What´s hot in SAR?
Alex Miller and Vina Kerai, TUV Product Service/BABT
In the past, SAR testing has predominantly been performed on mobile phone devices, but with the ever-increasing need from telecommunication manufacturers and operators for SAR testing to be performed on a number of different type of devices - such as base station antennae, microwave equipment, private mobile radio and medical devices - more development of SAR standards, specifications and requirements will be underway.
Specific Absorption Rate (SAR) testing is a requirement for a wide range of standalone, independently-operated, portable transmitters and radiating structures which can be operated within 20cm proximity to the human head or body, and transmit a minimum of 20mW of radio frequency (RF) power. SAR, an index quantifying the rate of absorption of energy in biological tissue, is expressed in watts per kilogram (W/kg). SAR is generally quoted as a figure averaged over a volume corresponding to 1 gram or 10 gram of body or brain tissue equivalent material (TEM).
An increase in mobile telecommunications devices being used in close proximity to the human body has led to mobile device manufacturers and network operators, including T-Mobile, having devices assessed for the European market. In the absence of a specific test standard for body measurements in Europe, the test methodologies of FCC OET Bulletin 65 Supplement C Edition 2001 have been used, with a SAR limit of 2.0W/kg applied.
In many areas around the world, including Australia, Canada, China, Japan, USA and Europe, regulations are in place to ensure that the permissible levels of exposure of the public to RF power are limited. Committees are continually working on developing existing SAR standards, and new standards are also slowly being released to reflect new scientific research developments in the area of SAR and RF as well as new technological devices coming out into the market (Table 1).
Table 1: Current SAR standards around the world
Newly released standards
European standard EN 50392:2004
EN 50392:2004: Generic Standard to demonstrate the compliance of electronic and electrical apparatus with the basic restrictions related to human exposure to electromagnetic fields (0Hz - 300GHz). The object of this standard, which was released in January 2004, is to demonstrate the compliance of electronic and electrical apparatus for which no dedicated product, or product family, standard regarding human exposure to electromagnetic fields applies.
International standard IEC 62209-1
In February 2005, the International Electrotechnical Commission (IEC) published an international standard called IEC 62209-1: International Standard: 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 300MHz to 3GHz). The standard was developed for wireless communication devices to ensure they are compliant with internationally-recognised SAR limits. The standard harmonises different practices and methods used around the world, and specifies the methodology to use to test for SAR. However, it does not set the applicable SAR limits.
The IEC is also developing a new standard for hand-held and body-mounted devices which can be used in close proximity to the body. This standard, which is in the draft stage, is entitled IEC 62209-2: 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 30MHz to 6GHz Hand-held and Body-mounted Devices used in close proximity to the Body. This standard is planned to be released in the near future but no date has yet been fixed.1
Basic restrictions and reference levels
Restrictions on the effects of exposure are based on established health effects and are termed basic restrictions. Depending on frequency, the physical quantities used to specify the basic restrictions on exposure to electromagnetic fields (EMF) are current density, SAR, and power density. Protection against adverse health effects requires that these basic restrictions are not exceeded, and reference levels of exposure are provided for comparison with measured values of physical quantities (electric or magnetic field strength). Compliance with all reference levels given in the International Commission on Non-Ionizing Radiation Protection (ICNIRP) guidelines will ensure compliance with basic restrictions. If measured values are higher than reference levels, it does not necessarily follow that the basic restrictions have been exceeded, but a more detailed analysis is necessary to assess compliance with the basic restrictions.2
Hearing aid compatibility standard
Hearing aids commonly operate in two modes where acoustic signals are detected with a microphone and magnetic signals are detected with a telecoil (T-Coil). These modes are susceptible to interference from wireless devices. The FCC mandated that wireless handsets for sale in the USA are to be compatible with hearing aids. The FCC requires that digital wireless phones be capable of operating effectively with hearing aids based on performance measurement standards contained in the 2001 version of the ANSI standard ANSI C63.19: American National Standard for Methods of Measurement of Compatibility between Wireless Communication Devices and Hearing Aids. The standard, which applies to wireless devices and hearing aids, provides uniform methods of measurement and covers the frequency range 800MHz to 3000MHz.3 On April 13 2005, the ANSI Committee informed the FCC that it recently adopted and released a draft version of an updated hearing aid compatibility (HAC) standard, ANSI C63.19-2005 (PC63.19 (D3.6)).
