Applicability investigation of holographic back-projection of spherical near field measured data

Planar holographic back-projection of spherical near-field measurements has been available for some time. Due to finite resolution and truncation effects, the classical implementation of this technique has mainly been applied to high directivity antennas like reflectors or phased array antenna to investigate phenomena´s like feeding network anomalies, sources of cross polarisation or local faulting. This paper discusses the applicability of the holographic back-projection technique for low and medium gain antennas, alone or mounted on a structure. To investigate the applicability of the holography method a comparison has been performed with an equivalent source (ES) back-propagation technique on numerical examples evaluated by method of moments (MoM). In the ES formulation, planar fields are evaluated as a summation of field contributions from equivalent sources located on the near field sphere surrounding the antenna. By using numerical examples the evaluation of the spherical near field can be performed at any near field distance. If the near field sphere is very close to the antenna, also the reactive field components are included, reproducing the highest achievable level of detail of the radiating structure. Although purely theoretical and impractical for realistic antenna measurements systems the ES method represents a benchmark back-transformation solution. It also determines the maximum achievable resolution and can be used in the evaluation of the truncation error effect on the holography solution.