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Specific Absorption Rate (SAR) Testing for Wireless Devices
What is SAR Testing, and why is it important?
SAR (Specific Absorption Rate) testing measures the amount of radiofrequency (RF) energy that is absorbed by human tissue when a wireless device is operating close to the body. This testing is crucial to ensure that a device meets international safety standards, which protect users and facilitate market access in regions like Europe, the USA, Canada, Australia, and others.
Which devices typically require SAR testing?
Devices designed to operate close to the head or body, such as smartphones, tablets and wearable devices, usually require SAR testing if they use technologies which exceed specific power thresholds.
Typically, devices using low power technologies like Bluetooth Low Energy, Zigbee, ANT+… meet the exception limits and do not require SAR testing, even though they are used close to the head or body. On the other hand, devices using Wi-Fi, Cellular, or similar technologies will require SAR testing because they exceed the power threshold.
Is SAR testing always required for regulatory approval?
Yes, SAR testing or RF exposure evaluation is mandatory for many certifications, including CE, FCC, and ISED, especially for devices that operate close to the body and exceed certain power levels. Even low-power devices may need to demonstrate exemption through calculations or justification.
What are the regulatory SAR limits?
SAR limits are set by regulatory authorities in each country, defining the maximum RF energy that can be absorbed by human tissue. These limits vary based on averaging mass (1g vs. 10g), exposure scenario (head, body, or limbs), and transmission mode (single or simultaneous). The main SAR limits by region are as follows:
| Region/Country | Head/Body Limit | Limb Limit | Averaging Mass | Simultaneous Transmission |
| Europe (CE / RED) | 2.0 W/kg | 4.0 W/kg | 10 grams | The sum of the individual SAR value from all technologies that can transmit simultaneously must not exceed the applicable limit for each applicable exposure condition. |
| USA (FCC) | 1.6 W/kg | 4.0 W/kg | 1 gram / 10 grams (limbs) | |
| Canada (ISED) | 1.6 W/kg | 4.0 W/kg | 1 gram / 10 grams (limbs) | |
| Australia (RCM) | 2.0 W/kg | 4.0 W/kg | 10 grams | |
| Japan (MIC) | 2.0 W/kg | 4.0 W/kg | 10 grams | |
| South Korea (KC) | 1.6 W/kg | 4.0 W/kg | 1 gram | |
| Taiwan (NCC) | 2.0 W/kg | 4.0 W/kg | 10 grams | |
| China (SRRC) | 2.0 W/kg | 4.0 W/kg | 10 grams |
The head/body SAR limit applies when the device is used next to the ear (e.g., during a phone call) or close to the body (e.g., in a pocket or on a belt). The limb SAR limit applies when a device is used on extremities, like smartwatches or fitness trackers. The simultaneous transmission limit applies when multiple RF transmitters (e.g., Wi-Fi + LTE) are active simultaneously. Regulatory authorities require that the cumulative SAR from all active transmitters for each exposure condition (head, body, limbs) remain within the specified limit.
In summary:
- A limit of 1.6 W/kg over 1g (U.S., Canada, South Korea) is stricter than 2.0 W/kg over 10g (Europe, Australia, Japan, etc.) due to the smaller averaging mass.
- Limb limits are typically double the head/body limit because extremities are less sensitive.
- For simultaneous transmission, the SAR contributions from each antenna or transmitter must be combined, and the total must remain within the regulatory limit for each exposure condition.
How is SAR testing performed?
Testing involves using a human phantom filled with tissue-simulating liquid. The device is placed in realistic positions, such as next to the ear, the head, the body, or the wrist, while a robotic arm with a dosimetric probe measures the RF energy absorbed by the simulated tissue.
Can SAR be evaluated without physical testing?
In some cases, SAR can be estimated using simulation tools (e.g., FDTD modeling) or Fast SAR measurement systems. However, most authorities still require physical measurements and full SAR testing for final certification. DEKRA can guide manufacturers through the best approach based on their product features.
Can a desktop device require SAR testing?
Yes, while SAR testing is typically required for handheld or body-worn devices, desktop or stationary devices may also require SAR evaluation under certain conditions. If a device is intended to be used within 20 cm of the human body, operates at high RF power levels, or has an unusual installation configuration, SAR testing may be necessary. For example, a device placed on a nightstand with Wi-Fi or cellular connectivity that transmits continuously could expose a nearby user (like someone sleeping within 20 cm) to significant RF energy. In such cases, SAR assessment would be required to ensure user safety and regulatory compliance.
What’s the difference between SAR and RF Exposure evaluation?
RF exposure evaluation is a broad process used to ensure that a device complies with safety limits for human exposure to radiofrequency (RF) electromagnetic fields. SAR (Specific Absorption Rate) is just one method used in this evaluation.
RF exposure assessment involves various methods depending on the type of device, distance of use, frequency range, and applicable regulations. The main assessment methods include:
- Electric and Magnetic (E&H) Field Evaluation / Nerve Stimulation (NS): Primarily used for low-frequency exposures, where the applicable limits are based on electric and magnetic field strength.
- Specific Absorption Rate (SAR): Measures the rate at which RF energy is absorbed by human tissue, typically used for devices that operate close to the body (e.g., mobile phones, wearables).
- Absorbed Power Density (APD) and Incident Power Density (IPD): Applied to higher-frequency devices (above 6 GHz), where power density absorption is the primary concern.
- Maximum Permissible Exposure (MPE): A more general and conservative evaluation method used when a detailed measurement (like SAR) is not necessary, often for devices not used close to the body (more than 20 cm).
In summary, SAR is one specific technique within the broader RF exposure evaluation, which encompasses various methods depending on exposure conditions, operational frequency, and distance of use.
How long does SAR testing take?
The duration of SAR testing can vary based on several factors, including the type and shape of the device and its usage conditions (e.g., head or body), which determine the number of test positions required. Additionally, the wireless technologies or frequency bands it supports can impact testing time. Depending on the complexity of the device, testing can take anywhere from a few hours to several weeks.
Can DEKRA help determine if my device needs SAR testing?
Absolutely. DEKRA offers pre-evaluation services and regulatory reviews to clarify whether SAR testing is necessary. We will design the most efficient and cost-effective path to compliance for your products.