What Is SAR? Testing Methods and Latest Updates

The concept of Specific Absorption Rate (SAR) has existed for many years, and concern regarding Radio Frequency (RF) safety is not a new development. Most people are aware that microwave ovens can heat tissue (food) using RF energy, and it is this heating effect from other RF devices that causes concern from an RF safety perspective.

The human body resists local heating by thermoregulating through blood flow across the affected organs. The eyes and male testes are particularly susceptible to RF heating because they lack a direct blood supply and thus have no method to dissipate heat. The heating effect in biological tissue increases as the radio frequency increases, although the depth of penetration into the tissue is reduced. Microwave ovens operate at 2.4GHz, providing a balance between heating efficiency and tissue penetration depth.

Much of the RF safety concern focuses on RF absorption by the head, particularly from mobile handheld devices. RF exposure levels are related to exposure duration; maximum SAR is calculated on an average cycle over six minutes per 24-hour day. Many recent concerns have focused on effects other than heating; since the majority of communication systems are pulsed in nature, concerns have arisen regarding their influence on brain function, for example, the GSM frame rate at 8.33Hz, which is close to the characteristics of alpha waves in the brain.

See more: Capacity Profile Of Phuc Gia Laboratory Corporation

While conclusive evidence regarding these effects remains absent, significant research into Radio Frequency (RF) and its impacts is currently underway. This initiative largely originates from the report by the Independent Expert Group on Mobile Phones, chaired by Mr. William Stewart, published in April 2000.

Additionally, funding of approximately £7.4 million has been allocated by both the government and the industry for the Mobile Telecommunications and Health Research (MTHR) Programme (LINK) over a three-year period. The first call for research projects is complete, with these projects set to begin shortly, while the second call will target projects currently in implementation. Much of this research focuses on the actual biological effects of RF on the human body. At present, there is no widely documented research regarding RF effects on biological cells.

1. What Is SAR?

Specific Absorption Rate (SAR) is a quantitative metric indicating the rate at which biological tissue absorbs energy. SAR is expressed in watts per kilogram (W/kg) of biological tissue. It is typically cited as an average figure over a volume corresponding to 1g or 10g of body tissue.

The SAR of a wireless product is measured directly utilizing a body model, a robotic arm, and associated test equipment, or it is determined via mathematical modeling. Mathematical modeling of a product for SAR measurement can be highly costly and time-intensive, potentially lasting up to several months. Conversely, testing the SAR of a dual-band GSM 900 and GSM 1800 phone according to new standards using conventional methods takes approximately one day.

2. Origins of Current SAR Limits

The following organizations have established exposure limits for acceptable RF safety levels through the following SAR levels:

• The American National Standards Institute (ANSI), whose RF safety divisions currently operate as part of the Institute of Electrical and Electronics Engineers (IEEE), has recently authored one of the most critical publications for SAR testing methods;
• The International Commission on Non-Ionizing Radiation Protection (ICNIRP), established as an independent commission in May 1992;
• In the United Kingdom, the National Radiological Protection Board (NRPB).

SAR limits are established for two distinct groups of people: workers (occupational/controlled exposure) and the general population (uncontrolled exposure). The limits for the general population are considered ‘uncontrolled exposure’ and are five times stricter than the limits for workers.

Limits are defined for whole-body exposure, partial-body exposure (e.g., head and trunk), and for hands, feet, wrists, and ankles. SAR limits are based on a whole-body exposure level of 0.4 W/kg for workers and 0.08 W/kg for the general population. There is limited concern regarding exposure to hands, wrists, feet, and ankles, so the limits are less stringent. There are also significant issues regarding the practicality of measuring SAR in such body regions.

The majority of SAR tests relate to the head, where the current limit is 2 W/kg for SAR averaged over a 10g volume for Europe, and 1.6 W/kg for SAR averaged over a 1g volume for the US and certain other countries. The US limit is stricter because it is lower and the volume is averaged over a smaller mass. Australia, Canada, and New Zealand have adopted the stricter US limits of 1.6 W/kg for 1g averaged SAR, while Japan and Korea have adopted 2 W/kg for 10g averaged SAR as in Europe.

