electronics Article Miniaturized Antipodal Vivaldi Antenna with Improved Bandwidth Using Exponential Strip Arms Mohammad Mahdi Honari * , Mohammad Saeid Ghaffarian and Rashid Mirzavand Intelligent Wireless Technology Laboratory, Electrical and Mechanical Engineering Department, University of Alberta, 9211 116 Street NW, Edmonton, AB T6G 1H9, Canada; [email protected] (M.S.G.); [email protected] (R.M.) * Correspondence: [email protected] Abstract: In this paper, a miniaturized ultra-wideband antipodal tapered slot antenna with exponen- tial strip arms is presented. Two exponential arms with designed equations are optimized to reduce the lower edge cut-off frequency of the impedance bandwidth from 1480 MHz to 720 MHz, resulting in antenna miniaturization by 51%. This approach also improves antenna bandwidth without com- promising the radiation characteristics. The dimension of the proposed antenna structure including the feeding line and transition is 158 × 125 × 1 mm3. The results show that a peak gain more than 1 dBi is achieved all over the impedance bandwidth (0.72–17 GHz), which is an improvement to what have been reported for antipodal tapered slot and Vivaldi antennas with similar size. Keywords: antipodal Vivaldi antenna (AVA); tapered slot antenna (TSA); ultra-wideband (UWB) 1. Introduction Due to the rapid development of wireless communication, ultra-wideband anten- nas/systems are becoming highly attractive in many wideband applications, such as broad Citation: Honari, M.M.; Ghaffarian, band wireless communications, ultra-wideband interference, and imaging systems [1,2]. M.S.; Mirzavand, R. Miniaturized They can be used in electromagnetic compatibility (EMC) test and measurement of emerg- Antipodal Vivaldi Antenna with ing wireless technology devices and UWB security radar detection [3]. Different antennas Improved Bandwidth Using such as double ridge horn antennas, fractal antenna or planar Vivaldi antennas could be Exponential Strip Arms. Electronics 2021, 10, 83. https://doi.org/ utilized for mentioned applications. Due to the relatively large and expensive fabrication 10.3390/electronics10010083 process of horn antennas and poor radiation of fractal antennas, Vivaldi antennas seem to be a good choice because of their ultra-wideband performance with acceptable radiation Received: 15 November 2020 characteristics. Accepted: 31 December 2020 Vivaldi antennas which are also called exponential tapered slot antennas (ETSA) were Published: 4 January 2021 introduced by Gibson and Gazit as a new class of UWB antennas [4,5]. The dual exponential tapered slot antennas (DETSA) are a type of conventional Vivaldi antennas [5–7]. Recently, Publisher’s Note: MDPI stays neu- many types of TSA antennas such as linear TSA, conformal TSA, parabolic TSA, and tral with regard to jurisdictional clai- logarithmic TSA were investigated extensively in [4–11]. ms in published maps and institutio- Several modifications have been carried out on the basic co-planar TSA structure, nal affiliations. resulting in antipodal Vivaldi antennas (AVAs) which consist of two radiating arms on either side of the substrate with 180◦ differential phase excitation to obtain the best radiation characteristics. The AVAs possess some advantages such as ultra-wideband performance compared to other Vivaldi antennas. Various kinds of AVA structures have been reported Copyright: © 2021 by the authors. Li- censee MDPI, Basel, Switzerland. in [11–18] and some techniques have been proposed to reduce the size of AVAs and improve This article is an open access article their performance [15–19]. In these antennas, although the high edge of frequency band of distributed under the terms and con- interest could be increased to ultra-high frequencies, the lower edge cut-off is still limited ditions of the Creative Commons At- by the antenna effective aperture size. tribution (CC BY) license (https:// In this paper, a miniaturized AVA with improved impedance bandwidth is proposed. creativecommons.org/licenses/by/ Two exponential arms with designed equations are added to the conventional structure 4.0/). of antipodal Vivaldi antenna in order to miniaturize the whole size of the antenna. By Electronics 2021, 10, 83. https://doi.org/10.3390/electronics10010083 https://www.mdpi.com/journal/electronics Electronics 2021, 10, 83 2 of 12 adding two of these additional arms, it is demonstrated that the lower edge of impedance bandwidth starts from 0.72 GHz instead of its original value of 1.48 GHz, resulting in antenna miniaturization by 51% size reduction. The measured results show that the proposed antenna perfectly works over 0.72 GHz to 17 GHz. The paper is organized as follows: First, the antenna configuration and design pro- cedure are presented. By using antenna current distribution, the antenna radiation mech- anism is demonstrated to bring a physical insight into the improvement of the antenna performance. Then antenna design simulation with parametric study and measured results are investigated. Finally, a conclusion is given in the last section. 