The Office of Engineering and Technology (OET) has determined that applicants for HAC certification may rely on either the 2001 or draft 2005 version of ANSI C63.19. The updated standard incorporates a new nomenclature for specifying the radio frequency interference and inductive coupling ratings of wireless phones, which makes the rating easier for consumers to understand. The draft standard is expected to yield more consistent and reliable measurement results and allow the use of new measurement and rating procedures which should assist manufacturers and carriers in meeting the September 16 2005 deadline for providing handset models that comply with RF interference requirements of 47 C.F.R. 20.19(b). The draft standard is now in review stage by the IEEE.4
BABT SAR Mark
Research has shown that while considerable disquiet remains about the radiation effects of mobiles, retailers often lack the knowledge required to advise and reassure would-be purchasers. BABT launched its own Specific Absorption Rate (SAR) Approved mark in February 2004 to provide instant visibility and confidence to the market that mobile phones comply with recognised SAR standards.
Initially the scheme applies to mobile phones that comply with EN 50361, the standard developed for the European Union, but it will expand over time to cover other standards as well. A product with a BABT SAR Mark affixed complies with the current EU standard, and ongoing surveillance is carried out to ensure that the product continues to comply with the standard. The BABT SAR Mark includes the application of rigorous procedures to ensure its high credibility. SAR testing to EN 50361 by TUV Product Service/BABT is followed by a technical review by British Approvals Board for Telecommunications technical certifiers. To maintain a valid certificate, the manufacturer must seek BABT approval for any modifications to the equipment, ongoing evidence of test results must be demonstrated, and the manufacturing facility also needs to maintain a recognised BABT quality system certification. Once the mark has been awarded, the manufacturer can display it on its product, packaging, product related promotional material, and Web site.
Blue Angel SAR Mark
Germany’s Blue Angel, which provides environment-related labels for various products and services, introduced an environmental label for mobile phones in June 2002.5 The basic criteria for the award of the environmental label for mobile phones are that the mobile phone must be measured in accordance with EN 50361, and have a SAR value not exceeding 0.6W/kg averaged over a 10 gram average.6 Section 3.1.1 of RAL-UZ 106 in the Blue Angel Web site states that each applicant must submit a summary test report from an independent testing institute accredited according to EN ISO/IEC 17025 to make the measurements. The phones must also meet recycling requirements.
Some phone manufacturers are dismissing the Blue Angel label as ’inappropriate and nonsensical’ for different reasons as per reference.7 The highest permitted SAR level as stated in the current EN 50361 standard is 2.0W/kg for a 10 gram volume average. The limits, which are prescribed by the ICNIRP, have been adopted by national and international committees and are based on results of laboratory and epidemiological studies, basic exposure criteria, and reference levels for practical hazard assessment. The guidelines presented apply to occupational and public exposure.8 Tests for the SAR of mobile phones are conducted with the mobile phone transmitting at its highest certified power level in all tested frequency bands. The SAR limit of 2.0W/kg averaged over 10 grams of tissue equivalent material is set conservatively, and includes a safety factor correlating to a maximum temperature rise in the side of the head tissue less than 0.1°C.9
BASE STATION ANTENNA TESTING
There is currently an emerging need for SAR testing on small indoor base station antennae (Fig. 1) which may be accidentally touched by workers. Conditions for testing are similar to those required for mobile phones, where the antenna should transmit a minimum power of 20mW and be able to operate within 20cm proximity of the human head or body. Base station antennae are tested to obtain the compliance boundary and the ’Touch Safe’ level for each antenna at each operating band while maintaining compliance to current limits as defined in guidelines.
