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Analysis of Peculiarities of Determination of theElectromagnetic Field Parameters near Radiation Sources

Eugeniusz Grudzin'ski, Vitalij Nichoga, Petro Dub

Abstract - Some peculiarities of determination of the electricalE and magnetic H components of field strength near the fieldsource (in the inner zone) are considered in the report. It isshown that in such cases calculation methods do not take intoaccount many factors that can essentially distort the calculationresults.

Keywords - Electromagnetic field, Parameter determination,Computer, Measurement

I. INTRODUCTION

During last decades the dynamic development of systems ofpower energy, telecommunication, television, radio-locationand great number of electronic devices in medicine, industryand housekeeping have caused gradual rising of man-causedelectromagnetic field (EMF) intensity at working places, athome and in environment.

Increase of number and intensity of radiation sources leadsto more frequent and longer exposure of humans to man-caused EMF. Radio communication devices, such as radio andtelevision transmitters, radiotelephones and their broadcastingrelaying station, and various industry equipment are the mainsources of electromagnetic exposure in the 0,1+300 MHzfrequency band. But the greatest concentration of radiationsources (taking into account the band width) are in the300 MHz+300 GHz frequency range. A great number oftelevision transmitters, cellular telephone networks, homemicrowave ovens, radio communication and radio-locationequipment work in this range.

It is necessary to determine the EMF levels, that can beradiated at the places of people's work and life when installingnew emission sources. Calculation methods are used mostoften for known standard telecommunication equipment.Measurement methods of EMF strength determination areused for other telecommunication transmitting apparatuses.This problem is especially actual when conducting suchmeasurements near a radiation source, that is to say, in thenear or induction zone of a source.

Such determination of component intensity near the EMFsource is needed when necessity of comparison of realmagnitudes of measured field and permissible lomits, whichare regulated by the state sanitary standards, appears. Thesemeasurements, that are now conducted around radiationsources on earth, in air and during long-termed flights onspace ships, allow to estimate danger of enduring exposure toradiation of persons, that work permanently in near contactwith such sources [1-3].Eugeniusz Grudzin'ski - EM Environment Protection Laboratory,Technical University of Wroclaw, Wyspianskiego 27, 50-370Wroclaw, Poland, E-mail: eugeniusz.grudzinskigpwr.wroc.plVitalij Nichoga, Petro Dub - Karpenko Physico-Mechanical Instituteof the National Academy of Sciences of Ukraine, Naukova 5, 79601Lviv, Ukraine, E-mail: nichgah.ipm.lviv.ua

II. PECULIARITIES OF ELECTROMAGNETIC FIELDINTENSITY DETERMINATION

The used measurement equipment is not, in most cases,adapted for measurements near a source, and even if it is used,it can considerably distort the measurement results.Usage of certain type of antenna in the near or far zone is

determined by a distance R from the radiation source, that isconnected with the maximal source dimension D and theradiation wave length X.

The minimal distance R of an EMF source, for which wecan consider near or far zones, is determined by the

expressions R < - + D(D and R >.4 2 A

The examples of errors, that appear during measurementsusing antennas, that react to magnetic field and at the sametime is graduated in the electrical field units, near fieldsources, are shown in Fig. 1.

Fig. I. 'The examples of usage of a frame antenna, that reacts tomagnetic field, for measurement of the EMF electrical component in

the near zones of the sources

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We have to deal with plane wave for the case of far zonewhere, as it is well known, the electric field strength vector Eis perpendicular to the magnetic field strength vector H andtheir ratio determines the medium wave impedanceZo=E/H= I 20n=377 Ohm. It is possible to measure either E orH in this zone and then to calculate, using the ZO=120O ratio,H or E correspondingly.The medium wave impedance Z,-EIH can be bigger or

smaller than 120it in the near zone. So we can consider a fieldsource as a high impedance one with predominance of thefield electric component when ZW>120. And, on the contrary,we can consider a source as a low impedance one withpredominance of the field magnetic component whenZW<12On.We should measure both electric and magnetic field

components in every point of the near zone of a field sourcebecause Z,. is a complicated function of wave length, distancefrom the measuring point to the source and type (structure) ofthe field source itself. Besides this we have to remember thatthe field is essentially non-homogeneous in the near zone, theE and H component magnitudes are complex values (theyhave phase shift) and medium impedance 4w = E1H is alsoa complex value.

