Question E0A10:
When must an RF exposure evaluation be performed on an amateur station operating on 80 meters?  

Choices:  
A: An evaluation must always be performed  
B: When the ERP of the station is less than 10 watts  
C: When the station’s operating mode is CW  
D: When the output power from the transmitter is less than 100 watts    

Correct Answer is  A: An evaluation must always be performed  

Correct answer: An evaluation must always be performed.

Why this is correct:
- FCC rules require every amateur station to assess RF exposure compliance, regardless of band, power, or mode. On 80 meters, you must perform an RF exposure evaluation for your station setup. This can be a simple exemption check or a calculation, but it must be done and documented.

Why the other choices are incorrect:
- B: “When the ERP of the station is less than 10 watts” — Incorrect. There is no blanket 10 W ERP exception that eliminates the need to evaluate. You must at least determine (evaluate) whether you qualify for an exemption.
- C: “When the station’s operating mode is CW” — Incorrect. Mode does not determine whether you must evaluate. Mode only affects duty cycle (and thus average power) used in the calculation.
- D: “When the output power from the transmitter is less than 100 watts” — Incorrect. There is no across-the-board 100 W exception. You still must evaluate compliance.

Useful formulas and rules of thumb:
- Average (time-averaged) transmitter power:
  $$P_{\text{avg}} = P_{\text{PEP}} \times D_{\text{mode}} \times D_{\text{TX}}$$
  where:
  - $D_{\text{mode}}$ is the modulation duty factor (typical rules of thumb: SSB voice ≈ 0.2–0.25, CW ≈ 0.4, FM/RTTY/FT8 while keyed ≈ 1.0),
  - $D_{\text{TX}}$ is the fraction of time you transmit during the averaging interval (6 minutes occupational, 30 minutes general public).

- Convert to EIRP:
  $$P_{\text{EIRP}} = P_{\text{avg}} \times G_{\text{ant,lin}}$$
  where $G_{\text{ant,lin}}$ is antenna gain in linear terms (not dB).

- Far-field power density (useful as a rough check; 80 m is often near-field, so use a proper calculator when close to the antenna):
  $$S = \frac{P_{\text{EIRP}}}{4\pi r^2}$$

- Field relationships:
  $$S = \frac{E^2}{377} = 377\,H^2 \quad \text{(W/m}^2\text{)}$$

- MPE limits for 1.34–30 MHz:
  - General public (uncontrolled):
    $$E_{\text{MPE}} = \frac{824}{f} \text{ V/m}, \quad H_{\text{MPE}} = \frac{2.19}{f} \text{ A/m}$$
  - Occupational (controlled):
    $$E_{\text{MPE}} = \frac{1842}{f} \text{ V/m}, \quad H_{\text{MPE}} = \frac{4.89}{f} \text{ A/m}$$
  where $f$ is in MHz. Example at 3.5 MHz (80 m), uncontrolled:
  $$E_{\text{MPE}} \approx \frac{824}{3.5} \approx 235 \text{ V/m}$$

- Minimum separation distance (far-field approximation):
  $$r_{\min} \approx \sqrt{\frac{P_{\text{EIRP}}}{4\pi\,S_{\text{MPE}}}}$$

Study tip:
- Document a simple evaluation for your station (use ARRL/FCC calculators), update it when you change power, antenna, height, or location, and remember that lower duty cycle and greater separation both reduce exposure.