Design of platform-based HF direction finding antennas using the characteristic mode theory

This paper presents a method for utilizing the platform as the antenna array for the on-board HF direction finding (DF) system. In the HF band, due to the large wavelength of electromagnetic waves, practical antennas deployed on the platforms are electrically small, limiting the DF accuracy. To address this issue in platform-based DF systems, the platforms that have dimensions comparable with the wavelength can be used as the antenna elements in a DF array themselves. However, due to the complex shape and radiation features of the platforms, it is challenging to use them efficiently. We present a characteristic mode (CM) based approach that allows for selecting the subset of the platform's CMs with the lowest Cramer Rao lower bound. This approach can achieve the lowest DF error and considers the field of view and polarization of impinging waves quantitatively. We apply this approach to design and simulate an HF DF antenna array for a midsize aircraft at 10 MHz. Our results demonstrate that the selected subset has significant advantage over the others.