Hast du es mal mit `\draw (0,0) circle[radius=8];` versucht?
\documentclass{article}
\usepackage{amsmath}
\usepackage{tikz}
\DeclareMathOperator\real{Re}
\DeclareMathOperator\imag{Im}
\begin{document}
\begin{tikzpicture}[x=.3cm, y=.3cm, thick]
\draw[help lines] (-11,-10) grid[step=3mm] (11,10);
\draw[->] (-10,0) -- (10,0) node[right] {$\real$};
\draw[->] (0,-9) -- (0,9) node[above] {$\imag$};
\draw (0.5,8) -- (-0.5,8) node[left] {$1$};
\draw (0.5,-8) -- (-0.5,-8) node[left] {$-1$};
\draw (-8,0.5) -- (-8,-0.5) node[below] {$-1$};
\draw (8,0.5) -- (8,-0.5) node[below] {$1$};
\draw (0,0) circle[radius=8];
\end{tikzpicture}
\end{document}
![alt text][1]
---
Man kann auch was ansehnliches mit `pgfplots` erzeugen.
\documentclass{article}
\usepackage{amsmath}
\usepackage{pgfplots}
\DeclareMathOperator\real{Re}
\DeclareMathOperator\imag{Im}
\begin{document}
\begin{tikzpicture}
\begin{axis}[
axis equal,
axis lines=middle,
xlabel=$\real$,
ylabel=$\imag$,
xtick distance=1,
ytick distance=1,
minor tick num=7,
grid=both,
]
\addplot+[samples=80,domain=0:360,no marks] ({cos(x)},{sin(x)});
\end{axis}
\end{tikzpicture}
\end{document}
![alt text][2]
[1]: http://texwelt.de/wissen/upfiles/test_347.pnghttp://texwelt.de/wissen/upfiles/test_347.png
[2]: http://texwelt.de/wissen/upfiles/test_348.png
