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Specific absorption rate (SAR) Testing – 5 things for companies to consider

Does your product require SAR testing? Knowing the answer to this question in early stages of product development helps you with certification process and getting your device to market as quickly as possible. Surprisingly, many wireless devices require SAR testing.

By Oona | on 19.08.2020 at 14:35 GMT+0200. | Modified 27.08.2020 at 14:59 GMT+0200.

Specific absorption rate (SAR) Testing – 5 things for companies to consider

By reading this article, you will learn more about the SAR testing process of any wireless devices.

  1. SAR testing – The basics
  2. Why is it so important to test SAR?
  3. SAR testing – 5 things for companies to consider
  4. SAR testing equipment
  5. SAR testing process
  6. Additional information
  7. References

SAR testing – The basics

In our everyday lives, we are surrounded by different types of electromagnetic radiation (e. g. light, microwaves, x-rays, or mobile phone transmissions). Therefore, all radio products need RF exposure assessment to make sure they are safe to use. This assessment can be done by RF exposure evaluation, EMF measurements, or SAR testing.

Specific Absorption Rate (SAR) measures the amount of RF energy absorbed by a human or animal body. SAR testing must be done on all wireless devices that have radiating parts closer than 20 cm to the human body or head.

Mobile phone next to SAR testing phantom

Devices with low power make an exception. In Europe, all devices with the power of 20 mW or higher, need their SAR value to be evaluated. In Canada and the USA, whether the device’s SAR value needs to be evaluated depends on the distance between the user and the device, the power, and the frequency, in which the product radiates. The wireless device is a radio transmitter and receiver and therefore also needs to be tested for SAR. SAR value limits and guidelines vary from device to device. Exposure from a wireless device is localized depending on where the device is placed, and the guidelines specify the maximum level of Rf energy that can be absorbed by the head or body with a safety margin. SAR is expressed in units of watts per kilogram (W/kg). 

Why is it so important to test SAR?

SAR testing needs to be done to make sure that the product’s safety and compliance requirements are met. Different countries have introduced standards that limit the amount of RF exposure allowed from a variety of wireless devices. Avoiding natural and manmade sources of radiation is impossible. With SAR testing the amount of RF exposure from a mobile phone or any other wireless device can be measured. Also, SAR testing can be done in all stages during the product development cycle. An early-stage SAR testing allows the manufacturer to control and modify products more easily.

Expert mixing tissue simulating liquids
Verkotan’s SAR test laboratory

In some countries, the SAR test results must be displayed when the product is put up to the sale. For example, in France, this will be required from the 1st of July onwards. Manufacturers and retailers need to demonstrate that the products they are providing, meet the SAR requirements stated by the country or market.

The SAR measurement standards usually require that the product manufacturers include the most severe and worst-case operating conditions during the tests for all the frequency bands, then passing the requirements means that the device will never exceed the maximum levels of consumer RF exposure. It is good to remember that this does not indicate the normal use of the device.  

Device under mobile SAR testing
Mobile phone’s radiation during testing

SAR exposure limits differ between countries

Exposure limits of SAR testing vary between different countries. Authorities such as the Federal Communications Commission (FCC), European Union (EU) and Innovation, Science and Economic Development Canada (ISED) have regulations for SAR testing and SAR limits. In the United States and Canada, the mobile phone must meet a SAR limit of 1.6 watts per kilogram, averaged measured from 1 g tissue volume. This means that to receive FCC or ISED certification and be sold in the markets, mobile phones’ maximum SAR level must not exceed this level. In the European Union, to receive CE certification, mobile phones’ maximum SAR limit is 2.0 W/kg average measured from 10 g tissue volume.

Different countries and different certifications require different reports. In general, in Canada, the requirements are the most precise. Canada’s requirements also include requirements from CE and FCC certification and has some of its own, this makes it a little bit more challenging.

FCC, ISED and CE SAR limits

SAR testing – 5 things for companies to consider

  • SAR limits vary between countries and certifications 
  • Different countries and certifications require different reports 
  • In some countries, SAR test results must be displayed when the product is put up to the sale 
  • SAR testing can be done in all stages during the product development cycle 
  • An early-stage SAR testing allows the manufacturer to modify product more easily 

Also, regulations are constantly being updated so working with a certified test lab who is aware of current SAR regulations is recommended. For example, France has recently updated their SAR regulations, you can read more here. 

SAR testing equipment 

In this article, we introduce mobile phone’s SAR testing process. SAR testing process and used equipment vary depending on the tested device.   

