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Reflectarray Antenna based FMCW Radar System Development: High Isolation Radar Antenna Design and Feasibility of Affordable Switched-Beam for the UAV Detection
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This study aims to develop anti-drone applications. It is clear that radar is one of the most promising anti-drone detectors, as they are not significantly dependent on weather or light conditions. The cost of current anti-drone radars is extremelyhigh, thus there is a need for cost-effective radars that protect critical infrastructures. The goal of our research is to develop a high gain, low power (potentially battery operated), lightweight, cost-effective, large field of view (FoV), and potentially scannable radar system that can replace those high-cost radar systems. If successful, the high-performance radar may be mounted on unmanned aerial vehicles (UAVs) for autonomous detection/tracking or hand-carried by soldiers in the war field. To achieve this goal, it was necessary to adopt a passive gain boosting technique to satisfy the low power and high gain requirements. The reflectarray antenna technique was chosen for this purpose due to its lightweight, foldable and high gain boosting capability. This study first introduced a conventional centered reflectarray antenna using a numerical modeling technique based on SDMoM. Simulations and measurements showed that the presence of the reflectarray antenna dramatically increased the gain of the system by 21.18 dB, which increased the range coverage three-fold in the human detection test. However, this method significantly reduced the isolation between the transmitter and receiver antennas. It was because there exists a reflectarray specific signal leakage, where the radiated signal was directly reflected by the reflectarray antenna and re-radiated into the receiver. The low isolation is a significant drawback for any radar system since the high-intensity transmission signal can easily damage the receiving circuit. To resolve the isolation issue, two methods were proposed and tested. The first method was to design and adopt a new antenna assembly, in which the transmitters and receiver were placed back-to-back so that the receiver antenna did not face toward the reflectarray. The simulations and measurements proved that the maximum isolation of ∼50 dB was achieved, but the total gain was reduced by ∼16 dB compared to the previous model. The second solution was to modify the reflectarray design so that the feeder operates with an offset angle of 45°. In this design, the feeder was not placed at the center of the reflectarray antenna, hence the direct reflections did not propagate toward the receiver, increasing the isolation level. With this method, the desired gain (∼27 dBi for both antennas) was achieved, but the isolation was reduced to ∼30 dB, which can be improved with a better feeder design. Finally, a proof-of-concept study that resolves the challenges discussed above (gain and isolation) as well as enables a switched beam or beam scanning capability is introduced. In this study, a reflectarray technique called the bifocal design was adapted, and the antenna was fabricated and characterized. The initial numerical modeling results showed that a bifocal reflectarray using ±7.5 cm feeds resulted in an acceptable gain (24.3 dBi for both transmitter and receiver) and high isolation. The measurement results showed ∼12 dB lower gain than the numerical modeling, which may be associated with measurement techniques. The study proved that the increase in gain and range by reflectarray could lower the isolation, and hence a compensation between the gain and the isolation may be an essential design step for developing a reflectarray boosted radar system. This task can be achieved by modifying the reflectarray or feeder designs. It was also proved that beam scanning was feasible with a limited gain increase, and more in-depth study is required to achieve maximum gain and isolation. More research toward beam scanning is currently under investigation.