developments in SAR test methods are bringing stricter limits
and requirements, but more-accurate results.
about human exposure to radio frequencies (RF) is not new. Ensuring
the safety of RF devices is the primary motivation for new standards
and test methods. The concept of specific absorption rate (SAR)
has been around for many years, but recent developments have improved
test methods. This article provides an overview of the current
limits and test methods for SAR. Standards, specifications, and
requirements are also discussed.
are required to use the specific anthropomorphic mannequin
The heating effect from RF devices causes the most concern from
an RF safety point of view. The human body counters local heating
by thermoregulation (blood flow through the affected organs).
The eyes and male testes are particularly susceptible to RF
heating because these organs have no direct blood supply and,
hence, no way of dissipating heat. The heating effects in biological
tissue escalate with the increase in frequency, although the
heat's penetration depth decreases.
With the proliferation of cellular phones, most RF safety concerns
have focused on RF absorption by the head, particularly from
mobile handsets. The dose of RF exposure is linked to exposure
time: maximum SAR is normally averaged over a 6-minute period
during the 24-hour day.
Some concerns have focused on other effects of RF exposure.
Most communications systems are pulse-like in nature, and their
effects on brain function have been discussed recently. For
example, the global system for mobile communications (GSM) frame
rate, at 8.33 Hz, is close to that characteristic of alpha waves
in the brain. Although there is no conclusive proof of such
effects, considerable research is currently examining the effects
of RF. Much of the research in this area was sparked by a report
published by the Independent Expert Group on Mobile Phones,
chaired by Sir William Stewart. The report, released in April
2000, is also known as the Stewart Report.
In the UK, nearly £7.4 million ($11.7 million) has been allocated
from both government and industry sources to research the effects
of RF. The LINK Mobile Telecommunications and Health Research
(MTHR) Programme will be funded over a three-year period. The
Programme Management Committee (PMC) was set up to advise on this
research program. To date, PMC has published two calls for research
proposals, and the first group of the projects is now under way.
PMC has decided to issue a third call for research proposals.
Much of this program's research addresses the biological effects
of RF on the human body. Currently, widely reproducible studies
of RF effects on biological cells are not available.
SAR is an index that quantifies the rate of energy absorption
in biological tissue. SAR is expressed in watts per kilogram (W/kg1)
of biological tissue. SAR is generally quoted as a figure averaged
over a volume corresponding to either 1 g or 10 g of body tissue.
The SAR of a wireless product can be measured in two ways. It
can be measured directly using body phantoms, robot arms, and
associated test equipment, or it can be mathematically modeled.
Mathematical modeling of a product for SAR can be costly, and
it can take as long as several months. Using conventional SAR
test methods, a dual-band GSM 900 and GSM 1800 handset takes about
one day to test to current standards.
Several organizations have set exposure limits for acceptable
RF safety via SAR levels. The International Commission on Non-Ionizing
Radiation Protection (ICNIRP) was launched as an independent
commission in May 1992. This group publishes guidelines and
recommendations related to human RF exposure.
For the American National Standards Institute (ANSI), the RF
safety sections now operate as part of the Institute of Electrical
and Electronic Engineers (IEEE). IEEE recently wrote one of
the most important publications for SAR test methods.1
In the UK, the National Radiological Protection Board (NRPB)
sets SAR limits. SAR limits are expressed for two different
classes of people: workers (occupational/controlled exposure)
and the general population (uncontrolled exposure). Because
the general-population exposure is considered to be uncontrolled,
the limit for this group is five times more stringent than the
limit for the workers, whose environment and exposure can be
monitored and controlled.
The limits are defined for exposure of the whole body, partial
body (e.g., head and trunk), and hands, feet, wrists, and ankles.
SAR limits are based on whole-body exposure levels of 0.4 W/kg1
for workers and 0.08 W/kg1 for the general
population. Limits are less stringent for exposure to hands,
wrists, feet, and ankles. There are also considerable problems
with the practicalities of measuring SAR in such body areas,
because they are not normally modeled. In practice, measurements
are made against a flat phantom, providing a conservative result.
Most SAR testing concerns exposure to the head. For Europe, the
current limit is 2 W/kg1 for 10-g volume-averaged
SAR. For the United States and a number of other countries, the
limit is 1.6 W/kg1 for 1-g volume-averaged SAR.
The lower U.S. limit is more stringent because it is volume-averaged
over a smaller amount of tissue. Australia, Canada, and New Zealand
have adopted the more-stringent U.S. limits of 1.6 W/kg1
for 1-g volume-averaged SAR. Japan and Korea have adopted 2 W/kg1
for 10-g volume-averaged SAR, as used in Europe.
SAR testing was originally performed by measuring minute changes
in temperature at specific locations in a tissue-simulant material.
