By reading this article, you will learn more about the SAR testing process of mobile phones & other mobile devices.
- Mobile SAR value – The basics
- What mobile radiation actually is?
- Why is it so important to test the mobile SAR value?
- Mobile SAR value – The testing equipment
- SAR testing process for mobile phones
- Additional information
Mobile SAR Value – 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.
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 mobile phone is a radio transmitter and receiver and therefore also needs to be tested for SAR. Mobile SAR value varies from device to device. Exposure from a mobile phone is localized depending on where the phone is placed and the guidelines for mobile phones 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).
What mobile radiation actually is?
The type of radiation emitted from mobile phones is electromagnetic radiation. Mobile phones produce electromagnetic radiation because they use radio frequency (RF) waves to make and receive calls. Mobile phones communicate by transmitting radio waves through a network of fixed antennas, which are called base stations. These radiofrequency waves cannot break chemical bonds and cannot cause ionization in the human body.
Electromagnetic radiation travels in waves and the length of these waves can be short or long. When we talk about mobile phones, the wavelengths are long (called radio waves). Radio waves can pass through walls while shorter wavelengths like visible light cannot. Also, radio waves have a low frequency.
Ionizing or non-ionizing?
Mobile phones emit non-ionizing radiation. Radiation is classified as non-ionizing or ionizing. Non-ionizing radiation (e. g. radio and microwave energy) has a longer wavelength, frequency, and energy.
Energy absorbed from non-ionizing radiation speeds up the movement of atoms and molecules. For example, this movement is responsible for heating up the food in the microwave.
Ionizing radiation has a short wavelength, high frequency, and higher energy. Examples of ionizing radiation are x-ray and gamma-ray. Radiation measure process depends on where the radiation is measured from. Typically, measurements can be made from the air or body simulant.
How distance affects mobile SAR value
Mobile phones are low-powered radiofrequency transmitters and they operate at frequencies between 450 MHz and 6 GHz and their peak powers are from 0.1 to 2 watts. Mobile phone transmits power when it is turned on and the SAR value falls when increasing the distance to the user. For example, when using a mobile phone 30-40 cm away from the body (text messaging or using the internet), the exposure for radiofrequency will be lower than when holding the mobile phone against one’s head and body. Also, the number and length of the calls will reduce the exposure. SAR exposure values are averaged over a six-minute time period.
When mobile phone is used in areas where the reception is good, the exposure decreases as it allows the mobile phone to transmit at reduced power. Later in this article, we will go through how mobile SAR value is measured and what else the process includes.
Why is it so important to test the mobile SAR value?
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 product development cycle. An early stage SAR testing allows manufacturer to control and modify product more easily.
In some countries, the SAR test results must be displayed when the product is put up to sale. For example, in France this will be required from 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.
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, average 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.
Mobile SAR value – The testing equipment
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.
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.
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.
3. Evaluation process & measurements
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.
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.
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
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:
- System Checks are performed to verify the SAR test system functionality
- Measurements of antenna input power i.e. conducted power measurements
- Measurement of SAR in different positions at different frequency areas are performed, including body and head measurements
- Summation of exposure from multiple simultaneously transmitting radios and evaluation of the results
- The test report
Additional information about mobile SAR value evaluation
In addition to mobile phones, other wireless devices also 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.