What impedance does a 1/8-wavelength transmission line present to an RF generator when the line is open at the far end?
The correct answer is C: A capacitive reactance. A 1/8-wavelength transmission line open at the far end presents a capacitive reactance to an RF generator. The line appears as a capacitor.
Transmission lines shorter than 1/4 wavelength transform impedances progressively. An open line shorter than 1/4 wavelength appears capacitive. At 1/8 wavelength, the line presents capacitive reactance - it looks like a capacitor. As the line gets longer (approaching 1/4 wavelength), the reactance decreases. At 1/4 wavelength, it becomes very low impedance (short). The 1/8-wavelength open line is useful for providing capacitive reactance in matching networks or filters.
Exam Tip
1/8λ open line = Capacitive reactance. Remember: A 1/8-wavelength transmission line open at the far end presents a capacitive reactance to the generator. Open lines shorter than 1/4λ appear capacitive.
Memory Aid
"**1**/**8**λ **O**pen = **C**apacitive **R**eactance (think '1/8λO = CR')"
Real-World Application
You have a 1/8-wavelength piece of transmission line open at the far end. At the input, it appears as a capacitor with capacitive reactance. This can be useful in matching networks where you need capacitive reactance. As you make the line longer (toward 1/4 wavelength), the capacitive reactance decreases.
FCC Part 97.3Key Concepts
Why Other Options Are Wrong
Option A: Incorrect. The impedance isn't the same as the characteristic impedance. It's a reactive impedance (capacitive).
Option B: Incorrect. An open line shorter than 1/4 wavelength appears capacitive, not inductive. A shorted line appears inductive.
Option D: Incorrect. The impedance isn't infinite. At 1/8 wavelength, it's a capacitive reactance with a specific value. Infinite impedance would occur at 1/2 wavelength for an open line.
题目解析
The correct answer is C: A capacitive reactance. A 1/8-wavelength transmission line open at the far end presents a capacitive reactance to an RF generator. The line appears as a capacitor. Transmission lines shorter than 1/4 wavelength transform impedances progressively. An open line shorter than 1/4 wavelength appears capacitive. At 1/8 wavelength, the line presents capacitive reactance - it looks like a capacitor. As the line gets longer (approaching 1/4 wavelength), the reactance decreases. At 1/4 wavelength, it becomes very low impedance (short). The 1/8-wavelength open line is useful for providing capacitive reactance in matching networks or filters.
考试技巧
1/8λ open line = Capacitive reactance. Remember: A 1/8-wavelength transmission line open at the far end presents a capacitive reactance to the generator. Open lines shorter than 1/4λ appear capacitive.
记忆口诀
**1**/**8**λ **O**pen = **C**apacitive **R**eactance (think '1/8λO = CR')
实际应用示例
You have a 1/8-wavelength piece of transmission line open at the far end. At the input, it appears as a capacitor with capacitive reactance. This can be useful in matching networks where you need capacitive reactance. As you make the line longer (toward 1/4 wavelength), the capacitive reactance decreases.
错误选项分析
Option A: Incorrect. The impedance isn't the same as the characteristic impedance. It's a reactive impedance (capacitive). Option B: Incorrect. An open line shorter than 1/4 wavelength appears capacitive, not inductive. A shorted line appears inductive. Option D: Incorrect. The impedance isn't infinite. At 1/8 wavelength, it's a capacitive reactance with a specific value. Infinite impedance would occur at 1/2 wavelength for an open line.
知识点
Transmission line, Eighth-wavelength, Open line, Capacitive reactance
Verified Content
Question from official FCC Extra Class question pool. Explanation reviewed by licensed amateur radio operators.