2 use of electromagnetic energy and resultant noises

5SUGIURA Akira

1 Introduction

We have recently witnessed explosivegrowth in the use of 13-MHz band RF-ID sys-tems for automatic ticket gates and securitysystems. These types of devices are designedto offer a transmission range of up to onemeter. Power-line communication (PLC) isanother method currently under active consid-eration in many countries. PLC can enablehigh-speed communication through an exist-ing utility power grid through the transmissionof HF-band current. A variety of new commu-nication systems use this HF band (3 MHz to30 MHz).

However, concern is growing that unwant-ed radiation from these systems may interferewith radio services or affect electronic devicesand the human body. To address this problem,in 2005 the Ministry of Internal Affairs andCommunications (MIC) established the“Study Group on Broadband PLC”. Thisgroup studied the requirements entailed by fre-quency sharing with existing radio services［1］.This paper will discuss issues concerning vari-ous electromagnetic environments based on

the results of this study.

2 Electromagnetic environments

Sources of energy that create an electro-magnetic environment include natural phe-nomena such as thunder, as well as radioequipment, high frequency-based equipment,and other electric and electronic devices. Thediscussion below deals mainly with HF-bandelectromagnetic environments and artificialsources of electromagnetic waves.

2.1 Radio equipmentHF-band radio waves reflected by the

ionosphere are suitable for long-distance com-munication and have traditionally been used incommunications over seas—by passenger andcargo ships, warships, and deep-sea fishingboats. However, since ionospheric conditionsvary greatly depending on the time, season,solar activity cycle, and other factors, estab-lishing stable communication channels proveddifficult. HF-band communication thus beganto be superseded by satellite communicationsin the 1960’s. However, HF-band radio waves

2 Use of Electromagnetic Energy andResultant Noises

SUGIURA Akira

Recently, the advent of RF-ID systems and PLC systems have raised a new problem of elec-tromagnetic interference (EMI) between newly developed systems and other various systemsincluding radio stations using the shortwave band. In this connection, EMI limits for a variety ofequipment are reviewed and compared in terms of the radiated field strength. Investigations areextended to cover the environmental noises, indicating the necessity for surveying recent noiseenvironments in a large scale.

KeywordsElectromagnetic environment, Electromagnetic disturbance, EMI limit, Man-made noise,PLC

6 Journal of the National Institute of Information and Communications Technology Vol.53 No.1 2006

are still used for various purposes—for exam-ple, in aeronautical communication (includingemergency transmissions), maritime commu-nications (also including emergency transmis-sions), shortwave broadcasting, ham radio(3.5, 3.8, 7, 10, 14, 18, 21, 24 and 28 MHz),radio astronomy, fixed and mobile communi-cations, 27 MHz citizen band radio, radiomicrophones, and radio-controlled machines.

These radio services use electromagneticwaves of designated wattages; however, spuri-ous and out-of-band emissions caused bymodulation may interfere with other radio ser-vices, as shown in Fig. 1.

To address this problem, the Japanese gov-ernment restricts spurious emissions fromradio services. For example, if the basic fre-quency of a radio service is 30 MHz or lower,its spurious emissions must be lower than50 mW and the emissions strength must, inprinciple, be lowered by 40 dB or more relativeto average power at the basic frequency［2］. Inmany countries, low-power communicationequipment may be provided without a license;in these cases the field strength of electromag-netic waves can be limited to levels equivalentto those of unwanted radiation from PCs. InJapan, for example, if the basic frequency is322 MHz or lower, the field strength is limitedto 500μV/m at a distance of 3 m; if the basicfrequency is somewhere between 322 MHzand 10 GHz, the field strength is limited to35μV/m, as shown in Fig.2［3］. At frequen-cies below the HF band, it is consideredappropriate to measure strength at a distanceof 10 m instead of 3 m, as the electromagnetic

field presents complicated characteristics nearthe source.

