5 electromagnetic environment - nict · 2 measurement method for elec-tromagnetic disturbances...

151Yukio YAMANAKA et al.

5 Electromagnetic Environment

5-1 Recent Progress of Studies on EMC Relat-ing to Various Equipment

Yukio YAMANAKA, Shinobu ISHIGAMI, and Katsushige HARIMA

In order to realize the further development of the advanced information society, R&D inEMC technologies between electric, electronic and communication equipment and/or sys-tems are indispensable. The target of the EMC is to promote the electromagnetic environ-ment in which those equipment can be used without mutual interference. In this paper,recent EMC studies on various equipment in CRL are explained. CRL/EMC group mainlyfocus its research area on measurement method. Research outline and outcome on sometopics are summarized which include the radiated emission measurement method above 1GHz, GTEM cell and reverberation chamber which are promising measuring instruments forboth radiated emission measurement and radiated immunity measurement in the nearfuture.

Keywords Electromagnetic Compatibility (EMC), electromagnetic disturbance, immunity, rever-beration chamber, GTEM cell

1 Foreword

With the popularization of electronicequipment such as computers, as well as thetrend toward its fusion with various otherforms of equipment and integration to createan integral system, a wide array of complexproblems involving telecommunications andbroadcasts caused by electromagnetic distur-bances have arisen. Moreover, with the explo-sive increase in the use of cellar telephonesand wireless equipment, concerns are increas-ing that the radio waves they emit may causeelectronic and medical equipment to malfunc-tion. In addition, the influences thereof uponthe human body have received a great deal ofmedia attention. As we enter the 21st century,to ensure the continued development of ouradvanced information society, it is absolutelynecessary to conduct research and develop-ment of electronic equipment and communica-

tion equipment/systems that increase conven-ience and the quality of daily life, while con-ducting research and development of tech-nologies to ensure a good electromagneticenvironment that allows all equipment/sys-tems to operate normally is also indispensable.

In the Communications Research Labora-tory (CRL), research on electromagnetic envi-ronments has been underway since 1965 asone of basic technologies supporting wirelesscommunication. In particular, as a researchinstitute of the Ministry of Posts and Telecom-munications (the current Ministry of PublicManagement, Home Affairs, Posts andTelecommunications) in charge of the admin-istration of radio stations, CRL has conductedvarious studies to ensure electromagneticcompatibility (EMC) between radio stationsand between radio stations and electronicequipment. For example, CRL has been con-ducting research on measurement methods for

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electromagnetic disturbances caused by elec-tronic equipment for many years, and has con-tributed to the enactment of related standardsin Japan and the planning of internationalstandards such as CISPR. Further, CRL hasdetermined the actual state of our country’selectromagnetic environments, such as that inthe neighborhood of a large RF-power radiostations, and has contributed to the enactmentof guidelines for the protection of humanhealth and the prevention of equipment dam-age. Moreover, since 1996 CRL has beenresearching the influences on the human bodyof radio waves emitted from equipment usedin close proximity to the human body, such ascellar telephones, and is also conductingresearch on evaluation methods for compati-bility with the radio-radiation protectionguidelines. We have set as a goal the creationof electromagnetic environments in which var-ious pieces of equipment can be used withoutmutual interference, and in which the influ-ences thereof upon the human body are negli-gibly small, through a wide range of research,as exemplified above.

In July 1997, the EMC project was movedto the Yokosuka Radio CommunicationsResearch Center due to a revision of the inter-nal organization. Since 2001 the project hasbeen administrated on a consignment basis bythe Ministry of Public Management, HomeAffairs, Posts and Telecommunications, aspart of the movement of the CommunicationResearch Laboratory into an independentadministrative institution. Though the framework of CRL has changed, we feel it is neces-sary to further enhance and develop the con-tents of research activities, and to use them asa foundation for our ongoing work to accom-plish our mission as a public institute.

In this special edition, EMC projects aredivided into those that are equipment-relatedand those that are human-related. Also, andthe most recent main research results andfuture research topics are introduced for eachproject. Among the subjects relating to theequipment, measurement methods for theEMC are chiefly being examined. Among the

subjects relating to the measurement methodsare “electromagnetic-disturbance measure-ment methods” whereby electromagnetic dis-turbances from the equipment are measured,and “immunity measurement methods” where-by the tolerance of the equipment againstincoming electromagnetic waves is measured.Further, the measurement methods can beclassified into those for radiated electromag-netic disturbances (immunity) whereby cou-pling of the electromagnetic wave from (or to)an enclosure of the equipment concerned istreated, and those for conduction disturbances(immunity) whereby coupling of the electro-magnetic wave through a power-source line orcommunication lines is treated.

