This paper shows several applications of the diagnostic techniques for the reduction of some error or uncertainty factors in antenna measurements. The method is based in the calculation of the extremely near field from the far field using FFT (Fast Fourier Transform) Techniques, improved with the Gerchberg-Papoulis Algorithm. The classical applications of the diagnostic techniques are errors detection, like phase errors in arrays or conformal errors in reflectors. Therefore, they constitute an important antenna design tool. Also, they can be used for other applications whose aim is improve the measurements in anechoic chambers. This paper shows different process applied to reduce the effect of the reflections, the effect of the leakage from AUT (Antenna under test), to improve the signal to noise and to reduce the truncation error in the planar or cylindrical near field.