EMF Meters & Dogmas Pawel Bienkowski, Hubert Trzaska

EM Environment Protection Lab, Technical University of Wroclaw, Poland Wyspianskiego 27, 50-370 Wroclaw, Poland

e-mail: pawel.bienkowski@pwr.wroc.pl, hubert.trzaska@pwr.wroc.pl Abstract: The paper deals with the problem of the measurements accuracy, measure-ments performed mainly in the near field for labor safety and environment protection purposes. Apart from errors specific to the metrology a role in the accuracy budget is played by imperfections of meters offered on the market. The paper presents several of them and suggest simple ways of the meters checking by their users themselves. 1. Introduction In previous presentation [1] we warned against protection standards accepted in the Western World. Now similar warning is presented in the relation to wide offer of the meters available on the market.

Electromagnetic field (EMF) measurements performed mainly in the near-field, for labor safety, environment protection purposes and in laboratory research in the area of bioelectromagnetics, create many doubts and misunderstanding. Their accuracy is one of the lowest amongst physical magnitudes measurements. Sometimes the measurements are rather of qualitative character than of quantitative one. Apart from errors specific to the near field EMF metrology one of the most important reason of errors here is “dogmatic” approach to available EMF meters and inappropriate their use. Every one device is loaded with specific to it imperfections and limitations in its use. They are resulted from their designers faults, the design simplifications, a trend to lower cost of design and manufacturing, their limited area of application and others. Usually they are not mentioned in a manual of the device and it may lead to

inappropriate its use and additional measuring errors. The first, who has broken silence in the area, was Mild [2]. He compared results of several meters testing and found disagreement of their parameters in relation to that declared by their manufacturer. The problem is much wider, as was shown by Mild. Unfortunately, the subject is rarely presented in the literature and the majority of the meters users pin their faith on the parameters listed in the meters manuals. Not to add that the most critical parameters, in our applications, are often passed over in silence.

The authors experience in the area includes an involvement in the meters construction and in calibration and recalibration meters of different types. The latter, by the way, made it possible to them an investigation the meters in details and, as a result, to find their imperfections. Several imperfections and simple methods of testing them are presented in [3, 4]. The paper presents farter results of the investigations.

One of the most important parameter of the meter is its susceptibility to the EMF. A limitation of the susceptibility requires a precise screening of the device. In order to illustrate the authors tries, in Figure 1 is presented a MPE type E-field meter. Similar E- and H-field meters were prepared for Polish BC and TV stations more than 50 years ago. The feeder of the dipole antenna as well as the indicator are screened. All switches and other regulation organs are protected against the field pickup and enter this way the meter. Compare it to “plastic toys” available now.

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Fig. 1 A EMF meter type MPE on a tripod 2. Susceptibility

In an EMF meter only its probe (antenna, sensor) should pickup measured field. Any other way of the field penetration into the meter is a source of error. As it may be seen from Figure 1 it is not simple to protect the meter against the penetration, especially taking into account its work in strong EMFs. The effect of penetration is usually due to ineffective screening the indicating part of the meter. The effect, as a rule, increases with frequency due to increasing electrical sizes of the device and may by enhanced while the device is kept in a hand of its operator due to its role as a concentrator. Presence of the effect and its role may be simply checked by the way shown in Figure 2. A hand held transmitter antenna is moved along the tested device. Its indications should be proportional to the distance between the antenna and the meter’s probe. If any maximum appears,

TRX

Probe

Monitor

Fig. 2 Susceptibility testing

10 10001

100

E[V/m]

f [MHz]100 Fig. 3 Susceptibility of an EMF analyzer

while the antenna is near the meter, it means that the meter’s screening is insufficient.

As an example of an EMF meter’s susceptibility to external EMF may be given an EMF analyzer, available on the market. A susceptibility of the device is shown in Figure 3. The curve shows E-field intensity necessary to have indications 20 dB above the device’s noise level while an antenna of the device was disconnected and it’s input was loaded with a matched load. It is worth to notice here that the tested device, in E-field intensities above 100 V/m stopped it’s work at all.

Several cases of meter sensitivity to ELF fields were observed. During HF field measurements in close proximity to overhead, high voltage transmission lines strange indications of the meter were noted. Instead of expected indications at the level of 1 V/m the meter shoved above 100 V/m and the indications were a function of the meter positioning in relation to the line conductors, apart from the meter probe isotropic pattern. In order to test the phenomenon a laboratory measurements, using several types of meters, were performed. During the experiments a meter was placed parallel or perpendicular to E-field equipotential lines, as shown in Figure 4. Two metallic plates were fed from a power line frequency source (50 Hz). Results of comparative measurements of two meters indications (Eind) as a function of applied ELF field are shown in Figure 5 for both polarizations.

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Fig. 4 Susceptibility testing at 50 Hz

0,1

1

10

100

1000

0 2 4 6 10

A ||

EELF [kV/m]

Eind [V/m]

B || ⊥ B

⊥ A

Fig. 5 Results of comparative measurements

3. Linearity

Primary calibration and then meters’ recalibration is, as a rule, performed in standards excited from CW sources. This way calibrated meters indicate true RMS value measurements error (δrms) while achromatic field is measured. Comparative results of three meters measurements in the achromatic field are shown in Figure 6.

