How does the feed point impedance of a horizontal 1/2 wave dipole antenna change as the antenna height is reduced to 1/10 wavelength above ground?
The correct answer is B: It steadily decreases. How the feed point impedance of a horizontal 1/2 wave dipole antenna changes as the antenna height is reduced to 1/10 wavelength above ground is that it steadily decreases. As height decreases, ground effects increase, reducing the feed point impedance. For amateur radio operators, this explains why low dipoles have lower impedance. Understanding this helps when installing dipoles.
Exam Tip
Dipole impedance vs height = steadily decreases as height reduced. Think 'D'ecreasing 'H'eight = 'D'ecreasing 'H'eight impedance. As height decreases, ground effects increase, reducing feed point impedance. Not increases, not peaks, not unaffected - just steadily decreases.
Memory Aid
"Dipole impedance vs height = steadily decreases as height reduced. Think 'D'ecreasing 'H'eight = 'D'ecreasing impedance. As height decreases, ground effects increase, reducing feed point impedance. Important for dipole installation."
Real-World Application
A horizontal dipole: At high height (e.g., 1 wavelength), feed point impedance is about 73 ohms. As height is reduced to 1/10 wavelength, ground effects increase, and the feed point impedance steadily decreases (e.g., to about 50 ohms or less). Lower height means lower impedance due to ground effects.
Key Concepts
Why Other Options Are Wrong
Option A: Incorrect. Impedance doesn't steadily increase - it decreases as height is reduced. Increase is wrong.
Option C: Incorrect. Impedance doesn't peak at 1/8 wavelength - it steadily decreases as height is reduced. Peak isn't the behavior.
Option D: Incorrect. Impedance is affected by height - ground effects change impedance. No effect is wrong.
题目解析
The correct answer is B: It steadily decreases. How the feed point impedance of a horizontal 1/2 wave dipole antenna changes as the antenna height is reduced to 1/10 wavelength above ground is that it steadily decreases. As height decreases, ground effects increase, reducing the feed point impedance. For amateur radio operators, this explains why low dipoles have lower impedance. Understanding this helps when installing dipoles.
考试技巧
Dipole impedance vs height = steadily decreases as height reduced. Think 'D'ecreasing 'H'eight = 'D'ecreasing 'H'eight impedance. As height decreases, ground effects increase, reducing feed point impedance. Not increases, not peaks, not unaffected - just steadily decreases.
记忆口诀
Dipole impedance vs height = steadily decreases as height reduced. Think 'D'ecreasing 'H'eight = 'D'ecreasing impedance. As height decreases, ground effects increase, reducing feed point impedance. Important for dipole installation.
实际应用示例
A horizontal dipole: At high height (e.g., 1 wavelength), feed point impedance is about 73 ohms. As height is reduced to 1/10 wavelength, ground effects increase, and the feed point impedance steadily decreases (e.g., to about 50 ohms or less). Lower height means lower impedance due to ground effects.
错误选项分析
Option A: Incorrect. Impedance doesn't steadily increase - it decreases as height is reduced. Increase is wrong. Option C: Incorrect. Impedance doesn't peak at 1/8 wavelength - it steadily decreases as height is reduced. Peak isn't the behavior. Option D: Incorrect. Impedance is affected by height - ground effects change impedance. No effect is wrong.
知识点
Horizontal dipole, Feed point impedance, Antenna height, Impedance decrease
Verified Content
Question from official FCC General Class question pool. Explanation reviewed by licensed amateur radio operators.