1536-1225 (c) 2016 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission. See http://www.ieee.org/publications_standards/publications/rights/index.html for more information. This article has been accepted for publication in a future issue of this journal, but has not been fully edited. Content may change prior to final publication. Citation information: DOI 10.1109/LAWP.2016.2572064, IEEE Antennas and Wireless Propagation Letters Abstract— Ultra Wide Band (UWB) communications, various parameters are important such as high gain, bandwidth, linear phase center and end fire pattern. In this article, end fire taper slot Vivaldi has been noticed because of its high gain and wide bandwidth, however achieving linear phase center was a big problem. Although the phase of the antenna changes near the aperture area in horn antennas, we have suggested novel technique using composite aperture structure to make linear phase center. Therefore, in our proposed structure, we separated a part of Double Exponentially Tapered Slot Antenna (DETSA) from its dielectric with an angle of 8 degrees upwards in the y- axis. In this case, we accomplish increasing the antenna gain and solve the phase center problem. The prototype antenna with added aperture in order to operate in a wide bandwidth shows good performance in 6-18 GHz with VSWR less than 2 and the proposed antenna achieves higher gain, more than 2-4 dBi, in comparison with conventional tapered slot antennas. The antenna is fabricated on Rogers-4003 and experimental results confirm the simulations. Index Terms— Aperture, DETSA, phase center, tapered slot, UWB I. INTRODUCTION Tapered slot antennas (TSAs) generally have wide bandwidth, high directivity and are able to produce a symmetrical radiation pattern. They have also simple feed structure and trouble-free fabrication process. This type of antenna has been employed in satellite communications, ground penetrating radar (GPR) and etc [1]. The attractions of end-fire TSAs are producing a symmetrical beam in the E-and H-plane or controlling the beam width by changing the shape, length, dielectric thickness and dielectric constant that radiates preferentially from the open end of the notch along the axis of symmetry [2]. DETSA, which is a distinct form of the Vivaldi antenna with the outer edge exponentially tapered, was introduced for the first time in [3] with designing process and performance characteristics. It is also discussed as a possible UWB antenna [4-5]. Also, a coplanar waveguide–fed version of DETSA is explored in [6] for a UWB sub-band. In Vivaldi antenna, the Afsane Saee Arezoomand and Mohammad Naser Moghadasi are with Faculty of Eng., Science and Research Branch, Islamic Azad University, Tehran-Iran, (Corresponding Author: mn.moghaddasi@srbiau.ac.ir). R. A. Sadeghzadeh is with Faculty of Electrical Engineering, K.N Toosi University of Technology, Tehran-Iran, phase center is noticeable and many researches have been done about it recently. The phase center is the point from which the electromagnetic radiation spreads spherically outward, with the phase of the signal being equal at any point on the sphere. The phase center of UWB antenna has an impressive effect on its performance in the time domain [7]. Usually, the width of transmitting an impulse is about several hundreds of nanoseconds, so even fine phase center, drift will influence the phase of the far field much and lead to the waveform dispersion in the time domain [8]. Therefore, the phase center of the antenna should be at the center of the sphere, but the antenna may have the phase center or not because, it correlates to the beam widths and the different tangent planes so that, the phase center will distribute over the scope under the different beam widths and it has a non-linear form. For UWB antenna applications, the phase center should be stable and linear as the beam width and frequency varies at the same time. The distance between Phase center and antenna shows the radius of curvature in the equal-phased polar plot and it is used to calculate the phase center [9-10]. In addition, employing the aperture can increase the gain of the antenna as ~ f 2 . So, effective aperture (A e ) is proportional to antenna gain and operating wavelength. A e is determined by the voltage available across a load matching the antenna feed impedance for a given electromagnetic field strength density. For instance, a high gain antenna with a symmetrical aperture distribution will have a unique phase center located at its geometrical center. In horn antenna, effective aperture leads to achieve higher gain [11]. In this paper, a new model based on our proposed method is presented and have been compared with conventional TSA. Finally, the results show that the suggested antenna has magnificent advantages. Moreover, a comparison between proposed antenna and folded models determined some weaknesses of these models. Further, the results of improvements of gain and phase center have been shown. II. ANTENNA DESIGN Fig.1 (a) and (b) show the designed antenna top and side view, respectively. Further, the folded form of proposed antenna is presented in Fig.1 (c) and as can be seen, dielectric rises 8 degrees along the y-axis. The final TSA model contains two tapered lines and aperture with 8º on one side and the antenna is connected to the 50-Ω SMA connector as shown in Fig.1 (d). Novel techniques in tapered slot antenna for linearity phase center and gain enhancement Afsaneh Saee Arezoomand, R. A. Sadeghzadeh, and Mohammad Naser-Moghadasi