In addition, receivers for radio servicesgenerally use heterodyne detection and emitunwanted radiation. Therefore, incidental radi-ation from these receivers is limited to 4 nWunder the Radio Law［4］.

2.2 High frequency-based equipmentApart from radio services, various types of

devices use electromagnetic energy. Amongthese, certain equipment is designed to flowsignal current of 10 kHz or higher through awired system in which transmitting/receivingends are coupled electromagnetically; othertypes of devices include those that use electro-magnetic energy within a shielded space. Bothtypes are classified as “high frequency-basedequipment” in Japan［5］. The former type isreferred to as “communications equipment”.Figure 3 shows some specific examples ofsuch devices. Devices of the recently high-profile PLC fall under the category indicatedin Fig. 3 (a). The latter type mentioned aboveincludes induction cooking appliances andmicrowave ovens, all of which fall under thecategory of “medical devices, industrial heat-ing devices, and miscellaneous devices”,although internationally these devices areknown as “ISM” (industrial, scientific, andmedical) equipment.

The Radio Law in Japan restricts electro-magnetic waves leaked from high frequency-

Fig.1 Electromagnetic waves emittedfrom radio services

Fig.2 Permissible limits for low-powercommunication equipment(“B” refers to -6 dB bandwidth ofthe measuring instrument)

7SUGIURA Akira

based equipment that the Law views assources of interference with radio services.For example, inductive radio communicationequipment can be used without permissiononly if its electric field strength is limited to15μV/m or lower at a distance of λ/2π［6］.Permission is not required to use ISM equip-ment with RF output of 50 W or lower［7］.ISM equipment with RF output of higher than50 W is subject to authorization. However, ifRF output is 500 W or lower, for example, thefield strength of leaked electromagnetic wavesmust be 100μV/m or lower at a distance of30 m from the device［8］.

Since ISM equipment used for materialprocessing and the like requires extremelyhigh-power electromagnetic energy, the Lawgreatly eases permissible limits at specific fre-quency bands. Such a frequency band is gen-erally referred to as an “ISM frequency band”.For example, in the case of a microwave oventhat uses an ISM frequency band of 2,450 ±50 MHz, the permissible limit for in-bandelectromagnetic wave leakage is established at5 mW/cm2 (with a field strength of 138 V/m)［9］from the viewpoint of human body safety. Inthe HF band, the 13 MHz band is allocated toISM equipment, and this band is widely usedin dielectric heating equipment for dryinglumber and thermo-compression bonding ofplastics, among other applications. Since this

band can accommodate fairly high-level elec-tromagnetic waves, a number of current com-munication systems also use this band, includ-ing RF-ID systems.

2.3 Electric/electronic devicesPCs and many other electric/electronic

devices also emit unwanted radiation. TheRadio Law stipulates that in the event of prob-lems in radio services caused by unwantedradiation (i.e., disturbance) from a givendevice or equipment (other than radio or highfrequency-based equipment), legal action ispossible—for example, to halt the use of thedevice or equipment in question［10］. Thisreflects the importance of establishing mea-surement methods and permissible limits forunwanted radiation from various devices andequipment in protecting radio services andvarious electromagnetic environments.

At the global level, the International Spe-cial Committee on Radio Interference(CISPR) defines permissible limits for distur-bance caused by high frequency-based equip-ment and electric/electronic devices. Thedomestic committee in charge in Japan isknown as the CISPR Committee, under theauspices of the Telecommunications Council(established by the MIC). Figure 4 shows anorganizational chart of the CISPR. The sub-committees (SCs) define permissible limits forequipment in given individual categories, withthe aim of protecting broadcasting and radiocommunications. For example, the permissiblelimits shown in Figure 5 apply to IT devices witha frequency range of 150 kHz to 6 GHz［11］. At afrequency of 30 MHz or lower, leaked wavesof longer wavelengths propagate along con-ductors such as power cables connected to thedevice. Therefore, the CISPR standards limitthe disturbance current (or voltage) flowingthough the connected conductors. At frequen-cies higher than 30 MHz, the wavelength ofradiation from a device is close to the dimen-sions of the device itself, and direct radiationfrom the body becomes more noticeable.Therefore, in these cases the standards definepermissible limits for field strength.

