Reading a Smith Chart

The Smith chart is a powerful tool for visualizing complex impedance. This tutorial explains how to read the NanoVNA’s Smith chart display.

What You Will Learn

• The structure of the Smith chart
• How to read resistance and reactance
• Identifying common impedance conditions
• Using Smith chart marker formats

Why Use a Smith Chart?

The Smith chart displays impedance in a way that makes matching problems intuitive:

• A perfect match (50 ohms) is at the center
• Mismatches move away from center
• Inductive loads are in the top half
• Capacitive loads are in the bottom half
• Transmission line effects trace circles around the chart

Smith Chart Anatomy

The Horizontal Axis (Resistance)

The horizontal line through the middle represents pure resistance (no reactance):

Position	Impedance	Meaning
Far left	0 ohms	Short circuit
Center	50 ohms	Perfect match
Far right	Infinite	Open circuit

Resistance Circles

The circles that touch the right edge represent constant resistance:

• Smaller circles = lower resistance
• The center point is on the 50-ohm circle (normalized to 1.0)
• Circles are labeled in normalized values (multiply by 50 for ohms)

Reactance Arcs

The curved lines (arcs) represent constant reactance:

• Top half: Positive reactance (inductive, +jX)
• Bottom half: Negative reactance (capacitive, -jX)
• Horizontal axis: Zero reactance (purely resistive)

Enabling Smith Chart Display

1. Select a trace

   Touch the trace indicator (colored number) or go to DISPLAY > TRACE.

2. Set format to SMITH

   Go to DISPLAY > FORMAT S11 (REFL) > SMITH

3. Choose marker format

   The Smith chart supports multiple marker readout formats. Touch MARKER and select a marker, then the readout shows impedance.

Smith Chart Marker Formats

Access marker formats through the menu when Smith chart is active. The firmware supports these formats:

Format	Display	Best For
R+jX	Resistance + Reactance	General impedance reading
R//LC	Parallel R, L, or C	Matching network design
G+jB	Conductance + Susceptance	Admittance analysis
Lin Mag	Linear magnitude	Reflection coefficient
Log Mag	dB magnitude	Return loss

1. Change marker format

   With Smith chart displayed, select a marker and touch the readout area, or navigate to the marker smith format menu.

2. Select your preferred format

   Choose R+jX for standard impedance readings in ohms.

Reading Impedance on the Smith Chart

Finding Your Point

When you enable the Smith chart, the trace shows how impedance varies with frequency:

1. Locate the marker

   The marker appears as a small triangle or dot on the trace.

2. Read the position

   • Distance from center = magnitude of mismatch
   • Angle from horizontal = phase of reflection coefficient

3. Read the marker values

   The marker readout shows the actual impedance at that frequency.

Common Impedance Conditions

Perfect Match

Location: Center of chart

Impedance: 50 + j0 ohms

Meaning: No reflected power, ideal condition

SWR: 1:1

Short Circuit

Location: Far left edge

Impedance: 0 + j0 ohms

Meaning: All power reflected, 180-degree phase shift

SWR: Infinite

Open Circuit

Location: Far right edge

Impedance: Infinite

Meaning: All power reflected, 0-degree phase shift

SWR: Infinite

Inductive Load

Location: Upper half of chart

Impedance: R + jX where X > 0

Meaning: Load has inductive reactance (antenna too long)

Fix: Add series capacitor or shorten antenna

Capacitive Load

Location: Lower half of chart

Impedance: R - jX where X < 0

Meaning: Load has capacitive reactance (antenna too short)

Fix: Add series inductor or lengthen antenna

The Constant SWR Circles

Imagine circles centered on the chart center. These represent constant SWR:

• Center point: SWR = 1:1
• Small circle around center: SWR = 1.5:1
• Larger circle: SWR = 2:1
• Edge of chart: SWR = infinite

Tip

A good antenna match keeps the trace within a small circle around the center across your operating frequency range.

Transmission Line Effects

When you add transmission line length, the impedance point rotates clockwise around the chart:

• One complete rotation = half wavelength of cable
• The rotation follows constant SWR circles
• Cable loss causes the trace to spiral inward

This is why calibrating at the antenna (not at the radio) gives more accurate results.

Example: Analyzing an Antenna

1. Connect the antenna and set frequency range

   Cover your band of interest with some margin.

2. Enable Smith chart display

3. Place marker at your operating frequency

4. Read the impedance

   Example reading: 52.3 + j12.5 ohms

5. Interpret the result

   • Resistance (52.3 ohms) is close to 50 ohms - good
   • Reactance (+j12.5 ohms) is positive - slightly inductive
   • The antenna is slightly too long or needs a small series capacitor

Matching Network Design

The Smith chart excels at designing matching networks:

Moving Along the Chart

Component	Movement on Chart
Series inductor	Clockwise along constant R circle
Series capacitor	Counter-clockwise along constant R circle
Shunt inductor	Clockwise along constant G circle
Shunt capacitor	Counter-clockwise along constant G circle

Using the L/C Match Function

The NanoVNA has a built-in L/C matching calculator:

1. Enable L/C Match

   Go to MARKER > MEASURE > L/C MATCH

2. Place marker on your frequency

3. Read the suggested component values

   The display shows suggested inductor and capacitor values to achieve a match.

Tips for Using the Smith Chart

Zooming the View

The NanoVNA displays the entire Smith chart by default. For loads close to 50 ohms, most of the chart is unused. Use:

• Marker readouts for precise values
• Multiple traces if needed (LOGMAG + SMITH)

Frequency Direction

As frequency increases, the trace typically moves clockwise (for inductive loads) or counter-clockwise (for capacitive loads). Mark your start and stop frequencies to understand the direction.

Multiple Resonances

Antennas often cross through the center of the chart (good match) at multiple frequencies. Use markers to identify each resonance and choose the one at your desired operating frequency.

Next Steps

• Your First S11 Measurement - Practical antenna measurement
• L/C Matching Networks - Design matching circuits
• Tuning HF Antennas - Apply Smith chart knowledge