Deep Dive: E9A09
The correct answer is B: Radiation resistance divided by total resistance. Antenna efficiency is radiation resistance divided by total resistance. Efficiency = R_radiation / (R_radiation + R_loss), where R_loss includes all loss resistances. Antenna efficiency tells you what percentage of input power is actually radiated versus lost as heat. Radiation resistance represents power that's radiated, while total resistance includes both radiation resistance and loss resistances (ground losses, conductor losses, etc.). Higher efficiency means more power is radiated and less is lost. A 100% efficient antenna would have only radiation resistance and no loss resistance. Efficiency is a key parameter for understanding antenna performance.
Why Other Answers Are Wrong
Option A: Incorrect. 'Transmission resistance' isn't a standard term. The correct formula uses total resistance (radiation + losses). Option C: Incorrect. This is the inverse of the correct formula. Efficiency is radiation resistance divided by total, not total divided by radiation. Option D: Incorrect. ERP divided by transmitter output gives a different ratio that includes gain, not just efficiency. Efficiency is about resistance ratios, not power ratios.
Exam Tip
Antenna efficiency = Radiation resistance / Total resistance. Remember: Efficiency = R_radiation / (R_radiation + R_loss). This tells you what fraction of power is radiated versus lost.
Memory Aid
**A**ntenna **E**fficiency = **R**adiation / **T**otal (think 'AE = R/T')
Real-World Example
Your antenna has 50 ohms radiation resistance and 10 ohms loss resistance (ground losses, conductor losses). Total resistance = 60 ohms. Efficiency = 50 / 60 = 83.3%. This means 83.3% of your input power is radiated, and 16.7% is lost as heat. To improve efficiency, you'd reduce loss resistance (better ground system, lower-loss conductors).
Source & Coverage
Question Pool: 2024-2028 Question Pool
Subelement: E9A
Reference: FCC Part 97.3
Key Concepts
Verified Content
Question from the official FCC Extra Class pool. Explanation reviewed by licensed amateur radio operators and mapped to the E9A topic.