This paper will outline and discuss theresearch results concerning (1) a measurementmethod for radiated electromagnetic distur-bances above 1 GHz, and (2) a GTEM celland reverberation chamber that will be avail-able in the near future for the measurement ofradiated electromagnetic disturbances andradiation immunity, respectively.

2 Measurement method for elec-tromagnetic disturbancesabove 1 GHz

(1) IntroductionIn recent years, with the development and

popularization of wireless communication sys-tems in frequency bands above 1 GHz and theenhanced performance of digital circuitstoward higher operating frequencies, the EMCproblem in these frequency bands has grownin magnitude. CISPR (International SpecialCommittee on Radio Interference) is currentlyexamining a measurement method for radiatedelectromagnetic disturbances above 1 GHz,part of which (1-18 GHz) has already beenstandardized. CISPR16-1 2nd edition (1999)[1] prescribes measurement receivers, meas-urement antennas, and measurement sites asfundamental devices for measuring electro-magnetic disturbances. Further, CISPR16-2Amendment 1 (1999)[2] prescribes measure-ment methods such as measuring arrange-

Journal of the Communications Research Laboratory Vol.48 No.4 2001

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ments and measurement procedures. Outlinesof these prescriptions are shown in Fig.1. Itcan be said that a basic structure has beenbuilt, but work remains to be done.

In this section, the problems with the spec-trum analyzers commonly used as measure-ment receivers, measurement antennas, andmeasurement sites are discussed, along withCRL’s efforts for the problems.

(2) Measurement apparatus

The basic requirements for the spectrumanalyzer specified in CISPR are as follows[1].

1) Detection method: Peak detection2) Resolution bandwidth (RBW): 1 MHz

± 10%3) Video bandwidth (VBW): 1 MHz or

moreHere, it should be noted that the RBW is

defined with the impulse bandwidth Bimp. Thisis due to the fact that, in the measurement ofthe peak, the measured value for pulse noise is

Yukio YAMANAKA et al.

Conceptual diagram of the measurement method for electromagnetic disturbances at1-18 GHz

Fig.1

Spectrum analyzer VBW [MHz] A(t)max [dBm] Difference [dB]

Company A, No.1 (B3)



Company B, No.1 (B3)

Company B, No.2-1 (B3)

Company B, No.2-2 (B6)

Company C (B3)

1313131-131313

-21.5-17.7-19.7-16.6-19.4-16.3-30.2

--24.1-21.6-24.8-23.5-31.2-28.6

3.8

3.1

3.1

-

2.5

1.3

2.4

Variations in measured values (peaks) by model and video bandwidth (VBW)RBW=1MHz B3 : 3-dB Bandwidth B6 : 6-dB Bandwidth

Table 1

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in proportion to Bimp. However, the RBW ofcommercial spectrum analyzers is definedwith an attenuation bandwidth of 3 dB or 6dB, and no measurement is taken of theimpulse bandwidth.

Moreover, arbitrary settings, such as 1MHz, 3 MHz, and the like, can be made forthe VBW. In addition, weighted measurementachieved by reducing the value of the VBW isalso prescribed.

Thus, in EMI measurement using the spec-trum analyzer at above 1 GHz, the characteris-tics of the RBW and VBW significantly affectthe measurement results; therefore, we sur-veyed the characteristics of typical spectrumanalyzers commercially available at present.The results are shown in Table 1[4].

As can be seen from the table, when thesame pulse is input, the measured values (peakvalues) differ significantly by model (differ-ence of up to 11.8 dB under a constant VBWof 1 MHz). This is due to the fact that thetype and characteristics of each IF filter differ.Moreover, even with the same model, the peakmeasured value changes significantly (by upto 3.8 dB) when the setting of the VBW ischanged. Therefore, in the actual measure-ment of electromagnetic disturbances as well,the VBW-dependence described above may beexhibited in the case of pulse electromagneticdisturbances. Moreover, the measured valuesof Bimp for company A and company B areshown in Table 2.

