Testing of IEEE 802.11a/b/g Devices
Sargent and Chris Zombolas
worldwide are making progress toward harmonizing standards covering
portable communication devices used close to the body as well as
close to the head.
setting limits for the specific absorption rate (SAR) of radio-frequency
(RF) transmitting devices used in close proximity to the human body
have been mandated in many countries. Lately, though, the proliferation
of wireless local-area network (WLAN) and wireless fidelity (Wi-Fi)
devices in portable digital instruments and laptop computers has
called for an update of the current SAR testing methodology and
procedures. Current methods of measurement were intended only for
devices used at the ear and cannot always be applied in the testing
of devices meant to be used near other parts of the body.
The broad scope
of human-exposure regulations mandated by the Federal Communications
Commission (FCC) in the United States and by the Australian Communications
Authority (ACA) captures most RF transmitting devices, including
devices employing Institute of Electrical and Electronics Engineers
standards IEEE 802.11b and IEEE 802.11g (2.4 GHz) and IEEE 802.11a
(5.2/5.8 GHz) as well as those used at the ear or other parts of
the body. Owing to the lack of harmonized SAR measurement standards
covering these devices, and also to the technical difficulties involved
with SAR testing above 3 GHz, FCC introduced SAR measurement guidelines
in its 2001 document OET 65C 01/01.1
The FCC SAR measurement guidelines were largely adopted by ACA as
part of the Australian regulations limiting human exposure to RF
fields from portable RF transmitting devices.
describes the application of FCC and ACA SAR measurement methods
in the testing of devices employing IEEE 802.11a/b/g, Bluetooth,
and similar transmitters. It focuses on issues related to SAR measurements
at 2.4–5.8 GHz while providing a general background in SAR
Limits, Regulations, and Standards
SAR is a measure
of the rate at which energy is absorbed by a certain unit mass of
human tissue, usually expressed in watts per kilogram but also in
milliwatts per gram. The rate of temperature increase of the body
is proportional to the SAR. SAR limits vary among regulators (see
Table I). FCC has
a limit of 1.6 W/kg, measured in a 1-g cube of tissue mass, while
in Europe the SAR limit is set at 2 W/kg, measured in a 10-g cube
The basic European
restrictions on human exposure are given in the recommendations
of the International Commission on Non-Ionising Radiation Protection
The SAR limits are given in the European Norm (EN) standard EN 50360,
with measurement specified by the methodology described in EN 50361.3,4
While European and other international standards set SAR limits
for exposure to all or parts of the body, current standards include
SAR measurement methodologies only for devices used at the ear,
such as mobile phones. No published standards exist for SAR measurements
on body-worn devices.
In the United
States, FCC specifies both SAR limits and methods of measurement
in OET 65C.
the ACA electromagnetic radiation standard known as EMR Standard
has adopted the basic restrictions given in the Australian Radiation
Protection and Nuclear Safety Agency (ARPANSA) standard,6
which has adopted the ICNIRP recommendations for the basic restrictions.
The SAR measurement methodologies are given in Schedules 1 and 2
of ACA EMR Standard 2003, which also calls up EN 50361.7.8
However, Schedule 1 expired on March 1, 2005, and may no longer
Measurement Standards. The scope of current standards EN
50361 and IEEE 1528 for SAR measurement has been limited to devices
used at the ear, and the applicable frequency range is 300 MHz to
There are no harmonized measurement standards specifically for measurements
of the SAR of body-worn devices, or for devices not used at the
ear. For frequencies above 3 GHz, the limitations have been due
primarily to difficulties in designing SAR measurement probes of
sufficient accuracy and spatial resolution, and, to a lesser extent,
to difficulties in formulating tissue-simulating liquids.
While SAR probes
are now available to cater to the recently introduced 5.2/5.8-GHz
devices, the development of SAR measurement standards is lagging
for these frequencies, as well as for devices used in positions
other than at the ear. Although regulators such as FCC and ACA have
mandated human-exposure standards to limit the SAR of devices used
in close proximity to the human body, the lack of appropriate SAR
measurement standards for frequencies above 3 GHz has forced the
authorities to introduce preliminary methodologies and guidelines
that are not as prescriptive as industry standards. The lag in publication
of suitable harmonized standards and the lack of SAR probe technology
in the 5 GHz band meant that FCC was not prepared to devolve the
responsibility for the certification of IEEE 802.11a (5.2/5.8-GHz)
devices to its authorized telecommunications certification bodies
(TCBs). (Industry generally prefers to certify devices by means
of the more commercially driven and less bureaucratic TCB route.)
