A Small-Aperture, Ultrawideband HF/VHF Direction-Finding System For Unmanned Aerial Vehicles

A small-aperture HF/VHF direction-finding (DF) system is presented. The antenna aperture occupies a cubical volume with electrical dimensions of 0.0107 lambda(0) x 0.0107 lambda(0) x 0.0033 lambda(0), where lambda(0) is the free-space wavelength at its lowest frequency of operation and is used for DF over a 100 : 1 bandwidth. The antenna array is composed of four hexagonal-cone-shaped monopoles on a finite ground plane. The antennas are loaded with top hats and exploit the near-field parasitic elements to obtain the realized gain values close to the theoretical limits over their entire frequency band of operation. A simple, yet robust null-steering technique is used both on its own and also in conjunction with the multiple signal classification (MUSIC) algorithm, as signal-processing methods, to detect the direction of arrival of the wave over the entire 100: 1 bandwidth. A scaled prototype of the antenna was fabricated and used to perform DF experiments in the field in the VHF-UHF band. The DF system demonstrates azimuthal hearing accuracies comparable to the Cramer-Rao-Lower-Bound values calculated for the measured received signal-to-noise ratios over the whole frequency range. The DF performance of the full-scale version of this antenna, when mounted on a representative unmanned aerial vehicle (UAV), is also examined through computer simulations. It is demonstrated that the proposed DF system can be effectively used in a typical UAV to perform DF in the HF, VHF, and low-UHF bands.