Low detectable JEM signals in radar echoes from a helicopter with composite blades

Theoretically, the rotating blades of helicopters can produce typical micro-Doppler, that is, strong Jet Engine Modulation (JEM) signals in their radar echoes. However, our X-band radar data indicate that there are only weak JEM signals with an approximate signal-to-clutter ratio (SCR) of 5 dB, similar to background clutter, in radar echoes from a helicopter of AS350 with composite blades. In contrast, radar echoes from a medium helicopter MI17 and a small drone DJI MATRICE 600 have strong JEM signals with SCR values of over 12 dB. The comparison indicates that radar dwell time and the size of blades are not the reason for the low detectable JEM signals in radar echoes from AS350, but the composite blades of AS350 may be responsible for it. The SCR of the blades is 18 dB smaller than that of AS350's body, making the identification range of AS350 35% of the detection range. The composite blades may provide an idea for designing stealth military helicopters and "stealth" turbine blades in wind farms.