ground plane.

For both situations the transmitter antenna was placed at a fixed height of 2 meters from the ground plane. According to the definition given in CISPR-16, the Open Area Site Attenuation (OASA) is defined in equation (1) as:

=

(1)

Where VTX and VRX are the voltage measured at both 50Ohms inputs from both, transmitter and receiver antennas. The parameter NSA (Normalized Site Attenuation) is defined in expression (2) shown below as:

Figure 12 - The mesh used on the ground plane.

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=

=

(2)

AFTX and AFRX are the transmitter and receiver Antenna Factors, respectively.

the frequency of 678.7 MHz, therefore an interpolation was used between the nearest two points of the table 2.

The two scenarios were simulated, Figures 14 and 15 shows the voltages expected at both receiver and transmitter antennas.

Table 2. The theoretical normalized site attenuation provided in CISPR-16 Frequency [MHz] 600 700

NSA [dB] -18.3 -19.7

The interpolated straight line between both points results in a NSA of -19.4dB. Table II compares the simulated and theoretical NSA results. Table 3 - Comparison of simulated and theoretical NSA Results Simulated VTX/VRX [dB]

Figure 14. Simulated voltages in the receiver antenna placed at a height of 1 meter from the ground plane.

Figure 15. Simulated voltages in the receiver antenna placed at a height of 4 meters from the ground plane.

RX antenna 1 meter height RX antenna 4 meters height

Analyti cal AF

Computed NSA [dB]

Theoretical NSA from CISPR-16

30.40

24.73

-18.80

-19.40

40.66

24.73

-8.53

-19.40

It can be seen that, for the first case where the receiver antenna was mounted 1 meter above the ground plane, the computed result lies close to the theoretical value, but at 4 meters it is different, with more than the +/- 4 dB out of the error margin. Further investigations led to the possible cause of the discrepancy for the 4 meters RX antenna case. By looking at the Fig.16 it is possible to notice that the coupling parameter S21 for both cases differs from about 10dB. That accounts for the large difference between the computed and theoretical NSA’s. One possible solution would be employ another antenna, maybe with smaller directivity to make this difference smaller.

From the results presented in Figures 13 and 14 it is possible to compute the OASA parameter. However, it is necessary to determine the Antenna Factor parameter in order to calculate NSA. CISPR-16 provides an analytical formulation for the Antenna Factor, when the radiant system is a tuned half-wavelength dipole:

= 20 log( ) − 31.9

(3)

Where f is the frequency in MHz. This equation is valid for frequencies near the resonance (when its input impedance is approximately 73 Ohms). Table 2 synthetizes the comparison between the simulated and theoretical NSA values, only for the frequency of 678.7 MHz. The first column VTX/VRX is taken from the simulations: Figures 13 and 14. The computed NSA in turn is the ratio between the VTX/VRX and twice the Analytical AF parameter (since both antennas are similar). The theoretical normalized site attenuation is provided in the CISPR-16, table E-2. The simulated dipole resonates at Page 5 of 9

Figure 16 - S12 results, transmittance, for both scenarios

B. Absence of a non-perfect ground plane In order to further investigate the real world effects of a counterpoise ground plane, the same simulation scenarios were evaluated but this time with a lossy ground plane, with a conductivity set to 3.56E6 S/m, one tenth of the Aluminum

usual value. Table 3 shows the computed results for the case where a lossy ground plane is taken into account. Table 4 - Comparison of the simulated and theoretical NSA Results

Simulated VTX/VRX [dB] RX 30.19 antenna 1 meter height RX antenna 41.9 4 meters height

Analytic al AF

Computed NSA [dB]

Theoretical NSA from CISPR-16

24.73

-19.19

-19.40

24.73

-7.29

-19.40 Figure 18. Horizontal dipole over a lossy ground plane, at a height h.

The fields generated by the antenna will be similar to an array, where the other element is its image [13]. It can be seen that the problem persisted. For the case where that RX antenna is 1 meter above the ground plane the computed result lies close to the theoretical value whereas for 4 meter it is not, with a value more than +/- 4 dB out of the error margin. Figure 17 shows that by looking into the S21 parameter (the system transmittance), the difference between a perfect and a lossy ground plane did not significantly change the response.

The Array Factor function for this particular problem can be written as:

= 2 sin ( ℎ(

))

(4)

The angle θ is defined as shown in the fig. 17.The overall gain of the system will be equal to the squared absolute value of AF times the individual element gain, defined in equation (5) as:

=|

| .

(5)

For the specific case of a dipole antenna resonant at 678 MHz placed at a height h=2m, the array factor can be written in dB, as defined in equation (6):

= 20 log [2. sin 28.4(

Figure 17 - S12 results, transmittance, for both scenarios with a conductivity set to 3.56E6 S/m

This scenario is still not in accordance with CISPR-16 regulation that specifies +/- 4 dB for the maximum difference between the Computed NSA and the Theoretical NSA [8].

C. Analysis of Half-Wave Dipole Antenna

Over a Losses Ground Plane In order to know the behavior of a half-wave dipole antenna over a lossy ground plane, a horizontal dipole placed above a conducting ground plane was studied, Fig. 18 illustrates this condition. It generates a perfect image of itself, but 180° out of phase. Page 6 of 9

) ]

(6)

Figure 19 shows the comparison of the simulated CST (solver I that uses the Method of Moments) far field and the theoretical AF [12] using the antenna showed in figure 9. The difference in amplitude between both curves is the antenna element gain. It can be seen a good correlation in both plots, and as expected, for angles θ larger than 90° the fields are null due the presence of the ground plane with value of - 200 dB, the limit value defined in the simulator.

