Direct infrared measurements of phased array near-field and far-field antenna patterns

Abstract

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 large phased array radar antennas and to determine the near-field distributions and the far-field antenna pattern. This thermal technique is based on infrared (IR) measurements of the heating patterns produced in a thin, lossy detector screen made from a carbon loaded polyimide film placed in the plane over which the field is to be measured. The temperature rise in the screen material is related to the intensity of the field incident on the screen. An experimental calibration table was developed at NIST/Boulder to convert the temperature rise at any point on the screen into an equivalent incident radiated field strength. This thermal imaging technique has the advantages of accuracy, simplicity, speed, and portability over existing hard-wired probe methods and produces a 2D picture of the near field or the far field. These IR measurements, therefore, can be performed on-site at the remote location of the antenna in-the-field to produce an image of the radiating field of the array, which can be used to determine the overall radiation characteristics of the array, i.e., the radiation pattern of the combined array elements (gain and beam-width) and the condition of the electronic switching circuits (phase shifters and attenuators). Therefore, the overall “state of health” of the array and the need for repair can be determined in-the-field using the IR technique to avoid the expensive and time consuming alternative of dismantling the array and shipping it to a maintenance depot for testing, calibration, and repair on a standard, planar, near-field antenna test range. In this paper, the IR technique is tested in a controlled environment to determine the feasibility of using the IR images as an array diagnostic tool i) to measure the radiated field of large phased array radar antennas (near-field or far-field patterns in a transverse plane parallel to the plane of the array), ii) to measure the transition of the field from the near field to the far field (in an axial plane perpendicular to the plane of the array), and iii) to test the switching of the array from a scan mode to a target tracking mode.