What must be done to prevent standing waves on a feed line connected to an antenna?
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.
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 Application
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.
Key Concepts
Why Other Options 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.
题目解析
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.
考试技巧
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.
记忆口诀
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.
实际应用示例
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.
错误选项分析
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.
知识点
Standing waves, Feed line, Impedance matching, Feed point impedance
Verified Content
Question from official FCC General Class question pool. Explanation reviewed by licensed amateur radio operators.