These results revealed that some models ofcommercially available spectrum analyzersachieve the impulse bandwidth of 1 MHz ±10% prescribed by CISPR, whereas somemodels fall far short of that mark. Therefore,when a spectrum analyzer that does not satisfy

that specification is used, the measured resultmay differ significantly from that of a specifi-cation-compliant spectrum analyzer, particu-larly in terms of impulse noise (wide-bandnoise having a bandwidth wider than themeasurement bandwidth). It will therefore benecessary in the future for the spectrum ana-lyzer used in the measurement of electromag-netic disturbances to explicitly indicate thevalue of the impulse bandwidth. Moreover, itis preferable to perform the measurement atthe same VBW value (such as 3 MHz), due tothe possibility that the measurements willyield different results only under the CISPRprescription directing that “the VBW shall be1 MHz or higher.”

Here, it should be noted that when thevideo bandwidth VBW of the spectrum ana-lyzer is set to a value lower than the modula-tion bandwidth of the measured signal, themeasured value becomes a value correspon-ding to the average level of the measured sig-nal, and weighted measurement can be per-formed in such a way that noise occurringcontinuously results in a high indicated valueand noise occurring intermittently results in alow indicated value. The display indication ofthe spectrum analyzer features a Log modeand a Linear mode and, in the case of weight-ed measurement, the values in the Linearmode become greater than the values in theLog mode.

CISPR11[3] prescribes weighted measure-ment with the VBW set to 10 Hz in the meas-urement of radiated electromagnetic distur-bances of 1 GHz to 18 GHz using a class-B,group-2 ISM device capable of operating atfrequencies equal to or higher than 400 MHz,which is intended to be performed in the Log


Spectrum analyzer Method 1[MHz]

Method 2[MHz]

Average [MHz]

Peak measurement errorwhen pulses are input [dB]

Company A, No.2

Company A, No.3

Company B, No.2-1

Company B, No.2-2

1.64

1.69

0.92

0.74

1.50

1.66

0.88

0.79

1.57

1.68

0.90

0.75

3.9

4.5

-0.9

-2.5

Comparison of impulse bandwidth (Nominal Value: RBW=1MHz VBW=3MHz)Table 2

CISPR Standard of impulse bandwidth: 1MHz±10% (0.9～1.1MHz)

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mode.Consequently, we examined the factors

that affect the measurement results in weight-ed measurement[4]. As a result, it was foundthat the values obtained in weighted measure-ment in the Linear mode were always greaterthan those obtained in the Log mode. Forexample, in the measurement of pulses with aduty ratio of 50%, the measured value in theLinear mode is decreased from the peak by 6dB, whereas the measured value in the Logmode is decreased therefrom by as much as 30dB. Incidentally, the weighted measuredvalue in the Linear mode indicates the actualaverage; the corresponding value in the Logmode differs from the actual average and isdifficult to define.

Therefore, when weighted measurementthat uses the VBW is adopted, its applicationmust be re-examined by a pertinent productcommittee (for each waveform to be meas-ured). Moreover, as a weighted-measurementmethod that can be substituted for this meas-urement, Japan has proposed a measurementmethod using APD (Amplitude ProbabilityDistribution)[6] based on research results[5]obtained by CRL and the like, which is nowbeing examined.

(3) AntennaAs the antenna used for electromagnetic

disturbances of 1-18 GHz, the following itemsare prescribed[1]:

- A calibrated antenna having linear polar-ization shall be used.

- The main lobe of the antenna (defined asa 3-dB width) shall cover the equipment undertest.

- The maximum size D of the antenna,measurement wavelengthλ, and measurementdistance Rm shall satisfy the following condi-tion:

Rm ≥ D2/2λ (1)

In the derivation of this formula, the mainradiation for the equipment under test is con-sidered to be that coming from a point source.

Typical antennas include various horn anten-nas, such as a double-ridged guide horn,pyramidal horns, and a standard-gain horn.Although the basic measurement distance Rm

is 3 m, measurement can be performed at adifferent distance if necessary due to the sur-rounding conditions and lack of measurementsensitivity. In such a case, the value is thenconverted according to the rule that the meas-ured value is in reverse proportion to the dis-tance.