62209. For the United States, FCC, while having different
SAR limits, generally adopts IEEE/ANSI [American National Standards
Institute] standards that aim to be harmonized with the International
Electrotechnical Commission (IEC) SAR measurement standards. In
Australia, ACA has indicated that it will adopt international SAR
measurement standards as they become available. This should lead
to the adoption of IEC 62209-1, already published, and IEC 62209-2
(now in draft), when it is published.10,11
published IEC 62209-1, applicable for devices used at the ear and
operating within the frequency range of 300 MHz to 3 GHz, will probably
be adopted as an EN standard to replace EN 50361.
publication of IEC 62209-2 relating specifically to body-worn devices
will extend the scope of standards for measurement of SAR to the
frequency range of 30 MHz to 6 GHz. It will include an accurate
and reproducible methodology for SAR measurement for two-way radios,
palmtops, laptops, desktop computers, and body-mounted wireless
communication devices, designed for determining compliance with
localized SAR limits in RF human-exposure standards. (The actual
exposure limits are set by other standards.) The standard will be
applicable to both public and occupational exposures, and will cater
to concurrent exposures from multiple radio devices. IEC 62209-2
is intended to be used in conjunction with IEC 62209-1 and will
provide detailed specifications for SAR measurement instrumentation
and protocols and for phantom simulation of the human body for devices
not used at the ear.
1. A diagram of the SAR system shown in Figure 2.
Equipment and Measurements
SAR systems use small, precision, isotropic electric-field (E-field)
probes to scan a phantom containing tissue-simulating liquid. A
precision multiaxis robot positions the probe within the phantom
at a large number of grid points. A precision positioner allows
the handset to be placed underneath the phantom, facilitating scanning
of the entire volume of the phantom (see Figures 1 and 2).
Phantom. The specific anthropomorphic mannequin (SAM) is
the phantom specified for devices used at the ear and is called
up by EN 50361, IEEE 1528, and IEC 62209-1. The SAM head phantom
is based on selected dimensions from a database of human males.
Thickness of the phantom shell must be 2.0 mm ± 0.2 mm except at
the ear reference point, where it must be 6.0 mm ± 0.2 mm. The material
must be of sufficiently low loss and low permittivity. The SAM head
phantom is also specified by the FCC and ACA Schedule 2 measurement
2. A typical commercial SAR system, the DASY4, from Schmid &
Partner Engineering (Zurich, Switzerland).
Phantoms. A flat phantom is specified for body-worn devices.
It must be constructed from low-loss dielectric material of 2.0-mm
thickness and have a uniformity of ±0.2 mm. ACA and FCC require
that the length and width of the flat phantom be at least twice
the corresponding dimensions of the device under test (DUT), including
its antenna. The liquid depth should be 15.0 cm ± 0.5 cm. The flat-phantom
dimensions for IEC 62209-2 are not yet defined, but they are likely
to be smaller than the FCC and ACA dimensions.
Liquid. The dielectric properties of brain- and muscle-tissue-simulating
liquids are shown in Table
dielectric values for the center frequency of the transmission band
should be within 5% of the target value. Linear interpolation is
used to obtain the dielectric properties at other intermediate frequencies.
Examples of recipes for obtaining the stated values are given in
the relevant standards. Deviations of the permittivity (e) and conductivity
(s) must be within 5% of the target values.
EN 50361, IEC 62209-1, and IEEE 1528 do not include SAR testing
of body-worn devices within their scope. Therefore, these standards
do not give dielectric parameters for muscle tissue. Muscle-tissue-simulating
liquids are used for body-worn devices and all devices not used
at the ear. The dielectric properties for muscle-tissue-simulating
liquid are given by current FCC and ACA regulations and will be
provided in the new version of IEC 62209-2 when it is published.
in Table II should be linearly interpolated when testing at other
frequencies within the band. As it is difficult to achieve the 5%
tolerance at some frequencies, FCC and ACA allow the use of 10%
tolerance as an interim measure. This relaxation is generally given
only for frequencies in the 5 GHz band.
Scans. A typical SAR scan comprises a power reference measurement,
surface check, area scan, zoom scan, and power drift measurement.
3: Typical area scan for handset.