The tissue simulant had to be extremely viscous to prevent convectional
currents from producing errone-ous results. SAR probes can still
be calibrated by this method.
Several key developments have been made in SAR test methods.
Manufacturers are required to use a new head phantom called
the specific anthropomorphic mannequin (SAM) phantom. SAM is
based on the 90th percentile of a survey of American male military
service personnel and represents a large male head. The SAM
phantom, which has human features (ears, nose, etc.), replaces
the featureless generic twin phantom. SAM has extremely well-defined
dimensions, particularly for parameters such as phantom shell
Fluid properties for SAR testing are now well defined. The methods
are also well defined for making and measuring fluids for the
most common frequencies used in testing. The IEEE P1528 specification
contains excellent references for fluid properties and methods.
It is essential to verify that fluid properties are within the
tolerances of the specifications.
Measurement uncertainties are defined in the specifications.
Overall measurement uncertainties must be below 30% for a 95%
confidence level. An uncertainty in measurements of 30% may
seem a bit high, but this percentage is small in decibel terms.
EN 50361 lists 21 individual uncertainty contributions.2
Depending on the setup, additional contributions may be required.
The new methods present a more pragmatic approach to handset testing,
reducing the number of positions required. Testing is performed
at the top, middle, and bottom channels of the DUT, but only at
the position of highest SAR at midfrequency. New methods have
a well-defined system-check requirement that must be performed
regularly. This system check indicates any drift in either the
properties (such as the fluids) or in the devices (such as the
SAR robot positional accuracy) used in the SAR testing. SAR robot
positional accuracy must be better than ±0.2 mm.
Most SAR probes now measure E-field in volts per meter (V/m1),
which allows SAR to be calculated. In addition to the E-field
present, SAR is also dependent on the conductivity and permittivity
of the tissue simulant. The equation used to calculate temperature-change
SAR relates directly to the one used in current measurements.
SAR probes must be physically small. They must also have good
spherical isotropy (i.e., measure equal amounts of E-field regardless
of the angle or direction that the probe points toward the radiation
source). In addition, SAR probes and their associated test setups
must be designed so that they have an insignificant effect on
the RF field.
For newer test methods, the probe is positioned at various points
within either a phantom head or body filled with an appropriate
tissue-simulant liquid. Head and body phantoms, in general,
can only represent the shape of the human body; they do not,
for example, mimic bone structure. Phantom heads have been produced
that mimic the tissue structure of a human head with skin, bone,
muscle, and brain tissue. However, these tissue phantoms are
not practical for SAR testing. The probe cannot be moved within
them, hence, the use of homogeneous phantom shells filled with
tissue-simulant liquids. The phantoms do not take into account
natural body thermoregulation by bloodflow; therefore, the rates
of temperature rise within the body deduced from SAR measurements
include a safety margin.
Because no known recipes for fluids are representative of body
tissue at all frequencies, different tissue simulant fluids
are required for different frequencies (e.g., 900 MHz for GSM
900 and 1800 MHz for 1800 products). The brain simulant must
be calibrated to ensure that the permittivity and conductivity
are correct for the frequency being tested. Fluids are often
made from a mixture of distilled water, sugar, and salt. Some
frequencies, however, require other chemicals to obtain the
SAR testing is performed on handset devices by placing them
at various positions on both sides of the phantom head. The
tip of the SAR probe is moved to exact points in a three-dimensional
grid within the tissue simulant. A complex mathematical formula
then calculates the volume-averaged SAR using extrapolation
and interpolation processes.
All current specifications require testing to be performed at
the maximum power of the device under test (DUT). The use of maximum
power is intended to represent the DUT's worst-case scenario.
However, depending on their location in relation to base stations,
mobile phones do not always transmit at maximum power. SAR probes
average the duty cycles for radio devices that do not transmit
continuously. For example, a GSM mobile phone transmits for only
about one-eighth of the time, so a SAR probe measures one-eighth
of the peak power from such devices.
Several groups have pushed recently to standardize test methods
for SAR testing, including uncertainty calculations. Although
new standards for measurement have been issued, the overall
SAR limits have not changed. CENELEC and IEEE have produced
similar specifications because the majority of people involved
in writing them were on both boards. The CENELEC standard, EN
50360, has recently been published in the Official Journal of
the European Communities as a harmonized standard. EN 50360
references EN 50361, which contains the test methods. SAR test
method specification IEEE P1528 is already in draft format and
should be due for release shortly.
In Europe, a key problem with the CENELEC standard is that it
is only concerned with devices held next to the human ear, that
is, handset testing next to a phantom head. EN 50360 is applicable
to all RF devices that are "to be used in close proximity to
the human ear."3 The standard does not contain the
actual limits. Actual limits can be found in either the ICNIRP
Guidelines (April 1998) or Council Recommendation 1999/519/EC
Annex II.4,5 EN 50360 applies to devices transmitting
with an average power greater than 20 mW and in the frequency
range of 300 MHz to 3 GHz.
