|
RF Exposure: SAR
Standards and Test Methods
Part 2
Cell Tower
Life Bluetube Headsets
Cell Phone Towers Health Effects
EM Field Meter
Cell Phone Sensitivity
Test Methods
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 thickness.
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.
SAR Probes
Most SAR probes now measure E-field in volts per meter
(V/m¬1), 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 required properties.
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.
Standards
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 required
testing.
SAR Data
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.
In 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.
Protection Devices
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.
Conclusion
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.
RF Exposure: SAR Standards and Test
Methods Part 1
Nicaragua Managua
Malta Valletta
Libya Tripoli
Mexico Mexico City City
Fontana California USA
Greece Athens
Serbia Belgrade
Ukraine Kiev
Blue Mountains Australia
Palestinian State
http://www.emfnews.org/store |
