ifft in VNA

Sir, I have a bandpass complex data spectrum in 2-20 GHz obtained from Vector Network Analyser i need to take its inverse Fourier Transform how to follow in such a bandpass case. I have worked it out for lowpass case where i took the negative frequencies to be the complex conjugate of the positive and then make a vector containing the both hence the results were found correct but in Bandpass case they are not coming like this please help