Regarding the above conditions, the dou-ble-ridged guide-horn antenna that is generallyalso used in EMC measurement was evaluated[7]. As a result, it was confirmed that the mainlobe had a minimum 3-dB width of 30 degreesup to 15 GHz, and that beyond this frequencythe pattern changed and the 3-dB widthdecreased. Moreover, from the calculation ofthe size of the equipment under test that iscovered by the main lobe when this antenna isused, it is shown that the maximum size of theequipment under test is 1.7 m for the basicmeasurement distance of 3 m and 0.6 m for adistance of 1 m up to 15 GHz, indicating thatthere is no problem in putting it to practicaluse, except in large equipment. Note that forfrequencies exceeding 15 GHz, the standardgain horn has a wider beam width than thedouble-ridged guide horn and becomes advan-tageous in measurement. In addition, it wasconfirmed that the double-ridged guide hornfulfilled the conditions of Eq. (1) at the basicmeasurement distance, which was prescribedto be 3 m, for all frequencies from 1-18 GHz.

In general, the measurement of antennacharacteristics is performed at a distance thatsatisfies the far-field condition which is shownin the next equation.

R′m ≥ 2D2/λ　　　　　　　　　　 (2)

Therefore, at a distance that satisfies Eq.(1), the antenna gain decreases from the gainin the far field. From our measurements, itwas found that, when the gain at the far fieldwas used as is in measurement at the distancespecified in Eq. (2), there was the possibility


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of error of up to 2-3 dB. Consequently, evenif Eq. (1) is satisfied, it is necessary to positionthe antenna apart from the equipment undertest by at least 1 m.

(4) Measurement siteFor the measurement site at 1-18 GHz,

only the following have been specified:- A standard test site shall be a free-space

open site with no reflection.- An arbitrary test site that fulfills the free-

space conditions may be used as an alternativetest site.

The allowable deviation for ideal free-space conditions and the compatibility confir-mation procedure are under consideration.Outlines of the most recent draft[8] are givenbelow.- Free-space conditions and allowable devia-tionThe normalized site attenuation AN in an idealfree space is given by the following formula:

AN[dB]=20log(D)－20 log(F)＋32 (3)

where D: distance [m] between the transmit-ting and receiving antennas and F: frequency[MHz]

In the measurement arrangement shownbelow, the normalized site attenuations at thesite concerned (for horizontal and verticalpolarizations) are measured and, if the differ-ence between the value and the above-men-tioned theoretical value is within ±4 dB, thesite is determined to be in conformity with thefree-space conditions.- Measurement arrangement of normalized siteattenuation・Receiving antenna: The same antenna as

is used to measure the electromagnetic distur-bance・Transmitting antenna: Antenna with a

gradual directivity (3-dB width exceeding 40degrees)・Distance between transmitting and

receiving antennas: Distance at which theelectromagnetic-disturbance measurement isperformed (normally 3 m)

・Heights of transmitting and receivingantennas: Both are set at an equal height, andno scanning is performed.

Specifically, the height is basically 1.0 m(from a metallic ground). However, in casesin which the receiving antenna is made to scanin the height direction when performing themeasurement due to the beam width of thereceiving antenna being narrower than that ofthe equipment under test, the space therebe-tween is scanned. For example, in cases inwhich the receiving antenna is made to scanfor 1-3 m at the measurement distance of 3 m,the measurement is performed every 0.5 mover the measurement distance.

In the future, it will be necessary to con-firm the validity of the above-mentioned draft.We are conducting an examination focusingon what type of antenna should be used as thetransmitting antenna, how the antenna factorsof the transmitting and receiving antennas willbe obtained, whether it is necessary to performthe measurement with the position of thetransmitting and receiving antennas changed,and the like, and are making pertinent propos-als. Further, the uncertainty in the calibrationof the measurement receiver and the measure-ment antenna must be evaluated. Moreover,how the free-space conditions will be satisfiedfor floor-standing equipment remains a prob-lem.

(5) Future problemsThe measurement system used to measure

radiated electromagnetic disturbances above 1GHz was evaluated and examined. First, sincethe results of the characteristics evaluation ofthe spectrum analyzers revealed that there aremodels satisfying the impulse bandwidth pre-scribed by CISPR, as well as models withcharacteristics that differ considerably fromthe prescription, the impulse bandwidth mustbe described explicitly in the specifications ofthe equipment. Further, since the measuredlevel (peak) differs according to the setting ofthe video bandwidth VBW, it is preferable tounify the VBW in order to improve repro-ducibility. In the weighted measurement


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achieved by reducing the size of the VBW, thevalue in the Log mode becomes smaller thanthe value in the Linear mode and deviates sig-nificantly from the true average. Regardingweighting using the VBW, the characteristicscannot be fully understood and their relation-ship to its limit is not clear. In the future, itwill be necessary to grasp the characteristicsas much as possible and to investigate othermethods as well.

