\documentclass[tikz]{standalone}

\usepackage{pgfplots}

\pgfplotsset{compat=1.14}

\begin{document}

\begin{tikzpicture}
    \begin{axis}[xmin=0, xmax=4, width=12cm, height=5cm, xtick={0,1,...,4}, xlabel={$p$}, ylabel={$|\underline H|$}]
        \addplot expression [domain=0:4, mark=none, samples=200] {sqrt(((2*x*0.2)^2+1)/((2*x*0.2)^2+(1-x^2)^2))};
        \legend{$q=0.2$}
    \end{axis}
\end{tikzpicture}

\end{document}
