Which of the following describes a log-periodic antenna?
The correct answer is A: Element length and spacing vary logarithmically along the boom. What describes a log-periodic antenna is that element length and spacing vary logarithmically along the boom. Log-periodic antennas have elements that get progressively longer and spacing that increases logarithmically. For amateur radio operators, this is the defining characteristic. Understanding this helps when identifying log-periodic antennas.
Exam Tip
Log-periodic description = element length and spacing vary logarithmically. Think 'L'og-'P'eriodic = 'L'ength and 'P'spacing vary 'L'ogarithmically. Log-periodic antennas have elements that get progressively longer and spacing that increases logarithmically. Not impedance variation, not gain variation, not SWR variation - just logarithmic length and spacing.
Memory Aid
"Log-periodic description = element length and spacing vary logarithmically. Think 'L'og-'P'eriodic = 'L'ogarithmic variation. Log-periodic antennas have elements that get progressively longer and spacing that increases logarithmically. Defining characteristic."
Real-World Application
A log-periodic antenna: Elements get progressively longer from front to back, and spacing between elements increases logarithmically. This logarithmic variation is what makes it 'log-periodic' - the ratio of element lengths and spacing follows a logarithmic progression. This is the defining characteristic of log-periodic antennas.
Key Concepts
Why Other Options Are Wrong
Option B: Incorrect. Impedance doesn't vary periodically as a function of frequency - element length and spacing vary logarithmically. Impedance variation isn't the description.
Option C: Incorrect. Gain doesn't vary logarithmically as a function of frequency - element length and spacing vary logarithmically. Gain variation isn't the description.
Option D: Incorrect. SWR doesn't vary periodically as a function of boom length - element length and spacing vary logarithmically. SWR variation isn't the description.
题目解析
The correct answer is A: Element length and spacing vary logarithmically along the boom. What describes a log-periodic antenna is that element length and spacing vary logarithmically along the boom. Log-periodic antennas have elements that get progressively longer and spacing that increases logarithmically. For amateur radio operators, this is the defining characteristic. Understanding this helps when identifying log-periodic antennas.
考试技巧
Log-periodic description = element length and spacing vary logarithmically. Think 'L'og-'P'eriodic = 'L'ength and 'P'spacing vary 'L'ogarithmically. Log-periodic antennas have elements that get progressively longer and spacing that increases logarithmically. Not impedance variation, not gain variation, not SWR variation - just logarithmic length and spacing.
记忆口诀
Log-periodic description = element length and spacing vary logarithmically. Think 'L'og-'P'eriodic = 'L'ogarithmic variation. Log-periodic antennas have elements that get progressively longer and spacing that increases logarithmically. Defining characteristic.
实际应用示例
A log-periodic antenna: Elements get progressively longer from front to back, and spacing between elements increases logarithmically. This logarithmic variation is what makes it 'log-periodic' - the ratio of element lengths and spacing follows a logarithmic progression. This is the defining characteristic of log-periodic antennas.
错误选项分析
Option B: Incorrect. Impedance doesn't vary periodically as a function of frequency - element length and spacing vary logarithmically. Impedance variation isn't the description. Option C: Incorrect. Gain doesn't vary logarithmically as a function of frequency - element length and spacing vary logarithmically. Gain variation isn't the description. Option D: Incorrect. SWR doesn't vary periodically as a function of boom length - element length and spacing vary logarithmically. SWR variation isn't the description.
知识点
Log-periodic antenna, Element length, Spacing, Logarithmic variation
Verified Content
Question from official FCC General Class question pool. Explanation reviewed by licensed amateur radio operators.