Design of Platform-Based HF Direction-Finding Antennas Using the Characteristic Mode Theory

A systematic method for utilizing the characteristic modes (CMs) of a platform to design a direction finding (DF) system operating at the high-frequency (HF) band is presented. In this band, due to the large wavelengths, practical antennas used for DF are electrically small and have limited bandwidths. When such a DF array is mounted on a platform, however, the platform's presence can be exploited to considerably improve the performance of the platform-based DF system compared to that of a stand-alone DF system. Our proposed approach relies upon selecting and exciting a group of the platform's CMs that provides the lowest Cramer-Rao bound for the DF system's accuracy. This approach allows for designing DF systems that can accurately detect the direction of arrival and the state of polarization of the wave over wide fields of views. This approach is used to design and simulate an HF DF system for an airborne platform. A scaled-model prototype of this platform-based DF array is fabricated and characterized. The measurement results of this array are used to perform DF experiments in an emulated environment. Measured results are found to be in good agreement with simulations, confirming the efficacy of the proposed design approach.