Deep Dive: G9A07
The correct answer is D: The antenna feed point impedance must be matched to the characteristic impedance of the feed line. What must be done to prevent standing waves on a feed line connected to an antenna is that the antenna feed point impedance must be matched to the characteristic impedance of the feed line. When impedances match, no power is reflected, so no standing waves form. For amateur radio operators, this is why impedance matching is essential. Understanding this helps when designing antenna systems.
Why Other Answers Are Wrong
Option A: Incorrect. DC ground potential doesn't prevent standing waves - standing waves are caused by impedance mismatch, not DC grounding. DC ground isn't the solution. Option B: Incorrect. Odd number of quarter wavelengths doesn't prevent standing waves - standing waves exist whenever there's impedance mismatch, regardless of length. Length isn't the solution. Option C: Incorrect. Even number of half wavelengths doesn't prevent standing waves - standing waves exist whenever there's impedance mismatch. Length isn't the solution.
Exam Tip
Prevent standing waves = match antenna impedance to feed line impedance. Think 'P'revent 'S'tanding 'W'aves = 'P'roper 'S'ignal 'W'ithout reflection (impedance match). When impedances match, no power is reflected, so no standing waves. Not DC ground, not length - just impedance matching.
Memory Aid
Prevent standing waves = match antenna impedance to feed line impedance. Think 'P'revent 'S'tanding 'W'aves = 'P'roper match. When impedances match, no power is reflected, so no standing waves. Essential for low SWR.
Real-World Example
An antenna feed point has 50-ohm impedance, and the feed line is 50 ohms. The impedances match, so no power is reflected, and no standing waves form. If the antenna were 100 ohms and feed line 50 ohms, there would be impedance mismatch, causing reflected power and standing waves. Impedance matching prevents standing waves.
Source & Coverage
Question Pool: 2023-2027 Question Pool
Subelement: G9A
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 G9A topic.