3. SAR Testing Methods

Initially, SAR testing was performed by measuring small temperature changes at specific locations within a tissue-simulating material. The tissue-simulating material for this test must be extremely viscous to prevent convective flow, which would generate erroneous results. SAR probes can still be calibrated using this method.

Most current SAR probes measure the electric field in volts per meter (V/m), allowing for the calculation of SAR. SAR depends on the conductivity and permittivity of the tissue-simulating material, as well as the current electric field. The equation used to calculate SAR from temperature change is directly related to the equation used in current measurements. SAR probes must be small in size and possess good spherical isotropy (meaning they measure the same amount of electric field regardless of the angle/direction they face the radiation source). The SAR probes and associated test setup must also be designed to have a negligible influence on the RF field.

The probe is placed at different points within a head model or body model filled with brain/tissue-simulating material. Head models and body models can only generally represent the human body; for example, they cannot mimic the bone structure. Head models have been produced to mimic the tissue structure of the human head with skin, bone, muscle, and brain tissue, but these simulated substances are impractical for SAR testing. Additionally, the body model does not account for natural body thermoregulation via blood flow, although this is advantageous as it provides an additional margin of safety.

Because there is no “formula” for a liquid that represents body tissue at all frequencies, different tissue-simulating liquid materials are required for different frequencies (e.g., 900MHz and 1800MHz for GSM 900 and GSM 1800 products). The brain-simulating material must be calibrated to ensure the correct permittivity and conductivity for the frequency being used. The liquid is typically made from a mixture of distilled water, sugar, and salt, although some frequencies require other chemicals to achieve the necessary properties. These include glycol, which unfortunately damages the plastic used in the SAR probe and the head model.

SAR testing is performed on handheld devices by positioning them at different locations on both sides of the head model, with the tip of the SAR probe being moved to precise points in a three-dimensional grid within the tissue-simulating material. A complex mathematical formula then calculates the volume-averaged SAR using extrapolation and interpolation processes.

Another important factor is that all current technical specifications require testing to be performed at the maximum power of the device under test, which represents the worst-case scenario. However, mobile phones in reality do not always transmit at maximum power, depending on their location relative to the base stations. The SAR probe averages the duty cycle of radio devices that do not transmit continuously. For example, a GSM mobile phone only transmits for about 1/8 of the time, so the SAR probe measures 1/8 of the peak power from such devices.

4. New Standards

The standards referenced in the table are primarily intended for mobile phones and similar devices; SAR standards for other transmitting devices are currently under preparation.

Read more: The State of Specific Absorption Rate (SAR) Management Worldwide

5. Developments in Testing Methods

The main developments in testing methods are:

• Manufacturers are required to use a new head model, the Specific Anthropomorphic Mannequin (SAM) Model, based on a survey of 90% of US servicemen, representing a large male head. The SAM model includes features (ears, nose, etc.) and replaces the featureless Generic Twin Phantom model.
• The properties of the liquid are now well-defined, as are the methods for creating the liquid and measuring them for the most common frequencies used in testing. IEEE 1528 Technical Document contains excellent reference guidance for this information.
• The overall measurement uncertainty must be below 30% for a 95% confidence level and is clearly defined in the technical specifications. An uncertainty of 30% in measurements may seem somewhat high, but anyone involved in calculating uncertainty for RF radiated emissions will recognize that this ratio is quite small compared to the decibel unit. There are 21 uncertainty factors listed in the technical document EN 50361, which may require additions depending on the setup.
• A more practical approach to testing handheld devices recommends reducing the number of required positions. Testing is performed at the highest, middle, and lowest channels of the device under test, but the highest SAR is typically found at the middle frequency.
• Routine testing must be performed regularly to detect any deviations in all characteristics (such as the liquid) and equipment (such as the positioning accuracy of the SAR robot) used in SAR testing.
• The positioning accuracy of the SAR robot must be better than ±0.2mm.