Fig. 1: T-Mobile Kathrein Omni-directional In-Building Antenna Module Type 738-454
The European standard EN 50383:2002 Basic standard for the calculation and measurement of electromagnetic field strength and SAR related to human exposure from radio base stations and fixed terminal stations for wireless telecommunications system (110MHz - 40GHz) outlines the calculation and measurement of EM field strength and SAR related to human exposure from radio base stations and fixed terminal stations for wireless telecommunications system operating within frequencies 110MHz and 40GHz. The standard specifies the method for assessment of compliance distances according to basic restrictions related to human exposure to RF EM fields. The standard does not, however, outline the method for Touch Safe assessment, or provide methods for whole-body SAR measurements which are performed if the conditions of section 7.1.2 of EN 50383:2002 are not met. The specification used for base station antenna testing is EN 50385:2002: Product standard to demonstrate the compliance of radio base stations and fixed terminal stations for wireless telecommunication systems with the basic restrictions or the reference levels related to human exposure to radio frequency electromagnetic fields (110MHz - 40GHz) - General public. For testing radio base stations or fixed terminal stations intended for occupational exposure to RF electromagnetic fields, the EN 50384:2002 Product standard to demonstrate the compliance of radio base stations and fixed terminal stations for wireless telecommunication systems with the basic restrictions or the reference levels related to human exposure to radio frequency electromagnetic fields (110MHz - 40GHz) - Occupational standard applies.
The following test description outlines the methods to determine the compliance boundary and the Touch Safe level for SAR testing for localised SAR measurement of antennae for microcell or picocell application. The EN 50383 standard is designed around large and complete base stations. The method described here is adapted to micro and pico application of base station antennae.
Determination of worst case frequency and location of maximum transmitted power
Prior to SAR testing, each antenna is connected to a 250mW continuous wave (CW) source and tested for its voltage standing wave ratio (VSWR) across the antenna transmission frequency ranges, and a 50 ohm termination is connected to unused port(s) of the antenna, if any, for impedance matching purposes. A closed loop calibration, with the signal generator set to the centre frequency of the base station, is required. In order to obtain a reference plot and to take into account insertion losses in the system, an open loop calibration is then performed. This involves applying a level of 250mW across the frequency range of the cable, and the reflected signal output is measured by a spectrum analyser. A specific frequency for maximum power transfer is noted for SAR testing with each antenna. At the appropriate frequency, and using a spectrum analyser with an RF sensitive probe, the antenna module is swept over to ascertain the point of maximum RF concentration.
The procedure is repeated with each antenna placed against a flat phantom filled with head tissue equivalent material. The dielectric properties of the tissue equivalent materials to be used for SAR testing are measured and checked to ensure they are in accordance with the requirements for the dielectric property targets specified in 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. The frequency range of the antenna is swept and, if necessary, the sweep is stopped and adjustments are made for the step size, number of steps or dwell time on the signal generator to give a continuous output on the spectrum analyser. The lowest point on the trace is the minimum reflected power, giving the frequency at which maximum power is transmitted. The SAR probe is positioned near the inner side of the phantom. The effective E-field, Eeff (V/m), in the medium is monitored while the position of the antenna module is adjusted until the position which gives rise to the maximum E-field reading is found. This position is marked on each antenna prior to SAR testing, and the orientation and positioning of the antenna is photographed and recorded to ensure repeatability. For each antenna, the frequency at which the antenna transmitted maximum power is used to carry out SAR testing.
For base station antennae, the procedure in the EN 50383:2002 standard for localised SAR measurement is valid for radiating structures which are less than 60cm by 30cm. The phantom must be made from material with dielectric properties similar to those of body tissue (skin, muscle and head) with a phantom thickness of 10mm or less. Section 18.104.22.168.2 of the standard also specifies a phantom size of 80cm long, 50cm wide and 20cm high (inner dimension). However, smaller phantom structures (Fig. 2) can be used for SAR testing of base station antennae given that the overall size of the antenna does not exceed the size of the phantom and the entire high energy area is measured. This can be monitored by observing the e-field distribution generated in the two-dimensional scan process. For certain larger base station antennae, if the active area is measured as significantly smaller, due to the nature of the frequencies being employed in the modules, and can be adequately measured with the same consideration to coverage of the entire SAR distribution, a phantom can be used which is smaller than that recommended in the standard. The SAR distribution, which is observed after an area scan (2D) process has been completed, should show a tapering-off effect of the e-field on the outer regions of the phantom shell. This provides confidence that the maximum SAR distribution is fully represented and measured, thus enabling a zoom scan to capture the highest RF energy area.
