We present a method to enhance the accuracy of platform-based, high-frequency (HF) direction-finding (DF) systems using dynamic mode selection. To improve bandwidth and efficiency, the metallic platform supporting the DF array is used as the primary radiator by employing electrically small antennas to excite linearly independent combinations of its characteristic modes (CMs). However, the radiation characteristics of these modes vary dramatically over the HF band, making it difficult to achieve consistently good DF accuracy with a fixed system. To alleviate this, we propose a dynamic mode selection strategy to achieve enhanced DF accuracy. This strategy is based on the assumption that the number of available CMs of the platform is greater than the number of available coherent receive channels. Thus, dynamic mode selection allows for choosing the optimal antenna combination to obtain the best DF accuracy at each frequency. This strategy is demonstrated for an airborne DF system employing five electrically small antennas and up to four coherent receive channels. We demonstrate the efficacy of the proposed approach using computer simulations and scaled-model experiments. Simulation and measurement results show that dynamic mode selection can significantly enhance the DF accuracy of platform-based HF DF systems using a limited number of coherent receive channels.
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