What antenna matching system matches coaxial cable to an antenna by connecting the shield to the center of the antenna and the conductor a fraction of a wavelength to one side?
The correct answer is A: Gamma match. The gamma match is an antenna matching system that matches coaxial cable to an antenna by connecting the shield to the center of the antenna and the conductor a fraction of a wavelength to one side. This creates an impedance transformation.
In a gamma match, the coaxial shield connects to the center of the antenna (or a symmetric point), and the center conductor connects to a point offset from center (a fraction of a wavelength to one side). This offset connection, combined with a series capacitor, creates the impedance transformation needed to match the antenna to 50-ohm coax. The gamma match is commonly used with Yagi antennas where the driven element can be connected to the boom (the shield connection). It's a popular matching method because it's relatively simple and works well.
Exam Tip
Gamma match = Shield to center, conductor to side. Remember: Gamma match connects coaxial shield to antenna center and conductor a fraction of wavelength to one side, with a series capacitor for impedance matching.
Memory Aid
"**G**amma **M**atch = **S**hield **C**enter, **C**onductor **S**ide (think 'GM = SCCS')"
Real-World Application
You're matching a Yagi to 50-ohm coax using a gamma match. You connect the coax shield to the center of the driven element (or to the boom if the element is connected). You connect the center conductor to a point offset from center (maybe 1/8 wavelength to one side). You add a series capacitor to complete the match. This transforms the antenna impedance to 50 ohms.
FCC Part 97.3Key Concepts
Why Other Options Are Wrong
Option B: Incorrect. Delta match is a different technique that uses a triangular connection, not the shield-to-center, conductor-to-side configuration described.
Option C: Incorrect. T-match uses a different configuration with connections at different points, not the gamma match configuration.
Option D: Incorrect. Stub match uses a parallel stub, not the series connection configuration of a gamma match.
题目解析
The correct answer is A: Gamma match. The gamma match is an antenna matching system that matches coaxial cable to an antenna by connecting the shield to the center of the antenna and the conductor a fraction of a wavelength to one side. This creates an impedance transformation. In a gamma match, the coaxial shield connects to the center of the antenna (or a symmetric point), and the center conductor connects to a point offset from center (a fraction of a wavelength to one side). This offset connection, combined with a series capacitor, creates the impedance transformation needed to match the antenna to 50-ohm coax. The gamma match is commonly used with Yagi antennas where the driven element can be connected to the boom (the shield connection). It's a popular matching method because it's relatively simple and works well.
考试技巧
Gamma match = Shield to center, conductor to side. Remember: Gamma match connects coaxial shield to antenna center and conductor a fraction of wavelength to one side, with a series capacitor for impedance matching.
记忆口诀
**G**amma **M**atch = **S**hield **C**enter, **C**onductor **S**ide (think 'GM = SCCS')
实际应用示例
You're matching a Yagi to 50-ohm coax using a gamma match. You connect the coax shield to the center of the driven element (or to the boom if the element is connected). You connect the center conductor to a point offset from center (maybe 1/8 wavelength to one side). You add a series capacitor to complete the match. This transforms the antenna impedance to 50 ohms.
错误选项分析
Option B: Incorrect. Delta match is a different technique that uses a triangular connection, not the shield-to-center, conductor-to-side configuration described. Option C: Incorrect. T-match uses a different configuration with connections at different points, not the gamma match configuration. Option D: Incorrect. Stub match uses a parallel stub, not the series connection configuration of a gamma match.
知识点
Gamma match, Coaxial matching, Impedance transformation, Yagi matching
Verified Content
Question from official FCC Extra Class question pool. Explanation reviewed by licensed amateur radio operators.