For the power
reference measurement, a single E-field measurement is performed
in a defined position above the DUT. This is done at the end of
the test in order to check for source drift. If power drift is excessive,
the measurements should be repeated or the uncertainty should be
reevaluated. Either optical or mechanical surface detection is used
to identify and map the phantom surface with a high degree of precision.
This is the surface check.
In the area
scan, a coarse measurement grid is used to determine the approximate
location of the peak SAR values. Figure 3 shows a typical area scan
for a head position, and Figure 4 shows typical area scans for a
laptop position. The zoom scan is a fine-resolution volume scan
performed at the peak SAR location determined previously by the
area scan. The zoom scan volume is used to evaluate the spatial-peak
average SAR for a 1- or 10-g cube of tissue. Following the zoom
scan measurement comes performance of an extrapolation from the
closest measured points to the surface and an interpolation to a
finer resolution between all measured points (see Figure 5).
4. Typical area scan for a laptop PC.
System Verification. FCC OET 65C and ACA EMR Standard 2003
Schedule 2 verification tests are identical. They are necessary
for checking and tracking the performance of the SAR measurement
system. A flat phantom and precision dipole radiating source are
specified to determine whether the measurement system meets its
performance requirements. The verification frequency must be within
100 MHz of the device transmit frequency. System verification checks
should be performed daily within each operating frequency band of
the test sample. The measured 10-g SAR must be within 10% of the
expected target values for the specific phantom and validation dipole
transmit antenna. System verification must be performed 24 hours
prior to a compliance test in order to ensure that any setup or
system errors are identified. The general verification setup is
shown in Figure 6.
Probe Design. The E-field probe must be a minimally perturbing
structure containing either single or multiple electrically small
E-field sensors, along with the components necessary to transform
the sampled RF signal into a proportional direct current or voltage.
An elemental E-field probe consists of a thin dielectric substrate
containing a sensor, such as an electrically short (in tissue) dipole;
a diode to rectify the RF signal; and a balanced high-resistance
transmission line to extract the rectified signal. The dipole elements
and the high-resistance leads are usually applied to the substrate
by means of thin-film techniques. Diodes and any other discrete
components are bonded to the thin-film elements.
5. A SAR area scan and zoom scan during a 900-MHz validation.
consists of three such devices arranged in an I beam (or H beam),
or a delta (triangular) beam configuration—the latter being
preferable—with the axis of each dipole orthogonal to the
axes of each of the others, for example, aligned along the diagonals
of a cube (see Figure 7).
depends on the following factors:
polarization and the direction of incidence.
field gradients at the vicinity of a measurement point where the
probe tip is located.
dielectric properties and boundaries between media in the vicinity
of the probe.
frequency, modulation, and power level of the source, that is, the
field sources, such as noise, static fields, extremely low-frequency
fields, and so on, from the ambient and nearby equipment.
6. General SAR measurements system verification setup.
other physical influences, including temperature and humidity, can
affect probe performance.
test applications, the highest integrated SAR value is the main
concern. These values can be found mostly at the inner surface of
the phantom and cannot be measured directly because of the sensor
offset in the probe. To extrapolate to the surface accurately, the
distance between the probe tip and the surface must be known with
some accuracy. Most E-field probes employ both optical and mechanical
surface-detection methods to resolve these issues.
7. Typical E-field probe structure.
of optical surface detection is that it measures the surface distance
from the center of the probe tip. This gives the correct and precise
distance to the phantom surface, even for angled surfaces. This
is important for head scans where the surfaces are irregular. Unless
the precise distance from the surface to the probe tip is known,
the SAR measurement uncertainty will be degraded.
surface detection uses the proximity sensor built into the probe.
In angled surfaces, the probe detects the surface and then moves
back until the touch condition is cleared. However, there will still
be some uncertainty in the final probe position if the angle of
incidence is not known. This method results in increased uncertainty
for head phantoms but is not a problem for flat phantoms. Mechanical
surface detection is highly repeatable when used with a flat surface.