All rules of measurement in the near zone can be useddirectly for measurement in the far zone, but the oppositeprocess never can be realised.

So, what do we measure or determine? Or, how we haveto measure according to the theory and how is it on practice?

According to the theory - measurements should be pointones.On practice - measurements are carried out using antennas

with finite dimensions.Measurement units: V/m for electric field, A/m for

magnetic field, V/m for field electric component ofEMF, A/mfor magnetic field component of EMF, W/m2 for powerdensity S in microwave frequency range in far field for theproblems of environmental protection.

Antennas that are used for conducting measurements:for measuring the field electric component E - electrically

short dipole antennas, for measuring the field magneticcomponent H - electrically small frame antennas (for thespecified frequency range).

Hall or fluxgate antennas can be used for measurement oflow frequency and constant magnetic field. Taking intoaccount essential field non-homogeneity in the field sourcenear zone it is impossible to use resonance antennas whichdistort additionally the measured field. Usage of coaxialcables for connecting antennas with pre-amplifiers isconsidered as deleterious.

In many cases detectors are connected to the outputs ofelectric and magnetic field antennas and signal transmitting tomeasuring instrument is conducted using direct current [4].Measurement of the power density S:Determination of S requires, from the point of view of

theory, preliminary determination of E, H and phase shiftbetween them.

But in practice determination of S is carried out bymeasuring E or H and calculation of S using expressions thatare correct for far zone: S=Sr-SHE2/Zo=H2Zo.

III. CALCULATION AND MEASUREMENTOF EMF INTENSITY

There are many methods and computer programmes thatallow to calculate analytically radiation intensity of sometransmitting termninals or antenna systems. Unfortunately,such calculation are possible only for some widely usedemitters and very often their results do not coincide withconclusions made on the basis of practical measurements.Also there are too little data in literature concerningverification of experimental values of EMF by theoreticalcalculations.

According to existent norms, practical measurements ofEMF intensity are mainly conducted when a measuringantenna is situated at a height of 2.5 m above the earth surface[1].The universal wide-band EMF strength measurer MEH- 1

with possibility of remote reading of EMF horizontal andvertical components was elaborated in Technical University ofWroclaw (Poland) to realise practically EMF measurementsincluding the source near zone [4].The additional universality of this measurer consists in

possibility of conducting measurements both on the earthsurface and above it in wide ranges of frequency and values ofmeasured strengths by replacement of correspondingantennas. This measurer was used in practice for verificationof experimental data and analysis of EMF distribution (thefield structure) in the 20 Hz -50 GHz frequency range atheights from 2.5 till 400 m above the earth surface neardifferent emitting objects in Poland (Figs. 2 and 3) [11.

Fig. 2. Field measurement at a height of 2.5 mabove earth near an antenna system

Fig. 3. Field measurement in vertical ection using a balloon

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The MEH-1 gives an opportunity to investigate directlyinfluence of such factors as weather, buildings, trees,infrastructure, presence of transmission lines and so on in thesource area on the field structure of different field sources.

Unfortunately, these factors are essential ones and often itis very complicated to take them into account in the cases ofconducting theoretical calculation analysis. The results ofcalculations and measurements for vertical direction for twodifferent EMF sources are presented in Fig. 4.

20; 8 /

x0 10 20 30 [V/mI so

Fig. 4. The calculation and experimental results of determination ofthe field electrical component value changes for two measurement

variants shown in Figs. 2 and 3 correspondingly (solid lines -theoretical calculation, x - the experimental data)

One can see from Fig. 4 that very good coincidence ofmeasurement and calculation results take place only for bigdistances from the field source. But transmission lines,buildings, trees and so on influence on the field distributionnear the source and increase the difference between theoryand practice.

Such situation take place also at very high frequency, forexample, at frequencies used by cellular communication.Results of theoretical analysis of the base station fielddistribution and the real field structure near the station do notcoincide. The experimental results of investigation of the field

power density S changes of the base station antenna at adistance up to 300 m are shown in Fig. 5. The measuringantenna in these experiments moved along the earth surface atheights 0.7 and 1.8 m.

