What Yagi driven element feed point impedance is required to use a beta or hairpin matching system?
The correct answer is A: Capacitive (driven element electrically shorter than 1/2 wavelength). To use a beta or hairpin matching system, the Yagi driven element feed point impedance must be capacitive (driven element electrically shorter than 1/2 wavelength). The beta match works with capacitive reactance.
The beta (hairpin) match is designed to work with a capacitively reactive feed point. This means the driven element must be slightly shorter than resonance (electrically shorter than 1/2 wavelength), making it capacitive. The hairpin (U-shaped conductor) provides inductive reactance that cancels the capacitive reactance, creating a match. If the element were longer (inductive) or at resonance (resistive), the beta match wouldn't work properly. The capacitive feed point is a requirement for beta/hairpin matching.
Exam Tip
Beta/hairpin match requires = Capacitive impedance. Remember: Beta or hairpin matching requires the Yagi driven element to have capacitive feed point impedance (element shorter than 1/2 wavelength).
Memory Aid
"**B**eta **M**atch **R**equires = **C**apacitive (think 'BMR = C')"
Real-World Application
You're designing a Yagi with a beta match. You make the driven element slightly shorter than 1/2 wavelength, making it capacitive. The beta (hairpin) match provides inductive reactance that cancels this capacitive reactance, creating a good match. If you made the element longer (inductive), the beta match wouldn't work properly.
FCC Part 97.3Key Concepts
Why Other Options Are Wrong
Option B: Incorrect. Inductive impedance (element longer than 1/2 wavelength) doesn't work with beta match. Beta match requires capacitive impedance.
Option C: Incorrect. Purely resistive impedance (at resonance) doesn't work with beta match. Beta match needs capacitive reactance to cancel.
Option D: Incorrect. Purely reactive could mean either capacitive or inductive, but beta match specifically needs capacitive, not just any reactive impedance.
题目解析
The correct answer is A: Capacitive (driven element electrically shorter than 1/2 wavelength). To use a beta or hairpin matching system, the Yagi driven element feed point impedance must be capacitive (driven element electrically shorter than 1/2 wavelength). The beta match works with capacitive reactance. The beta (hairpin) match is designed to work with a capacitively reactive feed point. This means the driven element must be slightly shorter than resonance (electrically shorter than 1/2 wavelength), making it capacitive. The hairpin (U-shaped conductor) provides inductive reactance that cancels the capacitive reactance, creating a match. If the element were longer (inductive) or at resonance (resistive), the beta match wouldn't work properly. The capacitive feed point is a requirement for beta/hairpin matching.
考试技巧
Beta/hairpin match requires = Capacitive impedance. Remember: Beta or hairpin matching requires the Yagi driven element to have capacitive feed point impedance (element shorter than 1/2 wavelength).
记忆口诀
**B**eta **M**atch **R**equires = **C**apacitive (think 'BMR = C')
实际应用示例
You're designing a Yagi with a beta match. You make the driven element slightly shorter than 1/2 wavelength, making it capacitive. The beta (hairpin) match provides inductive reactance that cancels this capacitive reactance, creating a good match. If you made the element longer (inductive), the beta match wouldn't work properly.
错误选项分析
Option B: Incorrect. Inductive impedance (element longer than 1/2 wavelength) doesn't work with beta match. Beta match requires capacitive impedance. Option C: Incorrect. Purely resistive impedance (at resonance) doesn't work with beta match. Beta match needs capacitive reactance to cancel. Option D: Incorrect. Purely reactive could mean either capacitive or inductive, but beta match specifically needs capacitive, not just any reactive impedance.
知识点
Beta match, Hairpin match, Capacitive impedance, Driven element length
Verified Content
Question from official FCC Extra Class question pool. Explanation reviewed by licensed amateur radio operators.