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