E9G09: Smith chart: What third family of circles is often

Q: 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. 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.

Updated: Dec 9, 2025 | Source: 2024-2028 Question Pool | Topic: E9G

E9G09E9G

What third family of circles is often added to a Smith chart during the process of designing impedance matching networks?

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Deep Dive: E9G09

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.

Why Other Answers 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.

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 Example

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.

Source & Coverage

Question Pool: 2024-2028 Question Pool

Subelement: E9G

Reference: FCC Part 97.3

Key Concepts

Smith chart Constant-SWR circles SWR Impedance matching

Verified Content

Question from the official FCC Extra Class pool. Explanation reviewed by licensed amateur radio operators and mapped to the E9G topic.