# UAV Detection Using Thrust Engine Electromagnetic Spectra
## 3 Results and Discussion
Figure 8: Recorded signature of a 92 kHz electromagnetic spectrogram, generated by the thrust engines of the multi-rotor DJI Mavic 2 Enterprise UAV. Figure 6: Spectrum of the multi-rotor Eachine Tyro UAV at the fifth operating mode while engines work at 50% of its maximum power. Figure 7: Spectrum of the multi-rotor Eachine Tyro UAV when the thrust motors are spinning at the minimal idle rotation speed. Again, the 30 s window of the spectrogram is presented in Figure 8. The essential areas of the spectrogram are: 1--the UAV's thrust motors are turned off; 2--the UAV's thrust motors are spinning at a minimal idle rotation speed; 3--the UAV's thrust motors are turning at 40% of the full power. Forced, equal and repetitive destabilization movements in the pitch axis are applied to the body of the UAV. Each forced movement is generating a proportional frequency shift of the signal in an electromagnetic spectrum. 4--the UAV's thrust motors are spinning at 80% of full power, takeoff static motor thrust is generated in stabilization mode; 5--the UAV's thrust motors are turning at 30% of full power, equal repetitive forced destabilization movements in the pitch axis are performed. Each forced movement is generating a proportional signal's frequency shift in an electromagnetic spectrum. 6--the UAV's thrust motors are turning at 30% of full power. Forced repetitive body destabilization movements in the pitch, yaw, and spin axes are performed. The last spectrogram is presented of the motor that is not from the drone, but from the DEWALT DCF809 impact driver (see Figure 9). The time window of the spectrogram is 30 s. The spectrogram of the Dewalt DCF809 impact driver has five essential working modes: 1--the BLDC impact driver motor in slow speed impact mode under the load, hammer in action; 2--the BLDC impact driver motor driving a bolt without impact hammer operation; 3--the BLDC impact driver motor under load condition, with pretension of the hammer spring; 4--the BLDC impact driver motor under load with the hammer in action; 5--the BLDC impact driver motor switched off. The physical experiments have shown that every set of engines of the drone could be recognized using the proposed and investigated tunable magnetic antenna. For example, the DJI drones have a specific signature in the spectrogram obtained during engine start-up, which allows the drone type/manufacturer to be identified from the spectrogram. All the transitions from minimum to maximum revolutions of the engines were visible in the spectrograms with their unique signatures. A clearly visible signature in the spectrogram was obtained in the fundamental and other higher harmonics of the spectrum while measuring the drone signature in the spectrogram positioning (stabilization) mode. Figure 9: Recorded signature of a 40 kHz electromagnetic spectrogram, generated by the single BLDC motor of the Dewalt impact driver. The higher harmonics could be investigated by shifting the resonant frequency of the antenna. For example, a precision tunable variable inductance magnetic antenna shifted the frequency from 23 kHz to 46 kHz and back to 23 kHz (see Figure 10). As it is seen from the recorded spectrum, the amplitude of the second harmonic increased and became visible for recognition. Comparing different harmonics of the same signal and continuous tracking of the ES signature enables new capabilities of AI-powered UAV detection systems. A precision tunable variable inductance magnetic antenna can be used together with the AI systems for more reliable ES signature detection and recognition. The advantage of the antenna is that it can track the movement of the ES signature in ES continuously or jump to any other frequency with precalculated harmonics for signature evaluation or comparison. In the case of a situation when the investigated harmonic at the particular frequency of the UAV's ES signature is with high disturbances, variable inductance magnetic antenna can instantaneously change reception frequency to other harmonics for signature reception and comparison.