Fig. 2: IndexSAR Flat Phantom, showing the T-Mobile Kathrein Omni-directional Type 738-454 with antenna module in 80mm separation position from liquid surface
The radiation pattern for base station antennae mounted on towers can be likened to a thin pancake centred around the antenna system, while power decreases rapidly with movement away from the antenna.10 Fig. 3 shows a 2D scan of a base station antenna, where the blue regions are areas of very low (close to zero) SAR values.
Fig. 3: 2D SAR Area Scan; e-field fully measured with close to zero SAR distribution on the outer regions of the phantom
SAR measurement process
SAR testing is initially performed with each antenna connected to a 250mW CW source and placed against the side of a flat phantom with the point of maximum transmitted power, as previously determined, in the centre of the flat phantom. An essential requirement of the standard includes taking into account the phantom shell thickness, confirmed as 2.0mm for this measurement process. The SAR measurement process for base station antennae involves an area scan, to establish the maximum energy location, and a zoom scan centred at the maximum energy location to determine the volume averaged 10 gram SAR level. Once all possible hot spots have been measured, interpolation and extrapolation techniques are implemented to generate a 1 gram and 10 gram SAR average. This local SAR value is then multiplied by a safety margin correction factor of two as stated in the EN 50383:2002 standard in Section 22.214.171.124.7 and detailed in Clause 1 entitled General Considerations of the standard. The methodology for test is outlined in Section 7 of EN 50383.
Evaluation of the compliance boundary and Touch Safe limit
In order to evaluate the compliance boundary, successive SAR tests must be performed with the antenna in the same orientation and at the same power level, with the separation distance between the antenna and the liquid surface increasing by a set interval, for example by 20mm, 25mm or 30mm, depending on the antenna being tested. Thus, for an interval distance of 20mm, the antenna would be positioned 18mm away from the phantom outer surface, which accounts for the 2.0mm phantom wall thickness. In section 126.96.36.199.4 of EN 50383:2002, the recommended interval distance is 25mm, from zero mm from the phantom liquid surface to 400mm, which would allow linear interpolation. However, it has been shown that fewer intervals, and using third- or fourth-order polynomial extrapolation, will give accurate, if not improved, readings. This has been utilised in SAR interpolation and extrapolation post-processing techniques. The type of polynomial extrapolation that is used, for example third or fourth order, depends on the best fit for the data. From experience, the minimum number of intervals for test is four, while other factors such as the current distribution on the antenna, magnetic field coupling, far-field or near-field, input power level and the frequency under test, may require a larger or smaller number of intervals.
Testing conducted on a T-Mobile ’Kathrein’ Omni-directional Wide Band Antenna Type 738-454 necessitated an overall test coverage distance of 120mm when performing GSM 1800 SAR testing. Testing was conducted at five positions, and the data from the measurements used to extrapolate the Touch Safe maximum power limit to achieve a SAR value of 2.0W/kg at the phantom liquid surface - 0mm (Table 2).
Table 2: Data for the Kathrein Antenna Type 738-454 (Omni); Tested at 1829MHz (1800MHz Band Tuned Frequency against lossy liquid). Calculated ’Touch Safe’ value includes a correction factor of two in accordance with EN50383:2002 Clause 188.8.131.52. 7.
Chart 2 shows the measured 10 gram SAR values from Table 2 using the ’0mm’ SAR value found in Chart 1 with x set to zero in the third-order polynomial equation. From Table 2 it can be seen that from extrapolated results to the ’Touch’ position at 0mm, the maximum Touch Safe power level required to give 2.0W/kg was 112mW.
