Question E0A04:
When evaluating a site with multiple transmitters operating at the same time, the operators and licensees of which transmitters are responsible for mitigating over-exposure situations?  

Choices:  
A: Each transmitter that produces 20 percent or more of its MPE limit in areas where the total MPE limit is exceeded  
B: Each transmitter operating with a duty cycle greater than 25 percent  
C: Each transmitter that produces 5 percent or more of its MPE limit in areas where the total MPE limit is exceeded  
D: Each transmitter operating with a duty cycle greater than 50 percent    

Correct Answer is  C: Each transmitter that produces 5 percent or more of its MPE limit in areas where the total MPE limit is exceeded  

Correct answer: C

Why C is correct
- FCC guidance (OET Bulletin 65) and Part 97 require shared responsibility at multi-transmitter sites when the total exposure exceeds the MPE.  
- Any station contributing at least 5% of the MPE at the location of concern must take corrective action.  
- “5%” is the de minimis threshold: below that, a transmitter’s contribution is considered too small to require mitigation responsibility.

Why the others are incorrect
- A (20%): The threshold is not 20%; it is 5%.  
- B (duty cycle > 25%): Responsibility is based on percentage of MPE contributed, not on a fixed duty-cycle cutoff.  
- D (duty cycle > 50%): Same issue—duty cycle matters only as part of time-averaged exposure, but there is no 25% or 50% duty-cycle trigger for responsibility.

Useful formulas and rules of thumb
- Multiple sources add linearly in terms of fraction of MPE:
  $$\sum_i \frac{S_i}{\text{MPE}_i} \le 1 \quad \text{(compliant)}$$
  where $S_i$ is the power density (or equivalent field-based exposure) from source $i$ at the location, and $\text{MPE}_i$ is the applicable limit at that frequency and exposure category.
- In percent form:
  $$\text{Total % of MPE} = \sum_i \left(\frac{S_i}{\text{MPE}_i}\times 100\%\right)$$
  If this sum exceeds 100%, each transmitter with
  $$\frac{S_i}{\text{MPE}_i}\times 100\% \ge 5\%$$
  must mitigate.
- Duty cycle matters only via time averaging. Compute $S_i$ using time-averaged power:
  $$P_{\text{avg}} = P_{\text{peak}} \times \text{duty cycle}$$
  but the mitigation trigger is based on percent of MPE, not a duty-cycle threshold.