What is SAR and what are the limits?
Specific absorption rate (SAR) is the rate at which RF energy is absorbed per unit mass of tissue, SAR = σ|E|²/ρ in W/kg, where σ is tissue conductivity, E the rms internal field and ρ the mass density. The US limit for portable devices used by the general population is 1.6 W/kg averaged over any 1 g of tissue (4 W/kg over 10 g for extremities, 0.08 W/kg whole-body),[1] while ICNIRP guidelines, used in Europe and many other countries, set 2 W/kg over 10 g for head and trunk and 4 W/kg for limbs.[2]
How is SAR measured?
A robotically scanned E-field probe measures fields inside a phantom filled with tissue-simulating liquid while the device transmits at maximum power in defined positions (against the head, at body-worn separation). IEC/IEEE 62209-1528 specifies the procedure, and the FCC adds KDB guidance on test reduction and exclusions.[3][4]
How does antenna design affect SAR?
- SAR is dominated by near-field current density close to tissue; spreading current over a larger area and moving the antenna away from the user lower peak SAR.
- Antennas at the bottom of phones (away from the head) and ground-plane design matter.
- Proximity sensors trigger power back-off when the body is close.
- At mmWave, absorbed or incident power density replaces SAR as the metric.[2][5]
Which devices need SAR evaluation?
Portable transmitters used within 20 cm of the body generally require SAR assessment unless they fall under exemption thresholds based on power, frequency and separation distance defined in FCC rules and guidance.[1][4]
Frequently asked questions
Is SAR a measure of safety margin?
Limits include safety factors below thresholds for established heating effects.
Does SAR apply to base stations?
No; fixed transmitters are evaluated against field strength or power density limits.
What is the 1 g vs 10 g difference?
The averaging mass; 10 g averaging smooths peaks, giving lower values than 1 g.
See also
References
Online sources accessed October 21, 2026; printed works cited by edition. See all sources.
- “47 CFR 2.1093 — Radiofrequency radiation exposure evaluation: portable devices”. U.S. FCC, eCFR. current text.
- “Guidelines for Limiting Exposure to Electromagnetic Fields (100 kHz to 300 GHz)”. ICNIRP, Health Physics, vol. 118, no. 5, pp. 483-524. 2020.
- “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”. IEC/IEEE. 2020.
- “KDB 447498: RF Exposure Procedures and Equipment Authorization Policies for Mobile and Portable Devices”. U.S. FCC Office of Engineering and Technology. current revision.
- “Modern Small Antennas”. K. Fujimoto and H. Morishita, Cambridge University Press. 2013.