# Circularly Polarized Vivaldi Antennas Integrated with Septum-like Polarizer
## 1 Introduction  
Circularly polarized (CP) antennas are advantageous due to their robustness against multi-path reduction, fading, and polarization mismatch between two antennas, compared to antennas with linear polarization (LP) [1; 2]. A variety of antennas such as microstrip patch antennas, wire antennas, slot antennas, and horn antennas have been used for realizing CP antennas [1; 3; 4; 5]. Among them, the Vivaldi antennas firstly proposed by [PERSON] are becoming more popular due to their high gain, broad bandwidth, and ease of impedance matching [6; 7; 8; 9; 10; 11; 12; 13]. In recent years, there has been advancement to offer advantages of diverse polarizations employed in the Vivaldi antennas. Especially, two or more assemblies of coplanar Vivaldi antennas have been widely used to meet the requirements in terms of different types of polarization [14; 15; 16; 17]. Dual-polarized antennas have been obtained through two co-planar LP Vivaldi antennas in an orthogonal configuration [14; 15]. With the emergence of modern wireless communication, the use of CP antennas has been ever increasing. Coplanar CP Vivaldi antennas have been presented using a feed network involving a power divider to split the power equally and a 90\({}^{\circ}\) phase shifter [16; 17]. However, within the feed network, circuitries with several electronic components are required and therefore introduce dielectric losses at high frequencies and increase the complexity of the feed network [18; 19]. Therefore, an efficient CP Vivaldi antenna with the simple design of a low-loss feed network is desired. Another alternative is a septum polarizer, which has been used in waveguide systems. A septum polarizer based on a common wall of two rectangular waveguides has been widely employed, where most studies have focused on optimizing the contour's shapeof the wall. A high quality of CP wave can be obtained using a septum polarizer, minimizing undesired losses [20; 21; 22; 23; 24; 25]. A septum polarizer that has been often used in large waveguide and horn antennas has extended its applications to low profile antennas [26; 27]. Some effort has been made to implement a septum polarizer for a multi-layered substrate integrated waveguide (SIW) instead of a planar antenna [26]. A septum polarizer with a low profile waveguide has been studied for use in the dual polarized antennas used in satellite communications [27]. A septum polarizer that is similar to the teeth of a wood saw has been obtained through optimizing its single tooth using a full-wave simulation tool, HFSS. This septum is advantageous due to its applicability to antennas with a low profile; however, its applications are confined to narrowband antennas, compared to the previous works. The CP Vivaldi antenna can be used in radar and satellite communication applications to enhance the probability of target detection and communication efficiency. For some high-altitude applications, the feed network using electronic components may be affected by the severe environment in the atmosphere [28]. Therefore, a feed network with robustness against the effects of the external circumstances is more desirable. This paper presents a circularly polarized Vivaldi antenna operating at 11 GHz incorporated with a septum polarizer, which has been frequently used in feed networks of waveguide antennae. In this work, the proposed antenna consists of vertical and horizontal Vivaldi antennas, and either is combined with the septum polarizer. A septum polarizer shaped like a pulse train is presented, which makes it possible to guide the wave until it arrives at an aperture of the antenna and produces a proper phase difference between the two orthogonal antennas. Optimizing the dimensions of the pulse train was performed using a full-wave simulation based optimization technique. In addition, in this work, non-uniform spacing between each pulse train was utilized to obtain a wide bandwidth. The prototypes of the two horizontal and vertical Vivaldi antennas were constructed and firmly attached together.