# Functionality Investigation of the UAV Arranged FMCW Solid-State Marine Radar
## 5 Results of the Secondary Radar  
The term of secondary radar usually means equipment or a system, which allows to determine the positions of targets by receiving information transmitted from the targets. Examples of secondary radars are the AIS in maritime and the ADS-B system in aviation. Due to low weight, low cost, and possibility to integrate into OpenCPN software, the RTL-SDR dongle was chosen as the AIS receiver. It is a multipurpose SDR receiver not optimized for AIS reception. We used an electrically short (10 cm long) rubber antenna from a VHF hand-held radio station. The antenna was installed as presented in Figure 15.  
Figure 14: The radar response plotter view where (**a**) is the 78 kHz signal from generator, (**b**) is the second and third harmonic of the 78 kHz signal, (**c**) is the response produced by the low-frequency electronic circuit and (**d**) is the response generated by the power supply. Despite the fact that the rubber antenna was well tuned to the AIS frequency when reflection loss was less than \(-16\) dB (Figure 16), we have noticed that the signal level with this antenna was more than \(-10\) dB lower in comparison to conventional half wave dipole antennas. Those losses were caused by the lower efficiency of electrically small antennas and by the scattering of waves caused by surrounding equipment. For further experiments full-sized antennas (quarter wave monopole and half wave dipole) were chosen. Due to the presence of high currents of frequencies in the order of kilohertz, the probability of interferences in radio frequency is significant. Interferences can penetrate the receiver via power supplying chains or via antenna input. At first we measured the interference level via power supply chains when the antenna was disconnected from the receiver. We used the SDR open source software to monitor signal levels around the AIS frequencies. No significant interferences were obtained in this case, as presented in Figure 17. The pink and blue arrows are at AIS frequencies. The peak on the right side corresponds to the tuning frequency of SDR receiver. Figure 16: Reflection loss of the rubber antenna. Figure 15: Installation of the first version of an antenna for AIS reception. During the second step, we measured the interferences caused by the drone's electric systems when the antenna was connected. We connected a quarter wave wire whip antenna instead of a rubber antenna. As presented in Figure 18a, we can see that the noise level increased by 25 dB in comparison to when the antenna was disconnected. After connecting drone battery, two strong and wide interference peaks on 161.65 MHz and 162.65 MHz appear (Figure 18b). Fortunately, those peaks are not on the AIS frequencies. In order to reduce interference caused by the drone's electric system, taking into account the recommendations in [24] we have made a dipole antenna--a quarter-wavelength choke-sleeve antenna--as presented in Figure 19. The lengths of the vibrator and the counterweights are equal to the quarter wavelength--462.5 mm for 162 MHz. The whole antenna's structure, which consists of a cable, counterweights and a vibrator was arranged between two ribbons of 50 mm width of armed adhesive tape. The antenna was hung approx. 1 m below the bottom of the drone. The vertically arranged dipole antenna provides minimal amounts of radiation with patterns in axial directions. Due to this radiation property and increased distance from the targets to the drone we can expect a lower level of interference caused by the drone. Figure 17: Noise in the receiver when antenna is disconnected: (**a**) the batteries of the drone are disconnected, (**b**) the batteries of the drone are connected. Figure 18: Noise in the receiver when the monopole antenna is connected: (**a**) the batteries of the drone are disconnected, (**b**) the batteries of the drone are connected. As presented in Figure 20, we obtained a significantly lower level of interference in comparison to the monopole antenna.