6. Applicability of Standards

In Europe, the main issue with the actual CENELEC standard is that it only pertains to devices held next to the human ear, which is the test for handheld devices near the head model. EN 50360 is applicable to all RF devices “used near the human ear.” It applies to devices that transmit with an average power greater than 20mW and within the frequency range of 300MHz to 3GHz. Devices transmitting at or below 20mW are “presumed to comply with the basic restrictions without testing.” This results in the practical reality that no other standards are suitable besides devices like mobile phones and cordless phones. Manufacturers must still adhere to the EU SAR limits for devices like PDAs with integrated RF modules for GSM. Such devices are tested against flat models, which are used to simulate “parts of the body.” EN 50360 does not contain the actual limits—these are found in the ICNIRP Guidelines (April 1998) or Annex II of Council Recommendation 1999/519/EC.

In the US, the required application references and limits are outlined in the Federal Communications Commission (FCC) Code of Federal Regulations 47 (CFR 47), Section 2.1093, for mobile devices with transmitters within 20cm of the user’s body. The full explanation of the relevant sections, SAR limits, and SAR testing methodology is contained in FCC OET Bulletin 65 Supplement C. CFR 47 Section 2.1093 has an almost comprehensive list of applicability for radio products, depending on their output power.

A recent development comes from the Australian Communications and Media Authority (ACMA), which has proposed expanding the scope of SAR testing to cover all radio products except emergency beacons. Again, there is an issue with the lack of testing methods available to perform some of the necessary tests.

7. Publication of SAR Data

The Stewart Report recommended that information regarding SAR values for mobile phones should be readily accessible to consumers at the point of sale, with information on the box, on a leaflet available in stores providing explanatory and comparative information, as an option on the phone’s screen, on a sticker on the phone, and on a national website.

In the US, the Cellular Telecommunications Industry Association (CTIA) announced that any mobile phone certified by them must be sold with explanatory information confirming that it has passed the FCC safety standards, and must include the Specific Absorption Rate data applicable to that phone, as well as an explanation of how SAR testing is performed.

Meanwhile, members of the Mobile Manufacturers Forum (including Alcatel, Ericsson, Mitsubishi Electric, Motorola, Nokia, Panasonic, Philips, Siemens, and Sony) committed to reporting SAR values globally. This includes providing SAR information on all new mobile phone models, as well as on existing models still in production, utilizing user manuals or in-box leaflets, and company websites.

8. SAR Protection Devices and Hands-Free Equipment

Some devices are being marketed as RF/SAR protective equipment, but until official testing procedures are established and results for these products are published, it is very difficult to comment on their benefit. There is one public report that suggests hands-free kits may actually increase SAR levels in the human brain, but the testing methods used for this report have been questioned, and these effects have never been replicated. There are numerous public SAR test reports from various researchers showing that hands-free kits significantly reduce SAR levels.

9. The Future of SAR Testing

It is predictable that SAR testing will evolve significantly as knowledge of radiation effects and legislation matures. For example, in Europe, new standards will be adopted by CENELEC to cover products such as GSM base stations, anti-theft gates, and low-power radio equipment. And in the US, the FCC has warned that Supplement C may be further revised before the draft standard is adopted by the IEEE.

Source: TUV Product Service Limited

On December 31, 2024, the Ministry of Information and Communications (MIC) (now merged with the Ministry of Science and Technology) issued QCVN 134:2024/BTTTT, the National Technical Regulation on Specific Absorption Rate for Handheld and Body-Worn Radio Equipment, with the following roadmap:

• Effective February 15, 2025: QCVN 134:2024/BTTTT shall be applied voluntarily in testing, conformity certification, and conformity declaration.
• Effective July 01, 2026: Terrestrial mobile phones must comply (mandatory compliance) with QCVN 134:2024/BTTTT before circulation on the market.
• Effective July 01, 2027: Products and goods under the scope of QCVN 134:2024/BTTTT (Cordless telephone equipment, DECT subscriber extension type – applicable to handset only; Laptops; Tablets) must comply (mandatory compliance) with this regulation before circulation on the market.

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