Evaluation of compliance with SAR limits on the basis of external … · 2014. 4. 24. · SAR...

SÄTEILYTURVAKESKUS • STRÅLSÄKERHETSCENTRALENRADIATION AND NUCLEAR SAFETY AUTHORITY

Evaluation of compliance with SAR limits on the basis of external RFEM- field and induced current

measurements

Current trends in health and safety risk assessment of work-related exposure to EMFsMilan, February 14-16, 2007

Kari JokelaNon-Ionizing Radiation Laboratory

STUK, Radiation and Nuclear Safety Authority (Finland)


Content

• Exposure limit values and action values

• Exposure assessment for non-homogeneous RF-fields in broadcast tower

• Exposure assessment for non-homogeneous RF fields in the vicinity of microwave antennas

• Assessment of SAR and induced body current in a reactive nearfield of a dielectric heater

• Localized exposure from a small source near the body


Exposure limit values for RF fields(Directive 2004/40/EC)

50----10 - 300 GHz

-20100.4-0.01 - 10 GHz

-20100.4f/100( f in kHz)

0.1 - 10 MHz

Power density

(W/m2)

Localised10 g average

SAR forlimbs

(W/kg)

Localised10 g average

SAR forhead and

trunk(W/kg)

Whole body

average SAR

(W/kg)

Current density for head and

trunk

(mA/m2,rms)

Frequency range

SAR values are 6 min time averages


SAR in uniform plane wave


Action values for RF-fields(Directive 2004/40/EC)

--500.361372 - 300 GHz

--f/400,008f½3f½400 - 2000 MHz

--100.1661110 - 400 MHz

10040100.166110 – 110 MHz

-40-1.6/f610/f1 - 10 MHz

-40-1.6/f6100.1 - 1 MHz

Limb induced current

IL(mA)

Contact current

IC(mA)

Equivalent plane wave power density Seq(W/m2)

Magnetic field strength H(A/m)

Electric field strength E(V/m)

Frequency range


The reference levels (action values) are spatiallyaveraged values over the body with the importantproviso that the basic restrictions (exposure limitvalues) on localized exposure are not exceeded

ICNIRP Guidelines 1998


Source characteristics (frequency / waveform,

power, etc.)

Low power?

Assess fields / currents at user position

Below action values

Meets other product -specific compliance criterion?

Further assessment exposure limit values

Below exposure limit values

Compliant Non-compliant

Yes

Yes

Yes

Yes

No

No

No

No

Occupational exposure limits for

RF Fields

Action values

• External electric and magnetic field strength

• Induced body current

Exposure limit values

• Peak SAR (10 g)• Whole body average SAR• Induced current density (< 10

MHz)

Flow chart for exposure assessment

(EN 50392)


http://www.elmagn.chalmers.se/~elfrk/work/dosimetry/dosimetry.html

measured in homogeneousliquid phantom

computed in realistic head model

SAR assessment for a localized exposurefrom a mobile phone

Tim Toivo and Tommi Toivonen, STUK


RF exposure sources in a FM/TV mast


0

10

20

30

40

50

-2 0 2 4 6 8 10 12 14 16 18 20

Distance from the bottom of the antenna [m]

Eq

uiv

ale

nt

pla

ne

-wa

ve

po

we

r d

en

sit

y [

W/m

2]

Measurement results with the average transmission power of 19.5 kW

Averaged over the whole body with the average transmission power of 19.5 kW

Reference level for the occupational exposure

Measurement results scaled to normal average transmission power of 50 kW

Top of the

antenna

E-field measurement in FM-radio antenna

100 MHz

E field

STUK measurements by Jokela, Puranen and Toivo 2004


0

50

100

150

200

250

300

-2 -1 0 1 2 3 4 5 6 7Distance from the bottom of the antenna [m]

Eq

uiv

ale

nt

pla

ne

-wa

ve

po

we

r d

en

sit

y [

W/m

2]

Measurement results with the average transmission power of 5 kW

Averaged over the whole body with the average transmission power of 5 kW

Measurement results scaled to normal average transmission power of 15 kW

Reference level for the occupational exposure

Top of the

antenna

E-field measurement in Digi TV antenna

DVB-T antenna

E field

STUK measurements by Jokela, Puranen and Toivo 2004

572 MHz


RF EXPOSURE ASSESSMENT IN THE FM/TV

MAST

• Routine SAR assessment impossible in so complexexposure environment

• Technical problems with body current measurements

• Electric and magnetic field measurements with isotropicbroadband probes at distances not less than 20 cm fromthe metallic structures

• Body average of E2, or SE is a good exposure indicator.


