Which of the following increases the resonant frequency of a dipole antenna?
The correct answer is C: Shortening it. Shortening a dipole antenna increases its resonant frequency. Antenna resonance is related to physical length - shorter antennas resonate at higher frequencies, longer antennas resonate at lower frequencies. For a half-wave dipole, the length is approximately 468/frequency (in MHz) in feet. For amateur radio operators, understanding this relationship helps when tuning antennas - to raise the resonant frequency, make the antenna shorter. To lower it, make it longer.
Exam Tip
Increase resonant frequency = shorten antenna. Think 'S'horter = 'S'ame frequency 'S'hifts higher. Shorter antennas resonate at higher frequencies. Lengthening, coils, and capacitive loading all lower frequency.
Memory Aid
"Increase resonant frequency = shorten antenna. Think 'S'horter = 'S'ame frequency 'S'hifts higher. Shorter physical length = higher resonant frequency. Standard antenna tuning method."
Real-World Application
Your 20-meter dipole resonates at 14.050 MHz, but you want it to resonate at 14.200 MHz (higher frequency). You shorten the antenna by trimming a few inches from each end. The shorter length raises the resonant frequency to 14.200 MHz. This is the standard way to tune a dipole - shorten to raise frequency, lengthen to lower frequency.
Key Concepts
Why Other Options Are Wrong
Option A (Lengthening it): Incorrect. Lengthening a dipole lowers the resonant frequency, not increases it. Longer antennas resonate at lower frequencies.
Option B (Inserting coils): Incorrect. Inserting coils (loading) lowers the resonant frequency by increasing electrical length. It doesn't increase resonant frequency.
Option D (Adding capacitive loading): Incorrect. Adding capacitive loading to the ends also lowers resonant frequency by increasing electrical length. It doesn't raise the frequency.
题目解析
The correct answer is C: Shortening it. Shortening a dipole antenna increases its resonant frequency. Antenna resonance is related to physical length - shorter antennas resonate at higher frequencies, longer antennas resonate at lower frequencies. For a half-wave dipole, the length is approximately 468/frequency (in MHz) in feet. For amateur radio operators, understanding this relationship helps when tuning antennas - to raise the resonant frequency, make the antenna shorter. To lower it, make it longer.
考试技巧
Increase resonant frequency = shorten antenna. Think 'S'horter = 'S'ame frequency 'S'hifts higher. Shorter antennas resonate at higher frequencies. Lengthening, coils, and capacitive loading all lower frequency.
记忆口诀
Increase resonant frequency = shorten antenna. Think 'S'horter = 'S'ame frequency 'S'hifts higher. Shorter physical length = higher resonant frequency. Standard antenna tuning method.
实际应用示例
Your 20-meter dipole resonates at 14.050 MHz, but you want it to resonate at 14.200 MHz (higher frequency). You shorten the antenna by trimming a few inches from each end. The shorter length raises the resonant frequency to 14.200 MHz. This is the standard way to tune a dipole - shorten to raise frequency, lengthen to lower frequency.
错误选项分析
Option A (Lengthening it): Incorrect. Lengthening a dipole lowers the resonant frequency, not increases it. Longer antennas resonate at lower frequencies. Option B (Inserting coils): Incorrect. Inserting coils (loading) lowers the resonant frequency by increasing electrical length. It doesn't increase resonant frequency. Option D (Adding capacitive loading): Incorrect. Adding capacitive loading to the ends also lowers resonant frequency by increasing electrical length. It doesn't raise the frequency.
知识点
Dipole resonance, Antenna length, Frequency tuning, Antenna adjustment
Verified Content
Question from official FCC Technician Class question pool. Explanation reviewed by licensed amateur radio operators.