What is the feed point impedance of an end-fed half-wave antenna?
The correct answer is D: Very high. The feed point impedance of an end-fed half-wave antenna is very high. End-fed half-waves have high impedance at the end (thousands of ohms) because current is minimum and voltage is maximum at the ends. For amateur radio operators, this requires impedance matching. Understanding this helps when using end-fed antennas.
Exam Tip
End-fed half-wave impedance = very high. Think 'E'nd-'F'ed = 'E'nd has 'F'ull voltage = 'V'ery 'H'igh impedance. End-fed half-waves have high impedance because current is minimum and voltage is maximum at ends. Not very low, not 50Ω, not 300Ω - just very high.
Memory Aid
"End-fed half-wave impedance = very high. Think 'E'nd-'F'ed = 'V'ery 'H'igh impedance. End-fed half-waves have high impedance because current is minimum and voltage is maximum at ends. Requires impedance matching."
Real-World Application
An end-fed half-wave antenna: The feed point is at the end where current is minimum and voltage is maximum. This creates very high impedance (typically 2000-5000 ohms). An impedance matching network (tuner or transformer) is required to match this high impedance to the transmitter. This is why end-fed antennas need matching networks.
Key Concepts
Why Other Options Are Wrong
Option A (Very low): Incorrect. End-fed half-wave doesn't have very low impedance - it has very high impedance because current is minimum at the ends. Very low is wrong.
Option B (Approximately 50 ohms): Incorrect. End-fed half-wave doesn't have 50 ohms - it has very high impedance (thousands of ohms). 50 ohms is for center-fed dipoles.
Option C (Approximately 300 ohms): Incorrect. End-fed half-wave doesn't have 300 ohms - it has very high impedance (thousands of ohms). 300 ohms is too low.
题目解析
The correct answer is D: Very high. The feed point impedance of an end-fed half-wave antenna is very high. End-fed half-waves have high impedance at the end (thousands of ohms) because current is minimum and voltage is maximum at the ends. For amateur radio operators, this requires impedance matching. Understanding this helps when using end-fed antennas.
考试技巧
End-fed half-wave impedance = very high. Think 'E'nd-'F'ed = 'E'nd has 'F'ull voltage = 'V'ery 'H'igh impedance. End-fed half-waves have high impedance because current is minimum and voltage is maximum at ends. Not very low, not 50Ω, not 300Ω - just very high.
记忆口诀
End-fed half-wave impedance = very high. Think 'E'nd-'F'ed = 'V'ery 'H'igh impedance. End-fed half-waves have high impedance because current is minimum and voltage is maximum at ends. Requires impedance matching.
实际应用示例
An end-fed half-wave antenna: The feed point is at the end where current is minimum and voltage is maximum. This creates very high impedance (typically 2000-5000 ohms). An impedance matching network (tuner or transformer) is required to match this high impedance to the transmitter. This is why end-fed antennas need matching networks.
错误选项分析
Option A (Very low): Incorrect. End-fed half-wave doesn't have very low impedance - it has very high impedance because current is minimum at the ends. Very low is wrong. Option B (Approximately 50 ohms): Incorrect. End-fed half-wave doesn't have 50 ohms - it has very high impedance (thousands of ohms). 50 ohms is for center-fed dipoles. Option C (Approximately 300 ohms): Incorrect. End-fed half-wave doesn't have 300 ohms - it has very high impedance (thousands of ohms). 300 ohms is too low.
知识点
End-fed half-wave, Feed point impedance, Very high, End feeding
Verified Content
Question from official FCC General Class question pool. Explanation reviewed by licensed amateur radio operators.