# UAV Detection Using Thrust Engine Electromagnetic Spectra
## 2 Materials and Methods
### Measurement Equipment  
The antenna is the most important device of the overall engine signature detection systems. The antenna must have the following characteristics. First of all, it should be selective. There should be a possibility to receive the signal from 1 to 100 kHz. The resonant frequency range of the antenna should be from 1 kHz to 100 kHz, because different manufacturers use different motors that have their own programmed frequency of the pulse width modulation. Two drones and an electric-powered impact driver were used during experiments, all of which were powered by a similar type, size, and acceleration-capable brushless electric motors that have their own unique signatures on the electromagnetic spectrum. The ferrite rod of the widely used radio magnetic antenna has standard physical dimensions: diameter 10 mm, length 200 mm, and ferrite type 600 NN. The widely available standard ferrite rod type 600 NN (\(\Phi\)600) has a maximum working frequency of 4 MHz, which satisfies the condition of signal reception at the interval of 1-100 kHz. The ferrite rod also satisfies physical conditions (diameter--10 mm, length--200 mm) for implementation of stepless induction change using the precision linear motor actuator. The coil's parameters were selected according to the specification of the ferrite rod, which satisfies the stepless reception frequency range from 1 kHz to 100 kHz. The antenna setup for the experiment was chosen according to the future prospects of the development of the multi-rod tunable antenna. Several rods with the same physical dimensions and characteristics will be used in order to increase the sensitivity of the reception of the signature from the electromagnetic spectrum and in order to realize the capability of the direction finding. Selectivity is a must-have property in this kind of system because it is necessary to investigate not only the main harmonics falling into this interval but also additional harmonics. Additional harmonics can be useful in order to avoid environmental interference when environmental noise does not allow unambiguous identification of the drone from the main harmonic. Additionally, the property of selectivity allows isolating and analyzing several harmonics separately. This functionality allows verifying the assumption that the drone has been detected more accurately. The uniqueness of the proposed antenna is that it is a magnetic antenna that is insensitive to the electrical component of the signal. This is its advantage in comparison with electromagnetic antenna, in which the electrical component adds interference that interferes with detection. There could be many different sources of environmental disturbances. For example, the sources of interference in the city could be outdoor lighting, ultrasonic sensors, power lines of trolleybuses, and others. Another outstanding advantage of such an antenna is the stepless change of resonant frequency while maintaining maximum interference with ambient noises and high selectivity. The proposed model of the antenna is presented in Figure 2. The resonant frequency of the antenna could be changed by varying the inductance and/or capacitance in the antenna matching circuit. There was no option to apply the variable capacitor of large capacity which is required in such type of antennas. Therefore, this antenna was realized by varying the inductance. The ferrite rod and the motorized actuator that were used affect the capacitance and change the resonant frequency of the proposed magnetic antenna. Such a tunable antenna allows scanning the environment (electromagnetic spectrum) and detecting the trace of the motors of drones in the spectrogram. The trace of the motor in the spectrogram could be additionally verified by analyzing the higher harmonics of the spectrum. The dimensions of the antenna are determined by several factors. In order to achieve higher sensitivity, it is necessary to manufacture a longer and wider ferrite rod. Therefore, larger dimensions are aspirational.