AdvancedTopic 96 of 106Electrical Engineering
Smith chart
Graphical tool for impedance matching on transmission lines.
In depth
The Smith chart maps normalised impedance onto a circular chart with constant-resistance and reactance circles. Moving along a line corresponds to rotating around the chart. It enables graphical design of matching networks and reading VSWR, reflection coefficient and admittance.
Key points
- Centre is matched
- Wavelengths toward generator clockwise
- Normalise by Z₀
ExampleA load of 100+j50 Ω on 50 Ω line is plotted at 2+j1.
What is Smith chart?
Graphical tool for impedance matching on transmission lines.
Why is it important to understand smith chart?
The Smith chart puts impedance and reflection information on one diagram, which speeds up matching design by hand.
What does the Smith chart show?
Normalised impedance mapped onto a circle diagram, where the distance from the centre gives the reflection coefficient magnitude.
Why is it called a Smith chart?
It was devised by Phillip Smith in 1939 to simplify transmission line calculations, and it remains a standard tool in RF design.
How is Smith chart used in practice?
Typical applications include designing an L-network to match an antenna to a radio, finding the length of a stub to tune a transmitter output and reading the standing wave ratio from a measured impedance.
What should I learn before Smith chart?
Study "Transmission line theory" first (topic 95 of 106 in Electrical Engineering). It covers the groundwork this topic builds on.
What are the key points of Smith chart?
Centre is matched. Wavelengths toward generator clockwise. Normalise by Z₀.
Can you give an example of Smith chart?
A load of 100+j50 Ω on 50 Ω line is plotted at 2+j1.
What comes after Smith chart?
The next topic is "Waveguides and antennas", which builds directly on this one.
How difficult is Smith chart?
It is classed as Advanced in Electrical Engineering (topic 96 of 106). It is an advanced topic. It assumes you are comfortable with the intermediate topics in Electrical Engineering.