Uncertainty. The degree of E-field probe uncertainty depends
on several factors, including:
8. Hemispherical and axial isotropy.
isotropy, which is the ability of a field probe to respond to a
transverse electromagnetic (TEM) plane wave impinging from a direction
along the probe axis equally regardless of axial orientation (see
isotropy, which is the ability of a field probe to respond equally
to TEM plane waves impinging, with arbitrary polarization, from
directions around the surface of a hemisphere (see Figure 8).
resolution, which is the ability of a probe to discriminate between
two SAR peaks in close proximity. For spatially averaged SAR measurements
(up to 2.45 GHz), the error is generally small as long as the probe
tip diameter is less than 8.0 mm. At 5.8 GHz, the tip diameter should
be at least 3 mm.
effect, where, in the vicinity nearest to the inner surface of the
phantom shell, the sensitivity of the probe deviates from that defined
under normal calibration conditions. This occurs because of the
capacitive coupling between the probe and the medium boundary at
the liquid and phantom-shell surface. The effect can be largely
error, a category that encompasses errors from the assessment and
compensation of the diode compression effects for continuous-wave
and pulsed signals with a known duty cycle.
System detection limits, which are a source of uncertainty when
the measured field strength is too close to the detection limit
of the probe.
Signal response time, evaluated as the time required by the system
to reach 90% of the expected final value after cycling of the power
Integration time, which (along with the discrete sampling intervals
used in the probe electronics), if not synchronized with the modulation
characteristics of the measured signal, may mean that the RF energy
at each measurement location is not fully or correctly captured.
Probe positioning, the procedures for which affect the tolerance
of the separation distance between the probe tip and the phantom
on the Measured SAR
number of things can influence the results of SAR measurements,
including probe calibration and response, the properties of tissue-simulating
liquids, and test sample positioning.
SAR is the measure of the rate of energy absorption per unit mass
of tissue at a specific location in the tissue medium. This absorption
rate is proportional to the rate of temperature increase in a tissue,
as well. It is calculated as follows:
= σ/ρ • |E|2,
σ is the conductivity of the tissue in siemens per meter, ρ
is the mass density of the tissue in kilograms per cubic meter,
and E is the root-mean-square electric-field strength in
volts per meter.
Actual Body. In most handheld transmitters, the antenna
radiates within 1–2 cm of the user's head or body. Even at
low power levels, relatively high field strengths would be expected
near the antenna. The actual field strength is highly dependent
on the location, orientation, and electromagnetic characteristics
of adjacent objects, including the user's body.
user of the handset is normally in the reactive near-field region
of the antenna where the electromagnetic field is mostly nonpropagating.
The energy absorbed by the user mainly derives from the electric
fields induced by magnetic fields generated by the current flowing
along the antenna and other radiating structures of the device.9
RF energy is scattered and attenuated as it propagates through the
body tissues. Maximum energy absorption occurs in the more absorptive
high-water-content tissues near the surface of the head and near
the surface of other parts of the body. To account for near-field
energy coupling effects, portable transmitters are evaluated with
dielectrically realistic head and body models—the phantoms.
Measurement Issues. SAR testing of portable computers fitted
with IEEE 802.11b and 802.11g (2.4-GHz) WLANs is practical with
current SAR probes and tissue-simulating liquids. Now, the recent
proliferation of WLAN devices using IEEE 802.11a protocol has introduced
a need for practical SAR measurement equipment operating over the
frequency bands 5.2 GHz and 5.8 GHz. Unfortunately, such high frequencies
exacerbate a number of problems that also exist in the lower frequency
ranges but are not so troublesome there.
main measurement issues at 5.2 and 5.8 GHz are extremely steep field
gradients close to the phantom surface, the physical limitations
of current E-field probe design, and the absence of published recipes
for tissue-simulating liquid for frequencies above 3 GHz. These
have the consequence of greater measurement uncertainty. This increase
in uncertainty is due to the availability of only imprecise extrapolation,
interpolation, and integration algorithms for maximum-SAR evaluation
and to the degraded positional repeatability of the DUT with respect
to the test phantom.
general, compliance testing of portable RF transmitting devices
must be performed if exposure is at a range of less than 20 cm from
the body and the power threshold is exceeded.
Equipment. WLAN and Wi-Fi transmitters installed in laptop
computers and handheld devices are classified as portable equipment
under the general-public/uncontrolled-exposure category. They often
exceed the power threshold specified by the ICNIRP, FCC, and ARPANSA
requirements and might be used within 20 cm of the human body. Because
it is not possible to prove their compliance by checking against
the reference levels, an SAR test most likely will be necessary.
