VHF helicopter antennas that incorporate the airframe and reduce rotor modulation

Abstract

Antennas at VHF frequencies mounted to small (with respect to wavelength) helicopter platforms are usually limited to whip and loop antennas. Antenna placement and rotor position can drastically change antenna performance due to induced currents on the platform. This work illustrates deliberate antenna-platform interactions that cause induced currents to reinforce and improve the radiating system performance. Additionally, rotor modulation effects can be simultaneously limited. This is accomplished by using oppositely placed loop antennas at a location on the airframe that is nominally one wavelength in circumference. The resulting broadcast pattern admits low cross-polarization radiation and low copolarized rotor modulation, but is not uniform. The dipole moment of the loops dictates this nonuniformity. Double-loop antennas are used to create a more uniform broadcast pattern without sacrificing the desired low cross-polarized radiation and low copolarized rotor modulation.