# An Ultra-Wideband Vivaldi Antenna System for Long-Distance Electromagnetic Detection
## 1 Introduction  
Ultrawideband (UWB) antennas have been increasingly applied in wireless communication, biomedical detection, and radar systems in recent years [1; 2; 3; 4; 5; 6]. As a tapered slot antenna, the Vivaldi antenna is well-known for its high gain, directive radiation pattern, planar structure and fairly wide bandwidth [7], and it is one of the best options for the UWB technology [8; 9]. Its small transverse spacing makes it a good candidate for antenna arrays [10]. The Vivaldi antenna can be classified into the category of end fire traveling wave antennas, has theoretically infinite bandwidth [11], and can be used to form an ultra-wideband antenna system [12]. However, the Vivaldi antenna requires a large antenna size to achieve excellent performance in the low-end working band [13; 14]. The width of a Vivaldi antenna should reach at least one half-wavelength so as to achieve effective radiation, according to the research work in [15].  
In order to reduce the size of the Vivaldi antenna and to further optimize it, some scholars have put forward many new strategies. To be specific, in [8], the feeding part is improved to realize miniaturization. The antenna in [8] aims to increase the area of feeding part and adopt feed with stepped structure. In [16; 17], radiation parts are improved. The antenna in [16] is seen to open unequal semicircle slots, while that in [17] can be witnessed to open multiple rectangular slots on the edge of the radiation part, which helps improve antenna gain. The antenna in [18] reflects the method of increasing the length of the dielectric substrate to improve the directivity, and the antenna in [19] has an enhanced gain over whole frequency band by adding an additional structure that supports spoof surface plasmon polaritons (SSPP) as the parasitic element. When compared with several antennae, the antennae in [8; 20] perform better with miniaturization, but their performance with regard to the direction and gain is far from scholars' satisfaction. The antennae in [16; 17; 18; 19] have higher antenna gain, while they cannot meet scholars' expectations in miniaturization.  
This article designed a set of receiving and transmitting antennas based on Vivaldi antennae, and built a long-distance electromagnetic detection system. In the target detection test, the transmitting antenna is used as illumination feed, and the receiving antenna obtains the signal reflected by the target from scanning angle to find the target and the azimuth angle. Then, with the Time Domain Transmissometer (TDT) measurement results, the target position is judged.  
The working frequency band of the detection system is 2.5-4 GHz. According to the working principle of the detection system, the beamwidth of the transmitting antenna needs to cover a wide angle to ensure that the target under test can receive and reflect the signal radiated by the transmitting antenna. Therefore, the half-power beamwidth (HPBW) of the transmitting antenna should be larger than 90\({}^{\circ}\). Correspondingly, in order to quickly and accurately determine the azimuth angle and judge the position of the target, the beamwidth of the receiving antenna is required to be as narrow as possible and have good directivity. Therefore, the receiving antenna should be a typical one that has narrow HPBW and good directional performance and that should also be miniaturized. To achieve long-distance detection, the target detection system should have a high transmitting power, and the antennae should be characterized by boasting better impedance matching and higher gain performance [21].