Near field of a base station antenna

Hansen et al., Univ. of Wuppertal

900 MHz1 W

vertical

horizontal

1 m


Computed SAR near a base station

Cooper et. al. Bioelectrom.23:429-443, 2002

935 MHz

• For distance exceeding 30 cmexposure is limited by the wholebody average (WBA) SAR.

• At GHz range WBA-SAR iscorrelated with body averagepower density


Planar layer model for calculation of SAR in surfaceabsorption range

k

Einc

Hinc

ICNIRP guidelines 1998

Power density limit from 10 to 300 GHz

50 W/m2 averaged over 20 cm2

1000 W/m2 averaged over 1 cm2for 10 g tissue cubed=2.2 cm


Concentration of SAR in the surface of the bodyat microwave frequencies

0 5 10 15 200

0.1

0.2

0.3

0.4

0.5

3 GHz, 10 W/m2

(b)

Distance from the surface [mm]

SA

R [

W/k

g]


Effect of averaging of local peakSAR in the body surface

Frequency (GHz)

Loca

lpea

kS

AR

(W

/kg)

10 W/m2

planar layer model

simple formula

.

Jokela K. EMC & SAFETY OF MULTIMEDIA COMMUNICATION TERMINALSStandard Evaluation Report, report 2000


Relaxation of the reference level for spatial peakpower density above 1 GHz.

Peak power density may exceed at least 6 dB the body average power density, if the distanceto the antenna exceeds 30 cm


Non-uniform exposure near an industrialdielectric heater

Electric field induces longitudinalRF-currents in the body

27 MHz


Research problem

Estimate the whole body average andlocal maximal SAR from the measuredRF current


Measurement of induced current


SAR simulation

Kännälä et al. 2006


Electric field

Distance 0.3 m

measured

computed


Measured and computed body current


Current density in pelvic cross-section

(heterogeneous model)

vertical (longitudinal) horizontaldistance 0.3 m


Relation of SAR to current and electric field

5.3 (in ankle)0.062931.0

5.2 (in ankle)0.0792120.5

7.3 (in arm)0.1303780.3

SAR10g

[W/kg]

(20)

SARwba

[W/kg]

(0.4)

Etot,max

[V/m]

(60)

Distance [m]

Current in the ankle = 100 mA


For non-uniform exposure to near

electric field from a dielectric heater

• induced body currents are longitudinal and distributed evenly along the high conducting tissues below the skin,

• currents depend on the body average electric field E,

• hot spots in the ankles and wrists are most critical,

• whole body and local peak SAR can be estimated from the measuredcurrent for electrode distance > 0.3 m,

• for distance <0.3 m, critical hot spot may arise in the tissue most adjacentto the electrode,

• SAR limits are not exceeded when current < 100mA and distance >0.3 m.


CONCLUSIONS

• Below 30 MHz the measurement of induced body currents and bodyaverage electric field is a good practice for assessing exposure toelectric field (high impedance sources).

• From 100 to 1000 MHz the equivalent electric field averaged over the whole body is probably sufficient exposure indicator in the FM/TV mastwhere the fields are relatively evenly distributed over the whole body.

• Above 1 GHz the local power density may exceed 6 dB the whole bodyaverage.

• At distance less than 20 cm determination of SAR is necessary for realisticexposure assessment.


Ackowledgments

I wish to thank my colleagues

Lauri PuranenTim ToivoTommi ToivonenHeidi NybergAri-Pekka SihvonenSami Kännälä

for the data provided for thispresentation

Evaluation of compliance with SAR limits on the basis of external … · 2014. 4. 24. · SAR...

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