<title>Phaseless measurements of antenna near fields from infrared images using holographic phase retrieval</title>

Abstract

Complex (magnitude and phase) measurements of the near field of a radiating antenna over a known surface (usually a plane, cylinder, or sphere) can be used to determine its far-field radiation pattern using near-field to far-field Fourier transformations. Standard gain horn antennas or open-ended waveguide are often used to probe the near field. This requires the time-consuming positioning of the probe antenna to several thousand positions in the radiating near field of the antenna under test. Experimental errors are introduced into the near-field measurements by mechanical probe position inaccuracies and electrical probe interactions with the antenna under test and probe correction errors. A minimally perturbing infrared (IR) imaging technique can also be used to map the near fields of the antenna. This measurement technique is much simpler and easier to use than the probe method and eliminates probe position errors and probe correction errors. Current IR imaging techniques, which have been successfully used to rapidly map the relative magnitude of a radiating field at many locations (mXn camera pixels per image captured) over a surface, however, suffer from an inability to determine phase information. This paper describes a method for determining the absolute magnitude and relative phase data from these phaseless IR measurements by using techniques derived from optical holography. Form a pair of microwave holograms and magnitude only measurements, the complete near field (magnitude and phase) of the antenna can be determined. Once obtained, this data can then be used to determine the antenna far field pattern by conventional Fourier near-field to far-field transformation techniques.