# Ultrawideband Low-Profile and Miniaturized Spoof Plasmonic Vivaldi Antenna for Base Station
## 1 Introduction  
The fifth-generation (5G) mobile communication systems have found various applications, such as autonomous driving [1], telemedicine [2], and virtual reality [3]. Considering radio wave propagation and available bandwidth, the bands between 3 and 5 GHz have been allocated for 5G services in many regions, such as 3.4-3.8 GHz in Europe, 3.7-4.2 GHz in the USA, and 3.3-3.6 and 4.8-4.99 GHz in China [4]. Hence, in the 2G/3G/4G/5G base station applications, the wide bandwidth covers the 2G, 3G, and 4G bands (1.7-2.7 GHz) and the new sub-6 GHz 5G frequency bands. It is urgent to investigate and design ultra-wideband antennas to cover the aforementioned bands and maintain good impedance matching within the entire frequency bands of interest, with a stable half-power bandwidth (HPBW) and a smaller structure.  
Crossed dipole antennas are widely used in wireless communication systems due to their advantages of wide bandwidth, stable radiation patterns, and ease of fabrication [5]. Various crossed dipoles were utilized to achieve a wide impedance bandwidth, such as magnetoelectric dipole antennas [6], bow-tie dipole [7], fan-shaped dipole [8], octagonal loop dipole [9], and double-loop dipole [10]. For most of them, the wide impedance bandwidth only covers 2G/3G/4G bands. Furthermore, the antenna profile is not low due to the necessary broadband feeding part.  
The microstrip antennas have advantages of low profile, light weight, and ease of conformability. However, their impedance bandwidth and isolation are not good enough. Some attractive techniques have been proposed to broaden the impedance bandwidth by using parasitic patches [11], loading H-slot [12] and U-slot [13], using a multiresonant structure [14], and using a fractal structure [15]. Although the shorted-dipoles printed antenna fed by integrated baluns [16] and the dual-band folded dipole antenna [17] cover 2G/3G/4G/5G frequency band, the gain and radiation pattern of these antennas are unstable over the operating frequency band.  
Another typical ultra-wideband antenna is based on tapered slot technique. As one of the most typical types, Vivaldi antenna was firstly introduced in [18] and has found many applications [19; 20].  
However, its size is still large. Surface plasmons (SPs) are collective electron-photon oscillations confined to the metal surface, which are characterized by subwavelength confinement and field enhancement in optical spectrum [21]. Spoof SPs could be obtained by constructing periodic metal surfaces to get the characteristics of SPs in the microwave frequencies [22; 23]. They have been explored in many antenna applications such as holo-graphic antenna [24; 25], metasurface planar lenses [26; 27], and metasurface superstrate [28; 29]. It has been shown that by loading plasmonic metamaterials, the antenna can be efficiently miniaturized [30].  
Here, by loading plasmonic metamaterials, a modified antipodal Vivaldi antenna (AVA) is designed and measured, which is composed of the AVA and the loaded periodic slots. The designed operating frequency is from 1.8 GHz to 6 GHz, which covers 2G/3G/4G/5G frequency band. The simulated results and measured results agree well. It shows that the radiation pattern of the plasmonic AVA is stable and the average gain is 7.24 dBi over the operating frequency band.