Understanding SAR Testing - When is it Required?

Introduction:

As our dependence on wireless devices continues to grow, the complexities of those devices and their national regulatory requirements also grow.

Specific Absorption Rate (SAR) testing plays a crucial role in assessing and ensuring the safety of electronic devices that emit RF radiation. In this article, we will explore what SAR testing is, why it is important, and when it is required.

SAR refers to Specific Absorption Rate, which measures the rate at which the human body absorbs RF energy from a wireless device. SAR testing determines the amount of RF energy absorbed by the body, particularly in areas where the absorption is highest, such as the head and torso. The results are expressed in watts per kilogram (W/kg), representing the amount of RF energy absorbed per unit of body mass. For regulatory requirements the SAR levels are averaged over a 10g mass typically for European regulatory requirements and a 1g mass typically for North American regulatory requirements.

The primary concern associated with RF radiation exposure is the potential for thermal effects, where the absorbed energy can lead to an increase in body temperature. SAR testing helps establish whether a device emits RF energy at levels that may contribute to thermal effects.

Regulatory Compliance:

The simplest way to know if SAR testing is required is to ask “is the device to be used within 20cm of a user?”. If the answer to that question is “yes” and the output power of the device is generally above 20mW (for Europe for example) then you would most likely have to carry out some SAR testing.

SAR testing is a regulatory requirement in many countries to ensure that electronic devices comply with established safety standards. Different regions have specific SAR limits that devices must adhere to in order to be deemed safe for use. For example, the Federal Communications Commission (FCC) and Innovation, Science and Economic Development (ISED) in North America, and the European Union all have stringent SAR regulations.

When is SAR Testing Required?

Mobile Phones and Smartphones:

SAR testing is mandatory for mobile phones and smartphones. Given the proximity of these devices to the user's body, especially during calls, SAR testing ensures that the emitted RF energy is within safe limits. Mobile phones need to be considered for all their various user cases; for example, they are typically used against the head and the body. The regulatory requirement for these conditions may be different. The devices may also operate at different output power levels for those use cases. This needs to be considered both during the testing and in defining a test plan, as testing may need to be repeated for each use case. Body testing has become every bit as important as head testing particularly following the introduction of “always on” data services. These technologies can be classed as an un-attended service meaning that the device can be placed anywhere about the body with the user in most cases being unaware of the transmit status of the device. The device could be doing an update or sending a large file or additionally being used by a third party as a wifi hotspot, for example. Following the introduction of this technology the amount of regulatory testing that is required increased significantly and the allowable separation distance between the device and the body reduced significantly. These changes made the SAR testing of mobile handsets much more demanding. Download your copy of our guide to the challenges of SAR testing in modern mobile phones.

Wireless Consumer Products:

Devices like tablets, smartwatches, and other wireless communication devices that are used in close proximity to the body are subject to SAR testing. This includes devices with wifi, Bluetooth, and cellular connectivity. Some of the devices in this category can be used in the hands or on limbs. This might allow those devices a slightly relaxed SAR test limit. This limit is  because limbs and extremities do not contain any major organs and are primarily made up of skin, bone, and muscles. Once again though it’s important to consider all of the use cases. For the example of a smartwatch, this could be classed as limb use only but if you can speak into the smartwatch then you must consider the proximity of the user’s  face to the device when speaking into it. This front of face condition could result in a head SAR use case condition with a separation distance of 10mm. This would need to be tested at this separation distance to the more stringent head (and body) SAR limits as well as testing the limb conditions.

Medical Devices:

Certain medical devices that use wireless technology, such as wearable health monitors or implantable devices, may also require SAR testing to national regulatory requirements.

Wireless Infrastructure:

In addition to individual devices, SAR testing is sometimes applicable to wireless infrastructure components like antennas and base stations to ensure that the overall RF exposure levels are within acceptable limits. As the use of small (Pico-cell) office-based base station infrastructure technologies is becoming more widespread, how they are installed in the workplace must be considered. In the case of a base station, it’s often the device's compliance distance which is important. If the minimum distance a user can get to a device and still meet the regulatory requirements is known, then the placement of that device in the workplace can be easily assessed. If the base station is compliant at 0mm separation distance, then it can effectively be placed anywhere for example. Alternatively, If the separation distance is relatively large then this could be mitigated by specifying a minimum height at which the base station could be mounted on an office wall.

Further Considerations

As can be seen from the various descriptions above, it’s vitally important to understand and define how a product will be used to understand when and how SAR tests will be applied.

Another key requirement is to understand and apply all “reasonably foreseeable use” conditions. Examples of this would be for something like a mobile phone where a manufacturer has stated that it’s used in a body condition at a separation distance of 15mm when in reality most users are placing the device in their pocket. In this case it is the pocket use case at 0mm to 5mm that must be assessed. Similarly, if a device is classified as only for use in the hands when it can be placed in a coat or hoodie type garment pocket while it is transmitting, then body conditions would need to be applied also.

The other conditions that are important to consider is whether the device under test is for public use or for occupational use.

If a device is for occupational use the SAR limits are less stringent. The reason behind this is that professional users can be given training on the operation of their equipment and on how to limit their exposure to the RF radiation from those devices. This would result in an awareness of the equipment with regards to what it is doing and how to use it.

If a device is classed as occupational use, it may have been designed to the limits associated with occupational use. Manufacturers need to be sure in this case that the device cannot be easily obtained by the general public. If this is the case, then the device must be tested to the general public limits. This could be problematic for a product that is designed to the occupational limits and would suggest that the device would not be compliant for the general public.

An issue like the one described above would be particularly difficult to deal with as SAR testing is a physical test that must be carried out on hardware that is representative of that which is to be placed on the market. Any issues that need to be fixed at the end of a product development cycle would be far more costly to rectify than if found earlier in the development cycle. 

In most cases a fix for SAR compliance issues is to turn down the output power of a product. This will affect the performance of a device in a negative way. It is therefore important to understand when a device that is being developed needs to be SAR tested, and ensure that a practical and realistic SAR test programme has been thought out, and carried out as soon as possible in a products’ development cycle.

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