0.004

jW/m21

.0

00 1.8m

.0

000'1 0.7m,

00 50 100 150 200 250 300

d [ml

Fig. 5. The changes of the field power density S above the earthsurface when the distance from the antenna to the base

station of cellular communication increases

The described changes of the field intensity round the basestation are different for different directions. The followingsituation is a result of this: the so-called self-moving cellularphones appear, that is an evidence of essential rise of EMFstrength levels in some points around the source as a result ofbad antenna configuration, influence of surrounding objects orre-emission of some elements of the base station [5].One more illustration of inconsequence in interpretation of

determined values ofEMF intensity is decision about buildingup the areas that are adjacent to radio-locating stations andother power emitters on the basis of determination of the fieldstrength at a height of 2.5 m above the earth surfaceindependently of the real conditions of the area, its relief,presence of trees, buildings, season and so on. Unfortunately,sometimes such situation can take place when in winter (whentrees have no leaves) the field intensity essentially increasesnear a source and at the same time theoretical calculation givequite satisfactory results that do not exceed the sanitarypermissible standards [6].

IV. CONCLUSIONS

Not depending on computer technology development intelecommunication and antenna systems, we cannot realiseaccurate analytical determination of field strength levels inspecific places because of absence of information aboutradiation peculiarities near its source or because of fieldtemporal changes, that influence essentially on wavepropagation conditions and on field strength determination.Practice of EMF strength measurement shows that its resultsdepend considerably on many factors, that has been confirmedwhen carried out measurements on the same objects in variousseasons.

Such factors, on which the measuring results depend, are:1. Profile of the rout between the field source and the

measuring point.2. Height of measuring antenna location above the ground

(on the opened area) with direct visibility of the antennaand the field source.

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3 Height of measuring antenna location above the ground(on the opened area) without direct visibility of theantenna and the field source.

4. Atmosphere conditions, soil cultivation conditions,presence of trees on the measuring route and near themeasuring points.

5. Type and configuration of the built area, peculiarities ofits elements.

Computer analysis allows to conduct determination ofinfluence of some factors on the measurement results. But, inspite of availability of corresponding computers programmes,that enables an opportunity of conducting theoreticalcalculation of EMF strength levels and comparing theobtained magnitudes with acceptable limits (that arenormalised by standards or other documents), there areconsiderable difficulties in realisation of such analysis in thenear fields of most EMF sources. That is why the final moreaccurate determination of the field distribution around a fieldsource should be conducted by measurement methods with theabove mentioned remarks.

REFERENCES

[1] E. Grudzifiski, H. Trzaska: "General Public Protectionagainst Electromagnetic Radiation", InternationalSymposium on Electromagnetic Compatibility,Symposium Proceedings, Vol. 2, pp. 742-746, Nagoya,Japan, September 8-10.1989.

[2] V. Nichoga, V. Shabelnikov, P. Dub, G. Trokhym,0. Ostap, "The "Kaskad" Information-Measuring Systemfor Diagnostics of Low-Frequency Magnetic Fields onthe "Mir" Space Station" (in Ukrainian), Vidbir i obrobkaInformatsii, Vol. 18 (94), pp. 46-52, 2003.

[3] V. Nichoga, V. Shabelnikov, P. Dub, L. Sopilnyk,"Diagnostics of Low-Frequency Alternating MagneticFields in Inhabited Modules of the "Mir" Orbital Station",10th International Symposium on Electromagnetic Fieldsin Electrical Engineering (ISEF'2001), SymposiumProceedings, pp. 547-552, Cracow, Poland, September20-22, 2001.

[4] E. Grudzinski, "Laboratorium Wzorc6w i Metrologii PolaElektromagnetycznego w swietle nowych unormowan",Przeglad Telekomunikacijny, No 8-9, S. 541-549, 2001.

[5] E. Grudzifiski, A. Florek, K. Rozwalka, R. Wroczynski,"Pomiarowa i komputerowa analiza p61elektromagnetycznych w srodowisku czlowieka",Konferencja PTZE 2002, pt: "Kompatybilnoscelektromagnetyczna ukladow biologicznych", ZielonaGora-Dychow, Polska, 16-18. 10. 2002 r.

[6] E. Grudziski, T. Rogowski, "Ochrona przedoddzialywaniem pola elektromagnetycznego - przeztelefon! ! !", XIV Konferencja Naukowo-TechnicznaBezpieczenstwo Elektryczne ELSAF'2003, Wroclaw, 10-12.09.2003.

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