## 4 Components and System
### Antennas  
#### 4.2.1 Antenna Selection  
The choice of the antenna type determines decisive properties for the presented radar system, such as the mechanical size of the antenna array, the scalability to N channels as well as the achievable resolution in range and cross-range direction. In order to clearly define the sectors and the overlap areas, the antenna should have a strong main lobe and side or  
Figure 6: (**a**) Simulation model and (**b**) manufactured notch filters with end launch SMA connectors. Figure 7: Simulated and measured S-parameters of the proposed notch flifilter. back lobes that are as well suppressed as possible. The width of the main lobe depends on the application and the desired cross-range resolution or the number of antennas. The half-power beamwidth (HPBW) should be set so that targets in the overlap area are still sufficiently detected. However, the side lobes should already be clearly suppressed in the main lobe of the neighboring antenna in order to clearly distinguish the radar scenarios shown. In the later shown measurements for cross-range detection, an angle of up to 90\({}^{\circ}\) should be detected. Therefore the antennas are to be aligned at an angle of 60\({}^{\circ}\) to each other. In order to still sufficiently illuminate the overlap area between the antennas, the HPBW in the cross-range direction is also to be selected at 60\({}^{\circ}\). The evaluation requires time gating of the received signals. To ensure that the time window is sufficiently small and targets can be separated from each other in range direction, the corresponding radar bandwidth must be ensured. In combination with an overall simple system design, this usually requires a high relative bandwidth, which cannot be realized with all antenna types. On the one hand, the antenna should exhibit sufficient matching within the bandwidth used in order to minimize reflections within the system. On the other hand, a reasonably flat gain is desirable so that constant system behavior can be assumed for the coding in the frequency domain. The radar system used to perform the later measurements operates in the frequency range of the X-band from 8.12 GHz. Thus, the absolute bandwidth is 4 GHz and the relative bandwidth is 40 %. The most common antenna shape for ultra-wideband radar systems of highest performance is the horn antenna. It belongs to the group of aperture antennas and represents a modified open waveguide radiator. The waveguide end is extended with a horn structure, which gradually matches the wave impedance of the waveguide to the free space impedance. On the one hand, this reduces reflections at the end of the antenna. On the other hand, it increases the radiating aperture and thus the gain of the antenna. The pitch of the horn structure and the size of the aperture can be used to adjust the directivity and find a compromise between performance and mechanical size. The losses in the antenna are very low because air is used as dielectric and the metallic walls can be realized with good conductivity and high surface quality. Due to the well-defined aperture area, horn antennas usually have very low back lobes, which means high isolation between adjacent antennas. The horn antenna is usually fed via a waveguide adapter, so simple coaxial cables can be used for the connection. While waveguides and couplers are widely available, it may be difficult to obtain horn antennas with suitable parameters. Special fabrication is possible, but this increases the already high costs due to the necessary mechanical precision. Especially for the application in the presented antenna array, these costs are an important aspect of antenna selection. Nevertheless, the main disadvantage of horn antennas is their mechanical size and weight. An alternative is planar antenna structures, which can be realized on printed circuit boards and thus can be manufactured in common PCB processes. For RF applications special substrates are used which have a defined permittivity and low losses up to the 10 GHz range.