# Simple Compact UWB Vivaldi Antenna Arrays for Breast Cancer Detection
## 1 Introduction  
The benefits of ultra-wideband (UWB) technology, in terms of a simple transceiver, low profile, wide bandwidth (BW), low power consumption, and low interference with the coexisting narrow band frequency technologies, are exploited in the recent wireless communication system applications to satisfy consumer demand and reduce the system budget [1]. These applications include an UWB see-through wall, microwave imaging, ground penetrating radar, ground and space communication, vehicular wireless communication, radio astronomy or telecommunication, water communication, and microwave moisture detection. To achieve these characteristics, Vivaldi or exponentially tapered slot antenna (VTSA or ETSA) with a low profile, wide BW, directive radiation patterns, easy integration with other circuits, and low cost is considered the best candidate for UWB technology among other broadband antennas [1]. Generally, a group of individual elements (array) can be used to enhance the antenna gain [2]. Various types of Vivaldi arrays are proposed by doubling slots or increasing the number of single elements with or without feeding networks. The Wilkinson power divider(WPD) is the most well-known microwave passive component, particularly as a feeding network in antenna arrays. Harmonic suppression and compactness techniques for WPD are addressed in [3]. Unequal split microstrip WPD is used as an alternative to a broadband coupler and a phase shifter in designing a microwave distribution network such as in antenna array beamforming [4]. Instead of adding a director to the Vivaldi antenna, its slot is doubled (DSVA) to enhance the BW and directivity [5, 6]. For UWB, target detection, and high-speed 5G communication applications, BW and impedance matching improvement are achieved by etching slots on the proposed DSVAs' edges in [5, 7, 8]. Enhanced directivity and gain required for UWB imaging and radar systems are achieved in the proposed DSVAs in [9, 10] by adding zero-index metamaterial (ZIM) unit cells and a parasitic element, respectively. For the microwave imaging system, further improvement in gain and BW is achieved by loading the DSVAs with parasitic and slot edges [11, 12, 13]. Based on a low-loss substrate integrated waveguide (SIW) binary divider, a 1 \(\times\) 8 Vivaldi array with enhanced performance is proposed for see-through-wall imaging, indoor localization systems, and breast tumor detection applications [14]. A wide scan angle (45\({}^{\circ}\)) with enhanced BW is achieved in [15] using a 1 \(\times\) 8 phased array based on a dual-parabola-shaped tapered slot antenna fed by WPD. For underwater communication and long-distance detection applications, the gain is improved in [16, 17] using 2 \(\times\) 4 and 1 \(\times\) 8 VTSA arrays fed by Wilkinson and T-junction dividers, respectively. The feeding topology in [18] combined with T-junction PD, frequency-independent phase shifter, and T-branch MS lines is used to reduce the size and enhance the BW and gain of the 1 \(\times\) 6 VTSA array proposed for microwave sensing and communication applications. Further, 1 \(\times\) 4 H-plane VTSA arrays are designed in [19, 20] using T-junction and flexible Microstrip-Slotline-Microstrip (MSM) power dividers to improve the BW and gain for directional UWB system and satellite communication applications, respectively. In [21], a cavity absorber material with corrugations is used to mount the 3 \(\times\) 4 VTSA array for protection and performance enhancement. In [22, 23], a 3 \(\times\) 4 metal Vivaldi antenna array with wide BW (5 GHz), improved gain (14.84 dBi), and reduced side lobe level (\(<\)\(-\)20 dB) are designed for airborne applications. Authors in [24] designed a 4 \(\times\) 4 (1.21-1.57 GHz) VTSA array fed by WPD with improved gain for radar application (wind profiler). In [25], two compact VTSAs with greater directivity are proposed to improve base station diversity.