Deep Dive: G9B02
The correct answer is B: Slope the radials downward. A common way to adjust the feed point impedance of an elevated quarter-wave ground-plane vertical antenna to be approximately 50 ohms is to slope the radials downward. Sloping radials downward increases the feed point impedance from the typical 36 ohms (horizontal radials) to about 50 ohms. For amateur radio operators, this is a common impedance matching technique. Understanding this helps when designing vertical antennas.
Why Other Answers Are Wrong
Option A: Incorrect. Sloping radials upward decreases impedance, not increases it - downward slope increases impedance. Upward is wrong direction. Option C: Incorrect. Lengthening radials beyond one wavelength doesn't adjust impedance to 50 ohms - radial length affects other things, not impedance matching. Length isn't the adjustment. Option D: Incorrect. Coiling radials doesn't adjust impedance to 50 ohms - coiling affects inductance, not impedance matching. Coiling isn't the adjustment.
Exam Tip
Adjust ground-plane to 50Ω = slope radials downward. Think 'S'lope 'D'ownward = 'S'lightly 'D'ifferent impedance (increases to 50Ω). Sloping radials downward increases feed point impedance from 36Ω to about 50Ω. Not upward (decreases), not lengthen, not coil - just slope downward.
Memory Aid
Adjust ground-plane to 50Ω = slope radials downward. Think 'S'lope 'D'ownward = 'S'lightly 'D'ifferent. Sloping radials downward increases feed point impedance from 36Ω to about 50Ω. Common impedance matching technique.
Real-World Example
An elevated quarter-wave ground-plane vertical: Horizontal radials give about 36-ohm impedance. Sloping the radials downward (e.g., 45° angle) increases the feed point impedance to approximately 50 ohms, matching standard feed lines. This is a common technique to match ground-plane antennas to 50-ohm feed lines without a matching network.
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.