Regarding the antenna, the procedure forusing antennas in the near field and standardi-zation and the like of the calibration methodwill be important issues in the future.

Regarding the measurement site, severalproblems remain to be solved, including theallowable deviation from the ideal free-spaceconditions and the compatibility confirmationprocedure; therefore, further examination willbe necessary in the future.

3 GTEM cell

(1) IntroductionThe GTEM (giga-hertz transverse electro-

magnetic) cell, a type of TEM waveguide, isan apparatus designed to allow the immunitytest and the radiated electromagnetic-distur-bance test to be performed in the microwavefrequency band. Its external appearance is asshown in the photograph in Fig.2. Other typesof TEM waveguides include a TEM cell, astrip line, and the like. For the standardizationof several test methods and the like that usethese devices, these methods are presentlyunder discussion at a Joint Task Force (JTF) of

IEC SC77B and CISPR/A, where examinationto make them an international standards isunderway.

To this point (December 2001), a commit-tee draft for a vote (CDV) CISPR/A/343/CDV[9] continuing the international standard docu-ment IEC61000-4-20 has been sent to thenational committees of the respective coun-tries. Each country votes for or against thecontents of this draft and, if the affirmativevotes represent the majority, the draft makesgreat strides toward its establishment as aninternational standard document.

The GTEM cell has the structure as shownin Fig.3. The internal electric field becomesalmost vertical between the septum and thefloor conductor. In the immunity test and theradiated electromagnetic-disturbance test, theequipment under test (EUT) is placed betweenthese two conductors. An EM-absorber and aresistor board on the rear side are installed forimpedance matching and to absorb electro-magnetic waves incident on the rear side.

(2) Uniformity evaluationTo perform a reproducible test in the

immunity test and the radiated electromagnet-ic-disturbance test, the electric field must beuniform in the space in which the EUT wasplaced. The CDV proposes the followingrequirements: (1) the space in which the testcan be performed shall be 0.6W×0.33d (W:width of the septum; d: distance between thefloor conductor and the septum); (2) the varia-tion in the primary electric-field component(vertical component) within the above-men-tioned region on the transversal cross sectionof the GTEM cell shall be－6 dB or less in an


Structure of the GTEM cellFig.3

External appearance of the GTEM cellFig.2

158

empty cell state in which no EUT is placed;and (3) electric-field components other thanthe primary component (components in a hori-zontal direction and in a propagation directionof the electromagnetic wave) shall be－6 dBor less than the primary component. To exam-ine whether the region is appropriate, we haveconducted an experimental and theoreticalexamination of each electric-field componenton three axes[10] and proposed an evaluationmethod, part of which has been reflected inthe CDV.

For a sensor for measuring the electric-field distribution of each axis in the above-mentioned area, an optical electric-field sensor(with a measuring frequency of up to 1 GHz)is used that, comparatively speaking, hardlydisturbs the surrounding electric field. Theresults are shown in Fig.4. Calculation resultsobtained by the FD-TD (finite-difference timedomain) method are shown in Fig.5. Theregion enclosed with a white line is thatmarked out by the measurement shown inFig.4. It was confirmed that the measurementresults were in fairly good agreement with thetheoretically calculated results, and hence themaximum usable area proposed by the CDVwas appropriate.

(3) Influence of EUT sizeIn the immunity test, the EUT is placed in

the GTEM cell. When the EUT is loaded intothe cell, the state of the electric field in the cellchanges to the electric distribution measuredwhen the cell is empty. On the other hand,there is an immunity test that uses a full ane-choic chamber as the conventional method. Inboth the conventional method and a methodusing the TEM waveguide, such as the GTEMcell, the electric field is adjusted so as to havethe same electric-field level as when the EUTis absent. However, when the EUT is placedin position, the influence of the EUT on thescattered wave differs between the two meth-ods, and as a result the test results may differbetween the two methods. Therefore, it isimportant to determine to what extent theimmunity test results differ between the con-

ventional method and the method using theGTEM cell, and whether there is any correla-tion between the two results. As such mattersare considered to be dependent on the EUTsize and the position within the cell at whichthe EUT is located, a theoretical examinationis being conducted with these factors assumedas parameters.

