This paper gives a theoretical solution to the problem of determining the radar cross section and doppler spectrum of a helicopter rotor as presented to a radar operating in the decametric wave band (3-30 MHz). At such frequencies for all practical rotors the scattering regime is either Rayleighan or resonant. The solution proceeds by modeling the rotor as an equivalent set of radial wires on which the incident wave is assumed to excite sinusoidally distributed currents. It is shown that, subject to certain simplifying assumptions, the doppler spectrum has a form similar to that associated with tone modulation of a frequency modulated bearer. The theoretical work is confirmed by experiment.
