Rotor modulation of helicopter antenna characteristics

The effect of rotor-blade rotation on a communication link between a helicopter and a satellite or a helicopter and a ground station could be severe depending on the type of antenna and the frequency of operation. For a transmitting or receiving antenna in close proximity to the rotor blades, the rotation itself may create harmonics that are dependent on the angular frequency of the rotor as well as the number of blades. This Doppler effect may impose severe limitations on the communication channel. In addition, rotor-blade rotation may alter other antenna characteristics including input impedance and polarization of the radiated field. This paper investigates the effect of rotor-blade rotation on the radiation characteristics of a 14-ft. high-frequency (HF) antenna mounted on a scaled helicopter model. Specifically, a quasi-stationary approach suggested by Van Bladel (1976) is used in conjunction with a full-wave method of moments code (NEC) to examine rotor modulation on the gain patterns and input impedance of this HF (2-12 MHz) antenna in the presence of a rotating rotor. The objective is to quantify the maximum modulation/variation of the gain and input impedance as a function of rotor orientation.