The WLAN standards IEEE 802.11a/b/g cover the frequency ranges and
modulations outlined in Tables III and IV.
a laptop computer or portable digital device incorporates multiple
transmitters, then each mode must be assessed. Device modulation,
power output, and duty cycles must be taken into account to ensure
that the device is operating in a worst-case mode. The intent is
to make sure that the SAR evaluation is conservative.
compliance of multifunction, multiband devices is becoming very
difficult to achieve. Some personal portable computers use three
bands—for example, 2.45, 5.2, and 5.8 GHz—and some also
include the Bluetooth function. If each WLAN module has an antenna,
then the cumulative SAR levels should be determined. Using preapproved
transmitter modules does not guarantee compliance, because it is
not known how the original module was configured for the initial
compliance test. In some cases, different antennas are used, or
different modules using different modes may operate simultaneously.
to near-field coupling effects, small changes in device positioning
can lead to unexpected changes in energy absorption. In the case
of devices used at the ear, the effect of device positioning on
the SAR levels must be considered. Current published standards such
as EN 50361, IEC 62209-1, and IEEE 1528 precisely define the positioning
relative to the ear.
OET 65C and ACA EMR Standard 2003 specify procedures for SAR measurements
of devices not used at the ear. The phantom dimensions are related
to the DUT dimensions and the wavelength of the DUT transmitter.
Radios and Portable Phones. Portable telephones operating
at 2.4/5.2/5.8 GHz are now commonplace. To comply with European
(ICNIRP) and ACA requirements, these phones must be tested at the
head and in the body-worn position if supplied with hands-free kits
and exceeding 20 mW. The power threshold for FCC compliance is slightly
9. The belt-clip position for testing hands-free portable telephone.
the belt-clip position, the transceiver is placed underneatha flat
phantom and suspended such that the belt clip touches the phantom
(see Figure 9). If the device incorporates a headphone socket, then
testing with the hands-free earpiece/microphone connected is required.
10. The face position for testing two-way PTT radios for compliance.
test a PTT (push-to-talk) two-way radio in the face position, the
device is placed 2.5 cm from the phantom (see Figure 10). This position
is deemed equivalent to the device being placed in front of the
nose during use.
and Tablet Computers. SAR test methods are constantly being
updated by FCC. The OET 65C guidelines define a variety of test
positions for laptop and tablet personal computers (PCs).
11. The lap-held position simulates a tablet PC on a person's
lap-held position simulates a laptop/tablet PC being used on a person's
lap (see Figures 12 and 13). This position provides a conservative
estimate of the actual SAR that a typical user would experience.
The bottom side of the laptop is pressed against a flat phantom.
a tablet PC has display-mounted antennas and an interactive screen
display, then the arm-held (interactive-display) test configuration
may be applicable (see Figure 14). The face of the tablet screen
is pressed against the flat phantom. This position is designed to
evaluate forearm exposure.
12. The lap-held position for testing a tablet PC.
edge position simulates the use of tablet PCs, which are held along
the edges of the screen surround (see Figures 15 and 16). The location
of antennas in these regions necessitates SAR compliance. Hand exposure
testing is not typically required, but such positions are designed
to evaluate forearm exposure.
addition, the back-of-lid position can be employed to account for
occasional exposure to the arm or torso region.
13. The arm-held position for testing a tablet PC.
is not generally necessary to test for exposure to the hands or
wrists, because the SAR limit for these areas is much higher. The
highest measured SAR levels generally correspond to the location
of the antennas and their proximity to the body.
14. Lap-held position for testing a tablet PC.
Compliance Requirements. In Australia, the procedures for
testing laptop and tablet PCs for compliance are prescribed in Schedule
2 of the ACA EMR standard. The need to evaluate the SAR depends
on where the antenna is located on the portable PC—on the
bottom, along the front, or on the lid—and whether it can
operate with the lid closed.
15. An edge position test setup for a tablet PC.
a laptop is used on the lap in what is considered a typical use
position and the antenna is located along the front of the device,
then exposure would occur at less than 20 cm from the stomach or
groin. For this reason, SAR compliance
is necessary for WLAN devices, laptops, tablet PCs, and the
S1 in the ARPANSA standard grants a test exemption for general-public
exposure (unaware users) when the mean power is less than the threshold
level in Table S2 and the separation between the device and the
body is more than 20 cm.6 Table S2, listing threshold levels for
testing, gives the nominal mean output power, expressed in watts,
for the frequency range 450–1500 MHz as 3150 divided by the
frequency in megahertz. Thus, at 2500 MHz, the threshold mean output
power would be 3150/2500 W, or 1.26 W.
exemption holds only if the manufacturer can prove that it is not
possible for the device to be within 20 cm of the body in normal
use. If the device is used at a distance closer than 20 cm and the
power threshold of Table S2 is exceeded, then the only option is
to carry out an SAR test as if testing for compliance. Also, in
the case of a device used within 2.5 cm of the body and having output
power exceeding 20 mW, SAR evaluation must be performed.
