Helicopter-Mounted HF Direction-Finding Antenna With Rotor-Blade Modulation Using the Characteristic Mode Theory

A single helicopter-based high frequency (HF) antenna is proposed for direction finding (DF) using rotor-blade modulation. Without integrating additional antenna structures, a landing gear is utilized as a coupling element to excite a combination of significant characteristic modes of the helicopter through characteristic mode (CM) analysis. Different from traditional DF systems where multiple antennas required, incoming signal receptions are achieved by a single radiator in this work. The diversity in antenna radiation patterns is realized by incorporating main rotor-blade rotations. The incident signals can be measured as the blade rotates from 0<inline-formula><tex-math notation="LaTeX">$^\circ $</tex-math></inline-formula> to 75<inline-formula><tex-math notation="LaTeX">$^\circ $</tex-math></inline-formula>, where the helicopter-mounted antenna with each rotation angle is equivalent to one antenna element in a conventional DF antenna array. Correlation coefficients of complex CM and excited radiation patterns between the antennas with different blade rotation angles are calculated to demonstrate pattern diversity. Both Cramer–Rao bound and multiple signal classification algorithm are investigated using the complex CM and excited radiation patterns to validate the azimuthal DF capability of the helicopter-integrated single antenna.