Fig.3 Suitable configuration based


2.4 Comparison of permissible limits The previous section described permissi-

ble limits for low-power radio services and forunwanted emissions by radio services, andpermissible limits for disturbance generatedby various types of equipment. However,these permissible limits cannot be simplycompared with one another because measure-ment methods and conditions differ dependingon the type of equipment. For example,ground-plane effects differ greatly dependingon the frequency, height of the equipment,height of the measurement point relative to theground, measurement distance, polarization,etc. In this section, I will compare various per-missible limits by converting them into field-strength values measured at a distance of 10 min free space (in which there are no groundeffects).

The strength and radiated power of anelectromagnetic wave generated from an infin-itesimal element ∆z of current I shown in Fig-ure 6 are given for a distance of “r ” by thefollowing equations, where β = 2π/λ:

(1)

(2)

(3)

The above equations hold true if the sizeof the wave source ∆z satisfies conditions “∆z≪ r” and “∆ z≪λ” and these equations areused to convert electric field strength valuesmeasured at a distance of 3 m into those mea-sured at a distance of 10 m. A loop antenna isused to measure field strength at a frequencyof 30 MHz or lower. In this frequency band,the magnetic field strength obtained by Equa-tion (1) is used to determine the equivalentelectric field strength (by multiplying themagnetic field strength by 377Ω). Withrespect to an electromagnetic field generatedby current flowing through an infinite-lengthwired system, the equivalent electric fieldstrength is determined using the followingequation, which is obtained by Ampere’s law:

Fig.4 International Special Committee onRadio Interference (IEC/CISPR)

Fig.5 Permissible limits for disturbancecaused by IT devices(CISPR 22-2003, Class B)

Fig.6 Electromagnetic wave emittedfrom micro current element

9SUGIURA Akira

(4)

Figure 7 shows permissible limits for vari-ous types of devices and equipment convertedinto field strength values measured at a dis-tance of 10 m. Specific conversion proceduresare described below: (a) Low-power radio service (JP/low): Electric

field strength values measured at a distanceof 3 m are converted into those measuredat a distance of 10 m, using Equation (1)when the frequency is 30 MHz or lowerand Equation (2) when the frequency ishigher than 30 MHz.

(b) Incidental radiation from receiver (JP/Rv):I∆z is determined from Equation (3). Thisvalue is then substituted into Equation (1)or (2) to calculate the field strength mea-sured at a distance of 10 m.

(c) Inductive radio communication equipment(JP/IC): Equation (4) is used to determinecurrent I corresponding to the permissiblelimit; the equivalent electric field strengthis then calculated for a distance of 10 m.

(d) ISM equipment (JP/ISM): Electric fieldstrength values measured at a distance of30 m are converted into those measured ata distance of 10 m, using Equation (1)when the frequency is 30 MHz or lowerand Equation (2) when the frequency ishigher than 30 MHz.

(e) Permissible limits set by CISPR (CIS/QP,Av, Peak): When the frequency is 30 MHzor lower, the permissible limit for terminalvoltage is divided by 50Ω/50μH to deter-mine the current value. This value is thensubstituted into Equation (4) to calculatethe equivalent electric field strength at adistance of 10 m. At frequencies between30 and 1,000 MHz it is not necessary toperform conversions, as permissible limitsare defined at a distance of 10 m. When thefrequencies are 1,000 MHz or higher, per-missible limits at a distance of 3 m areconverted into values at a distance of 10 musing Equation (2).As shown in Fig. 7, permissible limits for

unwanted radiation from various types ofequipment with a frequency range of 3 MHz