Fig. 6 Error of RMS value measurement in achromatic EMF with the use of three EMF probes

The situation is much worse while AM and pulse modulated fields are measured. Figure 7 presents

comparative results of

measurements of three different EMF probes in 100% AM modulated field as a function of the field intensity. The figure allows a conclusion that meter C has really square-law detector while meter A is equipped with a diode detector that causes a transition in the meter’s response from the square-law in lower measuring ranges to the linear one for higher field intensities. Dynamic characteristics of meter B suggests a diode detector with a short time constant that limits indications increase, due to modulation, in higher intensity ranges.

1 10 100 1000E [V/m]

peak value detection

square-law detection

A

B

C

Fig. 7 Three EMF probes in 100% AM EMF

Figure 7 suggest simple method of the

characteristic’s checking. If tested meter immerse in quite strong field (right side of Fig. 7) and regulate depth of modulation from

m = 0 to m = 100% indications of the meter should not increase more than about 1/4 if the characteristics is square-law, and the

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[db]

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δrms

EELF EELF

a) b)⊥ || - parallel - perpendicular

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indications may increase even two fold while the characteristics is linear.

Fig. 8. Pulse-modulated field measurement error of selected types of commercial EMF probes.

Results of comparative measurements of RMS value measuring error (δpulse) of pulsed EMF, versus pulse duration time τ [in %], are shown in Figure 8.

4. Temperature In order to illustrate the problem of a diode temperature sensitivity several measurements were performed. The results of temperature coefficient (δt) measurements for an electric field probe with a germanium diode and similar with a Schottky barrier diode, for temperature changes from 5oC to 50oC, are presented in Figure 9. The temperature coefficient shows a difference in voltages that should be lead to a diode in different temperatures in order to have identical detected output voltage; the temperature dependence vanishes while output voltages of the diode are in linear part of it’s characteristics, i.e. above about 0.2 V for germanium diodes and about 0.7 V for silicon ones. Unfortunately, in the most sensitive ranges of an EMF meter the dependence is remarkable, as it may be deduced from Figure 9. 5. Directional pattern

Probes of omnidirectional (isotropic) pattern there is a specificity of the near field

EMF measurements, where three spatial components of measured field may appear. Examples of measured pattern are illustrated in figures 10 and 11. Patterns presented in figure 10 allows to conclude that at relatively low frequency

(1 GHz) the pattern is almost ideal. Fig. 9. Measured temperature coefficient δt

for a probe with a germanium diode (solid line) and with a Schottky barrier diode (dashed line)

-3

-2

-1

0

1

2 dB

f=1GHz

f=18GHz

f=10GHz

E=10V/m

Figure 10. Directional pattern of a omnidirectional E-field probe measured at three frequencies and E = 10 V/m

However, with frequency increase, increases electrical size of the set and, as a

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δ pulse

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δt[dB]

germanium diode

Schottky barier diode

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result, the pattern deformation reaches ± 2 dB.

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-2

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0

1

2 dB

E=4V/m

E=140V/m

E=80V/m

f=1GHz

Fig 11. Pattern of a E-field probe measured at 1 GHz for three E-field intensities Patterns presented in figure 11 reveals a principle of the probe design. Output voltages of three antennas, loaded with diodes, are summed. In square law part of the diodes characteristics (E = 4 V/m), the directional sines of separate antennas are summed with square, it leads to almost deal omnidirectional pattern. In the case of higher E-field intensities there is a linear sum of the sines, that leads to the pattern deformation at the level of ± 1 dB.

Any of the presented phenomenon may be checked even during measurements by the way of the probe rotation. 6. Final comments

The authors have tested several types of commercially available EMF meters. The subject of the studies there were: susceptibility, linearity, temperature sensitivity, directional pattern, frequency response and other parameters. Several results of the measurements are presented in the paper. As it may be seen there are devices that do not fulfill the basic requirements. We do not indicate types of

the meters and their manufacturers as it is not presentation of a commercial character. The aim of the authors is to call attention of the meters users to parameters that usually are even not mentioned in a meter’s manual. A requirement here is an understanding the electromagnetics and phenomena accompanying EMF measurements, especially in the near field. Several summarizing comments: - Every one measuring device is not absolutely accurate; it is loaded with a measuring error. - A meter can not be more accurate than standard applied to calibrate it. - Every one electric and electronic device is susceptible to presence of external electromagnetic field. - The main goal of a manufacturer is to sell his product with no regard to its quality, and rather advantages are emphasized in the manual. - A proverb says: “cheap meat eat dogs”, usually similar approach may be necessary in relation to offered products (meters). - The last, but not least: an understanding and knowledge may allow to obtain correct results even using poor devices (and contrary!). Acknowledgement Presented analyses are a part of the research within frames of Polish Ministry of Science and Higher Education grant Nr 1765/B/T02/2009/37, a participation in the meeting is possible due to the Ministry grant Nr 332/N-COST/2008/0. The support is appreciated very much. References 1. H. Trzaska - Keep out the Western Protection Standards. Proc. 3rd Intl. Symp. on EMC, Beijing 2002, pp. 35-38. 2. R. Bostroem, K.H.Mild, G. Nilsson – Calibration of commercial power density meters at RF and microwave frequencies, IEEE Trans. vol. IM-35, No.2, June 1986, pp. 111-115. 3. P. Bienkowski, H. Trzaska – EM Measurements In the Near Field. SciTech 2011. 4. E. Grudzinski, H. Trzaska – EMF standards and exposure systems. SciTech (in print).

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