How does radiation resistance of a base-fed whip antenna change below its resonant frequency?
The correct answer is B: Radiation resistance decreases. As a base-fed whip antenna operates below its resonant frequency, the radiation resistance decreases. Below resonance, the antenna is shorter than optimal, reducing its radiation resistance.
Radiation resistance is the resistance that represents power radiated as RF. For a given antenna type, radiation resistance depends on the antenna's electrical length. As you go below resonance (shorter antenna), the radiation resistance decreases. This is because a shorter antenna is less efficient at radiating - it has less effective length to create radiation. The radiation resistance of a short vertical is much lower than a full-size quarter-wave vertical. This is why electrically short antennas often have low radiation resistance and need good ground systems to minimize other losses.
Exam Tip
Below resonance = Decreased radiation resistance. Remember: As a base-fed whip operates below its resonant frequency, radiation resistance decreases because the shorter antenna is less efficient at radiating.
Memory Aid
"**B**elow **R**esonance = **D**ecreased **R**adiation **R**esistance (think 'BR = DRR')"
Real-World Application
You have a base-fed whip that's resonant at 14 MHz (full quarter-wave). The radiation resistance is about 36 ohms. You operate it at 7 MHz (below resonance, electrically shorter). The radiation resistance is now much lower (maybe 10-15 ohms) because the shorter antenna is less efficient at radiating. This is why short antennas need loading coils and good ground systems.
FCC Part 97.3Key Concepts
Why Other Options Are Wrong
Option A: Incorrect. Radiation resistance doesn't increase below resonance - it decreases. Shorter antennas have lower radiation resistance.
Option C: Incorrect. Radiation resistance doesn't stay the same. It changes with frequency/electrical length.
Option D: Incorrect. Radiation resistance doesn't become purely reactive. It's still resistive, just lower in value. The antenna also has reactive components, but radiation resistance itself is still resistive (just smaller).
题目解析
The correct answer is B: Radiation resistance decreases. As a base-fed whip antenna operates below its resonant frequency, the radiation resistance decreases. Below resonance, the antenna is shorter than optimal, reducing its radiation resistance. Radiation resistance is the resistance that represents power radiated as RF. For a given antenna type, radiation resistance depends on the antenna's electrical length. As you go below resonance (shorter antenna), the radiation resistance decreases. This is because a shorter antenna is less efficient at radiating - it has less effective length to create radiation. The radiation resistance of a short vertical is much lower than a full-size quarter-wave vertical. This is why electrically short antennas often have low radiation resistance and need good ground systems to minimize other losses.
考试技巧
Below resonance = Decreased radiation resistance. Remember: As a base-fed whip operates below its resonant frequency, radiation resistance decreases because the shorter antenna is less efficient at radiating.
记忆口诀
**B**elow **R**esonance = **D**ecreased **R**adiation **R**esistance (think 'BR = DRR')
实际应用示例
You have a base-fed whip that's resonant at 14 MHz (full quarter-wave). The radiation resistance is about 36 ohms. You operate it at 7 MHz (below resonance, electrically shorter). The radiation resistance is now much lower (maybe 10-15 ohms) because the shorter antenna is less efficient at radiating. This is why short antennas need loading coils and good ground systems.
错误选项分析
Option A: Incorrect. Radiation resistance doesn't increase below resonance - it decreases. Shorter antennas have lower radiation resistance. Option C: Incorrect. Radiation resistance doesn't stay the same. It changes with frequency/electrical length. Option D: Incorrect. Radiation resistance doesn't become purely reactive. It's still resistive, just lower in value. The antenna also has reactive components, but radiation resistance itself is still resistive (just smaller).
知识点
Radiation resistance, Below resonance, Base-fed whip, Antenna length
Verified Content
Question from official FCC Extra Class question pool. Explanation reviewed by licensed amateur radio operators.