HF antennas for NVIS applications mounted to helicopters with tandem main rotor blades

Antennas mounted to helicopters for HF near-vertical incidence skywave (NVIS) communication suffer from severe rotor modulation at frequencies where the blades are nearly resonant. Helicopters with tandem main rotors such as the US Army Chinook cargo helicopter are considered. It is determined that there are two effects contributing to rotor modulation: the parasitic radiation from the rotor blades, and the induced current on the hubs. It is shown here that two towel-bar (loop) antennas, one on each side of the airframe and centered with respect to the hubs, can be used to reduce the severe drop in gain due to hub currents. However, the variation in gain due to parasitic radiation from the blades remains.