16. An edge position test setup for a tablet PC.
of the steeper gradients of the field distribution in the 5–6
GHz frequency range, improved scanning methods and better dosimetric
E-field probes are required. The draft standard IEC 62209-2 requires
that probes exhibit the following minimal performance:
A probe tip diameter no greater than 8 mm for frequencies below
2 GHz and no greater than 16 mm divided by the frequency in gigahertz
for frequencies above 2 GHz (for example, no larger than 2.8 mm
for 5.8 GHz).
Maximum sensor displacement from the bottom of the probe no greater
than 4 mm for frequencies below 2 GHz and no greater than 8 mm divided
by the frequency in gigahertz for frequencies above 2 GHz (for example,
no farther than 1.4 mm for 5.8 GHz).
typical 5–6-GHz probe has a smaller tip diameter than a standard
900/1800-MHz E-field SAR probe, and the sensors are located 2 mm
from the tip. Leading manufacturers of SAR equipment are currently
developing 5–6 GHz probes with tips less than 2 mm in diameter
and a distance from probe tip to sensors of approximately 1 mm.
The major benefit of an optimized probe is better spatial resolution.
GHz Issues. Tissue-simulating liquids used in SAR testing
at 5–6 GHz have to be made of special materials because of
the unsuitability of the usual base ingredient, water, at such high
frequencies. Formulating the dielectric properties required for
tissue simulation at 2.4/5.8 GHz presented challenges. Much experimentation
and research is necessary to identify nontoxic, environmentally
strong decay of the fields in the frequency range of 5.2–5.8
GHz requires that the zoom scan cube be physically smaller than
recommended in current OET 65C guidelines. The x-y-z dimensions
of a 5–6 GHz measurement cube are 30 X 30 X 21 mm, with 7
X 7 X 8 points. This compares with the recommended 30 X 30 X 30
mm with 7 X 7 X 7 points. Figures 17 and 18 diagram the x-z plane
only for standard and high-frequency zoom scans.
17. Zoom scan for 900 MHz (a). Zoom scan for 5200/5800 MHz (b).
parameters for the 5–6 GHz cube represent a trade-off between
time and accuracy.
Uncertainty. One of the major factors influencing measurement
uncertainty in the 5–6 GHz band is the rapid decay of fields
close to the phantom surface. Figure 18 shows the decay of E-fields
from the phantom surface in the frequency range of 450–5800
18. Comparison of E-field penetration depth.
data displayed in Figure 18 were derived from the results of system
validation using precision dipoles at 450, 900, 1800, 2450, 5200,
and 5800 MHz. The graph illustrates the steep field gradient present
in the frequency range of 5–6 GHz. If a standard cube of 30
X 30 X 30 mm were used at 5800 MHz, the SAR level at the top edge
of the cube (the z-axis) would be approximately 50 dB down from
the peak. This would cause gross errors in the final averaged SAR
steep field gradients increase measurement uncertainty in several
ways. One involves device positioning. Small variations in device
placement can greatly affect the final spatial-averaged SAR level.
A variation of a few millimeters can reduce the final averaged SAR
level by 4 dB (approximately 130%) in the 5.8 GHz range, whereas
the same variation in the 2450 MHz band would correspond to about
1 dB (a reduction of approximately 25%).
steep gradients increase the level of error in the interpolation,
extrapolation, and integration algorithms used to calculate the
final spatial-averaged SAR. The algorithms may add uncertainty due
to general assumptions about field behavior; therefore, they may
not perfectly predict the E-field distribution in the tissue. The
uncertainty is directly proportional to the spatial resolution chosen
in the measurement and postprocessing methods. It is increased from
only 1% for frequencies below 3 GHz, but increases to 20% for 5–6
GHz measurements. All SAR standards limit the maximum length of
a scan to 30 minutes, so the accuracy of the measurement uncertainty
is time dependent. Newly marketed optimized field probes have reduced
the boundary effect is increased from 1% for frequencies less than
3 GHz to 2% for 5–6 GHz measurements. The increase is due
to placement of the probe tip closer to the phantom surface.
