Proceedings CEEM'2009/Xi'an

Non-Stationary EMF Surveying Accuracy ImprovementPawel Bienkowski, Hubert Trzaska

EM Environment Protection Lab, Technical University ofWroclaw, PolandWyspianskiego 27,50-370 Wroclaw, Poland

e-mail: pawel.bienkowski@pwr.wroc.pl.hubert.trzaska@pwr.wroc.pl

Abstract: The paper discusses accuracy ofnon-stationary EMF measurements. Theaccuracy is mainly limited by appliedmethods ofmeters calibration. New methodof calibration is presented. The methodallows a limitation of the calibrationinaccuracy to level ofIIdB.

1. IntroductionElectromagnetic Field (EMF)

surveying is usually done for labor safetyor/and environment protection purposes. Asa result of the measurements often legalmeasures are initiated. Thus, performedmeasurements should be as accurate aspossible. Unfortunately, accuracy of theEMF measurements is one of the lowest ascompare to the accuracy of other physicalmagnitudes measurements.

Apart from internal reasons of ameasuring device, one of the mostimportant reasons of the accuracy limitationis the accuracy of the EMF standards. Aninternational action of the standardscomparison [1] shown that inaccuracy of theworld-best EMF standards is on the level of5%. Thus, it gives inaccuracy of the bestEMF meters at the level of 1dB.

The inaccuracy is valid for far-fieldconditions and continuous-wave (cw) fieldmeasurements. In the near-field conditionsthe accuracy of measurements may beremarkably reduced and an error on thelevel of± 6dB must be sometimes accepted[2]. By the way it, well illustrates anabsurdum, in metrological sense, in manyvalid protection standards where exposurelimits are given with accuracy to foursignificant numbers [3]. Lets consider errorsdue to AM modulated fields measurements.

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2. Modulated fields' measurementsIt is evident that only modulations

that affect amplitude are here of concern.As an example we'll consider traditionalAM emission and continuous pulsedmodulation.

a. AM modulationIf assume that we would measure

only single AM modulated source, theelectric component of linearly polarizedelectric field E is given by:

E = A(l + m cos rot)cos Ot 1.where: A - amplitude,

m - modulation depth,co and 0 - angular frequency of

modulating signal and of the carrier wave.

The effective value of the E-field Erms is:

IT Ag2

Enns = - ff(t)dt = r,; 1+- 2.To ...;2 2

Now lets assume the maximal modulationdepth, i.e.: m = 1. It allows a conclusion thatin these conditions the amplitude of thefield increases in two while the rms value in25% in relation to a cw field (m=O).

b. Train ofpulsesNow we will repeat calculations

similar as above to infinite train of pulses, isshown in Fig. 1.

A

'-

t+----+

T

Proceedings CEEM'2009/Xi'an

Fig.3 Pattern fr(q» of a radar antenna

.LA------7 ....E

An example of the radiation patternof a radar antenna is shown in Fig. 3 whileE-field variations at a distance from the(rotating) antenna in FigA.

T

FigA E-field variations of a radar antenna

Presented in Fig.4 E-field variationsin a point of observation show the shape ofthe voltage that should be applied for astandard excitation. Pulses duration time ('t)and repetition time (T) should be selected atthe generator. Repetition time (Tr) of trainsof pulses is a function of the considered(real) antenna rotation velocity, and it is:

ITR =- 4.

kwhere: k - number of the antenna rotations.

The trains duration time ('t r) is afunction of the antenna radiation pattern fr ( q>)and rotation velocity:

I 2q> I q> r ,'t =---=--=- 5.

r k 360 0 k 1t 1t

where: q> - angle at which EmaxlE = 0.5.The main lobe of the radiation pattern of the

antenna, within frames -rp ::; 0 ::; +q>, may beapproximated in the form:

Efr(q» = ~ax = cos" q> 6.

where: m - a number dependent on the beamwidth.

a standa rd

Fig.2 Block diagram of non-stationaryexcitation system of an EMF standard

Fig.I A train of pulses

In this case the amplitude of the field isalways A; however, the effective value is:

E==~H 3.

