Question E0A06:
Why are there separate electric (E) and magnetic (H) MPE limits at frequencies below 300 MHz?  

Choices:  
A: The body reacts to electromagnetic radiation from both the E and H fields  
B: Ground reflections and scattering cause the field strength to vary with location  
C: E field and H field radiation intensity peaks can occur at different locations  
D: All these choices are correct    

Correct Answer is  D: All these choices are correct  

Why D is correct:
- Below about 300 MHz, many amateur antennas have significant near-field regions around them. In the near field, the E and H fields are not locked in the fixed plane-wave ratio, so each can peak at different places and strengths. Humans also couple differently to E and H fields. Because of all this, regulators set separate MPE limits for E and H fields. That makes all of A, B, and C true, so “All these choices are correct” is the best answer.

Why the other choices are not the best single answer:
- A: True. The body reacts to both fields: E-fields tend to induce surface charge and displacement currents; H-fields induce circulating currents (magnetically induced). But A alone doesn’t cover spatial variations and near-field decoupling.
- B: True. Reflections and scattering create standing waves and hot spots, so field strength varies with location. But B alone doesn’t explain why E and H must be limited separately.
- C: True. In the near field, E and H are not in a fixed ratio, so their peaks can occur at different locations. But C alone omits the biological and environmental reasons.

Useful formulas and rules of thumb:
- In true far-field plane waves, electric field E, magnetic field H, and power density S are related by the free-space impedance $Z_0 \approx 377\,\Omega$:
  $$S = \frac{E^2}{377} = 377\,H^2$$
  $$E = \sqrt{377\,S}, \quad H = \sqrt{\frac{S}{377}}$$
- Because near-field conditions often exist below 300 MHz around typical antennas, $E/H \ne 377\,\Omega$, so measuring one does not predict the other—hence separate limits.
- Common FCC MPE reference values (30–300 MHz):
  - General population/uncontrolled: $S \le 0.2\,\text{mW/cm}^2$ ($2\,\text{W/m}^2$), $E \le 27.5\,\text{V/m}$, $H \le 0.073\,\text{A/m}$.
  - Controlled/occupational: $S \le 1.0\,\text{mW/cm}^2$ ($10\,\text{W/m}^2$), $E \le 61.4\,\text{V/m}$, $H \le 0.163\,\text{A/m}$.
- Far-field rule of thumb (where plane-wave relations start to apply):
  $$R_{\text{ff}} \gtrsim \frac{2D^2}{\lambda}$$
  If you’re closer than this, E and H can behave independently—supporting the need for separate E and H MPE limits.