What third family of circles is often added to a Smith chart during the process of designing impedance matching networks?
The correct answer is A: Constant-SWR circles. A third family of circles often added to a Smith chart during impedance matching network design is constant-SWR circles. These circles show all impedances that have the same SWR value.
Constant-SWR circles are concentric circles centered on the chart. All points on a given SWR circle have the same SWR value. For example, all points on the 2:1 SWR circle have SWR = 2:1, regardless of their specific impedance. These circles are very useful for matching network design because they show you what SWR you'll achieve. You can see how close your impedance is to the center (1:1 SWR) and design matching networks to move your impedance onto lower SWR circles. Constant-SWR circles are essential tools for impedance matching work.
Exam Tip
Smith chart third family = Constant-SWR circles. Remember: Constant-SWR circles are often added to Smith charts - they show all impedances with the same SWR value, which is very useful for matching network design.
Memory Aid
"**S**mith **C**hart **T**hird **F**amily = **C**onstant-**S**WR **C**ircles (think 'SCTF = CSWC')"
Real-World Application
You're designing a matching network using a Smith chart. You see constant-SWR circles - maybe a 2:1 SWR circle, a 3:1 SWR circle, etc. Your antenna impedance is on the 3:1 SWR circle. You design a matching network to move it to the 1.5:1 SWR circle (closer to center). The constant-SWR circles help you see your progress toward a better match.
FCC Part 97.3Key Concepts
Why Other Options Are Wrong
Option B: Incorrect. Transmission line length circles aren't a standard third family. Length is represented by moving around the chart, not by circles.
Option C: Incorrect. Coaxial-length circles aren't a standard third family on Smith charts.
Option D: Incorrect. Radiation-pattern circles aren't part of Smith charts. Smith charts are for impedance, not radiation patterns.
题目解析
The correct answer is A: Constant-SWR circles. A third family of circles often added to a Smith chart during impedance matching network design is constant-SWR circles. These circles show all impedances that have the same SWR value. Constant-SWR circles are concentric circles centered on the chart. All points on a given SWR circle have the same SWR value. For example, all points on the 2:1 SWR circle have SWR = 2:1, regardless of their specific impedance. These circles are very useful for matching network design because they show you what SWR you'll achieve. You can see how close your impedance is to the center (1:1 SWR) and design matching networks to move your impedance onto lower SWR circles. Constant-SWR circles are essential tools for impedance matching work.
考试技巧
Smith chart third family = Constant-SWR circles. Remember: Constant-SWR circles are often added to Smith charts - they show all impedances with the same SWR value, which is very useful for matching network design.
记忆口诀
**S**mith **C**hart **T**hird **F**amily = **C**onstant-**S**WR **C**ircles (think 'SCTF = CSWC')
实际应用示例
You're designing a matching network using a Smith chart. You see constant-SWR circles - maybe a 2:1 SWR circle, a 3:1 SWR circle, etc. Your antenna impedance is on the 3:1 SWR circle. You design a matching network to move it to the 1.5:1 SWR circle (closer to center). The constant-SWR circles help you see your progress toward a better match.
错误选项分析
Option B: Incorrect. Transmission line length circles aren't a standard third family. Length is represented by moving around the chart, not by circles. Option C: Incorrect. Coaxial-length circles aren't a standard third family on Smith charts. Option D: Incorrect. Radiation-pattern circles aren't part of Smith charts. Smith charts are for impedance, not radiation patterns.
知识点
Smith chart, Constant-SWR circles, SWR, Impedance matching
Verified Content
Question from official FCC Extra Class question pool. Explanation reviewed by licensed amateur radio operators.