Fig.6 shows the spatial distribution of theratio of the electric-field value when the EUT


Measurement results for electric-fielddistribution

Fig.4

(Upper: Horizontal electric-field component Ex; Cen-ter: Primary electric-field component Ey; Lower:Electric-field component in propagation direction Ez)

159

is loaded and that when no EUT is loaded; thefigures on the left indicate that the ratio (a/d)of the EUT size (a) and the distance betweenthe septum and the floor conductor (d) is equalto 0.1, and the figures on the right indicate aratio of 0.3. Here, the EUT is assumed to be acube. The upper two figures are for the caseof the anechoic chamber, and the lower twofigures are for the case of the GTEM cell.

From Fig.6, it is apparent that the manner inwhich the distribution of the normalized elec-tric field changes spatially depends on theEUT size, and that it differs among measuringfacilities.

Fig.7 is a graph showing the averages ofthe electric fields on all surfaces of the EUT asa function of the frequency (frequencyresponse), for the case of the anechoic cham-ber and for the case of the GTEM cell. Thecase of a/d = 0.2 is shown as an example.

From the figure, it can be seen that the fre-quency characteristics of the anechoic cham-ber and of the GTEM cell are roughly identi-cal, but that the characteristics of the GTEMcell exhibit greater fluctuations. The width ofthese fluctuations tend to grow with anincrease in ratio a/d. Therefore, as the EUTsize increases, the possibility grows that thetest results obtained using the anechoic cham-ber will deviate from those obtained using theGTEM cell on a larger scale.

To this point, the EUT is placed in themiddle between the septum and the floor con-ductor. Fig.8 shows how the differencebetween the average electric fields on all sur-faces of the EUT in the anechoic chamber andthat in the GTEM cell changes when the posi-tion of the EUT is changed vertically by plot-ting the difference against the positionalheight of the EUT. In the figure, hEUT denotesthe distance between the bottom surface of theEUT and the floor conductor. This parameteris that proposed in the committee draft (CD),which recommends hEUT ≥ 0.05d. From thefigure, it can be seen that when hEUT = 0.05d,the difference in the average of the electricfields between the anechoic chamber and theGTEM cell is larger than at any other position.Further, when hEUT = 0.85d, which correspondsto a case in which the spacing between theseptum and the upper side of the EUT is equalto 0.15d, the difference is the second largest.Therefore, it was found from this result thatthe EUT shall preferably be set apart fromboth conductors by 0.15d or more.

(4) Future problems


Calculation results for electric-field dis-tribution

Fig.5

(Upper: Horizontal electric-field component Ex; Cen-ter: Primary electric-field component Ey; Lower:Electric-field component in propagation direction Ez)

160

Regarding the EUT to which the electricfield is applied by the GTEM cell, it is neces-sary to evaluate not only its surface electricfields but also other elements such as its cur-rent and magnetic field, and to investigate thedifference from the conventional method andthe correlation therewith. Further, if possible,it is also important to measure the electricfields and the like on the surfaces of the EUTin order to prove the validity of the theoreticalcalculation. Moreover, its application tomeasurement of electromagnetic disturbancerequires further exploration in the future.

4 Reverberation chamber

(1) Introduction

Generally, measurements of the radiatedelectromagnetic disturbance and the radiationimmunity of the electronic equipment[11] areperformed primarily at open test sites and inanechoic chambers. In recent years, the use ofthe reverberation chamber and TEM wave-guides such as a stripline or TEM cell is beingexamined by international organizations suchas IEC/SC77B and CISPR, as an alternative tothe conventional methods and as an independ-ent method[12]～[15]. Further, as can be seenfrom the growing use of portable wirelessequipment, the popularity of antenna-integrat-ed wireless equipment is increasing exponen-tially. The use of the reverberation chamber isbeing examined as a new measurementmethod of the radiated power of such wireless


Frequency characteristics of the aver-age electric field on all surfaces of theEUT

Fig.7Frequency characteristics of the aver-age of the electric fields on all sur-faces of EUT

Fig.8

Spatial distribution of the ratio of the electric field when EUT is loaded and when no EUT isloaded

Fig.6

161

equipment[16].The reverberation chamber is an apparatus

such that a stirrer composed of metallicimpeller blades and the like is installed insidea metallic chamber[17] to establish a statisti-cally uniform electromagnetic-field distribu-tion therein, by changing the boundary condi-tion inside the chamber using the stirrer[18][19][20]. In the measurement of radiatedpower using the reverberation chamber, theequipment under test (EUT) is placed in a testarea of the reverberation chamber, and theradiated power is estimated based on the aver-age received power obtained by continuouslyoperating the stirrers. Further, in the measure-ment of radiation immunity, the performanceof the EUT is monitored while the stirrers arebeing rotated stepwise. The uniformity evalu-ation is performed statistically based on thedistribution obtained from the electric probeplaced in the test area[14]. In any measure-ment, the non-uniformity of the electric-fielddistribution is a primary factor in the measure-ment error.

