Question E1B04:
[97.303(b)] What must the control operator of a repeater operating in the 70-centimeter band do if a radiolocation system experiences interference from that repeater?  

Choices:  
A: Reduce the repeater antenna HAAT (Height Above Average Terrain)  
B: File an FAA NOTAM (Notice to Air Missions) with the repeater system's ERP, call sign, and six-character grid locator  
C: Cease operation or make changes to the repeater that mitigate the interference  
D: All these choices are correct    

Correct Answer is  C: Cease operation or make changes to the repeater that mitigate the interference  

Correct answer: C — Cease operation or make changes to the repeater that mitigate the interference

1) Why C is correct
- In the 70-centimeter band (420–450 MHz), amateurs are secondary to federal radiolocation services (e.g., military radar). Under 47 CFR 97.303(b), a secondary service must not cause harmful interference to a primary service and must accept interference from it. If interference occurs, the amateur station must cease operation or modify operations to eliminate the interference.

2) Why the other choices are incorrect
- A: Reduce the repeater antenna HAAT — Reducing HAAT might help, but the rule does not prescribe this specific step. The requirement is to stop causing interference, which could involve any effective mitigation (frequency change, ERP reduction, filters, pattern/height changes) or ceasing operation.
- B: File an FAA NOTAM — NOTAMs are for aviation safety/hazards, not RF interference. There is no FCC requirement to file a NOTAM for repeater interference issues.
- D: All these choices are correct — Incorrect because A and B are not required or relevant actions per FCC rules.

3) Useful rules of thumb
- Secondary vs. primary: A secondary service must not cause harmful interference to a primary service. If it does, it must cease or change operations to stop the interference.
- Practical mitigation options:
  - Reduce ERP: $ERP = P_T \times G_A$
  - Change frequency within the band (avoiding affected segments)
  - Use filtering, change antenna pattern/azimuth/height, or relocate
  - As needed, cease operation until interference is resolved
- Power/dB quick tip: A 3 dB reduction halves power; a 10 dB reduction reduces power to one-tenth. Reducing ERP often quickly mitigates interference.