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