Question E0A03:
Over what range of frequencies are the FCC human body RF exposure limits most restrictive?  

Choices:  
A: 300 kHz - 3 MHz  
B: 3 - 30 MHz  
C: 30 - 300 MHz  
D: 300 - 3000 MHz    

Correct Answer is  C: 30 - 300 MHz  

Correct answer: C — 30 to 300 MHz

Why this is correct:
- The human body has a whole-body resonance roughly in the VHF range (about 70–80 MHz for an adult). At resonance, absorption and resulting SAR are maximized for a given field strength.
- Because absorption is highest here, the FCC sets the most restrictive MPE limits in this band.
- For general public/uncontrolled exposure, the FCC MPE is held at its lowest constant value across 30–300 MHz:
  - $$S_{\text{limit}} = 0.2\ \text{mW/cm}^2 \quad (30\text{–}300\ \text{MHz})$$
  - For occupational/controlled exposure, the limit is:
  - $$S_{\text{limit}} = 1.0\ \text{mW/cm}^2 \quad (30\text{–}300\ \text{MHz})$$

Why the other choices are incorrect:
- A: 300 kHz–3 MHz
  - Coupling is mainly via induced currents; whole-body absorption is relatively low. FCC limits here are expressed as higher allowed E- and H-fields and contact current limits, so not the most restrictive.
- B: 3–30 MHz
  - Still largely near-field/inductive coupling; whole-body resonance has not yet peaked. Allowed fields are higher than in 30–300 MHz.
- D: 300–3000 MHz
  - Above 300 MHz, allowed power density increases because absorption becomes more superficial and less whole-body. For the general public:
    - $$S_{\text{limit}} = \frac{f}{1500}\ \text{mW/cm}^2 \quad (300\text{–}1500\ \text{MHz})$$
    - $$S_{\text{limit}} = 1.0\ \text{mW/cm}^2 \quad (1500\ \text{MHz–}100\ \text{GHz})$$
  - These limits are higher than the 30–300 MHz limit, so they are less restrictive.

Useful formulas and conversions:
- Plane-wave power density and fields:
  - $$S\ (\text{W/m}^2) = \frac{E^2}{377} = 377\,H^2$$
- Unit conversion:
  - $$1\ \text{mW/cm}^2 = 10\ \text{W/m}^2$$
- Example field strengths for the general public at 30–300 MHz:
  - $$S = 0.2\ \text{mW/cm}^2 \Rightarrow S = 2\ \text{W/m}^2 \Rightarrow E \approx \sqrt{377 \times 2} \approx 27.5\ \text{V/m}$$

Rule of thumb:
- Human whole-body resonance occurs in VHF (around 70–80 MHz), so exposure limits are tightest across 30–300 MHz.