Fig.7 Comparison of permissible limits for unwanted radiation from various types of devices andequipmentConverted to field strength values measured at a distance of 10 m. JP/low: low-power radio service in Japan.JP/Rv: incidental radiation from receiver. JP/IC: inductive radio communication equipment. JP/ISM: ISMequipment (50 to 500 W). CIS/QP: quasi-peak value set by CISPR 22. CIS/Av: average value set by CISPR 22.CIS/Peak: peak value set by CISPR 22.


to 3 GHz are 30 to 50 dBμV/m. However, toachieve the common goal of protecting radiocommunication, it seems necessary to estab-lish further consistency among these permissi-ble limits. In particular, permissible limits forlow-power radio services at 322 MHz or high-er in Japan are extremely low; future revisionis thus considered necessary.

A Japanese government regulation stipu-lates the use of a measurement receiver with－6 dB bandwidth “B”, as shown in Fig. 2,when measuring electromagnetic waves emit-ted from a low-power radio service. Thesebandwidths are set in accordance with theCISPR standards［12］. However, in general theJapanese Radio Law does not include any pro-visions for specific bandwidths, nor does itaddress detection methods applied with thegiven measuring instruments. Therefore, dif-ferent instruments may provide different mea-surement results, which is particularly incon-venient in radio administration. To addressthis problem, in principle CISPR instrumentsare now used to measure spurious emissions.In Fig. 7, permissible limits related to theRadio Law are converted based on theassumed use of CISPR instruments in mea-surement.

As described above, permissible limits at afrequency range of 3 MHz to 3 GHz are 30 to50 dBμV/m. If the field strength of electro-magnetic waves emitted from a device is40 dBμV/m, this device emits unwanted radia-tion of approximately 33 nW per bandwidth Bof the measuring instrument. The bandwidthsof measuring instruments are 9 kHz at 3 MHzto 30 MHz, 120 kHz at 30 MHz to 1 GHz, and1 MHz at 1 GHz to 3 GHz. If a device emitselectromagnetic waves of 40 dBμV/m (mea-sured at a distance of 10 m) over the entirerange of 3 MHz to 3 GHz, the total radiatedpower of unwanted emissions from this devicewill be approximately 0.4 mW. However,there is no possibility that unwanted emissionswill occur over the entire frequency range, andthe actual value will be on the order ofmicrowatts (μW).

2.5 Ambient noiseThe previous section described levels of

unwanted radiation (radio noise) emitted fromvarious types of radio equipment and elec-tric/electronic devices. This section willbriefly describe ambient noise levels, whichcorrespond to the accumulation of such radia-tion.

Ambient noise includes natural noisecaused by sferics and man-made noise. In theHF and UHF bands, man-made noise is morenoticeable and occurs continuously. Figure 8shows the statistical results of large-scaleresearch on man-made noise environmentsconducted in the United States from 1966 to1971. The researchers conducted measurementat 103 locations in different environments dur-ing the daytime. While moving in a monitor-ing vehicle or at fixed measurement points,they received eight to ten frequencies within arange of 250 kHz to 250 MHz at the sametime, and measured radio noise［13］. In the fig-ure, “Fa” refers to a noise figure defined bythe following equation:

(5)

When a monopole antenna is used in mea-surement, the amount of received power willbe reduced by half relative to a dipole antenna.Therefore, received power is expressed by thefollowing equation:

(6)

Fig.8 Frequency characteristics of ambi-ent man-made noise (ITU-R P.372-8)

11SUGIURA Akira

By substituting “T = 290 K” and theBoltzmann constant “k” into Equation (5), weobtain an equation for field strength (medianvalue) and a noise figure from Equation (6). Inthis equation, “B” refers to the equivalentnoise bandwidth (Hz) and “Fa” refers to thereading (dB).

