How do you use a Smith chart for antenna matching?

The Smith chart, introduced by Phillip H. Smith in 1939, plots normalized impedance z = Z/Z₀ on the complex reflection-coefficient plane Γ = (z−1)/(z+1), with circles of constant resistance and arcs of constant reactance. For antenna work it shows the impedance locus versus frequency, makes the effect of each matching element visible as a movement along a circle, and displays VSWR as concentric circles around the center.^[1]

How do you read an antenna’s locus?

A resonant antenna crosses the horizontal axis (X = 0). A tight loop or kink indicates a second resonance or coupled mode; a small circle around the center indicates a broadband match. The VSWR = 2 circle has radius |Γ| = 1/3; points inside meet the specification.^[1]

How do matching elements move the impedance?

• Series inductor: clockwise along a constant-resistance circle (upward).
• Series capacitor: counter-clockwise along a constant-resistance circle (downward).
• Shunt capacitor: clockwise along a constant-conductance circle.
• Shunt inductor: counter-clockwise along a constant-conductance circle.
• Lossless line: clockwise rotation about the center (toward generator) by 2βl.^[1]

What is a practical workflow?

1. Calibrate the VNA and set the reference plane at the matching pads.
2. Measure the antenna in its final enclosure.
3. Choose topology from the region: e.g., if the impedance lies inside the 1+jx circle, start with a shunt element.
4. Pick standard part values, re-measure, iterate for the full band.^[2][1]

Frequently asked questions

See also

• Impedance matching
• VSWR and return loss
• Input impedance
• VNA measurement

References

Online sources accessed October 21, 2026; printed works cited by edition. See all sources.

1. “Microwave Engineering, 4th ed.”. D. M. Pozar, Wiley. 2012.
2. “Applying Error Correction to Network Analyzer Measurements (Application Note 1287-3)”. Keysight Technologies. application note.