Devices that transmit ¾20 mW are "deemed to comply with
the basic restrictions without testing." No standards have been
harmonized for devices other than those such as mobile phones
and cordless phones. However, manufacturers must still comply
with the EU SAR limits for devices such as PDAs that have an
integral RF module for GSM. Such devices are tested against
flat phantoms that simulate body parts.
In the United States, the limits and applicable products are
contained in Title 47 of the Code of Federal Regulations
47 CFR Part 2.1093, which covers portable devices with transmitters
within 20 cm of a user's body.6 It also includes
an applicability list that encompasses virtually all radio products,
depending on their output power. A full explanation of the relevant
parts, SAR limits, and SAR test methods is contained in FCC
OET Bulletin 65 Supplement C.7
A recent development in Australia has delayed plans for more-aggressive
SAR requirements. The Australian Communications Authority postponed
a proposal to extend the scope of SAR testing. That scope would
have included all radio products except emergency beacons. Test
methods have not yet been developed for implementing some of the
For the UK, the Stewart Report recommends that information on
SAR values for mobile phones should be readily accessible to
consumers at the point of sale. For example, the report recommends
that the information be printed on the product's box. The report
also suggests that stores provide leaflets with explanatory
and comparative information. Other recommendations include placing
the information on the phone's label, and making it available
via the phone's display. The report also recommends publishing
such data on a national Web site.
well-defined testing methods incorporate recent key developments.
the United States, the Cellular Telecommunication Industry Association
(CTIA) requires that any mobile phone it certifies be sold with
explanatory information. This information must confirm that
the phone has passed FCC safety standards. Manufacturers must
also include applicable SAR data for that phone and an explanation
of how the SAR testing was done.
The Mobile Manufacturers Forum (including Alcatel, Ericsson, Mitsubishi
Electric, Motorola, Nokia, Panasonic, Philips, Siemens, and Sony)
reports SAR values on its Web site (http://www.mmfai.org). The
site provides SAR information on all new models of mobile phones.
Information is also posted for existing models still in production.
Some devices are being marketed to protect users from RF or SAR,
but until formal test procedures are established and results are
published for these products, it is difficult to comment on their
effectiveness. One report found that hands-free kits may actually
increase SAR levels within the human brain, but the test methods
used for the report have fallen into question. These effects have
never been repeated.8 To the contrary, SAR test reports
from various test houses show that hands-free kits considerably
reduce SAR levels.
New developments in SAR testing can be expected as knowledge of
radiation effects increases. Improved standards and legislation
should follow. In Europe, standards are set to be adopted by CENELEC
that will cover products such as GSM base stations, antitheft
ports, and low-power radio devices. In the United States, FCC
has cautioned that further revisions to Supplement C can be anticipated
before it adopts draft standard IEEE P1528.
1. Draft Standard IEEE P1528, "Recommended Practice for
Determining the Peak Spatial-Average Specific Absorption Rate
(SAR) in the Human Body Due to Wireless Communications Devices:
Experimental Techniques," Institute of Electrical and Electronics
Engineers, New York, not yet published.
2. EN 50361, "Basic Standard for the Measurement of Specific
Absorption Rate Related to Exposure to Electromagnetic Fields
from Mobile Phones (300 MHz3 GHz)," European Committee
for Electrical Standardization (CENELEC), Brussels, 2001.
3. EN 50360, "Product Standard to Demonstrate the Compliance
of Mobile Phones with the Basic Restrictions Related to Human
Exposure to Electromagnetic Fields (300 MHz3GHz)," CENELEC,
4. Guidelines for Limiting Exposure to Time-Varying Electric,
Magnetic, and Electromagnetic Fields (up to 300 GHz), International
Commission on Non-Ionizing Radiation Protection (ICNIRP),
Munich, Germany, 1998.
5. Council Recommendation on the Limitation of Exposure
of the General Public to Electromagnetic Fields (0 Hz to 300
Official Journal of the European Communities, July 12,
6. Code of Federal Regulations 47 CFR Part 2.1093.
7. Supplement C to OET Bulletin 65 (Edition 9701),
"Evaluating Compliance with FCC Guidelines for Human Exposure
to Radio Frequency Electromagnetic Fields," FCC, Washington,
8. "Mobile Phone Safety," Consumers' Association "Which?"
Report, London, UK, January 1999.
Alex Miller is a SAR test engineer for TÜV Product Service
Ltd. (Hampshire, UK). For further information, e-mail info@tuvps.
co.uk or go to http://www.tuvps.co.uk.