Simultaneous Transmit SAR Evaluation Thresholds in Portable Radio Systems
Simultaneous SAR evaluations require summing standalone SAR or estimated values to prove combined exposure remains below regulatory limits.
SAR Exclusion Distance
Meaning
The physical separation required between a transmitter antenna and the human body allows a device to avoid complex laboratory evaluation if the transmit power is low. Product design teams establish a SAR exclusion distance to determine whether a device requires expensive compliance testing or is exempt. This boundary is calculated using the maximum output power of the radio and the operating frequency.
The exemption rules cease to apply if the device is held closer than this calculated threshold.
Calculation Method
Standard mathematical formulas defined by regulatory bodies convert the transmitter power and frequency into a minimum safe distance. If the physical separation exceeds this value, the device is automatically deemed compliant without laboratory scans.
Hardware Placement
Antenna engineers use this minimum spacing during the mechanical layout phase to position transmitters away from surfaces that are held by users. For example, placing a cellular antenna at the top of a tablet rather than the bottom increases the distance to the user’s hand during normal use. This physical layout helps avoid the need for complex power-reduction firmware or expensive compliance testing.
Mechanical designers must lock down these positions early in the CAD layout phase to avoid costly redesigns.
Risk Mitigation
Devices that cannot maintain this minimum separation must either run at lower power or undergo full laboratory testing. Proximity sensors are often used to detect when a hand or lap is close and dynamically reduce the transmit power. This approach allows the device to run at high power when placed on a table, yet remain safe and compliant when held.
Certification documents must clearly show the physical measurements and the sensor behavior to prove the exclusion limits are never breached during normal operation.