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
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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.