What is Specific Absorption Rate or SAR?

All the wireless devices emit radio waves which can be absorbed by the environment including our bodies. The measure of the rate of absorption, by which this RF energy is absorbed by our body while using a wireless device is called Specific Absorption Rate (SAR). The unit of SAR is Watts per Kilogram (W/Kg).

In our day-to-day life, we are constantly surrounded by different types of electromagnetic radiation (for example, light, Wi-Fi signals, microwaves, cellular signal, X-rays, etc.). A number of studies have concluded that RF wave exposure is harmful to our health and can lead issues like tissue heating or even damage to cells. SAR was introduced to limit the exposure to radiation since it is not possible to completely avoid using wireless devices. Technically, RF exposure assessment of all RF products is necessary to ensure that the products are safe to use. This assessment can be done by using SAR testing, EMF measurement, or RF exposure evaluation.

Usually, SAR is averaged either over the whole body or over a small sample (for instance, 1 g or 10 g of tissue). The SAR value can be calculated from the electric field (E) within the body tissue (while exposing to RF energy) using the following formula:

Where,

SAR = Specific Absorption Rate (in W/kg)

σ = Sample electricity conductivity (in S/m)

E = RMS electric field (in W)

ρ = Sample density (in kg/m3)

Calculate SAR using this online calculator

The estimation of the SAR value of the RF devices helps to understand their energy emission level. SAR testing is the estimation of the SAR value (i.e., the measure of the amount of RF energy absorbed by the body (using phantom) from an RF product that continuously transmits maximum power). Phantom is a human body model that simulates the RF absorption (electrical) characteristics of the human body or head.

Standards for SAR testing and SAR value:

SAR standards are developed by various institutions according to their specific requirements. The most common SAR standards have been developed by the Federal Communications Commission (FCC) for the US, European Union (EU), and Innovation, Science and Economic Development Canada (ISED). These standards describe regulations for SAR testing and SAR limits to limit the RF emission levels of wireless devices.

A wireless device can be introduced into the market if its maximum SAR value (max RF energy emission level) does not exceed the SAR limits described by the relevant SAR standards. Being certified by the FCC, EU or ISED means that the wireless device is safe to use (i.e., it will emit radiation that is lower than SAR limits mentioned by the corresponding authority of a country).

SAR exposure limits differ between the countries/regions, the following table provides mobile phones’ SAR limits of various countries.

The FCC (US) has set some guidelines to restrict the value of SAR below a particular level. It is mandatory for all wireless device manufacturers to follow these guidelines to get the FCC approval for their products. The FCC set SAR level for the general public is 1.6 watts per kilogram (1.6 W/kg) i.e., no mobile or wireless device must exceed the 1.6 W/kg SAR limit, in order to receive FCC certification. For example, the iPhone 6 has a Body SAR of 1.14 W/kg and Head SAR of 1.08 W/kg. The European Union has a slightly higher SAR limit as can be seen in the table above. Click here to see more SAR Exposure Limits by country.

What is SAR testing and why it is important?

Specific Absorption Rate (SAR) testing is necessary to ensure that the product’s safety and compliance are according to the standards. SAR testing can also be done during the product development stage so that it is easier to control and modify the products before they are finalized. SAR testing is a must for wireless devices that have radiating parts within 20cm from the human body or head. Usually, low-power devices have exceptions from this test. In Europe, the RF products that have a power level of 20mW or more must undergo the SAR test. In some countries, the SAR value of RF products needs to be displayed when the product comes into the market for sale.          

Testing equipment involved in the SAR testing:

SAR test inside a head phantom

SAR testing measures the heating effect on the human body due to the RF exposure by using a ‘phantom’ (that simulates the electrical characteristics of the human body or head). Phantom is a tub in standardized shapes (replicates human body model) and filled with liquids that simulate the RF absorption characteristics of human tissue.

Other equipment involved in the SAR testing include the high precision robotic system to measure and estimate the SAR value, robot controller to control the robot, a computer, near-field probe to measure RF level, and probe alignment sensor. 

How is a SAR test is conducted?

Let’s discuss the SAR testing of a mobile phone. Prior to starting the test, a system check is done to verify the SAR measurement system functionality. 

The DUT (device under test) is placed against the representation of the human head (a SAR phantom) in talk position. During the test, the mobile phone transmits maximum power continuously, and the robotic system scans the head or body phantom by using the near-field probe & sensor for the maximum radio signal. The computer system converts the measured radio signals to its corresponding SAR value and also records the maximum level. The mobile phone is tested for all frequency bands in which it operates and in various positions against the head or body that simulate how different mobile users hold the device while speaking.

Cheek position

Tilt position (15 degrees)

Typically, body testing is done with a 5mm distance between the phantom and mobile phone. Also, some countries (like France) require limb measurements as well. In case the device transmits multiple radio signals simultaneously, SAR measurements include the summation of all signals. The combined output also needs to meet the required SAR limit of 1.6-2.0 W/Kg.

The SAR measurement process differs between countries because of different standards. 4G devices require more testing because of the complexity of the radio access technology when compared to the 2G & 3G. SAR testing of 5G devices creates new challenges as it introduces multiple new features and technologies.   

After the SAR testing process, the measurement results are compared to the SAR limit requirements of the standard that applies to the various countries and then the SAR test report is prepared.    

SAR test report:

The SAR test report of a DUT contains the following information:

• Product information of the product (DUT) under the measurement.
• Results of the product being tested and all other measurement results.
• Scaled SAR results
• Conductivity and permittivity of used stimulant
• Uncertainty evaluation
• Images of measured product
• Plots of measurements
• List of used equipment and their calibration data. 

SAR testing- 5 things for companies to consider:

• SAR limits differ between the countries because of different standards
• Some countries require the SAR value of the products to be displayed when the product comes into the market for sale 
• SAR testing can be done during the product development stage
• Early SAR testing helps the manufacture modify the product to meet SAR limits.

Click here to see SAR Testing Equipment.

Click here to use the SAR Calculator on everything RF.