What is the purpose of making a Yagi’s parasitic elements either longer or shorter than resonance?
The correct answer is C: Control of phase shift. The purpose of making a Yagi's parasitic elements either longer or shorter than resonance is to control the phase shift of the currents induced in those elements. This phase shift determines whether the element acts as a reflector or director.
Parasitic elements (reflectors and directors) are not directly fed - they receive energy through mutual coupling. An element longer than resonance (inductive) has current that lags in phase, making it a reflector (pushes energy forward). An element shorter than resonance (capacitive) has current that leads in phase, making it a director (pulls energy forward). By adjusting the length (making elements longer or shorter than resonance), you control the phase relationship, which determines the element's function and the antenna's performance. This is fundamental to Yagi operation.
Exam Tip
Parasitic element length = Control phase shift. Remember: Making Yagi parasitic elements longer or shorter than resonance controls the phase shift of induced currents, determining whether they act as reflectors (longer) or directors (shorter).
Memory Aid
"**P**arasitic **L**ength = **C**ontrol **P**hase **S**hift (think 'PL = CPS')"
Real-World Application
You're tuning a Yagi. You make the rear element longer than resonance - it becomes a reflector (current lags, pushes energy forward). You make the front element shorter than resonance - it becomes a director (current leads, pulls energy forward). By adjusting lengths, you control the phase relationships that make the Yagi work properly.
FCC Part 97.3Key Concepts
Why Other Options Are Wrong
Option A: Incorrect. Making elements longer or shorter doesn't cancel wind torque. Wind torque is a mechanical issue, not related to electrical length.
Option B: Incorrect. Element length doesn't provide mechanical balance. The purpose is electrical - controlling phase shift.
Option D: Incorrect. Making elements longer or shorter doesn't minimize losses. The purpose is to control phase shift for proper Yagi operation.
题目解析
The correct answer is C: Control of phase shift. The purpose of making a Yagi's parasitic elements either longer or shorter than resonance is to control the phase shift of the currents induced in those elements. This phase shift determines whether the element acts as a reflector or director. Parasitic elements (reflectors and directors) are not directly fed - they receive energy through mutual coupling. An element longer than resonance (inductive) has current that lags in phase, making it a reflector (pushes energy forward). An element shorter than resonance (capacitive) has current that leads in phase, making it a director (pulls energy forward). By adjusting the length (making elements longer or shorter than resonance), you control the phase relationship, which determines the element's function and the antenna's performance. This is fundamental to Yagi operation.
考试技巧
Parasitic element length = Control phase shift. Remember: Making Yagi parasitic elements longer or shorter than resonance controls the phase shift of induced currents, determining whether they act as reflectors (longer) or directors (shorter).
记忆口诀
**P**arasitic **L**ength = **C**ontrol **P**hase **S**hift (think 'PL = CPS')
实际应用示例
You're tuning a Yagi. You make the rear element longer than resonance - it becomes a reflector (current lags, pushes energy forward). You make the front element shorter than resonance - it becomes a director (current leads, pulls energy forward). By adjusting lengths, you control the phase relationships that make the Yagi work properly.
错误选项分析
Option A: Incorrect. Making elements longer or shorter doesn't cancel wind torque. Wind torque is a mechanical issue, not related to electrical length. Option B: Incorrect. Element length doesn't provide mechanical balance. The purpose is electrical - controlling phase shift. Option D: Incorrect. Making elements longer or shorter doesn't minimize losses. The purpose is to control phase shift for proper Yagi operation.
知识点
Yagi parasitic elements, Phase shift, Reflector vs director, Element length
Verified Content
Question from official FCC Extra Class question pool. Explanation reviewed by licensed amateur radio operators.