(7)

Equation (5) is based on the assumptionthat ambient noise occurs over a wide frequen-cy band and that the measured field strength isproportional to the square root of the instru-ment’s bandwidth. It is likely that this assump-tion was derived from the fact that the instru-ment used in the measurement shown in Fig. 8featured a limited equivalent noise bandwidthof 4 kHz and the noise sources were at somedistance from the measurement points (andthus the central limit theorem held true). Wealso should consider the following when read-ing Fig. 8: since a man-made noise sourceoften generates periodic noise, it may havemany spikes in the spectrum measured nearthe source; the results of the measurementconducted on the street might have been influ-enced by passing automobiles; it is highlydoubtful whether the measurement resultsobtained more than 30 years ago are validtoday; there have been subsequent increases inpower consumption and the significantprogress in electric/electronic equipment.Given the above facts, it would seem neces-sary to conduct new research on ambient noisein Japan.

In the Study Group on High-Speed PowerLine Communication of MIC, we discussedthe issue of ambient noise in the HF band. Wemade an estimation of the field strength ofambient noise based on the data shown in Fig.8. Table 1 shows the results of this estimation.We found that ambient noise may exceed thesensitivity of various radio services that usethis frequency band［1］. Note that the estima-tion results in Table 1 are based on researchconducted in the United States more than 30years ago; it is safe to assume that current

noise levels are higher than these values.

3 Propagation of electromagneticwaves

In Section 2.4, I described calculations ofthe distance characteristics of electromagneticfield strength in free space (in which there areno ground effects) using an infinitesimal cur-rent element and current flowing though aninfinite-length wired system. However, thesetheoretical assumptions do not hold true in thecase of an ordinary source of unwanted radia-tion. In the sections below, we will discusspropagation of electromagnetic waves emittedfrom a power line and shielding effects ofbuildings to analyze possible effects of HF-band PLC.

3.1 Wave propagation over a shortdistance

It is possible to estimate easily and accu-rately the level of electromagnetic waves gen-erated from a finite-length wired system bythe moment method (MoM) calculations, pro-vided that the ground surface is flat and themeasurement point is located within a line-of-sight distance (within a few kilometers whenthe height of the transmitter is 2 m). This sec-tion will describe the calculation results foremissions from a power line of HF-band PLCusing NEC-2, a widely used moment methodprogram.

For the sake of simplicity, we picked uponly one type of indoor wiring and assumedthat a linear antenna measuring 20 m in length(L = 20 m) is placed horizontally at a height of2 m (Ht = 2 m) from the ground, as shown in

Table 1 Field strength of ambient noiseEn (dBμV/m): B = 10 kHz


Figure 9. At a height of 2 m (Hr = 2 m), wecalculated electromagnetic fields while vary-ing the distance “R”. Although this antenna isextremely long, it can be considered as a pointsource when seen from a distance of 50 m ormore in this frequency band. RF power is fedfrom the middle section of the antenna. How-ever, since this is a fixed-length wired system,standing wave current is generated along theantenna. For this reason, we fixed the maxi-mum value of the standing wave current“Imax” at 1 mA to determine the distancecharacteristics of the electromagnetic fields.We assumed that the ground is “wet” or“medium dry”, which generally reflects thesoil conditions in Japan.

(a) Electromagnetic field generated by currentthrough horizontally placed wired systemFigure 10 shows the attenuation character-

istics for magnetic field strength in the hori-zontal direction when electromagnetic wavesare emitted from a linear antenna (20 m long)placed horizontally at a height of 2 m fromwet ground. We calculated the field strengthfor maximum current on a single line of 1 mA.

When the ground is medium dry, the mag-netic field strength becomes less dependent onthe frequency due to reduced effects of groundreflection. There was little difference in thecalculation results between the cases of wet

and medium dry ground. We also calculatedthe electric field strength, and the ratiobetween the magnetic and electric fieldstrengths was close to a characteristic imped-ance of 377Ω. As shown in Fig. 9, the wiredsystem is placed horizontally, and horizontallypolarized waves reflected by the ground areout of phase with the direct wave. Therefore,at a distance of 10 m or farther from the sys-tem, electromagnetic field strength is attenuat-ed inversely to the square of the distance.

