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A thermal imaging technique has been developed to measure electromagnetic (EM) fields. This technique is applied in this paper to measure the EM fields radiated by phased array antennas. This thermal technique is based on infrared (IR) measurements of the heat patterns produced in a thin, lossy detector screen placed near the antenna in the plane over which the field is to be measured. A low-loss, planar detector screen was made from a carbon loaded polyimide film to measure the intensity distribution of the electric field radiated from the antenna. The Joule (conductive) heating in the screen causes the temperature of the screen to rise in proportion to the intensity of the wave incident on the screen. The temperature distribution on the surface of the film is captured with an IR imaging camera. The magnitude of the radiated field at each location in the detector screen (on a pixel by pixel basis) is determined directly from the temperature distribution in the IR thermogram of the field. The temperature rise in the screen material (over the ambient background temperature of the screen) was measured at NIST/Boulder when the screen was irradiated by a plane wave of known intensity. A calibration table of measured temperature rise versus expected field intensity was obtained for the detector screen. This thermal imaging technique has the advantages of simplicity, speed, and portability over existing hard-wired probe methods and produces a 2D picture (a pseudo-color image) of the field. In general, these images can be used for field diagnostics of the antenna (near-field or far-field patterns) and/or to evaluate the aperture excitation of the array. In particular, the source distribution in the aperture plane of the antenna was measured. This distribution can be compared to a standard 'test pattern', e.g., full power, equal-phase, broadside illumination, to determine the operational state of each individual element of the array, which controls the radiation pattern of the antenna. Phase shifters and/or attenuators which produce incorrect element phase magnitudes or phase shifts can be identified with this technique. Faulty elements, once located, can be repaired.
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