Electromagnetic magnitude and phase measurements from infrared thermograms

This paper describes a technique for determining the magnitude and phase of a radiating electromagnetic (EM) field from measurements taken using an Infrared (IR) camera. A thin resistive screen of low conductivity is positioned in the radiating field. The resistive screen absorbs energy from the radiating field and converts this energy into heat within the screen. An IR thermal picture is then taken of the heat distribution on the screen using an IR imaging camera (either a “scanning” array or “starring” focal plane array). The resulting 2D image is called an IR thermogram, i.e. an iso-temperature contour map of the data which is a representation of the absorbed electric field. The thermogram is processed to determine the intensity (magnitude) of the radiating field at each pixel location in the thermal image. This IR technique is described and comparisons are made between standard probe measurements and results from measured IR thermographs. The thermogram is also processed to determine the phase of the radiating field by the application of a plane-to-plane (PTP) iterative Fourier processing technique to the near-field IR thermographic images of the radiation, for the purpose of determining the complete complex near-field and far-field patterns of the radiating antenna. The PTP technique allows recovery of the phase by combining magnitude-only measurements made on two planes, both in the radiating near field of the antenna under test. The PTP technique is described and comparisons are made between predicted results and results from measured IR thermograms of the field of a 36 element patch array antenna operating at 4 GHz