Therefore, in a basic-characteristics evalu-ation of the reverberation chamber, the follow-ing were investigated through a computer sim-ulation using the FDTD (Finite-DifferenceTime-Domain) method; how the size of thereverberation chamber affects the uniformityof the electric field, the number of stirrers, thesize of the stirrers, and the installation loca-tions of the stirrers[21][22]. Further, the influ-ences of the stirrer and the number of probelocations on the uniformity evaluation wereexamined through actual measurements[23][24][25].

(2) Evaluation by simulationThe reverberation-chamber method

changes the boundary condition by rotatingthe stirrers, and generates a statistically uni-form electromagnetic field. When the rever-beration chamber is applied to the radiatedelectromagnetic-disturbance measurement andthe immunity measurement, the spatial unifor-mity of the electromagnetic field that isintended to exist in the reverberation chamber

becomes a problem. Therefore, the uniformityof the electric-field distribution in the rever-beration chamber was examined by the FDTDmethod[21][22].

Fig.9 shows an analytic model of thereverberation chamber. To enable the FDTDmethod to be applied thereto, the analyticmodel is composed of a primitive (Yee’s) cell[26]. As shown in Fig.9, the stirrer is made upof two metallic plates, and two stirrers areinstalled on the side walls. Incidentally, theTx point in the figure denotes a transmittingpoint, and the Rx point denotes a receivingpoint.

The reverberation chamber is specified tobe a rectangular parallelepiped chamber meas-uring 150 cm×138 cm×168 cm, and a dipoleantenna is placed at the transmitting point asan excitation source. It is specified that thespatial discrete spacing of the primitive cell be1.5 cm (=Δx =Δy =Δz), and that the timestep thereof be 25 ps (=Δt).

In general, regarding the electromagnetic-field problem involved in the use of a metalliccavity with a loss-free interior, if the walls areassumed to be a complete conductor, conver-gence in numeral calculation becomes difficultto attain. Therefore, certain electric constantsare given to the walls by arranging absorptionboundaries around the outer peripherals of the


Analytic model of the reverberationchamber

Fig.9

162

walls, so that a reflection coefficient is set forthe walls. The reflection coefficient R(θ) isshown as a value as in the case in which aplane wave is incident. Note that the metallicplates of the stirrer are assumed to be a com-plete conductor, and that the electric conduc-tivity in of the medium is set to zero.

Fig.10 shows an example of the calcula-tion results for the average electric-field distri-bution of Ey on the xz plane passing throughthe receiving point Rx for two cases in whichthe exciting frequencies are 1 GHz and 2 GHz,respectively. In the case in which the rever-beration chamber is not sufficiently large com-pared to the wavelength of the measurementfrequency, deterioration in the uniformity canbe predicted. Fig.11 shows the cumulativedistributions for the median of the receivedelectric-field strength at point Rx for an x-direction component or a y-direction compo-nent of the electric field. From the results, itcan be inferred that the instantaneous fluctua-tions in the electric field caused by the stirrersin the reverberation chamber follow aRayleigh distribution.

Thus, by calculating the electromagneticfield in the reverberation chamber by the

FDTD method, a basic examination was con-ducted on the influence of the size of theenclosure in the reverberation chamber andthe stirrers on the electric-field uniformity inthe reverberation chamber.

From the results, in order to minimize thenon-uniformity of the electric field in thereverberation chamber, we are able to inferthat the following are necessary: (1) the sizeof the reverberation chamber shall be set so asto be equal to 10 wavelengths or more; (2) two


Average electric-field distribution of Ey in a cross section of the reverberation chamber (y = 67, |R(0)| = 0.97)

Fig.10

Cumulative distributions for thereceived electric-field strengths atreceiving point (Rx) (at 2 GHz)

Fig.11

163

or more stirrers shall be installed, with eachhaving a different rotation speed; (3) the sizeof each stirrer shall be as large as 3 wave-lengths or more; (4) each stirrer shall be setapart from the walls by approximately 1 wave-length.