Chart 1: This graph shows how the 0mm separation logarithmic value was obtained. This log10 value of 0.9506 was extrapolated from plotted log10 values of recorded values at 20mm, 40mm, 60mm, 80mm and 120mm separation distances. Third-order polynomial extrapolation was performed to the liquid surface (0mm).
Chart 2: This graph shows the allowable distance (mm) against input power (W), i.e. for 2W input minimum separation for compliance is approximately equal to 44mm. Measured data does not include a correction factor of two in accordance with EN 50383:2002 Clause 184.108.40.206.7.
The compliance boundary of the antenna is determined by investigating the distance at which the SAR limit is ’safe.’ A sufficient number of measurements points were investigated in Table 2 to determine an accurate compliance boundary. The compliance boundary was deduced by scaling the results in Table 2 with a 2.0W input power. As shown in Chart 2, the minimum separation for compliance is approximately equal to 44mm for a 2.0W normalised input, and therefore the module has a compliance boundary limit of 44mm in the GSM 1800 band. Chart 3 shows the calculated 10 gram SAR (W/kg) for an input feed power of 2.0W.
Chart 3: This graph shows the calculated 10 gram SAR (W/kg) for an input feed power of 2W. Measured data does not include a correction factor of two in accordance with EN 50383:2002 Clause 220.127.116.11.7.
Because of the linear scaling quality of SAR with power, and assuming that all ’feed points’ are in the same location within the antenna (worst case), the total input power limit can be made up from any percentage contribution of the proposed input signals. For example, an antenna capable of transmission in GSM 1800 and UMTS would have 50 percent of the GSM 1800 maximum input limit plus 50 percent of the UMTS maximum input limit, which would give the total maximum input limit.
In line with SAR measurement theory, the SAR at any specific measurement point will scale linearly with input power (if nothing else changes) and this should be supported by measurements. Consequently, measured results can be used to infer the assessed SAR levels for different input powers in direct proportion to the level of the input power.
Improvements in SAR standards and test procedures are expected as a growing number of devices are manufactured which meet the current requirements for SAR testing, and with the growing understanding of radiation effects on humans. In the interim, in-house test methodologies can be used once there is scientific evidence of integrity in test methodologies, as well as maintained compliance with applicable SAR standards.
TUV Product Service/BABT, a leading total compliance services organisation, opened a new facility in March 2005, with two new shielded enclosures for SAR testing, enabling two tests to run in parallel. TUV Product Service/BABT also provides mobile device, EMC, safety and environmental test, certification and consultancy.
FCC: http://www.fcc.gov/cgb/sar/ and http://www.fcc.gov/oet/rfsafety/Welcome.html
Europa DG: http://europa.eu.int/comm/health/ph_determinants/environment/EMF/emf_en.htm
Europa Press Release: http://europa.eu.int/rapid/pressReleasesAction.do?reference=IP/01/1190&format=HTML&aged=0&language=EN&guiLanguage=en
Mobile Manufacturers Forum: http://www.mmfai.org/public/sar.cfm?lang=eng
1 International Electrotechnical Commission, URL: http://www.iec.ch
2 International Commission on Non-Ionizing Radiation Protection, URL: http://www.icnirp.de/documents/emfgdl.pdf
3 TCB Council Meeting on HAC and SAR Gerry Hayes, Technical Manager, Sony Ericsson Mobile Communications David Seabury, Business Development Manager, ETS-Lindgren
4 Federal Communications Commission News Media Information, URL: http://www.fcc.gov
5 Federal Ministry for the Environment, Nature Conservation and Nuclear Safety, URL: http://www.bmu.de/english/radiological_protection/pm/3423.php
6 Blue Angel, URL: http://www.blauer-engel.de/englisch/navigation/body_blauer_engel.htm
7 Powerwatch, URL: http://www.powerwatch.org.uk/news/news503.asp
8 International Commission on Non-Ionizing Radiation Protection, URL: http://www.icnirp.de
9 Australian Radiation Protection and Nuclear Safety Agency, URL: http://www.arpansa.gov.au/pubs/rfstand/rf_std_fct.pdf
10 US FDA of the Department of Health and Human Services, URL: http://www.fda.gov/cellphones/qa.html#8
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