\documentclass[crop,border=3,tikz,convert={outext=.svg,command=\unexpanded{pdf2svg \infile\space\outfile}},multi=false]{standalone}
\usepackage{ifthen}
\usetikzlibrary{math}
\usetikzlibrary{calc,through,shapes}

\pgfkeys{/tikz/savelength/.code 2 args={\global\edef#1{#2}}}

\pgfdeclarelayer{background}
\pgfdeclarelayer{foreground}
\pgfsetlayers{background,main,foreground}

\def\lines{40}
\def\omegat{20}
\def\PhaseDiff{359}

\begin{document}
  \begin{tikzpicture}[>=latex]
    \tikzmath{\D=\PhaseDiff;}
    \draw[gray,->] (0,-4) -- (0,4);
    \draw[gray,->] (-4,0) -- (4,0);
    \coordinate (A) at (0,0);
    \tikzmath{\Amp=5/\lines;}
    \coordinate (B) at (\omegat:\Amp);
    \foreach \a in {1,...,\lines}{
      \draw[ultra thick,->,blue] (A) -- (B);
      \coordinate (A) at (B);
      \coordinate (F) at (B);
      \coordinate (B) at ($ (B)+(\a*\D+\omegat:\Amp) $);
    }
    \node()[anchor=north east] at (5,5){\LARGE $\delta=\D^\circ$};

    \path let \p1=(F), \n1={veclen(\x1,\y1)} in [savelength={\n}{\n1}];
 %   \node at (2,2){\n};

    \ifthenelse{\lengthtest{\n < 8 pt}}{
      \draw[ultra thick,red,tips=proper] (0,0) -- (F);
      \ifthenelse{\lengthtest{\n < 1 pt}}{
        \draw[ultra thick] (0,0) circle (1);
        %\node at (-1.5,-1.5){Minimum};
      }{}
    }{
      \draw[ultra thick,red,->,tips=proper] (0,0) -- (F);
    }

    \begin{pgfonlayer}{background}
      \draw[fill=white] (-5.1,-5.1) rectangle (5.1,5.1);
      \path[fill=red!30] let \p1=(F), \n1={veclen(\x1,\y1)}  in (0,0) circle[radius=\n1];
    \end{pgfonlayer}
  \end{tikzpicture}
\end{document}