# Add circle to Smith chart using PGF

I'm using the Smithchart library with PGF/TikZ, and I'm trying to add a simple circle in the middle of the chart. However, the circle is plotted using the "smithchart coordinate system" so it looks flatted out.

Code:

\documentclass[preview]{standalone}
\usepackage{tikz}
\usepackage{pgfplots}
\usepgfplotslibrary{smithchart}
\pgfplotsset{compat=1.11}

\begin{document}
\begin{tikzpicture}
\begin{smithchart}
\end{smithchart}
\end{tikzpicture}
\end{document}


Result:

I tried digging into the PGFplots manual but found nothing similar.

• Do you need the circle for highlighting a particular point, or is the size of the circle meaningful in terms of the axis units? – Jake Nov 10 '14 at 19:06
• The size is meaningful. For example, I would like to draw the circle in which the arc crossing the x-axis at 1 is in. – Lorenzo Gasparini Nov 10 '14 at 20:32

Take a look at 5.10 Smith Charts of pgfplots manual, there are many examples and notes about your question.

Please note that by changing \path[draw=red] (0pt,0pt) circle (1.5cm); to \path[draw=red] (0,0) circle (1.5cm); you can use the smith's coordinates instead.

\documentclass[preview]{standalone}
\usepackage{tikz}
\usepackage{pgfplots}
\usepgfplotslibrary{smithchart}
\pgfplotsset{compat=1.11}

\begin{document}
\begin{tikzpicture}
\begin{smithchart}
\path[draw=red] (0pt,0pt) circle (1.5cm);
\path[draw=blue] (0.2,0.5) circle (0.75cm);
\path[draw=blue,fill=blue] (0.2,0.5) circle (0.05cm);
\end{smithchart}
\end{tikzpicture}
\end{document}

• What if I would like to plot circle for SWR = 1.5 similiar to yours like \path[draw=blue] (0,0) circle (1.5); with no cm handling? Is it possible too? – Jan Filip Dec 4 '17 at 14:53
• Doesn't work. The coordinates are plotted in the wrong coordinate system. Drawing a circle in the middle of the chart requires me to write the coordinate (0,0) and not (1,0). – GNA Oct 7 '18 at 16:44