how to IFFT a spectrum

hi fellows

I have a spectrum and I want to transform it to time domain by means of IFFT.but what I got, has different magnitude from its spectrum.

I am not sure if my code is correct or not.I already checked out similar problems but they didn't help me.

please help out

thanks

here is my code:

FRF_Z_Zb is my spectrum which is a vector
Fs = 2048;T = 1/Fs;
num_s = 20397;
df = Fs/(num_s-1)
t = (0:num_s-1)*T;
FRF_Z_Zb_flip = fliplr(FRF_Z_Zb);
FRF_T = [FRF_Z_Zb FRF_Z_Zb_flip];

figure  % generated spectrum for ifft
fr2 = [freq freq+max(freq)];
plot(fr2,abs(FRF_T))
xlabel ('Frequency [Hz]')
ylabel ('Amplitude [mm]')
title ('Spectrum2')

s_F = FRF_T * num_s/2 ;

inver = ifft(s_F,'symmetric');
signal = real(inver);

figure % recovered signal
tt=[t t+max(t)];
plot(tt,signal)
title('recovered signal')

then I have to convolute it with another signal