positioning is the final factor. Owing to the small tip diameter,
it is difficult to incorporate optical surface detection capability
in the probe tip. With curved surfaces, use of mechanical surface
detection rather than optical surface detection increases the uncertainty
by approximately 2.5%. This does not have a significant effect on
a flat phantom.
measurement standards limit the measurement uncertainty to a maximum
of 30%. While this has been achievable below 3 GHz, only the newly
developed probes meeting the aforementioned specifications for the
5.2–5.8 GHz frequency range will give a measurement uncertainty
of less than 30%.
the complications added to the testing process, SAR testing of 5.2-
and 5.8-GHz portable devices has proven to be practical, with only
a slight increase in the measurement uncertainty. The electrically
smaller probes being designed by major manufacturers of SAR equipment
enable measurement uncertainties at these frequencies to come in
below the 30% maximum.
no harmonized standards now exist for frequencies above 3 GHz, FCC
guidelines are constantly improving to meet the challenge of the
new and more demanding SAR measurement requirements. Also, the necessary
work is currently under way among international standards bodies.
It is expected that internationally harmonized testing methods and
procedures will soon be available to cover both head- and body-mounted
devices operating at frequencies up to 6 GHz.
Federal Communications Commission, "Evaluating Compliance
with FCC Guidelines for Human Exposure to Radiofrequency Electromagnetic
Fields," OET Bulletin 65, ed. 97-01; supp. C, ed.
01-01 (Washington, DC: Office of Engineering and Technology, FCC,
International Commission on Non-Ionising Radiation Protection,
"Guidelines for Limiting Exposure in Time-Varying Electric,
Magnetic, and Electromagnetic Fields (up to 300 GHz)," Health
Physics 74 (1998): 494–522.
EN 50360:2001, "Product Standard to Demonstrate the
Compliance of Mobile Phones with the Basic Restrictions Related
to Human Exposure to Electromagnetic Fields (300 MHz–3 GHz)"
(Brussels: CENELEC, 2001).
EN 50361:2001, "Basic Standard for the Measurement of
Specific Absorption Rate Related to Human Exposure to Electromagnetic
Fields from Mobile Phones (300 MHz to 3 GHz)" (Brussels: CENELEC,
"Radiocommunications (Electromagnetic Radiation—Human
Exposure) Standard 2003" (Melbourne: Australian Communications
Authority, March 2003).
"ARPANSA Radiation Protection Standard No. 3: Maximum
Exposure Levels to Radio-Frequency Fields—3 kHz to 300 GHz"
(Sydney: Australian Radiation Protection and Nuclear Safety Agency,
ACA EMR Standard Schedule 1, "Specific Absorption
Rate Test Method Using Phantom Model of Human Head."
ACA EMR Standard Schedule 2, "[SAR] Measurement Method
for Devices 20 cm or Less from the Human Body; Information for
IEEE 1528:2003, "Recommended Practice for Determining
the Peak Spatial-Average Specific Absorption Rate (SAR) in the
Human Head from Wireless Communications Devices: Measurement Techniques"
(New York: Institute of Electrical and Electronics Engineers,
IEC 62209-1, "Human Exposure to Radio Frequency Fields
from Hand-Held and Body-Mounted Wireless Communication Devices—Human
Models, Instrumentation, and Procedures—Part 1: Procedure
to Determine the Specific Absorption Rate (SAR) for Hand-Held
Devices Used in Close Proximity to the Ear (Frequency Range of
300 MHz to 3 GHz)" (Geneva: International Electrotechnical
IEC 62209-2 (Draft), "Human Exposure to Radio Frequency
Fields from Hand-Held and Body-Mounted Wireless Communication
Devices—Human Models, Instrumentation, and Procedures—Part
2: Procedure to Determine the Specific Absorption Rate (SAR) in
the Head and Body for 30 MHz to 6 GHz Hand-Held and Body-Mounted
Devices Used in Close Proximity to the Body" (Geneva: International
Electrotechnical Commission, unpublished).
Sargent is EMR/SAR test engineer for EMC Technologies (Melbourne,
Australia). He can be reached at email@example.com
firstname.lastname@example.org. Chris Zombolas is technical director
of the EMC Technologies group and is based in Melbourne, Australia.
He can be reached at email@example.com.