In this series, we will dive into the world of SAR testing, an essential part of assessing the safety of modern wireless devices. SAR, or Specific Absorption Rate, measures the amount of radiofrequency energy absorbed by the human body from wireless devices like mobile phones, tablets, and other electronic gadgets. SAR testing is crucial as it helps to ensure that these devices do not expose users to harmful levels of electromagnetic fields.

SAR testing involves a complex scientific process, examining how wireless devices affect the body at various frequencies and power levels. Throughout this series, we will explore what SAR is, how it is measured, and why it’s so important for our health. Our goal is to provide readers with a thorough understanding of the scientific principles behind SAR testing and its role in the development of safe and effective technology.

The purpose of Specific Absorption Rate (SAR) testing is to measure how much radiofrequency (RF) energy is absorbed by the human body. Since direct human testing is not feasible due to ethical and safety concerns, tissue-simulating liquids and phantoms are used to accurately mimic human tissues during SAR assessments. These materials play a vital role in ensuring that mobile devices comply with safety regulations and do not exceed permissible exposure limits.

Tissue Simulating Liquids – TSL’s

Tissue simulating liquids are specially formulated liquids to mimic the dielectric properties of biological tissues at specific RF frequencies.

· Relative permittivity determines how the fields propagate in a tissue.

· Conductivity defines how efficiently it absorbs and converts RF energy into a heat.

The interaction between the RF energy and biological tissues varies depending on the frequency. At lower frequencies, RF waves penetrate deeper into the body, requiring TSLs with higher permittivity and lower conductivity. At higher frequencies, RF energy is absorbed closer to the surface, requiring TSLs with lower permittivity and higher conductivity to accurately reflect the reduced penetration depth and increased surface absorption.

TSLs in SAR testing do not replicate individual tissue types (such as skin, muscle, or fat) but instead it represents an average medium that approximates human exposure characteristics that is defined by regulators and standardization organizations. Standards such as those set by the IEC and IEEE define the required target values for each frequency range, ensuring consistency of SAR assessments. For SAR testing, the measured dielectric properties must be

within ±5% of the target values. If a correction methodology is applied, a deviation can be within ±10% of the target values.

Temperature of the TSL can affect to the dielectric properties. Since SAR testing relies on precise dielectrics properties, it is important to maintain a stable and controlled temperature during testing.

Phantoms

Phantoms play a crucial role in SAR testing, providing a standardized physical structure that holds tissue-simulating liquids and thus replicates the human body’s interaction with radiofrequency (RF) energy. These phantoms allow for repeatable, controlled, and accurate SAR measurements. There are different types of phantoms used in SAR testing:

1. Head Phantoms – Used to test mobile phones, wireless headsets, and other near-head devices. Head phantom replicates the electrical properties of the human head to measure absorbed RF energy.

2. Body Phantoms – Representing the torso or limbs. Body phantoms are used for testing body-worn devices.

3. Whole Body Phantoms – Representing the whole body of a human. Whole Body phantoms are used for testing of base station antennas.

Phantoms are usually made from materials such as acrylic or fiber-reinforced plastics, which are RF-transparent and structurally stable. They are filled with TSLs before testing to simulate real-world conditions. Regulatory agencies such as FCC and ISED and standards organizations such as IEC and IEEE define specific protocols for SAR testing using standardized TSLs and phantoms.

Conclusion

Tissue-simulating liquids and phantoms are indispensable in SAR testing, providing a scientifically accurate method to measure RF absorption in the human body. By mimicking the dielectric properties of tissues, these materials enable reliable and standardized compliance testing, ensuring that wireless devices operate safely within regulatory limits.

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Stay tuned for the next article in our series, where we’ll continue exploring The Science Behind SAR.

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