Quantitative images of antenna patterns using infrared thermography and microwave holography

Abstract

ABSTRACT:In this paper, we apply microwave holography to infrared (IR) thermal images of electromagnetic (EM) fields to measure near-field and far-field radiation patterns of antennas. The phase of the field is retrieved from IR thermograms (magnitude-only thermal images of the field) measured in the near field of the antenna using microwave holography. One method to extract the phase is to use an iterative plane-to-plane (PTP) two-dimensional (2D) holographic phase retrieval method. After the phase is recovered, the near-field thermographic/holographic data can be processed to determine the complex intensity (magnitude and phase) of the field at any distance in front of the antenna under test (AUT). Of particular interest is the far-field radiation pattern of the antenna or, especially for phased array antennas, the aperture source-plane distribution. Numerical simulations were performed to determine the feasibility, accuracy, and sensitivity of these IR thermographic/holographic phase-retrieval techniques. The advantages and disadvantages of the technique are also discussed. To demonstrate the feasibility of the technique, actual IR thermograms were obtained at the Air Force Research Laboratory/ Rome Research Site (AFRL/RRS) in Rome, (New York) and holograms were derived from a simple planar 6 ϫ 6 phased array patch antenna using the PTP technique. The phase-retrieval results are presented and compared with the known results for this antenna, as measured on the near-field range at the National Institute of Technology and Standards (NIST) using standard hard-wired probes. The agreement between the results for this antenna is very good.

Wideband Validation of a Phase Retrieval Process Applied to Infrared Planar Near-Field Measurements

“…For instance, the camera resolution is 320 × 240 pixels leading to measurement plane of maximum size equal to 1.6 × 1.2 m 2 with a λ/3 discretization step at f = 20 GHz. The dynamic of the measurement is also a critical point [15]. In order to improve the latter, an averaging on several images is applied (typically 8000 images are captured).…”

“…Many phase retrieval techniques using magnitude only measurements have been developed for various application areas [3,4,6,7,9,10,[13][14][15][16][17]20]. In most cases, the known of the source geometry is used in combination with an optimization procedure.…”

“…The results presented in this paper are based upon the classical iterative PTP phase retrieval process [5,6,14,15] as far as no a-priori information related to the source geometry is available and because of the huge number of unknowns involved. To optimize the measurement configuration a parametric study has been performed on the distance between the acquisition planes and their sizes and also on the number of acquisition points per wavelength.…”

“…This approach has been validated for antenna characterization on a wide frequency range [0. [5][6][7][8][9][10][11][12][13][14][15][16][17][18][19][20] using data obtained from near-field measurements.…”

“…We will then focus on that aspect and present some numerical and experimental results to evaluate the minimal dynamic range needed for this kind of algorithm. This paper differs from [14] and [15] since a full parametric study is carried out and wide band application is considered.…”