What is the function of a loading coil in an electrically short antenna?
The correct answer is D: To resonate the antenna by cancelling the capacitive reactance. The function of a loading coil in an electrically short antenna is to resonate the antenna by cancelling the capacitive reactance. The coil's inductive reactance cancels the antenna's capacitive reactance, making it resonant.
An electrically short antenna (shorter than 1/4 wavelength for a vertical) appears capacitive - it has capacitive reactance. To make it resonant (purely resistive), you need to cancel this capacitive reactance with inductive reactance. A loading coil provides this inductance. At resonance, the inductive reactance equals the capacitive reactance, they cancel, and the antenna presents a purely resistive impedance. This allows efficient power transfer and proper antenna operation. Without the loading coil, the antenna would have high SWR due to the reactive component.
Exam Tip
Loading coil function = Cancel capacitive reactance for resonance. Remember: A loading coil resonates an electrically short antenna by providing inductive reactance that cancels the antenna's capacitive reactance.
Memory Aid
"**L**oading **C**oil **F**unction = **C**ancel **C**apacitive **R**eactance (think 'LCF = CCR')"
Real-World Application
You have a shortened vertical that's electrically short (maybe 1/8 wavelength). It appears capacitive. You add a loading coil. The coil's inductive reactance cancels the antenna's capacitive reactance, making the antenna resonant (purely resistive impedance). Now the antenna works properly with low SWR. Without the coil, the reactive component would cause high SWR.
FCC Part 97.3Key Concepts
Why Other Options Are Wrong
Option A: Incorrect. Loading coils don't increase SWR bandwidth - they decrease it. The coil's function is resonance, not bandwidth improvement.
Option B: Incorrect. Loading coils don't lower losses - they actually add some loss due to coil resistance. The function is resonance, not loss reduction.
Option C: Incorrect. Loading coils don't lower Q - they typically increase it. The function is to cancel capacitive reactance for resonance.
题目解析
The correct answer is D: To resonate the antenna by cancelling the capacitive reactance. The function of a loading coil in an electrically short antenna is to resonate the antenna by cancelling the capacitive reactance. The coil's inductive reactance cancels the antenna's capacitive reactance, making it resonant. An electrically short antenna (shorter than 1/4 wavelength for a vertical) appears capacitive - it has capacitive reactance. To make it resonant (purely resistive), you need to cancel this capacitive reactance with inductive reactance. A loading coil provides this inductance. At resonance, the inductive reactance equals the capacitive reactance, they cancel, and the antenna presents a purely resistive impedance. This allows efficient power transfer and proper antenna operation. Without the loading coil, the antenna would have high SWR due to the reactive component.
考试技巧
Loading coil function = Cancel capacitive reactance for resonance. Remember: A loading coil resonates an electrically short antenna by providing inductive reactance that cancels the antenna's capacitive reactance.
记忆口诀
**L**oading **C**oil **F**unction = **C**ancel **C**apacitive **R**eactance (think 'LCF = CCR')
实际应用示例
You have a shortened vertical that's electrically short (maybe 1/8 wavelength). It appears capacitive. You add a loading coil. The coil's inductive reactance cancels the antenna's capacitive reactance, making the antenna resonant (purely resistive impedance). Now the antenna works properly with low SWR. Without the coil, the reactive component would cause high SWR.
错误选项分析
Option A: Incorrect. Loading coils don't increase SWR bandwidth - they decrease it. The coil's function is resonance, not bandwidth improvement. Option B: Incorrect. Loading coils don't lower losses - they actually add some loss due to coil resistance. The function is resonance, not loss reduction. Option C: Incorrect. Loading coils don't lower Q - they typically increase it. The function is to cancel capacitive reactance for resonance.
知识点
Loading coil, Resonance, Capacitive reactance, Electrically short antenna
Verified Content
Question from official FCC Extra Class question pool. Explanation reviewed by licensed amateur radio operators.