What is SAR Testing and Why is It Important?

Specific Absorption Rate (SAR) testing measures how much radio frequency (RF)

Energy the human body absorbs when exposed to a wireless device. Regulatory bodies like the FCC (Federal Communications Commission) in the U.S. and ICNIRP (International Commission on Non-Ionizing Radiation Protection) in Europe have set safety limits for SAR, ensuring that wireless devices do not expose users to harmful levels of RF energy.

SAR testing is particularly important for devices used close to the human body, where RF energy can be more concentrated. Devices that are intended to be held or worn within 20 cm of the body—such as mobile phones, wearables, and medical devices—must undergo SAR testing to ensure compliance with these safety limits. This testing is crucial for manufacturers to gain regulatory approval and to demonstrate their commitment to user safety.

Different Types of Devices and Pricing Considerations

Not all devices require SAR testing, but for those that do, the testing process varies depending on the device type and its complexity. Devices like smartphones, tablets, smartwatches, medical devices, and IoT gadgets have different SAR testing requirements based on their proximity to the body and the number of wireless technologies they use (such as  Bluetooth, Wi-Fi, 4G, and 5G).

• Mobile phones and tablets often require extensive SAR testing, as they typically support multiple frequency bands and are held very close to the body.
• Wearable devices, such as smartwatches or fitness trackers, also require SAR testing due to their constant proximity to the skin, though they tend to operate at lower power levels.
• Medical devices, particularly those that are implanted or worn on the body (e.g., wireless pacemakers or insulin pumps), undergo specialized SAR testing due to the sensitive nature of their use.
• IoT devices and routers generally require SAR testing only if they are intended to be used within 20 cm of the body. Devices placed farther away often undergo Maximum Permissible Exposure (MPE) testing instead, which has different criteria.

How Materials Affect RF Performance in SAR Testing

The materials used in a device’s design can significantly affect its SAR values. For example, devices made of plastic tend to perform better in SAR testing because plastic is non-conductive and does not significantly interact with RF energy. This allows the RF signal to pass through more predictably, minimizing its absorption by the body.

In contrast, metallic materials can reflect or concentrate RF energy, which can result in higher SAR values in certain areas of the device. The choice of material can also affect the radiation pattern of the device’s antenna, which directly impacts how RF energy is emitted and absorbed.

In SAR testing, phantom models are used to simulate human tissue, with materials designed to mimic the dielectric properties of the human body. These models ensure that SAR measurements reflect how the device will interact with actual human tissue, making material choice a key consideration in product design.

Timing Considerations to Avoid SAR Testing

While SAR testing is crucial for ensuring devices comply with RF exposure limits, there are certain operational timing factors that may help avoid it:

1. Low Duty Cycle Operation: Devices that transmit RF signals for less than 10% of the time (low duty cycle) may be exempt from SAR testing, as they expose users to RF energy for only brief periods.
2. Short Transmission Duration: Devices that transmit data in short bursts, like Bluetooth Low Energy (BLE) devices, may not require SAR testing due to minimal RF exposure during short transmission times.
3. Idle or Standby Modes: Devices spending long periods in idle or low-power modes with minimal RF activity, such as wearables or fitness trackers, may have reduced SAR testing requirements.
4. Power-On Time: Devices that transmit only when powered on or interacting with the user, like those syncing periodically, can avoid SAR testing during periods of inactivity.
5. Low Output Power: Devices operating at very low power levels (e.g., below 10 mW) may be exempt from SAR testing, depending on regulatory thresholds.
6. Antenna Duty Timing: Managing antenna transmission time can reduce RF exposure, helping to lower or eliminate SAR testing during idle periods.
7. Distance from the Body: SAR testing is typically required when devices operate within 20 cm of the body. Devices designed to transmit at greater distances may avoid SAR testing altogether.

Key Considerations for SAR Testing: A Summary

To successfully navigate SAR testing, manufacturers must keep several key factors in mind:

1. Device Type and Usage: Devices intended for use within 20 cm of the body, such as mobile phones, wearables, and medical devices, require SAR testing to meet regulatory safety limits.
2. Material Composition: The choice of materials, such as plastic versus metal, can affect how RF energy is absorbed by the body and impact SAR results.
3. Operational Behavior: By optimizing the timing factors, such as low duty cycle, short transmission bursts, or low output power, manufacturers may reduce or avoid SAR testing requirements.
4. Regulatory Compliance: Each region has its own SAR limits (e.g., FCC in the U.S. and ICNIRP in Europe), so manufacturers must ensure their products are tested according to the applicable guidelines.

However, SAR testing is still very much a case-by-case process, as each device's design, usage, and RF characteristics can affect the testing requirements. It’s always recommended to consult with SAR testing experts to get a better understanding of the specific needs and regulatory obligations for your device.