Calculating Total Exposure Ratios for Co Located Transmitters in Handheld Equipment
Multi-radio handheld exposure compliance requires summing normalized SAR values across concurrent transmitters to verify the total exposure ratio stays below unity.
Localized SAR Limits
Meaning
Maximum permissible values for radio frequency energy deposition in a specific small volume of human tissue protect the user from thermal hazards. When a wireless device transmits near the body, localized SAR limits define the allowable energy absorption measured over a defined mass, typically one gram or ten grams of tissue. These limits are set by international commissions to ensure that localized tissue heating remains below levels that could cause physical harm.
Testing Methodology
Compliance with these limits is verified using robotic probes inside human head and body models during active transmission states. The probes scan the region of highest energy concentration to locate the maximum deposition value. This measured value is then averaged over the standard tissue mass to produce the final compliance metric.
Thermal Threshold
The biological basis for these values relies on preventing tissue temperature rises from exceeding one degree Celsius. Because certain organs such as the eyes are more sensitive to thermal changes, localized limits are stricter for head exposure than for limbs. This division ensures that the most vulnerable areas receive the highest level of protection during prolonged use.
National regulators enforce these values through mandatory market surveillance, meaning that devices found to exceed the limits can be recalled or barred from distribution.
Design Adjustment
If the device exceeds these boundaries during initial testing, engineers must modify the output power or redesign the antenna structure. Adding shielding materials or repositioning the antenna away from the external surface can reduce the local hot spot. This iterative process is necessary to achieve certification before the product is released to the market.