Amplitude and phase recovery using holographic imaging of antenna feeds

Abstract

It is possible to use wave-front reconstruction for imaging at millimetre wavelengths employing off-axis holography (a frequently used technique at visible wavelengths). We report on how the technique can also be used for imaging the phase centre of non-standard feed antennas at millimetre wavelengths such as planar lens antennas for example. Holography provides a method for recording a lens-less image of an object reducing loss of spatial frequency information important for maximum resolution. An experimental arrangement at 100 GHz based on a simple form of near-field off-axis holography was developed, with the object and reference beams derived from two radiating horn antennas fed by a single coherent source via a 3dB cross-guide coupler. The reference beam derived from a well understood and characterised horn was collimated using a large off-axis mirror, while the object beam was derived directly from the horn antenna whose pattern is to be measured. The hologram (or intensity pattern) resulting from the interference of the two beams was recorded over an area of 150 × 150 mm with a spatial resolution of 1 mm by a scanning detector and the object wave-front recovered by simulating the reconstruction through near-field diffraction of the reference beam. It is possible to model the propagation of the recovered object beam back towards the horn and recover the object horn fields in the vicinity of the waist (the effective phase centre of the horn). This is a useful inexpensive experimental method for recovering the phase centre position of non-standard feeds.