You must
- move your sum inside the loop
- increment your summation variable
Note that your vector is complex and plot displays only the real part
trangeF = 0:0.001:1.6; % Time range
% initialization
sums = zeros(size(trangeF));
for n = -2:2
if n == 0
s0 = 312.5.*((-0.08481.*1j.*exp(-0.08481.*1j.*n)/-7.8539*1j)-((-7.8539.*1j.*n)/(-61.6837.*(n^2)+400)).*(exp(-0.08481.*1j.*n)-1));
xt = (s0).*(exp(7.8539.*1j.*n.*trangeF));
else
s1 = 312.5.*(((exp(-0.08481.*1j.*n)-1)/-7.8539.*1j.*n)-(((-7.8539.*1j.*n)/(-61.6837.*(n^2)+400)).*(exp(-0.08481.*1j.*n)-1)));
xt = (s1).*(exp(7.8539.*1j.*n.*trangeF));
end
% sum in the loop
sums = sums + xt;
end
figure
plot(trangeF, sums,'b')