In the direction of height, an electromag-netic field (generated by a horizontally placedwired system) is attenuated inversely to thefirst power of Hr (height of the receivingpoint) if the height of the receiving point isgreater than a distance ofλ/2π (Hr≫λ/2π)and is sufficiently greater than the length ofthe antenna (Hr≫L). At a closer location, theelectromagnetic field is attenuated inversely tothe square of the distance. In the vicinity of afrequency fMHz = 75/Ht [MHz], the strength ofemitted waves may increase nearly twofolddue to ground reflection. As shown in Fig. 11,the level of the electromagnetic field variesgreatly depending on the frequency. Whenviewing Fig.10 for comparison, we can seethat the amount of attenuation is smaller in thevertical direction than in the horizontal direc-tion. (b) Electromagnetic field generated by current

through vertically placed wired systemWe also performed calculations of electro-

magnetic fields when high-frequency currentis passed though a vertically placed wired sys-tem, as shown in Fig.12. In this case, thewired system was a single conductor measur-ing 5.6 m in length (L = 5.6 m) and the middlesection of this conductor was located at aheight of 3.2 m. Power was fed from the mid-dle section and the maximum current on thesystem “Imax” was 1 mA. We selected anantenna length of 5.6 m assuming installationof a PLC system in a two-story house. We per-formed moment method calculations for thecases of wet and medium dry ground.

Figure 13 shows the calculation results.Compared with the values for Fig. 10, the

Fig.9 Magnetic field generated by cur-rent through a horizontally placedwired

13SUGIURA Akira

field generated by the vertically placed wirebecome relatively noticeable because its atten-uation is less than that of the horizontal wirefield. At a greater distance, however, theground reflection coefficient of verticallypolarized waves approaches －1. Therefore,the electromagnetic field strength is attenuatedinversely to the square of the distance, as inthe case of horizontally polarized waves.

3.2 Wave propagation over a mediumto long distance

When studying propagation in the HFband over a medium to long distance, we mustconsider not only ground waves but also elec-tromagnetic propagation by sky-wave (i.e.,those reflected by the ionosphere). Therefore,field strength is expressed as a function ofionospheric characteristics, which change in acomplicated manner. For example, if electricpower Pt is input to a transmitting antenna

with a gain of Gt in free space, the fieldstrength of electromagnetic waves at a dis-tance “r” is given by the following equation:

(8)

In the case of ionospheric propagation,however, “r” is the distance of apparentoblique propagation. Many other factors mustalso be taken into consideration, includingreflection, absorption, and scattering of wavesby the ionosphere and ground reflection loss.The calculation procedure soon becomesextremely complicated. We used a commonlyaccepted software program for a propagationmodel based on ITU-R RecommendationP.533, “HF propagation prediction method”.Figure 14 shows the calculation results for thedistribution of field strength in and aroundJapan, when a 5-W source located at NICTemitted electromagnetic waves at a frequency

Fig.10 Distance dependence of mag-netic field generated by currentthrough horizontally placed wiredsystem (Wet ground, Ht = 2 m)

Fig.12 Electromagnetic field generatedby current through verticallyplaced wired system

Fig.11 Height dependence of magneticfield generated by currentthrough horizontally placed wiredsystem (Wet ground, Ht = 2 m)

Fig.13 Distance dependence of electricfield generated by currentthrough vertically placed wiredsystem (Wet ground, Hr = 6 m)


of 13.39 MHz. As shown in the figure, thefield strength of sky-waves peaks at a distancefrom the emission point and this peak locationvaries depending on the frequency, incidentangle, and ionospheric conditions. A “hotspot” is sometimes generated at this peak loca-tion, but here electromagnetic field strength isnot high because the waves are attenuated dueto the oblique propagation distance describedin Equation (8). Naturally, the electromagneticfield strength becomes highest in the vicinityof the transmission source, within the range ofthe ground waves.

