Deep Dive: G9B07
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.
Why Other Answers 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.
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 Example
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.
Source & Coverage
Question Pool: 2023-2027 Question Pool
Subelement: G9B
Reference: 2023-2027 Question Pool · G9 - Antennas and Feed Lines
Key Concepts
Verified Content
Question from the official FCC General Class pool. Explanation reviewed by licensed amateur radio operators and mapped to the G9B topic.