Needs["Calculus`FourierTransform`"];
f=If[x>-0.25 && x<0.25,1,0];
reihe=NFourierTrigSeries[f,x,10,AccuracyGoal->4]
bild=Plot[{f,reihe},{x,-0.5,0.5},
PlotStyle->{Dashing[{0.01}],{}},AxesLabel->{"x","y"},
ImageSize->{400,300}];