In this paper we report on the improvements in holographic techniques developed for applications in the millimeter-wave and terahertz range of the electromagnetic spectrum. An experimental arrangement, adapted from off-axis near-field holography at visible wavelengths, was employed that utilizes a raster scanning detector to record the holograms digitally. The object and reference fields were based on the beams from a pair of radiating antennas fed by a single coherent source via a cross-guide coupler. Using phase retrieval methods, the recorded holographic interference pattern can be used to determine the effective phase centers of radiating feed antennas, including non standard radiators such as planar lens antennas. By numerically propagating the recovered object beam back to the source plane the object beam in the vicinity of the waist (the effective phase center) can be recovered. Among the issues investigated was improvement in the accuracy of the phase retrieval process by taking account of the non-perfect reference beam. The technique has also been applied to the investigation of increased co-polarisation levels in the scattering of radiation from surface features of dielectric materials on millimeter-wave radiation.