EnvironmentalEnvironmental ProtectionProtection AgencyAgency ofof PiedmontPiedmont RegionRegion
(ARPA Piemonte)(ARPA Piemonte)
Radiofrequency Exposure Assessment: a synthetic review of environmental
fields measurements with a focus on induced currents.
Laura Anglesio
Sino-Italian Cooperation Project
Exposure to uniform RF electromagnetic fields can be assessed simply by measuring the electric, magnetic field strength or power density in one point occupied by the body (body removed) and comparing the results to the reference levels ( ICNIRP 1998)
EXPOSURE EVALUATION
If the exposure takes place in the far-field in free space, the emitted electric field can be assumed uniform on the whole body.
Measurement procedure
The first step in defining the measurement method is the choice of the type of instrumental chain: broad band or narrow bandDepending on the purpose of measurement campaign or on the type of signal to be detected it may be more useful to carry out a simple broad band measure, evaluating the global field level, or a narrow band measurement which provides detailed information on the harmonic content of electromagnetic signal
� metallic cables connecting sensor to meter should be oriented tobe perpendicular to the expected electric field vector to minimize cable interaction with the field;
� during measure it shall not be signal interfering with instrument;
� instrument shall not be placed on a conductive surface;
� the body of operator shall be at a distance from the instrument greater than 3 m;
� if an isotropic sensor is used, it is convenient to verify its isotropy by mounting it on the principal axis of the instrument;
� in the case of indoor measures, measurement point has to be chosen at a distance from the walls greater than 3 times the
dimension of the sensor or antenna, so that the measured values are valid over all room.
Measurement procedure
Measurement procedure
Since standing-wave effects and multiple-field interactions must be accounted for, it is necessary to scan a volume of space in the zone of interest. If possible, optical fibre or high resistance cable should used between the probe and remote readout. This practice will reduce errors due to cable reflections and pick-up
The accurate measurement of near fields is dependent upon the availability of a probe with an electrically small antenna array, since large gradients exist in near fields and spatial resolution is critical. In addition, a small antenna array produces minimal perturbation of the field under study, and the source’s radiation characteristics are not modified by the alteration of reactive fields
Measurement procedureFor an unknown near-field, an isotropic probe should be used.
Alternatively, measurements with three orthogonal orientations of a single axis probe shall be performed to ensure that all components of the field have been accounted for. If this latter approach is used, ensure that the field being measured is time invariant.
Due to the large spatial variations which occur in the near field of a radiating source, a series of continuous scan shall be performed to map the field over the area of interest in order to find the point of maximum intensity.
Reflections from cables, operator’s hands or the readout meter should be considered. These are considered relatively insignificant error sources when separated from the probe sensor by an adequate distance or located farther from the source than the sensor
Measurement procedureFor compliance with standard limits the following aspects have to be taken into account:� measurements has to be made in areas where it is possible the
access for workers or population but without the presence of subjects because of standard limits refer to unperturbed field;
� it is necessary to subdivide the area in uniform parts to make anumber of measurements statistically significant, so to permit time and spatial distribution of the fields. The number is dependent on the type of monitored area and on the number of exposed subjects;
� after measurements are carried out, characterization of area it is obtained calculating more adequate statistical parameter (mean, standard deviation etc) also depending on destination of considered area.
Measurement procedure
To evaluate compliance with exposure limits, it is necessary to obtain time and spatial average of measured field values.
Spatial distribution
To take into account of field variations with height above ground, it is necessary to evaluate field values along a surface equivalent to vertical section of human body.
It is necessary to consider a number of measuring point adequate to radiation wavelength and probe dimensions, so that measurement results can give information on maximum and minimum values in the investigated area.
Measurement procedure
Spatial distribution
In case in which the adopted antennas have small dimensions withrespect to a typical mean height of human body, it is suggested to make measurements at the following three different heights above ground or foot level: 1,1 m, 1,5 m and 1,9 m. At these heights can be exposed the more critical organs of adult person.
If antennas with great dimensions area used (for instance dipole or biconical antenna with length of about 1 m), it is sufficient only one measurement at a height of 1.5 m above ground. In this case measurement result represents an average value on antenna length.
Measurement procedureTime distribution
If the signal level is time constant, such as for FM broadcast transmitters, the time period of measurements can be strongly reduced (for instance less than 1 minute, taking into account of the time necessary for instrument stabilization).
The duration of measurement has to be adequate to characterize time variation of detected signal. In some cases protection standard define a duration of measurement equal to 6 minutes.
