VHF helicopter antennas that incorporate the airframe and reduce rotor modulation

Antennas at VHF frequencies mounted to small (with respect to wavelength) h elicopter 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 ante nna-platform interactions that cause induced currents to reinforce and impr ove the radiating system performance. Additionally, rotor modulation effect s can be simultaneously limited. This is accomplished by using oppositely p laced loop antennas at a location on the airframe that is nominally one wav elength in circumference. The resulting broadcast pattern admits low cross- polarization radiation and low copolarized rotor modulation, but is not uni form. The dipole moment of the loops dictates this nonuniformity, Double-lo op antennas are used to create a more uniform broadcast pattern without sac rificing the desired low cross-polarized radiation and low copolarized roto r modulation.