Explanation of Resonance vs Impedance based on Smith Chart

| January 12, 2026 | Amateur Radio (Ham Radio) | No Comments

The Smith Chart is the ultimate visual bridge between resonance and impedance matching. It allows you to see exactly where your antenna is “electrically” and how to move it to where your radio wants it to be. NanoVNA is a relatively inexpensive device to see the Smith Chart of your antenna.

1. Finding Resonance on the Chart

On a Smith Chart, the horizontal centerline is the “Pure Resistance” line.

  • The Rule: Any point on this line has zero reactance (X = 0).
  • Visualizing Resonance: When you sweep an antenna across a range of frequencies, it creates a trace (a line) on the chart. Every time that trace crosses the horizontal center line, the antenna is resonant.
  • The Trap: Resonance can occur anywhere along that line—from the far left (short circuit) to the far right (open circuit). Just because it’s on the line doesn’t mean it’s at the center.

2. Finding the Impedance Match

The exact center of the Smith Chart represents your system’s characteristic impedance (usually 50 Ω).

  • The Goal: Impedance matching is the act of moving your antenna’s current point to that center dot.
  • SWR Circles: You can visualize SWR as a circle centered on the middle of the chart. The smaller the circle, the lower the SWR.
    • A point on the 1.5:1 SWR circle means you have a decent match, even if the antenna isn’t perfectly resonant (i.e., it’s not on the horizontal line).

3. The “Prime” Difference: Moving the Points

The power of the Smith Chart is seeing how components move your “dot” toward the center:

  • Series Inductors/Capacitors: Move the point along Constant Resistance Circles (the circles that all meet at the right side of the chart).
  • Transmission Lines (Coax): Adding coax rotates the point in a circle around the center of the chart. This explains why changing coax length can change your SWR reading at the radio, even though it doesn’t change the antenna itself!

Summary Table

GoalAction on Smith Chart
Achieve ResonanceMove the point onto the Horizontal Center Line.
Achieve MatchingMove the point to the Exact Center Dot (1.0).
Reduce SWRMove the point into a Smaller SWR Circle.