Direct infrared measurements of phased array aperture excitations

Abstract

A thermographic imaging technique has been developed to measure electromagnetic (EM) fields. This technique is applied in this paper to measure the aperture plane fields of large phased array radar antennas and to determine the aperture (source) excitations of the array, i.e., the distribution of the radiated energy over the elements in the aperture plane of the array. These IR measurements, therefore, can be performed on-site at the remote location of the antenna in-the-field to produce a non-distorted image of the field excitations in the aperture plane of the array, which control the overall radiation characteristics of the array. In general, these images can be used for field diagnostics to evaluate the electrical characteristics of the elements of the array, i.e., the state (strength) of the aperture excitations and the condition of the switching circuits (phase shifters and attenuators), which control the radiation pattern of the antenna. The aperture field distribution can be compared to a standard “test pattern” to quickly determine the operational state of each individual element of the array. Individual attenuators and/or phase shifters that produce incorrect element intensities can be easily and quickly identified with this technique. Short-circuited elements at various positions in the array are used to simulate faults in the elements and to test the feasibility of the thermal technique to determine the operational state of the array. Therefore, the overall “state of health” of the array and the need for repair can be determined in-the-field using the IR measurement 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 to measure the aperture excitations of large phased array radar antennas.