What impedance does a 1/4-wavelength transmission line present to an RF generator when the line is shorted at the far end?
The correct answer is A: Very high impedance. A 1/4-wavelength transmission line shorted at the far end presents very high impedance to an RF generator. A quarter-wavelength line transforms a short (0 ohms) to an open (very high impedance) at the input.
Transmission lines transform impedances based on length. A shorted 1/4-wavelength line inverts the impedance - a short at the far end becomes an open (very high impedance) at the input. This is the opposite of an open 1/4-wavelength line, which transforms an open to a short. This property is useful for creating high-impedance points, blocking DC, or creating RF chokes. The 1/4-wavelength transformation is fundamental to many matching and filtering applications.
Exam Tip
1/4λ shorted line = Very high impedance. Remember: A 1/4-wavelength transmission line shorted at the far end presents very high impedance (essentially an open circuit) at the input because 1/4λ transforms a short to an open.
Memory Aid
"**1**/**4**λ **S**horted = **V**ery **H**igh **I**mpedance (think '1/4λS = VHI')"
Real-World Application
You have a 1/4-wavelength piece of transmission line shorted at the far end. At the input, you measure very high impedance (essentially an open circuit). The 1/4-wavelength length transforms the short at the far end to an open at the input. This is useful for creating RF chokes or blocking DC while passing RF.
FCC Part 97.3Key Concepts
Why Other Options Are Wrong
Option B: Incorrect. A shorted 1/4-wavelength line presents very high impedance, not very low. It transforms a short to an open.
Option C: Incorrect. The impedance isn't the same as the characteristic impedance. A shorted 1/4-wavelength line presents very high impedance (essentially an open circuit).
Option D: Incorrect. The impedance isn't the same as the generator output impedance. It's very high due to the 1/4-wavelength transformation.
题目解析
The correct answer is A: Very high impedance. A 1/4-wavelength transmission line shorted at the far end presents very high impedance to an RF generator. A quarter-wavelength line transforms a short (0 ohms) to an open (very high impedance) at the input. Transmission lines transform impedances based on length. A shorted 1/4-wavelength line inverts the impedance - a short at the far end becomes an open (very high impedance) at the input. This is the opposite of an open 1/4-wavelength line, which transforms an open to a short. This property is useful for creating high-impedance points, blocking DC, or creating RF chokes. The 1/4-wavelength transformation is fundamental to many matching and filtering applications.
考试技巧
1/4λ shorted line = Very high impedance. Remember: A 1/4-wavelength transmission line shorted at the far end presents very high impedance (essentially an open circuit) at the input because 1/4λ transforms a short to an open.
记忆口诀
**1**/**4**λ **S**horted = **V**ery **H**igh **I**mpedance (think '1/4λS = VHI')
实际应用示例
You have a 1/4-wavelength piece of transmission line shorted at the far end. At the input, you measure very high impedance (essentially an open circuit). The 1/4-wavelength length transforms the short at the far end to an open at the input. This is useful for creating RF chokes or blocking DC while passing RF.
错误选项分析
Option B: Incorrect. A shorted 1/4-wavelength line presents very high impedance, not very low. It transforms a short to an open. Option C: Incorrect. The impedance isn't the same as the characteristic impedance. A shorted 1/4-wavelength line presents very high impedance (essentially an open circuit). Option D: Incorrect. The impedance isn't the same as the generator output impedance. It's very high due to the 1/4-wavelength transformation.
知识点
Transmission line, Quarter-wavelength, Shorted line, Impedance transformation
Verified Content
Question from official FCC Extra Class question pool. Explanation reviewed by licensed amateur radio operators.