Discrete Fourier Transform problem

Hi All,

I am trying to do a discrete Fourier transform on a specific cell Vy(i+1,probe_j,probe_k). What is wrong with my code? I just get some dotted lines as output!

close all clear all clc

a = 0.02; d = 0.02;

source_j = 26; source_k = 26;

probe_j = 29; probe_k = 45;

L = (4*pi)*10^(-7); C = 0.5*(8.854*10^(-12)); del_L = 0.0002; vL = 1/sqrt(L*C); ZL = sqrt(L/C); del_t = del_L/(2*vL);

na = (a/del_L); nd = (d/del_L); nt = 1013;

Vy = zeros (nt, nd, na);

V1i = zeros(nt,nd,na); V2i = zeros(nt,nd,na); V3i = zeros(nt,nd,na); V4i = zeros(nt,nd,na);

V1r = zeros(nt,nd,na); V2r = zeros(nt,nd,na); V3r = zeros(nt,nd,na); V4r = zeros(nt,nd,na); f = linspace(1*(10^9),10*(10^9),100);

F=zeros(1013,na,nd);

%%excite just one cell V1i(1,26, 26) = 1; V2i(1,26, 26) = 1; V3i(1,26, 26) = 1; V4i(1,26, 26) = 1;

for i=1:nt-1

    %%%%%%%%%%%%%Scattering%%%%%%%%%%%%%%%%%%%%%%
    for j = 1:nd
        for k = 1:na
%             [V1i(i,j,k); V2i(i,j,k); V3i(i,j,k) ;V4i(i,j,k)] = 0.5*[-1 1 1 1; 1 -1 1 1; 1 1 -1 1; 1 1 1 -1]*[V1i(i,j,k); V2i(i,j,k); V3i(i,j,k); V4i(i,j,k)];
            V1r(i,j,k) = 0.5*(-V1i(i,j,k) + V2i(i,j,k) + V3i(i,j,k) + V4i(i,j,k));
            V2r(i,j,k) = 0.5*(V1i(i,j,k) - V2i(i,j,k) + V3i(i,j,k) + V4i(i,j,k));
            V3r(i,j,k) = 0.5*(V1i(i,j,k) + V2i(i,j,k) - V3i(i,j,k) + V4i(i,j,k));
            V4r(i,j,k) = 0.5*(V1i(i,j,k) + V2i(i,j,k) + V3i(i,j,k) - V4i(i,j,k));
            %
        end
    end
   %%%%%%%%%%%%%Connection%%%%%%%%%%%%%%%%%%%%%%
    %Boundary Cells :-
    k=1;
    for j=2:nd-1
        V1i(i+1,j,k)=-V1r(i,j,k);
        V4i(i+1,j,k)=V2r(i,j+1,k);
        V2i(i+1,j,k)=V4r(i,j-1,k);
        V3i(i+1,j,k)=V1r(i,j,k+1);
    end
    k=na;
    for j=2:nd-1
        V3i(i+1,j,k)=-V3r(i,j,k);
        V4i(i+1,j,k)=V2r(i,j+1,k);
        V1i(i+1,j,k)=V3r(i,j,k-1);
        V2i(i+1,j,k)=V4r(i,j-1,k);
    end
    j=1;
    for k=2:na-1
        V2i(i+1,j,k)=-V2r(i,j,k);
        V1i(i+1,j,k)=V3r(i,j,k-1);
        V3i(i+1,j,k)=V1r(i,j,k+1);
        V4i(i+1,j,k)=V2r(i,j+1,k);
    end
    j=nd;
    for k=2:na-1
        V4i(i+1,j,k)=-V4r(i,j,k);
        V2i(i+1,j,k)=V4r(i,j-1,k);
        V1i(i+1,j,k)=V3r(i,j,k-1);
        V3i(i+1,j,k)=V1r(i,j,k+1);
    end
   %Corner Cells:-
   %Left-Bottom corner:
   j=1;
   k=1;
   V1i(i+1,j,k)=-V1r(i,j,k);
   V2i(i+1,j,k)=-V2r(i,j,k);
   V3i(i+1,j,k)=V1r(i,j,k+1);
   V4i(i+1,j,k)=V2r(i,j+1,k);
   %Right-Bottom corner:
   j=nd;
   k=1;
   V1i(i+1,j,k)=-V1r(i,j,k);
   V4i(i+1,j,k)=-V4r(i,j,k);
   V3i(i+1,j,k)=V1r(i,j,k+1);
   V2i(i+1,j,k)=V4r(i,j-1,k);
   %Left-Top corner:
   j=1;
   k=na;
   V2i(i+1,j,k)=-V2r(i,j,k);
   V3i(i+1,j,k)=-V3r(i,j,k);
   V1i(i+1,j,k)=V3r(i,j,k-1);
   V4i(i+1,j,k)=V2r(i,j+1,k);
   %Right-Top corner:
   j=nd;
   k=na;
   V3i(i+1,j,k)=-V3r(i,j,k);
   V4i(i+1,j,k)=-V4r(i,j,k);
   V1i(i+1,j,k)=V3r(i,j,k-1);
   V2i(i+1,j,k)=V4r(i,j-1,k);
   %Intermediate Cells:-
   for j = 2:nd-1
        for k=2:na-1
            V1i(i+1,j,k) = V3r(i,j,k-1);
            V2i(i+1,j,k) = V4r(i,j-1,k);
            V3i(i+1,j,k) = V1r(i,j,k+1);
            V4i(i+1,j,k) = V2r(i,j+1,k);
        end
    end

    Vy(i+1,probe_j,probe_k) = 0.5*(V1i(i+1,probe_j,probe_k) + V2i(i+1,probe_j,probe_k) + V3i(i+1,probe_j,probe_k) + V4i(i+1,probe_j,probe_k));

F = fft(Vy(i+1,probe_j,probe_k));

end

% plot(1:nt,Vy(:,probe_j,probe_k));

   plot(f, abs(F));
%     plot(Vy(:,30,30));