Helicopter rotor-blade modulation of antenna radiation characteristics

This paper investigates rotor modulation on the gain and input impedance of an antenna mounted on a helicopter's fuselage, The rotor blades, which are rotating about an axis with a given angular velocity, continuously modulat e the signal transmitted by the antenna and adversely affect the establishe d communication links. In this study particular emphasis is placed on wire antennas operating within the high frequency (HF) and very high frequency ( VHF) bands. Specifically, the Doppler spectrum of the magnitude and phase o f the radiated field by a vertical and a horizontal short dipole underneath the rotors is computed and analyzed. The variation of the corresponding an tenna gain as a function of rotor angle and frequency is also investigated. The same type of analysis is then extended to a 14-ft towel-bar antenna mo unted on the tail boom of a 10:1 scaled helicopter model. In addition to ga in, the variation of input impedance versus angle and frequency is computed within the HF band. Finally, the bit error rate (BER) assuming a quadratur e phase-shift keying (QPSK) modulation is evaluated assuming linearly and c ircularly polarized receiving antennas.