Indicationsin the formula as in Fig.I .It may be concluded that for short

duration times the effective value of thetrain ofpulses may be very small. It leads tomeasuring error that may reach one order ofmagnitude or more (if applied meter wascalibrated in a cw field).

One more example ofpulsed EMF isa field generated by a rotating antenna, forinstance of a radar system. We call it: non­stationary field. In this case the accuracy ofmeasurement is behind of any control andresults of measurements, performed insimilar conditions with the use of differentmeters, calibrated in different conditions,give far ofagreement results. The errors areresulted by the use of meters calibrated incw fields. The Authors have proposed anew calibration method that would allowremarkable increase of the non-stationaryEMF meters calibration accuracy.

3. Non-stationary EMF standardThe idea of the standard is evident.

It requires a standard excitation with apulsed voltage that both the duration timeand the repetition time of pulsed voltage isidentical to that of a source that would bethen measured with the use of such a waycalibrated meter. Additionally the outputvoltage of the generator is modulated such away that the voltage fed to a standard hassimilar shape to time-variations of EMFobserved in a measuring point. A blockdiagram of the approach is shown in Fig.2

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Proceedings CEEM'2009/Xi'an

J1

lAl

- -- -- -- 1 - -- -- - ,, ,

i,, ,, ,, ,,,, , t,

.I't ; I.I

E

cp

2fA 2cos? <pd<p 2 4

E 0 In 2 m 4_r = ::::: 1--<p +_<pE i ~2A 2<p 3 10

Taking into account that for a half-beam isfulfilled relationship: cosm<p = 1/2, we may write:

E r IEj ::::: 0.87 9.

Fig.7 E-field intensity in a point of observationgenerated by ideal reference radiator

Fig.6 Pattern fie<p) ofthe reference antenna

The pattern of the reference antenna fi(<p) maybe expressed in the form:

f j (<p) =sin 2 <p + cos 2 <p 7.

Variations of the E-field intensity, as a functionof time, in a point of observations in discussedcase are shown in Fig. 7.

T.

In the considerations we assume thatduration time of trains of pulses 1:r and theirrepetition time Tr are identical in the case ofreal antenna, as shown in FigA, and referenceone, as shown above.

A comparison of figures 4 and 7 allowsa conclusion that although amplitude value ofthe field is in both cases identical, their rrnsvalues are different and the latter of thereference antenna majorizes that of the real one.

Lets calculate relationship of effectivevalues of E-field of the real antenna E, and thatof the reference one E, if assume that the bothenvelopes are filled with cw signals. It will beexpressed in the form of following formula 8.

1\1\\\\

"- ---2000

If assume that for half-beam the aboveformula should be equal 0.5 we may estimate mas a function of rp ; results of estimations areplotted in Fig. 5.

10000

Problem:As it may be seen from Fig.5, especially

for narrow-beam antennas, value of m is quitelarge. It means that E-field variations at thepoint of observation may be very rapid. It wouldrequire an application in system shown in Fig.2an attenuator that would be able to follow thevariations. A computer controlled attenuator is,the first of all, discrete one and usually onlyseveral steps are available, more over thesystem may be to slow to assure required herevelocity of switching. Thus, the attenuator maydamage the presented idea.

4000

m6000

8000

oo 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15

cp [deg)

Fig.5 Calculated values ofm as a function of <p

a. The first approximation solutionAs it was shown above an application in

practice of the presented idea of the calibratingsystem may be very limited, not to sayeliminated, by availability of appropriateattenuators. With no regard to it, it may be noteasy to achieve good agreement between shapeof pulse trains in the standard and that of asource . In order to simplify the approach, andmake it easier in practical use it is proposed alittle modification in the set shown in Fig.2.Namely: the attenuator is replaced by a switch.Thus: we will have train of rectangular pulses.The train is equivalent to continuous radiationpatter of an ideal reference antenna, withinframes -<p ~ 0 ~ +<p, presented in Fig. 6.

