Question E0A08:
What does SAR measure?  

Choices:  
A: Signal attenuation ratio  
B: Signal amplification rating  
C: The rate at which RF energy is absorbed by the body  
D: The rate of RF energy reflected from stationary terrain    

Correct Answer is  C: The rate at which RF energy is absorbed by the body  

To calculate what SAR measures:

- SAR = Specific Absorption Rate.
- It quantifies how quickly the body absorbs RF energy, expressed in watts per kilogram ($\text{W/kg}$).
- It is typically averaged over a specified mass of tissue and time (e.g., 1 g or 10 g).

Why C is correct:
- SAR directly measures the rate at which RF energy is absorbed by biological tissue. This is the parameter regulators use to limit localized human exposure.

Why the other choices are incorrect:
- A: Signal attenuation ratio — describes loss of signal strength through a medium, not biological absorption rate.
- B: Signal amplification rating — refers to gain in a device, not tissue absorption.
- D: Rate of RF energy reflected from stationary terrain — relates to radar/propagation reflections, not human exposure.

Useful formulas and notes:
- Definition:
  $$\text{SAR} = \frac{dP_{\text{abs}}}{dm} \quad (\text{W/kg})$$
- In tissue (using RMS electric field):
  $$\text{SAR} = \frac{\sigma\,E_{\text{rms}}^{2}}{\rho}$$
  where:
  - $\sigma$ = tissue conductivity (S/m),
  - $E_{\text{rms}}$ = electric field in tissue (V/m),
  - $\rho$ = tissue density (kg/m³).

Rule of thumb context:
- SAR is for near-field, localized human exposure.
- For far-field exposure assessment, we more often use power density and MPE:
  $$S \approx \frac{P_\text{out} G}{4\pi r^{2}}$$
  but this is not SAR; it’s a different exposure metric.

Key takeaway: SAR measures the rate at which RF energy is absorbed by the body (C).