Which of the following factors affect the feed point impedance of an antenna?
The correct answer is B: Antenna height. Antenna height affects the feed point impedance of an antenna. As antenna height above ground changes, the feed point impedance changes due to ground reflections and coupling effects.
When an antenna is close to ground, ground reflections affect the impedance. As height increases, the impedance changes. For example, a dipole's feed point impedance varies with height - it might be around 50-70 ohms at certain heights but different at other heights. The ground acts as a reflector, and the distance to ground affects how the antenna 'sees' its environment, changing the impedance. This is why antenna height is an important factor in antenna design and tuning.
Exam Tip
Feed point impedance affected by = Antenna height. Remember: Antenna height affects feed point impedance due to ground reflections and coupling. Higher or lower height changes the impedance.
Memory Aid
"**F**eed **P**oint **I**mpedance = **A**ffected by **H**eight (think 'FPI = AH')"
Real-World Application
You install a dipole at 20 feet height and measure 60 ohms feed point impedance. You raise it to 40 feet, and the impedance changes to 72 ohms. The height change affects how the antenna couples with ground, changing the feed point impedance. This is why antenna height is important in antenna design.
FCC Part 97.3Key Concepts
Why Other Options Are Wrong
Option A: Incorrect. Transmission line length affects the impedance seen at the transmitter end, but it doesn't change the antenna's feed point impedance itself. The antenna impedance is what it is; the line transforms it.
Option C: Incorrect. Antenna tuner settings affect the impedance match at the transmitter, but they don't change the antenna's actual feed point impedance. The tuner matches the line to the transmitter.
Option D: Incorrect. Input power level doesn't significantly affect feed point impedance. Impedance is a property of the antenna structure and environment, not power level.
题目解析
The correct answer is B: Antenna height. Antenna height affects the feed point impedance of an antenna. As antenna height above ground changes, the feed point impedance changes due to ground reflections and coupling effects. When an antenna is close to ground, ground reflections affect the impedance. As height increases, the impedance changes. For example, a dipole's feed point impedance varies with height - it might be around 50-70 ohms at certain heights but different at other heights. The ground acts as a reflector, and the distance to ground affects how the antenna 'sees' its environment, changing the impedance. This is why antenna height is an important factor in antenna design and tuning.
考试技巧
Feed point impedance affected by = Antenna height. Remember: Antenna height affects feed point impedance due to ground reflections and coupling. Higher or lower height changes the impedance.
记忆口诀
**F**eed **P**oint **I**mpedance = **A**ffected by **H**eight (think 'FPI = AH')
实际应用示例
You install a dipole at 20 feet height and measure 60 ohms feed point impedance. You raise it to 40 feet, and the impedance changes to 72 ohms. The height change affects how the antenna couples with ground, changing the feed point impedance. This is why antenna height is important in antenna design.
错误选项分析
Option A: Incorrect. Transmission line length affects the impedance seen at the transmitter end, but it doesn't change the antenna's feed point impedance itself. The antenna impedance is what it is; the line transforms it. Option C: Incorrect. Antenna tuner settings affect the impedance match at the transmitter, but they don't change the antenna's actual feed point impedance. The tuner matches the line to the transmitter. Option D: Incorrect. Input power level doesn't significantly affect feed point impedance. Impedance is a property of the antenna structure and environment, not power level.
知识点
Feed point impedance, Antenna height, Ground effects, Impedance variation
Verified Content
Question from official FCC Extra Class question pool. Explanation reviewed by licensed amateur radio operators.