2. Antenna Configuration and Design Figure1a shows conventional antipodal antennas with different widths of tapered Electronics 2021, 10, x FOR PEER REVIEW m 3 of 12 strips labeled as 1. To investigate the effect of tapered strips’ width of the antenna on cut-off frequency and antenna size miniaturization, four cases with different widths of tapered strips of 10 mm, 22 mm, 50 mm, and 80 mm are considered in Figure1. As shown in FigureIn order1b, by to changing further understand the width of the tapered behavior strips, of the the antenna AVA structure, cut-off frequency especially changes in the betweenlower frequencies, 1.55 GHz to the 1.4 current GHz. Although distribution m1 can of bethe used AVA for with antenna arms matching, (proposed this structure) variable cannotand without be obviously them (conventional utilized for antenna structure) size is miniaturization. shown in Figure In addition,3. As shown the antennain Figure gain 3a, forat 1.6 all GHz, four casesboth instructures Figure1b with/without shows that the arms, width approximately of tapered strips have in the antipodal same current antennas dis- affectstribution. mainly However, the antenna at 0.8 GHz, gain. shown It should in Figure be noted 3b, that the increasingarms change the the length current of the distribu- strips shiftstion significantly. the cut-off frequency At this frequency, of the antipodal the strip antennas arms in to the lower proposed frequency structure with thecreate cost an- of havingother resonance a larger antenna.since they In behave this manuscript, like a dipole we antenna investigate with theelectrical possibility length of of integrating a quarter- anotherwavelength. resonator In the into structure the antipodal without Vivaldiarms, no antenna, resonance which happens could at help 0.8 toGHz. reduce As a the result, size of antipodal Vivaldi antennas, without compromising the radiation characteristics. at low frequencies, the additional arms contribute to the antenna miniaturization. (a) (b) FigureFigure 1. AntipodalAntipodal Vivaldi Vivaldi antenna antenna configuration configuration with with (a ()a different) different widths widths of oftapered tapered strips, strips, and and (b) (theirb) their reflection reflection co- coefficientefficient and and gain. gain. Figure2 demonstrates the proposed antenna. The proposed antenna includes two main tapered strips (conventional structure) and two additional strip arms. We will investigate how these additional strip arms contribute to antenna size reduction. The 2 dimensions of the antenna are set to be 158 × 125 mm , which is approximately 0.38λ0 × 0.3λ0, where λ0 is the free space wavelength of 0.72 GHz. The proposed microstrip-fed antenna is designed on Taconic TLT substrate with dielectric constant and thickness of Figure 2. The proposed Vivaldi antenna configuration. Electronics 2021, 10, x FOR PEER REVIEW 3 of 12 In order to further understand the behavior of the AVA structure, especially in the lower frequencies, the current distribution of the AVA with arms (proposed structure) and without them (conventional structure) is shown in Figure 3. As shown in Figure 3a, at 1.6 GHz, both structures with/without arms, approximately have the same current dis- tribution. However, at 0.8 GHz, shown in Figure 3b, the arms change the current distribu- tion significantly. At this frequency, the strip arms in the proposed structure create an- other resonance since they behave like a dipole antenna with electrical length of a quarter- wavelength. In the structure without arms, no resonance happens at 0.8 GHz. As a result, at low frequencies, the additional arms contribute to the antenna miniaturization. Electronics 2021, 10, 83 3 of 12 2.55 and 1 mm, respectively. In order to design a compact ultra-wideband antenna, four exponential curves are employed. The equations of these curves are given as: 8 a x y(x) = a1 e 2 + a3 upper curve > b x , for main tapered strips < y(x) = b1 e 2 + b3 lower curve c x (1) > y(x) = c1 e 2 + c3 upper curve :> d x , for additional strip arms y(x) = d1 e 2 + d3 lower curve where a2, b2, c2, and d2 demonstrate the degree of the exponential curves and a1, b1, c1, (a) (b) d1, a3, b3, c3, and d3 are related to the length and width of the arms and depend on the Figure 1. Antipodal Vivaldiposition antenna ofconfiguration the origin ofwith the (a Cartesian) different coordinatewidths of tapered system strips, in the and antenna (b) their structure. reflection