Complex near-field antenna measurements using infrared (IR) thermograms

This paper describes the application of the plane-to-plane (PTP) iterative Fourier processing technique to infrared thermographic images of microwave fields to calculate the near-field and farfield patterns of radiating antennas. A resistive sheet is positioned in a radiating field, and a thermal picture is then taken of the heat pattern. Each pixel of this thermal image represents a measurement of the intensity (magnitude) of the field at the pixel location on the resistive sheet. The PTP technique allows recovery of the phase by combining measurements made on two planes, both in the radiating near field of the antenna under test. Starting with an estimate of the phase and the measured magnitudes, Fourier processing techniques are used to iteratively propagate between the planes to determine the correct phase distribution at each plane. We describe the technique and show comparisons made between the predicted results and results from measured IR thermograms of the field of a 36 element patch array antenna operating at 4 GHz using the University of Colorado, Colorado Springs (UCCS) Thermal Camera.