(3) Evaluation through actual measurementsIn the radiated immunity measurement

using the reverberation chamber, the equip-ment under test (EUT) is placed within the testarea in the reverberation chamber, and the per-formance of the EUT is monitored while thestirrers are rotated stepwise. The evaluationof the electric-field uniformity is performed insuch a way that the probe is placed within thetest area and the electric-field uniformity isevaluated statistically from the maximumelectric-field strengths obtained during onecycle of rotation of the stirrer[22]～[25]. At thistime, the number of stirrers installed in thereverberation chamber and the setting of thenumber of steps of the stirrer influence theuniformity[21][22]. Moreover, it is expectedthat the number of probe locations affects theuniformity evaluation[13]. We have examined,through actual measurements, the influence ofthe number of stirrers installed in the reverber-ation chamber and the number of steps on theuniformity, and the influence of the number ofprobe locations on the uniformity evaluation[24][25].

The electric-field distribution within thetest area in the reverberation chamber shownin Fig.12 was measured using an optical elec-tric-field probe for each direction component(Ex, Ey, and Ez) from 200 MHz to 3 GHz.The probe locations are 125 (= 5×5×5)points within the area. The uniformity withinthe test area was evaluated on the basis of thedeviation (E75%) of a 75% value (from 12.5%to 87.5%) of the cumulative distribution of theobtained data.

Fig.13 shows the variation in E75% of theelectric-field distribution obtained using threestirrers. The use of three stirrers provides themost uniform distribution. However, theeffect obtained by the increase in the number

of stirrers from two to three is not as great asthe effect obtained by the increase in the num-ber of stirrers from one to two. When the tol-erance 6 dB of the uniformity in the immunitytest like the same way conducted in the exist-ing anechoic chamber method is applied, theavailable frequency of the used reverberationchamber becomes 200 MHz or higher.

The variation in the E75% versus frequencywhen the number of steps necessary for thestirrer to return to the initial state is increasedfrom 10 to 400 steps is shown in Fig.14. Asthe number of steps of the stirrer is increased,the uniformity is improved. However, whenthe number is 100 or more, the effect obtainedby the increase in the number is small. More-over, even with as few as 10 steps, theobtained uniformity satisfies the tolerance (6dB) for frequencies above 400 MHz.

Fig.15 shows the variation in E75% evaluat-ed using the following number of probe loca-tions in the measurement area: 125, 45, 27,and 8 points. The uniformity evaluated using


Schematic diagram of test arrange-ment

Fig.12

164

8 probe locations differed from the valueobtained using 125 locations by ±1 dB. Toevaluate the variation in E75% with a differenceof approximately 0.5 dB from the valueobtained using 125 probe locations, probelocations at 27 points or more become neces-sary.

From the above results, we have experi-

mentally confirmed the following : (1) the useof two stirrers can yield a sufficiently uniformdistribution; (2) only a small improvement canbe obtained in the uniformity even when thenumber of steps of the stirrer is increased to100 or more; (3) the use of a smaller numberof probe locations increases the uncertainty inthe evaluation of uniformity.

(4) Future problemsRegarding the evaluation standard of the

electric-field uniformity in the reverberationchamber, further statistical examination mustbe conducted on the relationship between the75% value and the standard deviation. Fur-ther, it is necessary to examine the relationshipbetween the shape of the stirrer and the unifor-mity and the problems in the measurement ofactual equipment. Moreover, the applicationthereof to the measurement of electromagneticdisturbances requires further investigation inthe future.

5 Conclusions

Among the subjects of the equipment-related EMC research being conducted in theEMC group of CRL, the measurement methodfor radiated electromagnetic disturbancesabove 1 GHz, outlines of the GTEM cell andthe reverberation chamber, the researchresults, and the like were introduced. We areconsidering investigating the remaining prob-lems, and we also wish to properly address


Influence of the number of probelocations on uniformity evaluation

Fig.15

Influence of the number of stirrers onuniformity

Fig.13

Influence of the number of steps ofthe stirrer on uniformity

Fig.14

165Yukio YAMANAKA et al.

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Yukio YAMANAKA

Leader, Electromagnetic CompatibilityGroup, Yokosuka Radio Communica-tions Research Center

EMC

Shinobu ISHIGAMI, Ph. D.

Senior Researcher, ElectromagneticCompatibility Group, Yokosuka RadioCommunications Research Center

EMC

Katsushige HARIMA

Researcher, Electromagnetic Compati-bility Group, Yokosuka Radio Commu-nications Research Center

EMC

5 electromagnetic environment - nict · 2 measurement method for elec-tromagnetic disturbances...

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