If emissions from source are very time variable, it is preferable to carry out measurements in conditions of maximum emission. For instance in the case of Base Radio Station for mobile telephone may be useful to have information on day period of maximum traffic and, consequently, to make measurements in that period which corresponds to maximum emission
ELECTROMAGNETIC FIELD MEASUREMENTS
Broadband instruments
Continuous monitoring
Narrowband instruments
An example of monitoring in a complex urban site
4 plants with different systems: 18 GSM and DCS cells, 12 UMTS2100 cells, 4 UMTS900 cells, 3 LTE1800 cells
Measurements performed:
-Broadband measurements-Narrowband measurements-Continuous measurements with a monitoring station
An example of monitoring in a complex urban site
Broadband measurements to determine the point with maximum field level
Field meter characteristics:
100kHz – 3GHz0.5-800 V/m
An example of monitoring in a complex urban site
Narrowband measurements to determine the contributions of different systems in the maximum point
Spectrum analyzer (100kHz – 3GHz)Isotropic antenna (75MHz – 3GHz)Coaxial cable
An example of monitoring in a complex urban site
Continuous measurements with monitoring station (6 min average)
An example of monitoring in a complex urban site
Narrowband measurements: short-term results and extrapolations to maximum value
RSs
LTE EBF
NE ×=max
But exposure situations occur also in the close vicinity of the source where the (high level) fields are more or less nonuniform and localized to a small part of the body.
In this case, the maximal field strength may considerably exceed the reference levels for the external fields without exceeding the basic restrictions expressed in terms of Specific Absorption Rate (SAR) specified as a whole body average and localized maximum value
Spatial averaging of the unperturbed external field strength over the location of the body or part of the body may be used, but it is necessary to assess that the basic restrictions for local SAR and current density are not exceeded.
There is a clear need to define practical procedures for exposure measurements which full-fill these conditions but are not unnecessary restrictive (in term of field strenght or time to carry out measurements or modelization and numerical dosimetry)
Above 100 MHz usually we proceed with the definition of spatial averaging of the unperturbed electric and magnetic field strengths.
At lower radio-frequencies, the measurement of currents induced by the electric field in the body is a very useful procedure for the exposure assessment.
If the source of the fields is very localized and close to the body, closer than approximately 20 cm, the exposure assessment based on the reference levels becomes too conservative and the compliance with the limits for local SAR and current density must be verified by dosimetric measurements and numerical simulation
ρσ 2
2
1 inESAR =
E field conduction + polarization current
B field induced emf
E field B field
Ein complex function depending on externalfield, charge distribution, body shape and conductivity
Plane wave E field for 0,4 W/kg
different size
STAND-ON meter developed by Dipartimento di Ivrea of ARPA Piemonte Laboratory
Dynamic range: 0-2000 mA
Frequency range: 3kHz - 100MHz
Dimension: 30 X 30 cm
Induced current measurement by means of a “two parallel insulatedplates” meter:
Diode rectified current reading using a milliamperometer
Setting and characterization of the STAND ON meter
RF generator connected to meter plates(human feet equivalent area):known current injection and reading ofcorresponding value
Frequency linearity respondse (FM radio frequency range): for high levelof injected current a slightoverestimate occurrs
The meter realised in our laboratory was compared with a clamp-on commercial meter
CLAMP-ON Holaday HI-3702
Dynamic range: 1mA -1A (linearity ± 0.5dB)
Frequency range: 9kHz -110MHz ± 2dB
Induced current meter based on current transformer
Primary circuit currentinduces (ankle) induces a current in the secondarycircuit (thoroidal coil):
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XXRR
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I 202
202
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1 )()( +++=
RF sources causing quite high levels of induced currents:
Impianti per telecomunicazioni: FM radio(88-108 MHz)Outdoor
Indoor (workers)
Industrial heating apparatus(dielectric loss- froma few MHz to50MHz; magnetic induction fromhundreds of kHz to a few MHz), Medical apparatus(marconitherapy 27.12 MHz or 40.68 MHz)
Measurement campaign: broadcasting and industrial sites to obtainresults on different sources
Experimental measurement campaign withbroadbandanalysis of induced current
4 different sites with FM radio broadcaser (88-108 MHz) + 1 RF plastic-sealer industry RF (27.12 MHz) for tensile-structure tarps
8 people with different body-shapes and height
Variables: type of soil, subject insulation fromground, frequency and strenght of the external E field, subject height and posture, way ofmeter application, time variability and weather conditions
Investigated parameter: specific current = I(mA)/E(V/m) (thoroughtboth the two ankles)
Subject were placed at a distance ranging from10 m to 50m (4λ to13λ) from the transmitting antennas and at a distance of 1 to 3m (1/10 to 1/5λ) from RF sealers.
ResultsResults (1)(1)Type of soil and way of meter application
Measurements on teh same subject (bare-foot) in two different sites on 6 types of soil:
STAND-ON: some problems due to different way of soil adhesion of the lowerplate, ⇒ 80% variability between different foothold.In order to reduce this variability need to assure the grounding of the lower plate⇒meter palced on a copper sheet (90X90cm) ⇒ 12% variability.
CLAMP-ON: 10% variability(taking care to hold the sensoron a plan perpendicular to the limb)
ResultsResults (2)(2)
Ground insulation of the subject
Subject’s ground impedance variation⇒variation of the coupling with the field (resonance phenomena) resulting in variation in current intensity and distribution⇒ variation of the circuit characteristics for stand-on meter insertionand for coupling of clamp-on meter:introduction of a capacity estimated in about 80 picoFarad assuming C= ε (S/d) with εrubberabout 3F/m, d=0.5cm Sfoot about 150 cm2
Measurements on people wearing shoes (with different types of rubbersoles) and bare-footed with both meters
Variation of the coupling with the field as a function of the groundinsulation of the subject: numerical evaluation of the SAR
The SAR induced leveldecreases when the subject isbetter insulated from the ground; this effect reduces asfrequency grows, at f > 80-90MHz curves converge
Calculation of the ratio R [shoes-wearing measure/bare-foot measure] for the two meter
A) Measurements at the sameplace (FM broadcast site) withdifferent subjects (and shoes)
B) Measurements on the samesubject but in different places, FM broadcsat sie and RF sealingindustry.