3.

4. 5. 6.

7. Figure 19. Farfield plots for the element placed 2 meters above a perfect ground plane

In the Fig. 18 the blue curve is the analytical Antenna Factor function and the red curve the simulated MoM numerical solution (Antenna factor times individual element gain).

Conclusions

8. 9.

Yan Jie Guo, Li Fang Wang and Cheng Lin Liao, Modeling and analysis of conducted electromagnetic interference in electric vehicle power supply system Progress In Electromagnetics Research, Vol. 139, 193– 209, 2013 PAUL, C.R, Introduction to Electromagnetic Compatibility, John Wiley & Sons,Inc., New York, New York, 1992; SANTOS, K. M. G et al., Qualitative and Quantitative Analysis of the CISPR 25. In:II Colloquium SAE BRASIL de Eletro-Eletrônica Embarcada. Resende, 2009; CISPR 25: Edition 3.0 2008-03: Vehicles, boats and internal combustion engines - Radio Disturbance Characteristics – Limits and methods for the protection on board receiver; James McLean and Robert Sutton “The Determination of the Ultra-Wideband and Time-domain Behavior of Open Area Test Sites Using Frequency Domain Measurements and the Complex Antenna Factor Concept,” Electromagnetic Compatibility, 2003 IEEE International Symposium on , pp. 659-662, 18-22 Aug. 2003; CISPR 16-1-4: 2004-05: Specification for radio disturbance and immunity measuring apparatus and methods; part 1; W. G. Duff, Electromagnetic Compatibility in Telecommunications, Interference Control Technologies, Inc.Volume 7, Gainesville, Virginia, 1988. John S. Maas,“The Effects of Ground Screen Termination on OATS Site Attenuation”, IEEE 1989 Int. Symp. on EMC, pp 166-170; SADIKU,Matthew N.O., Numerical Techniques in Electromagnetics, CRC Press, Boca Raton FL, 2001. CST MICROWAVE STUDIO® -Overview-, “© 2009 CST ComputerSimulationTechnology”,, 26/02/2014. Constantine A. Balanis, Antenna Theory: Analysis and Design, Portuguese Translation: j. R. Souza - 3rd Edition LTC-2009.

The site located near the city of Capelinha, state Minas Gerais (Lat. 17.8651°S Long. 42.4313° W) is a good place to be an open area test site because the ambient emissions were more than 6 dB below the limit line. It was computationally verified the recommended tests of the CISPR-16 and it lies close to the theoretical value when the antenna is 1 meter above the ground plane but it failed when the antenna is 4 meters above the ground plane. OATS calibration is an important engineering job. The keywords for an open area test site calibration are repeatability of electromagnetic interference (EMI) measurements, accuracy and accreditation. An ideal OATS has an infinite area, is perfectly flat and good conducting, beside the absence of reflecting obstacles. The cost of a semi anechoic chamber for automotive EMC test is very expensive, one way for solve this problem is the use of an open area test site. Finally, this work demonstrates that it is possible to find low levels of ambient radio frequency in a real open area. It is clear that, there are a lot of other studies to be done, mainly about the ground plane, its properties and other factors in order to improve the results significantly and define all conditions necessary to perform tests in an open area test site. It is important to remark that, weather conditions such as excessive temperature and rain, can adversely affect equipment performance and measurements, so any OATS need to be fully protected by a shelter made of non-conductive materials.

10.

References

Marcela Silva Novo: Associate Professor at Federal University of Bahia, PhD in Electrical Engineering - PUC – Rio / Ohio State University, USA. [email protected]

1.

2.

SANTOS, K. M. G. et al., Measure of the Shielding Effectiveness in Coaxial Cables. In: XIV International Symposium on Electromagnetic Fields in Mechatronics Electrical and Electronic Engineering. ARRAS- France, 2009; SANTOS, K. M. G. et al., Measure of the shielding effectiveness in coaxial cables. Amsterdam. V. 1, p. 540548, IOS Press: ISSN:1383-728, 2010;

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11. 12. 13.

Contact Information Kenedy Marconi Geraldo Santos: Assistant Professor and EMC Researcher at IFBA - Federal Institute of Bahia, MsC. Engineering Federal University of Minas Gerais. Doctoral student, electric engineer, at the Federal University of Bahia. [email protected] Marcelo Bender Perotoni: Associate Professor at UFABC, Santo André /SP, PhD in Electrical Engineering - Polytechnic School USP, [email protected]

José Osvaldo Saldanha Paulino: Professor at Federal University of Minas Gerais, Doctor of Engineering University of UNICAMP. [email protected] Décio R. M. Faria: Project Engineer, Master’s degree student at UNIFEI: IEST. [email protected]

Caio Luminatti Andrade: EMC lab coordinator at SENAI Cimatec, Master's degree student at SENAI CIMATEC. [email protected]

AF

Antenna-factor

OASA

Open Area Site Attenuation

Thayane Almeida Alves: Student in electrical engineering from the Federal Institute Bahia. [email protected]

MoM

Method of Moments

MLFMM

Multi-Level Fast Multipole Method

FEM

Finite Element Method

FDTD

Finite Difference Time Domain

Acknowledgments The authors would like to thank to Fiat Automobile plant in Betim-MG, Brazil, for the use of broad band antenna, receiver and spectrum analyzer.

Abbreviations OATS

Open Area Test Sites

EMC

Electromagnetic compatibility

EMI

Electromagnetic interference

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