# UAV Detection Using Thrust Engine Electromagnetic Spectra
## 2 Materials and Methods
### Experimental Setup  
First of all, the multi-rotor UAV is permanently fixed on a holder during the experiments. The holder is implemented using the tripod interconnecting ball joint (see Figure 3). This kind of construction of the presented holder allows the free rotation of the UAV in three axes representing the yaw, pitch, and roll of the UAV during normal flight. The horizontal position of the UAV is obtained by using a counterweight that is connected to the tripod ball joint. When all the UAV's thrust engines are turned off, the counterweight stabilizes the UAV in a perfect horizontal plane, thus making it possible to apply any force  
Figure 2: Precision tunable variable inductance magnetic antenna. 1—antenna coil with cooper windings; 2—antenna coil holder; 3—linear motion actuated ferrite rod; 4—insulating textolite insert; 5—shaft of the precision linear actuator; 6—body of the precision linear actuator; 7—step motor of the linear actuator. The dimensions of the proposed and manufactured antenna are coil length—100 mm; coil diameter—13 mm; ferrite rod diameter—10 mm; ferrite rod length—200 mm; insulator length—50 mm; overall system length of the magnetic antenna—600 mm.  
direction that is produced by the UAV's thrust engines or any external force. The altitude of the UAV is fixed and stays the same during the experiment.  
A precision tunable variable inductance magnetic antenna is placed 1 to 5 m from the UAV. The antenna is connected to the SDR receiver, which sends data to the personal computer with dedicated software. Initially, the ES of the surrounding was recorded and examined in order to evaluate the presence of any artifacts before powering on the UAV thrust engines. The electric thrust motors of the UAV started to spin at a minimal speed. The control of the motors was established using the remote controller of the UAV. The signature of the ES was observed and recorded.  
Next, the thrust engines are powered up to 40% of the full throttle with the stabilization function activated while the UAV stays permanently fixed to the holder. The force to the UAV's body is applied in a such way that the stabilization on any of yaw, pitch, and roll axes is present and visible as an ES signature. To confirm that the ES signature is not an artifact, force to the UAV's body is applied in a repetitive way, which is also represented in the recorded UAV's ES signature. The connected counterweight system was used in the experiment as a pendulum for a repetitive external force generation to any of the UAV's axes except yaw.  
Any small applied force to the UAV's body leaves a proportional frequency change as the ES signature during the stabilization mode. This kind of experimental setup is an alternative to the UAV's natural free-flight conditions in order to simulate any wind gust, take off, landing, yaw, pitch, and roll axis movements.  
The physical experiments are based on collecting data on the operating modes of different engines from different drones (see Figure 4).  
Several engines were used as a source of investigation. First of all the engine from the DEWALT DCF809 impact drive was used in the investigation, because its spectrum is similar to the spectrums of engines from drones. The second engine used was from a DJI MAVIC 2 Enterprise drone. The last engine was from the ACHINE TYRO 109 multi-rotor drone.  
Figure 3: The experimental test bench of the tripod interconnecting ball joint holder: 1—roll axis of the UAV fixture; 2—pitch axis of the UAV fixture; 3—yaw axis of the UAV fixture; 4—the UAV’s fixture connecting ball joint; 5—the UAV’s fixture horizontal stabilizer connecting rod; 6—the UAV’s fixture horizontal stabilizer counterweight; 7—UAV’s thrust engine; 8—holder.