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Low Frequency SAR testing

Many consumer products like mobile phone and WIFI products work in radio frequencies above 700MHz. However, there are many products that operate at low radio frequencies under 700MHz. Do you know how these low frequency devices are tested? Continue reading to find out how Low Frequency SAR testing is done step by step.

By Riina | on 23.03.2021 at 12:58 GMT+0200.

Low Frequency SAR testing

By reading this article, you will learn the process of Low frequency SAR testing.

1. What is Low Frequency SAR testing?
2. What kinds of devices use low frequencies?
3. SAR Testing regulation and guidance
4. Laboratory requirements
4.1. Tissue simulating liquid dielectric parameter check
4.2. Test system check
4.3. SAR-measurement
5. Summary
6. References

SAR, Specific Absorption Rate, testing is done for the radio transmitting devices that operate above 4MHz frequency and are used closer than 20 cm to the human head or body. SAR is a measure that explains how much radio frequency energy is absorbed by the user’s or bystander’s body or head. At Verkotan we are able to test SAR for radio transmitters transmitting from 13MHz to 6GHz. This covers consumer, medical, healthcare and military products.

What is Low Frequency SAR testing?

Many consumer products like mobile phones work in radio frequencies above 700MHz. There is also a growing range of products working at low radio frequencies between 13MHz and 700MHz. At Verkotan we have the capability to do accredited SAR testing also for radio frequencies from 13MHz to 700MHz. In this context, low frequency range covers frequencies from 3MHz to 700MHz which is located at HF (high frequency), VHF(very high frequency) and UHF (Ultra high frequency) radio spectrum.

Low frequency range
Low Frequency Range

Devices working at HF, VHF and UHF frequencies under 700MHz are for example amateur radios, ppt radios, cordless telephones, walkie-talkies, military devices and Marine radio. Also, two-way land mobile radio systems, industrial remote controls, IoT devices, RFID tags and readers and wireless microphones are working under 700MHz.

The advantage for using these frequencies are wide coverage, improved voice, data and video services with broadband. These frequency ranges also support Cat-M and Narrowband-IoT devices.

What kinds of devices use low frequencies?

Picture below demonstrates different types of devices that work between 3 kHz and 300 GHz frequencies and between 100 km and 1 mm wavelengths. In this article, we will introduce HF, VHF, UHF frequencies and 450 MHz alliance in more detail.

Low Frequency range

HF frequency

Example of technology working in HF, 13.56 MHz frequency, is RFID. There are RFID readers that are transmitting on 13.56 MHz frequency in order to read RFID tags. RFID tags at 13.56 MHz frequency, called HF tags, use are quite inexpensive, making the utilization of the frequency tags cost effective. The uses of the RFID tags can be for example access control, patient tracking and credit cards. HF tags are currently the most wildly used tags.

Active RF components, like RF readers, need to have RF exposure evaluation, which depending on device and transmission is SAR testing or other methods of evaluation.

VHF and UHF Frequencies

As an example, amateur radio uses universally 50-54MHz and 144-146MHz frequencies. Other frequency allocations can vary country by country for example Maritime Mobiles are using fixed 130-135.7MHz frequency in Australia and in United States Marine radio is using 156-158MHz and Maritime mobile in 216-222MHz. Walkie-talkies can operate on 136 MHz to 900 MHz frequency range. In practice, the operating range is divided into smaller sections like 136-174 MHz and 400-512 MHz rather than the whole 766MHz bandwidth in one device.

The lower frequency the walkie-talkie uses, the longer the radio waves are. Then the device is able to cover longer distances with less output power which works well outdoors. For indoor use, the higher frequency like 400-512MHz is better as shorter waves travel through obstacles better.

UHF Frequency and 450MHz alliance

To reinforce the usage of 450 MHz band, the 450MHz alliance has been established. The 450MHz alliance members consists of wireless carriers and equipment manufacturers. The task of the 450MHz alliance is to promote allocation of the 450 MHz spectrum worldwide. According the 450MHz alliance prediction the evolvement toward LTE of the spectrum in 380 MHz to 512 MHz is expected to increase. Specially NB-IoT and LTE-M usage in 450MHz band is expected to grow. The devices at this frequency range consist for instance of handheld mobiles, routers, modules, utility meters, security monitoring, location tracking, medical metering and wearable devices. LTE bands operating in 400 range are LTE 31, 71, 72. LTE Bands 87 and 88 (410-430 MHz) were accepted by 3GPP in June, 2019.

SAR Testing regulation and guidance

Accredited SAR testing is guided by standards and regulations set by authorities of the country the product is going to be sold and used. Standards like EN 50566, Product standard to demonstrate the compliance of wireless communication devices with the basic restrictions and exposure limit values related to human exposure to electromagnetic fields in the frequency range from 30 MHz to 6 GHz: hand-held and body mounted devices in close proximity to the human body, for EU. For US markets, 447498 D01 General RF Exposure Guidance v06: Rf exposure procedures and equipment authorization policies for mobile and portable devices, state the requirements for the SAR testing.

In October 2020, IEC published an updated combination standard of two standards; IEC/IEEE 62209-1528: Measurement procedure for the assessment of specific absorption rate of human exposure to radio frequency fields from hand-held and body-mounted wireless communication devices – Part 1528: Human models, instrumentation, and procedures (Frequency range of 4 MHz to 10 GHz). As stated in that standard, SAR is a valid method of evaluating RF exposure from 4MHz. Previously the lowest applicable frequency was 30MHz as per IEC 62209-1 and 300MHz as per IEEE 1528. The extension of the applicability range to 4MHz means that more products can be assessed for compliance by SAR testing.

The SAR value limits are set by authorities and the product that is tested must be less than or equal to the limit in order to be compliant. The SAR limit varies in different countries, being 2.0W/kg for EU and 1.6W/kg for US market for general public.