Conclusion:If a standard is excited with pulses

identical to shown in Fig. 4 we may assume thatcalibrations conditions are identical with themeasurement ones and measuring error due tothe modulation may be reduced and lead to anestimable level.

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It means that we can use a rectangulartrain of pulses for the meter calibration and thenadd a correction factor -13%.

b. Second approximation solutionNow we will try to find such duration

time of trains of pulses from the referencesource (~i) that effective values of E-field fromreal source and that from the reference one areidentical, i.e.: relationship given by formula 8should be equal to unity. After estimations wehave:

9.

We may conclude that if duration timeof rectangular shaped train of pulses fulfilsrelationship 9, it is energetically equivalent tothat of the real antenna.

Question: In above estimations was assumedthat as envelope of real E-field, as that of thefield applied for a meter calibration, iscontinuously filled with cw signal (it means thatthe rotating antenna radiates noninterrupted cwsignal and similar one is fed from generator inthe standard considered. On the other hand inreal non-stationary systems the envelope isfilled with pulses. Moreover, it is impossible tosay what is location of separate pulses inrelation to the antenna main-lobe direction (inFig.3 the pulses are symmetrically locatedaround the maximum). The question is: what isan equivalence of presented concept andestimations to real sources?

In order to answer the question a seriesof calculations were performed for differentnumber of pulses (n) in a separate train ofpulses and their location within an envelope.The considerations show that for n>3 a role ofthe pulses position may be neglected with anaccuracy better than 1%. In presently appliedradars usually n~5.

4. Final commentsA new method of EMF meters

calibration, designated especially for non­stationary field measurements, is brieflypresented in the paper. A concept of the methodseems to be evident, but contrary to calibrationmethods used up to now, the new one must beperformed separately for any measuring case. Itis troublesome and requires a priori data of anysource to be measured. Some simplification ofthe method is proposed; however, it does not

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change necessary here individual approach.Moreover, presented estimations were basedupon conditions specific to the far-field, i.e.:single ray propagation. In the real conditionsmay appear rays reflected from the ground andother objects in the area, a role may be playedby sidelobes of the antenna pattern as well asthe pattern deformations and other phenomena.Of course, it is possible to eliminate their role inthe measurement accuracy degradation by theway of using in above estimations experimentaldata instead of those from a systemdocumentation. It is not to add that it leads tofarther complication of the approach.Unfortunately, it is a necessary expense of thepossibility to reduce measuring errors to level of"estimability".

Presented estimations were performedfor the case of rms E-field measurements. It isevident that it is only an illustration of themethod and necessary here procedures. Theestimations must be separately performed forany EMF parameter that will be measured then(H, S, amplitude, mean value) and, to remind,any source.

The estimations lead to a conclusionthat here is possible to obtain an inaccuracy of ameter calibration at level about ±ldB. In theinaccuracy of measurement budget estimationsshould be taken into account other factors thatwould affect the accuracy, as it is done in anyother measuring procedure.

Although the method is consideredespecially for measurements of non-stationaryfields it seems to be useful also in other caseswhere AM modulated fields measurementaccuracy may create any doubts.

AcknowledgementA presentation of the work was possible

due to the Polish Ministry of Higher Educationand Science grant nr. 332/N-COST/2008/0.

References:1. H. Trzaska - EMF measurements in the near

field, Noble Publ. 2001.2. M. Kanda and others - International

comparison GTRF-86-1 electric fieldstrengths, IEEE vol., nr2/2000, pp.190-205.

3. H. Trzaska - Keep out of the westernprotection standards, Proc. 3-rd Intl.Symp.onEMC, Beijing 2002, pp35-38.