The Smith Chart was conceived by Philip Smith, an engineer at Bell Laboratories in the 1930s – in an attempt to avoid the complex calculations associated with RF design and to obtain solutions using a more visual method. His solution was the Smith Chart which is still widely used today.

Why does it work?

Well basically, it’s just geometry.

You can take the impedance plane and bend the vertical axis downwards. This allows us to plot impedance in a convenient circular space. If you are familiar with reactances you can deduce that the upper part represents inductive influences and the lower part represents capacitive influences.

But you can do more than analyse a single component. Every introduction of a component to the circuit moves the point along the chart’s contours with the chart’s centre point representing 50 ohm (for a 50 ohm characteristic impedance). Known as the Z chart, it allows us to compute any impedance whose components are connected in series.
A second variation of this chart or the Y “admittance” chart allows the same calculations with components connected in parallel.

Example of using the Smith Chart

Creating matching networks

When you want to match load and transmitter impedance.

For more details..see our article

Finally…

For the design and tuning of RF circuits, plotting Smith Charts with pen and paper has become largely obsolete with modern CAD software. The automatic display of Smith Charts is a good visual aid to allow observation of impedance in real time and quickly identifying reactances as inductive or capacitive.

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