Laboratory requirements

From the SAR testing point of view, lower frequency SAR testing has its own demands; special verification antennas, special simulation liquids and phantoms are required. The accredited SAR testing requires the test system to be calibrated and validated. The testing consists of following parts;

  • Tissue simulating liquid dielectric parameter check
  • Test system check
  • SAR measurement

1. Tissue simulating liquid dielectric parameter check

For SAR testing, the phantoms are filled with tissue simulating liquid that has similar electric parameters of either head tissue or muscle tissue. The conductivity and permittivity of the tissue are dependent on the frequency. Similarly, the simulant liquid has electric parameters that alter with frequency. The conductivity and the permittivity of the tissue simulant are measured prior the SAR testing. The testing of the dielectric parameters is done by using Dielectric Assessment Kit, DAK and vector network analyzer.  If the dielectric parameters are not within specified limits of the target values specified in standards, the liquid is adjusted by adding water or other ingredients to meet the required specifications.

In Verkotan we have the capability to produce tissue simulating liquids from 30MHz to 6GHz. We offer head simulating liquids, body tissue simulating liquids, simulants for non-standard testing, and for research purposes.

ELI phantom filled with tissue simulating liquid
ELI phantom filled with tissue simulating liquid

2. Test system check

The system check is required to verify the measurement setup before the SAR testing of the device can be done. The purpose of the system check is verifying that a SAR measurement system operates within its specifications at the test frequencies. With system check the measurement system is detected for not having any test system component failures or drifts or any unfavorable conditions in environment such as RF interference.

The system check is done by feeding known signal to a dipole antenna or another standardized antenna placed below the phantom and by measuring the SAR value. The measured SAR value is compared to numerical target values and it should not deviate from the target value by more than ±10%.

The verification of the measurement system is done with dipoles up to 300MHz and with confined loop antennas (CLA) below 300MHz.

As the radio frequency gets lower the wavelength gets longer. For example, the wavelengths are at 1800MHz 0.17m, 700MHz is 0.43m, at 450 MHz is 0.67m, at 150MHz is 2.0m and at 30MHz is 10.0m.

Verification antennas

The length of the dipole is in relation to the frequency, the lower the frequency, the longer the dipole. The length of the dipole would be very long at frequencies below 300MHz. Thus, the CLA antennas are more practical in day-to-day use for low frequencies. The structure of the dipole is symmetrical with λ/4 balun and resonant loop antenna for CLA antennas.

In Verkotan we have 13MHz, 30 MHz, 64 MHz, 128 MHz, 150 MHz, 220 MHz CLA antennas. We also have dipoles for 300 MHz and 450 MHz, covering frequencies from 13MHz to 550MHz for which we can perform accredited SAR testing for products working in MF, VF and UF ranges. Verkotan has also the capability to do accredited SAR testing from 1350MHz to 5860MHz frequency range.

CLA antennas at Verkotan
CLA antennas at Verkotan
300MHz and 450MHz dipoles
300MHz and 450MHz dipoles

Phantoms for Low frequency testing

SAR testing
Dasy5 testing system & Base station child, BSTP-C phantom

The low frequencies, 13 MHz to 700 MHz, and long wavelength set also special specifications to phantom’s size that can be used for testing. The requirement for the flat phantom size is that the phantom’s flat bottom must fit an ellipse with length 0,6 λ and width 0,4 λ, where λ is the wavelength in air. For body SAR testing Verkotan have three different phantoms that can be used at these frequencies they are: ELI; elliptical phantom, Base station phantom adult; BSTP-A and Base station phantom child; BSTP-C. For against the head testing, the SAM phantom is used.

ELI phantom
ELI phantom

3. SAR measurement

SAR testing positions for a product depend on the product and how it is used while the transmission is on. If the product can be used next to the head, then head SAR is needed.

Head SAR
Head SAR is measured by placing the product under the SAM phantom. SAM (Specific Anthropomorphic Mannequin) phantom is a standardized phantom. The phantom has right and left sides of the head. Testing is done in two standardized positions. Cheek position, which has device touching phantom at the ear and cheek and tilt position, which is a 15° tilt away from the cheek while the device touches ear. For compliance, the tested SAR results of the device has to meet the exposure limits set by authorities.

Body SAR
A device that can transmit within 20cm of the body, such as lone worker alarms, body SAR needs to be tested. The body SAR is tested by placing the device under the flat phantom with specified separation distance from the phantom. There are requirements for the flat phantom size for the frequencies from 150 MHz to 800 MHz. The phantom flat bottom can have any shape that fits an ellipse with length 0,6 λ and width 0,4 λ, where λ0 is the wavelength in air. So, for example 450MHz frequency has wavelength of 0.67m thus the flat phantom needed for testing must fit the ellipse of length 0,4m and width 0,27m. SAR testing 450MHz body SAR can be done with ELI or Base station phantom adult.

SAR body test
SAR body test

Devices that are used in the front of the face, like push to talk devices, need to be tested for that configuration. The testing is done by placing the device under the flat phantom with 25mm separation between the device and the phantom. If the intended use of the device is in the hand, then limb SAR is tested. For the testing the device is placed in touch with the phantom and the testing is done for all six sides of the device.

Summary

There is a growing number of different products that need low frequency SAR testing, frequency range is then between 13MHz and 700MHz. The advantage for using these frequencies are wide coverage, improved voice, data and video services with broadband. From the SAR testing point of view, the lower frequency SAR testing has its own demands. It needs special verification antennas, special simulation liquids and phantoms are required.

To sum up the testing process:

  1. Tissue simulating liquid dielectric parameter check
  2. Test system check
  3. SAR measurement

References


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