How to find the start and end coordinates of main-lobe of rectangular window spectrum.

The rectangular window spectrum is found like

M=66;
r=rectwin(M);
n=512;
r_fft=fft(r,n);
r_fft2=fftshift(r_fft);
r_fftdB=20*log10(abs(r_fft2));
figure
plot(1:n,r_fftdB)
title(M)

The spectrum of the rectanular window spectrum (r_fftdB) is given by r_fftdB.

The plotted spectrum is

The spectrum is symmetrical about the peak of main-lobe. The starting of main-lobe is at coordinate 249. The ending of the main-lobe is at 265, The peak of main-lobe is at 257.

How to find the values of staring and ending coordinates of main-lobe (249,265 respectively)?

[KSSV]

r_fftdB([249 265])

[Palguna Gopireddy]

@KSSV, My question is to get the values of 249,265 ithout looking at the plot.

Answer 1

[Palguna Gopireddy]

I have tried this

M=66;
r=rectwin(M);
n=512;
r_fft=fft(r,n);
r_fft2=fftshift(r_fft);
r_fftdB=20*log10(abs(r_fft2));
[Max,I_max] = max(r_fftdB);
for i=I_max:-1:1
    if r_fftdB(i)>r_fftdB(i-1)
        start=i-1;
    else
        break;
    end
end
for i=I_max:n
    if r_fftdB(i)>r_fftdB(i+1)
        ending=i+1;
    else
        break;
    end
end
disp(start);

   249

disp(ending);

   265

Is there anyway we can get both the values in the same loop, so that the code-length and runtime minimizes?

Or

is there any other method to get these values?