SAR testing equipment  and set up

SAR testing equipment uses standardized models of the human head and body. These models are then filled with liquids that simulate the RF absorption characteristics of human tissue. Verkotan is providing these tissue simulating liquids as well.

employee mixing tissue simulating liquid

For the mobile SAR value evaluation process, Verkotan uses Dasy52 near field scanning system, manufactured by SPEAG. Also, the test system consists of a high precision robotics system (Staubli), robot controller, computer, near-field probe, probe alignment sensor, and a phantom containing the tissue-equivalent material.

Verkotan uses SAM Phantom, which has the left and right head section and the flat phantom section. This phantom conforms to all known tissue simulating liquids. Phantom simulates the electrical characteristics of the human head and body and the energy penetrating the phantom is monitored.

SAR testing process for mobile phones 

1. The system check

Mobile phone SAR testing process begins with a system check that is made to verify the test system functionality. In a system check, a certain amount of power is applied to the dipole and the results are verified to be as they are supposed to be.

SAR testing system check with dipole
The dipole and body test phantom

2. Conducted power measures

The next step is measuring the conducted power. This means that the real power going to the antenna is measured and then the measured SAR results can be scaled to correspond to the worst-case power, that the phone could ever transmit.

measurement of the conducted power
Measuring the conductive power

3. Evaluation process & measurements

permittivity and conductivity of the simulating liquid must be same as human tissue
Measuring permittivity and conductivity

Then starts the mobile SAR value evaluation process and measurements. SAR is measured from the simulating liquid. It is important that dielectric properties of tissue-equivalent liquid are comparable to the brain or body tissue. Permittivity and conductivity need to be the same as in the brain or body tissue.

SAR measurement processes vary between countries because of different standards. At the same time, different frequency areas are used. 4G devices require more testing due to the complexity of the radio access technology compared to 2G and 3G. Also, SAR testing for 5G devices creates new challenges.

Products are tested while operating the product with its highest power levels in all frequency bands in which it operates, and the product is tested in various positions against the head or body to simulate how different users typically hold the device. When evaluating mobile SAR value, different positions for head and body needs to be tested. For example, a cheek position, where a mobile phone is placed under the phantom in the way that the middle part of the acoustic output touches the center of ear and cheek at the same time. Or tilt position, where the angle is increased by 15 degrees from the cheek position. These positions are tested for left and right sides of the head. Also, other positions are for example front side, back side, top side and bottom side of the device towards the phantom.

Measuring mobile phone radiation at cheek position
Cheek position
Measuring mobile phone radiation at tilt position
Tilt position

Because we hold the phone in the pockets, SAR body test measurements are required. A big portion of the measurements provided by Verkotan are body measurements. Body testing is typically done with a 5mm distance between mobile phones and phantom. Also, some countries for example France requires limb measurements, the maximum SAR limit for limb is 4.0 W/kg average measured from 10 g tissue volume.

Measuring mobile radiation absorbed to body
Body test

4. Exposure from multiple sources simultaneously

One interesting point is that if multiple radio signals are transmitted from one device at the same time, SAR measures of the signals are summed. This combined output still needs to meet the required SAR limit of 1.6-2.0W/kg.  

The mobile phone uses a certain frequency bands that contain many frequencies where the mobile phone can communicate. International standards require SAR testing for the high, center, and low frequencies for the bands that the mobile phone works for. When SAR measurements are performed, the results are compared to the requirements. Finally, the test report is issued based on the measurements made.

5. SAR test report

Employee writing SAR test report

The information included in the test report differs depending on the geographical location of the place where the product is to be sold. When the testing is done, Verkotan provides a test report. The test report includes:

  • product information of the product under the measurement
  • results of the system check and all other measurement results
  • scaled SAR results
  • conductivity and permittivity of used simulant
  • uncertainty evaluation
  • images of the measured product
  • plots of measurements
  • list of equipment used and their calibration data

The declaration of conformance is also provided in the test report. This means that the results pass requirements.

The whole process in a nutshell:

  1. System Checks are performed to verify the SAR test system functionality  
  2. Measurements of antenna input power i.e. conducted power measurements  
  3. Measurement of SAR in different positions at different frequency areas are performed, including body and head measurements  
  4. Summation of exposure from multiple simultaneously transmitting radios and evaluation of the results  
  5. The test report

Download the Example SAR Test Report

Additional information about SAR testing

Many wireless devices need SAR testing. It is surprising how many devices are used closer than 20cm to the human head or body and therefore need testing.

The test process itself is straight forward but what makes the process challenging is the number of different products in these days. Also, the process requires great knowledge and accuracy to be able to follow the standards.  When you do not know the standards well, it is better to let the knowledgeable laboratory to take care of the measurements, and you can focus on what is most relevant.

References