FF-NF Back Projection

I was wondering if anyone had any experience in near-field to far-field back transformations. I have a simulated far-field pattern and I'd like to back project the fields onto the aperture of the array. I've gotten NF-FF transformations to work and also NF planar back to the aperture to work but I'm having trouble with back projecting far-field patterns in angle space back to the aperture. From what I've read, the first step is to interpolate onto a uniform grid in U-V space and then just take the inverse FFT. That doens't seem to be working. It seems like there needs to be some sort of focusing step to fucus the far-field grid on points back to a near-field plane in front of the aperture. Any help would be appreciated. Thanks!