## 2 UWB MIMO Radar System
### Low-Frequency UWB Miniaturized Vivaldi Antenna  
The Vivaldi antenna is a kind of UWB tapered slot antenna. It has the advantages of wide bandwidth, symmetrical pattern, stable gain, simple structure, low profile, and easy integration [23; 24]. Generally speaking, the size of antenna is determined by its working wavelength, and the wavelength \(\lambda\) can be obtained from the relationship (\(\lambda=c/f\)) between the wave velocity \(c\) and frequency \(f\) in free space. Therefore, it can be clearly seen from the above relationship that the lower the operating frequency of the antenna, the larger the size of the antenna required. According to the needs of some specific applications, such as a portable MIMO antenna array for TWR imaging in this paper, we plan to use the UWB Vivaldi antenna with low frequency characteristics, so it is necessary to study the miniaturization of Vivaldi antenna. Through a series of studies and discussions [23; 24; 25; 26; 34; 35; 36], it can be concluded that most antenna sizes are reduced but many aspects of performance will be affected, such as narrowing of the working frequency band and decreasing of gain. As a result, the research on antenna miniaturization is to ensure that the performance of the antenna is minimally changed. A miniaturization technique commonly used with Vivaldi antennas is to etch shorting slots with different shapes on the nonradiative sides of the metal patch [25; 26; 34; 35]. Firstly, according to the structural analysis of the conventional tapered slot Vivaldi antenna, the radiation mechanism consists of current flowing effectively along the edge of the mid-tapered slot line and radiating outward. However, surface current and edge current also exist in the nonradiating metal areas on both sides of the antenna, which will affect the antenna gain and directivity. Then, some regular rectangle slots are etched on the nonradiative sides and the direction of the rectangle slot is perpendicular to the direction of the current. In this case, the flow path of the current is significantly increased. The rectangle slot edge structure on both sides of the antenna has infinite impedance for surface waves. It can effectively suppress the radiation of surface waves at the nonradiation sides so that the current flows along the mid-tapered slot line. As a result, the operating frequency of the antenna can be lowered without changing the size of the antenna and the operating mode. Above all, the final dimensions of optimized Vivaldi antenna structure in the results simulated by HFSS 15.0 are 250 mm \(\times\) 200 mm \(\times\) 1.6 mm, which is designed to work across the desired bandwidth of 0.5-2.5 GHz. Figure 7 shows the final design and fabrication result of the Vivaldi antenna. It is made of the low-cost FR4 substrate for which the dielectric constant is about 4.6. The feeding of the antenna is achieved by a subminiature version A (SMA) connector. Figure 8 plots both the E-plane and H-plane radiation patterns of the designed antenna at 0.5 GHz, 1.5 GHz, and 2.5 GHz, which are simulated using HFSS 15.0. It can be seen that the proposed Vivaldi antenna has good end fire radiation behavior and acceptable directivity in bandwidth of 0.5-2.5 GHz. Then, the antenna is measured by  
Figure 6: Simulations for the MIMO sparse array by: (**a**) the time domain back projection (TDBP) algorithm; (**b**) the cross-correlation-based time domain back projection (CC-TDBP) algorithm. vector network analyzer (VNA) FieldFox N9925A. A comparison between the measured S11 with shorting slots and simulated S11 with and without shorting slots for the antenna is shown in Figure 9a. The \(-10\) dB frequency bandwidth of 0.5-2.5 GHz is apparent. The measured result agrees well with the designed wideband except for the slight degradation of the low frequency starting band and the band near 2.0 GHz, where the S11 parameter rises to be a slightly higher than \(-10\) dB.