3.3 Electromagnetic shielding effectsby buildings

If a PLC transmitting or receiving point issurrounded by a building or other structure,electromagnetic waves are shielded. In thissection, I will briefly describe the results ofcomputer simulation of such electromagneticshielding effects, citing a report from theStudy Group on High-Speed Power LineCommunication［1］［14］.

We performed simulations of three rectan-gular box-shaped structures made of a steelframe, wood, and reinforced concrete, respec-tively. These structures were all of the samesize: 3.0 m (W)×3.9 m (D)×3.2 m (H), andeach featured a door and window at the front

and back. To measure PLC wave leakage, weused a wave source consisting of a 6-m powerline routed in an angulated U-shape. We per-formed numerical analysis using commercial-ly available finite integration software.

Since the distribution and level of electro-magnetic fields differ markedly depending onthe presence of a building, we expressed theshielding effect as a ratio between fieldstrength values calculated in the presence of abuilding and in the absence of such a struc-ture. We obtained average values for locations10 m and 150 m away from the source. Table2 shows the calculated results. As you can see,reinforced concrete structures are expected toprovide shielding effects of 20 dB or higher inthe HF band while wooden structures provideshielding effects of only about 10 dB.

4 Measurement of HF-band elec-tromagnetic environment

To perform measurement of an HF-bandelectromagnetic environment, the strength ofthe electromagnetic field is determined using aloop antenna connected to a measuring receiv-er such as a spectrum analyzer, as shown inFig. 15. In this case, strength is calculatedbased on the loop antenna’s magnetic fieldantenna factor “AFH(dB(S/m))” and thereceiver’s reading “V (dBμV)”, using the fol-lowing equation:

(9)

To measure a low-level electromagnetic

Fig.14 Distribution of field strength in thecase of ionospheric propagation(Using 5-W, 13.39-MHz transmissionsource located at NICT)

Table 2 Electromagnetic shielding effectsof buildings

15SUGIURA Akira

field, an antenna with a small “antenna factor”is employed. Such an antenna needs to featurea large loop with a number of turns. However,when the loop becomes larger, the antenna’spositional resolution will decrease, and whenthe number of turns increases, resonance willoccur at a low frequency range, which shouldbe avoided. Therefore, we normally use a sin-gle-turn loop antenna 60 cm in diameter［12］.In a high-frequency magnetic field, the elec-tromotive force of a loop antenna changes inproportion to the frequency and loop area. Themagnetic field antenna factor AFH decreasesin proportion to the frequency, but the antennawill usually present more complicated charac-teristics because the inductance of the loopincreases in proportion to frequency. Certaincommercially available loop antennas have aresonant circuit to eliminate this inductance.

Monopole antennas are also available foruse in measurement in the HF band. However,this type of antenna is susceptible to theeffects of surrounding objects, including theindividuals performing the measurement.Therefore, loop antennas are generally used tomeasure electromagnetic fields in the HFband. These antennas provide measurementvalues of magnetic field strength [dB (A/m)],but it is still more common in Japan to useequivalent electric field strength [dB (V/m)],which is calculated by multiplying a measure-ment value by a characteristic impedance of377 Ω (The CISPR began to use magnetic

field strength about ten years ago.).A commercially available spectrum ana-

lyzer has an equivalent input noise level ofapproximately －140 dB (mW/Hz) in the HFband. With this level of noise, a quasi-peakvalue is calculated without input (B = 9 kHz).CW input is then calculated when the analyz-er’s indication increases by 6 dB—to arrive atapproximately 10 dBμV. In the case of a com-mercially available EMI measuring receiver,the value of a CW input corresponding to that ofthe above analyzer is approximately －5 dBμV.As you can see, the EMI measuring receiverdescribed here enables us to extend the rangeof measurable strength of electromagneticfields downward by 15 dB.

