Traceable SAR measurements - NPLresource.npl.co.uk/docs/networks/electromagnetics/... · N-type to...

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Traceable SAR measurements Benjamin Loader 29 Nov 2007

Transcript of Traceable SAR measurements - NPLresource.npl.co.uk/docs/networks/electromagnetics/... · N-type to...

Page 1: Traceable SAR measurements - NPLresource.npl.co.uk/docs/networks/electromagnetics/... · N-type to waveguide transformer ... CST simulation, 1.5T, 0.5W average power. Tuesday, 11

Traceable SAR measurements

Benjamin Loader

29 Nov 2007

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Summary• What is SAR?

• Calibration of probes in liquids

• Application of SAR standards

• Conclusions

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What is specific absorption rate?

ρσ 2E

SAR=

⎟⎠⎞

⎜⎝⎛=

dmdW

dtdSAR

tTcSARΔΔ

= Thermally

From electric field

SAR is the power absorbed per unit mass and has units of W kg-1

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What is Specific Absorption Rate?

tTcSARΔΔ

=

Thermally:

ρσ 2E

SAR=

From electric field:

⎟⎠⎞

⎜⎝⎛=

dmdW

dtdSAR

SAR is the power absorbed per unit mass and has units of W kg-1

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Temperature rise (approx 650Wkg-1 )

dTTcSAR Δ

=

21.6

21.8

22

22.2

22.4

22.6

22.8

0 2 4 6 8 10 12 14 16

Elapsed time (s)

Tem

pera

ture

(o C)

c = 3.25 Jg-1 0C-1

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Electric field probes for determining SAR

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Typical 3- axis E-field probe

High resistivity signal lines to meter

54.7o

6 mmDiodeDipole sensor

Δ beam

Protective cover

222zyx EEEE ++=

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Calibration in Liquids

Fluid waveguide section

Matching window

Air waveguide section

Side arm power sensor

Directional coupler Band-pass filter

Male to Male N-type adapter

N-type to waveguide transformer

δ

δ/2)(4 ZwV e

abPSAR −=

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SAR probe calibration system

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NPL SAR standards

Under development

Artefact Standard

n/a6-10 GHz± 9.2 %5.8 GHz± 9.2 %5.2 GHz± 6.5 %2.45 GHz± 7.1 %1.9 GHz± 7.1%1.8 GHz± 6.7 %900 MHz± 8.5 %380 to 450 MHz± 10 %?100 kHz to 380MHz

Uncertainty (k=2)

Frequency

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Applications of SAR standards

1. Product compliance testing- wireless communications devices

2. Human exposure assessment

3. Experimental dosimetry

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1. Product compliance testing

• IEEE Std 1528 “Recommended Practice for Determining the Peak Spatial-Averaged Specific Absorption rate (SAR) in the Human Body Due to Wireless Communication Devices: Experimental Techniques”

• BS EN50361 “Basic Standard for the measurement of specific absorption rate related to human exposure to electromagnetic fields from mobile phones (300 MHz to 3 GHz)”

• FCC OET65C “Evaluating compliance with FCC Guidelines for Human Exposure to Radio frequency Electromagnetic Fields”.

• IEC 62209-1,2 “Human exposure to radio frequency fields from hand-held and body-mounted wireless communication devices - Human models, instrumentation, and procedures”

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Compliance testing of mobile phones.

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2. Human exposure assessment.

• SAR is the basic restriction for human exposure to EMF given by ICNIRP 10 MHz (100kHz) to 10 GHz

• EU Directive 2004/40/EC will make these restrictions mandatory for workers.

Related standards• NEN-EN 50499 “Determination of workers

exposure to electromagnetic fields.”• NEN-EN 50413 “Basic standard on measurement

and calculation procedures for human exposure to electric, magnetic and electromagnetic fields (0 Hz to 300 GHz)”

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Example: Patient exposure in MRI CST simulation, 1.5T, 0.5W average power

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SAR distribution.

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SAR measurements in MRI using the Optical Electric Field Sensor (OEFS)

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Principle of OEFS Sensor

Laser source

Photodetector

Voltmeter

Spectrum analyser

Optical switch

dc rf

Incident E-fieldModulators1

2

3

Circulator

Structure of chip

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3T MRI at PTB in Germany

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Results of SAR measurements in simple phantom exposed to 3T head coil.

0

0.01

0.02

0.03

0.04

0.05

0.06

0.07

1 2 3 4 5 6 7 8 9

Measurement point number

Poin

t SA

R (W

kg-1

) for

1 W

ComputedOEFSThermal

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3. Calibration of in-vitro exposure system.

Waveguide calibration system

E-field probe

TEM cell exposure system (University of

Nottingham)

E-field probe

δδ

abzEXPPSAR inv )/2(4 −

=

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SAR distribution in petri-dish

123456A

BC

D0

0.0050.01

0.0150.02

0.0250.03

0.0350.04

SAR per watt (kg-1)

Transverse position

Axial position

0.00

0.02

0.04

0.06

0.08

0.10

0.12

0.14

0.16

0 2 4 6 8 10 12

Distance from bottom of sample (mm)

SAR

per

wat

t (k

g-1)

1 ml sample1.5 ml sample2 ml sample

Probe over-reads as it approaches

the surface

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Temperature measurements in sample

0

1

2

3

4

0 5 10 15 20Elsapsed time (hours)

Sam

ple

tem

pera

ture

rise

(o C)

Original exposure systemAfter modifications

0

0.05

0.1

0.15

0.2

0.25

0.3

0 5 10 15

Elapsed time (hours)

Sam

ple

tem

pera

ture

rise

(o C)

Original exposure systemAfter modifications Input power 10 W,

(SAR level 0.14 Wkg-1)

input power of 1 W (SAR level 0.014Wkg-1)

Power loss in the TEM cell can cause additional sample heating- this may

affect the results!!

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Conclusions

• NPL has standards for calibrating SAR probes in liquids from 380 MHz to 5.9 GHz.

• A system for calibrating SAR probes 100kHz to 400 MHz is under development.

• These standards have important application for human (worker) exposure assessment, product compliance testing, and experimental dosimetry.

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Future developments

• SAR standards for 100 kHz to 400 MHz (by April 2008).

• SAR standards for 6 to 10 GHz.

• New techniques for micro-dosimetry.