Points 1 and 2 nearby the RF sealers (27.12 MHz); points from 3 to 8 FM broadcast site
STAND-ONRaverage= 0.69 ± 0.11CLAMP-ONRaverage= 0.95 ± 0.04
STAND-ON: Raverage(FM)= 0.69 ± 0.04 (same subject); Raverage(27.12 MHz)= 0.6 CLAMP-ON:Raverage(FM)= 0.95 ± 0.08 (stesso soggetto) ; Raverage(27.12 MHz)= 0.7
Different results ⇔ different detection mechanism
CLAMP-ON is a current transformer which creates, due to induction in the coil, a current proportional to that actually flowing trought the ankle. If the load impedance of the sensor is fairly high, it couples with the subject in a manner not depending on his impedance to ground.
STAND-ON in series on the capacity due to the sole ( whose impedance is comparable with that of the sensor load) ⇒ generation of deplacementcurrents not detected by the sensor (“edge” current and ebnergy absorption in the sole) with subsequent decrease in the detected values.
ResultsResults (3)(3)
Subject Height
Measurements on 7 people ranging froma 164 to 178 cm in the same exposureconditions (same point) ⇒ maximum variability 43% with stand-on meter and 45 % with clamp-on meterCorrelation coefficient height- specific current≥ 0.9
Average measurements in different points for each subject (clamp-on meter)
For small people the measured levelsare generally lower than those obtainedfor tallest people; but the minimum level do not coincide at anymeasurement point to the smallestpeople
Due to the influence of individualmorphology and frequencycontents of the different fields. Empirical relation: Is = K0 h
2 f
Resonance frequencies for different subjects (height ranging from 164 to 178 cm) :42-46 MHz (grounded subject, λ/4)88-92 MHz (insulated subject, λ/2)
For the same people, the specific current value in the FM range is about the double of that measured in correspondance of 27.12 MHz sources. Due to nearfield exposure conditions and different current distribution along the body.
In the FM site with predominant frequency 94MHz (equalto 48% of total measured power) detection of rilevati specific current values higher (15% circa) than those detected at other FM sites (where about 60% of power was due to a 100MHz signal).
Frequency content of incident field
FindingsFindings (1)(1)
Results obtained with the two different meterare comparable within 25%more or less.Good agreement for E field level not too high (< 15-20 V/m), worst agreement (<35%) for higher levels (30 V/m) .
Results obtained with the two different meter are repeatable at sameexposure conditions, with a increased variability for the stand-onmeter due basically to:
Insulation to ground of theb subject (presence of shoes), greatest discrepanciesfor FM frequenciesposture hold by the subject and different adherence of the bare-foot to the plateClamp-on reading is more stable even if the probe is moved along the leg(maximum variation between ankle and kneee within 20%)
FindingsFindings (2)(2)Results obtained for different points and exposure conditions, but for the samefrequency range, measurement conditions and subject, have variability of about10% ⇒ most influencing parameters are the frequency content of the field and the subject morphology
The subject height has great influence on the induced current value (r=0.9); forheights ranging from 164 to 178cm the variability is about of 45%.
The induced current detected for each ankle during the measurement campaignresulted in some case comparable with the limit proposed for populationICNIRP (1998). In one case that limit was exceeded; for a 25 V/m field strenght and bare-footedsubjects we measured 50mA with clamp-on meter and 60mA withstand-on meterfor the tallest subject (178 cm)
FindingsFindings (3)(3)The measurement procedure has to be set up depending on the type of the usedmeter (stand-on o clamp-on), so as to reduce possible atrefacts due to the different responses
Environmental parameters influence: no correlation with the type of the soil, temperature, humidity and clothes worn; the main factors that influence resultsare the incident field characteristics (amplitude and frequency) and subjectmorphology.
Extrapolation techniques to obtain induced current level variation (due the above mentioned parameter) from a limited set of measures : empirical relations as a function of overall electric field level and subject’s height:I = a + (3.2 ± 0.4)EIspec = -1.6 + 0.00018h2
Using the proposed models, experimentally validated, theydemonstrate that to respect the actual limit on externalelectric field at 10 MHz (i.e.184 V/m), the induced current hasto be lower than 80 mA. Between 30 and 90 MHz, the limit on external electric field is61 V/m thus to respect those limits, the induced current in ankle has to be lower than 95 mA at 30 MHz and lower than180 mA at 50 MHz, 70 MHz and 90 MHz.
The actual limits on induced currents are more restrictivethan external electric field for frequencies above 40 MHz.
For frequencies below 40 MHz, the limits on external electricfield are more restrictive
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