The magnetic field antenna factor AFH of aloop antenna is between －10 and －20[dB(S/m)] in the HF band. Given the sensitivi-ty of the measurement system mentionedabove, it becomes possible to measure a mag-netic field (B = 9 kHz) featuring strength ofapproximately －10 dB (μA/m). This valuecorresponds to an equivalent electric fieldstrength of approximately 40 dB (μV/m).Using average detection, it is possible toextend the measurable strength level further,by approximately 10 dB downward.

However, to measure the strength of ambi-ent noise at the levels shown in Table 1, it isnecessary to improve sensitivity even fur-ther—by 20 dB or more. To achieve this, wemust use a variety of techniques, such as mea-surement of amplitude probability distribution(APD). Some commercially available loopantennas or measuring receivers feature a low-noise preamplifier to improve sensitivity.When using such an instrument, it is importantto note that a measurement value may be erro-neous due to the preamplifier’s nonlinearresponse when a strong broadcasting/commu-nication wave exists.

Under an ordinary method for measuringdisturbance, a loop antenna is placed 10 mfrom a device under measurement. Therefore,the field incident on the antenna is low andpreventing the effects of extraneous wave inci-dence is difficult. To address this problem, a

Fig.15 Measurement of HF-band electro-magnetic field (using a loopantenna)


“large loop antenna system” is used to mea-sure the magnetic fields of electronic devicesat frequencies below those of the HF band.This antenna consists of three orthogonalloops, as shown in Fig. 16.

5 Conclusions and acknowledge-ments

In recent years, new HF-band communica-tion systems have appeared, from RF-ID tagsto power-line communication (PLC) systems,all of which are expected to undergo explosive

growth in use. It is thus essential that weestablish electromagnetic compatibility amongvarious types of communication equipment(including radio equipment) and electric/elec-tronic devices.

This paper described a comparison of vari-ous noise sources that create electromagneticenvironments and their permissible limits at afrequency range of 150 kHz to 10 GHz. As aresult of these comparisons, we demonstratedthe necessity of ensuring consistency amongpermissible limits for various types of equip-ment and devices, and of conducting compre-hensive studies on today’s noise environment.This paper also described methods for measur-ing HF-band electromagnetic fields and men-tioned a number of issues concerning the mea-surement of ambient noise.

The discussion in this paper is mainlybased on reports by the Study Group on High-Speed Power Line Communication (theauthor is the leader of this group). I wouldlike to express my sincere gratitude to the fol-lowing NICT researchers who conductedresearch within this group: Mr. Yukio Yamanaka,Mr. Shinobu Ishigami, Ms. Kaoru Goto, andMr. Katsumi Fujii.

Fig.16 Large loop antenna system

References01 “Report of the study group on broadband PLC”, Ministry of Internal Affairs and Communications,

2005.12.

02 Ordinance Regulating Radio Equipment, Article 7.(in Japanese)

03 Enforcement of the Radio Law, Article 6.(in Japanese)


05 Radio Law, Article 100.(in Japanese)

06 Enforcement of the Radio Law, Article 44(in Japanese).

07 Enforcement of the Radio Law, Article 45.(in Japanese)


09 Enforcement of the Radio Law, Article 46-7.(in Japanese)

10 Radio Law, Article 101.(in Japanese)

11 CISPR 22, 2005.

12 CISPR 16-1-1, 2003.

13 ITU-R Recommendation P.372-8.

14 S. Ishigami, K. Goto, and Y. Matsumoto, “Numerical analysis of electromagnetic-field attenuation of

buildings in power line communications”, IEEJ Transactions on Electronics, Information and Systems,

OS1-4, pp.366, 371, 2005.

17SUGIURA Akira

15 CISPR 16-1-4, 2003.

16 T. Shinozuka, A. Sugiura, A. Nishikata, “Rigorous analysis of a loop antenna system for magnetic inter-

ference measurement”, IEICE Trans. Commun., E76-B, No. 1, pp. 20-28, 1993.

SUGIURA Akira, Dr. Eng.

Professor, Research Institute of Electri-cal Communication, Tohoku University

Communication Environmental Engi-neering

2 use of electromagnetic energy and resultant noises

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