A Direction Finding (DF) algorithm for small aperture DF systems is proposed. Traditionally, small aperture DF systems lack of beamforming capabilities and therefore require manual rotation, which may affect the Angle of Arrival (AOA) estimation accuracy. Based on Characteristic Mode (CM) Analysis, a Multi-Feed Structural Antenna (MFSA) is developed that utilizes an electrically small platform as a radiator. This paper chooses a Small Unmanned Aerial Vehicle (SUAV) as a design platform. The overlap of all Individual Element Radiation Patterns (IERPs) of the proposed MFSA covers the entire azimuth plane. In this way, beamforming and null steering of the MFSA on the azimuth plane can be achievedby linearly combining all weighted IERPs. A new method based on Vector Singular Value Decomposition (SVD) is proposed to determine the weight vector of beamforming (``Sum'' pattern) and null steering (``Difference'' pattern) in a specific direction. Based on the ``Sum-Difference'' delta method, the AOA of the Radio Frequency (RF) signal source can be estimated. A small aperture VHF DF system with a multi-channel digital-IF receiver is developed to experimentally verify the proposed concept. The evaluation results show that the AOA estimation RMS error is 1.55°, and the false detection rate is significantly improved.
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