sensors Article Compact Slotted Waveguide Antenna Array Using Staircase Model of Tapered Dielectric-Inset Guide for Shipboard Marine Radar Kyei Anim , Henry Abu Diawuo and Young-Bae Jung * Electronics Engineering Department, Hanbat National University, Daejeon 34158, Korea; [email protected] (K.A.); [email protected] (H.A.D.) * Correspondence: [email protected]; Tel.: +82-42-821-1136 Abstract: This paper presents a new configuration of a slotted waveguide antenna (SWA) array aimed at the X-band within the desired band of 9.38~9.44 GHz for shipboard marine radars. The SWA array, which typically consists of a slotted waveguide, a polarizing filter, and a metal reflector, is widely employed in marine radar applications. Nonetheless, conventional slot array designs are weighty, mechanically complex, and geometrically large to obtain high performances, such as gain. These features of the conventional SWA are undesirable for the shipboard marine radar, where the antenna rotates at high angular speed for the beam scanning mechanism. The proposed SWA array herein reduces the conventional design’s size by 62% using a tapered dielectric-inset guide structure. It shows high gain performance (up to 30 dB) and obtains improvements in radiation efficiency (up to 80% in the numerical simulations) and weight due to the use of loss and low-density dielectric material. Citation: Anim, K.; Diawuo, H.A.; Keywords: beam scanning; dielectric-inset guide; high gain; low weight; marine radar; radiation Jung, Y.-B. Compact Slotted efficiency; slotted waveguide array; X-band; size reduction; tapering Waveguide Antenna Array Using Staircase Model of Tapered Dielectric-Inset Guide for Shipboard Marine Radar. Sensors 2021, 21, 4745. https://doi.org/10.3390/s21144745 1. Introduction Marine radars have been proven useful in commercial and military settings to provide Academic Editors: Youchung Chung information about other seacrafts and land targets [1] for diverse applications, such as and Cynthia M. Furse coastal surveillance and surface target detection, collision avoidance, weather, and bird migration monitoring [2–5]. In this regard, X-band marine radars have become more Received: 24 May 2021 attractive as they offer a high spatial and temporal resolution, low cost, flexibility, and Accepted: 8 July 2021 installation ease [6]. The marine environment imposes severe constraints on antennas’ Published: 12 July 2021 performance and mechanical design. Thus, the marine radar antenna needs to obtain fan-beam radiation patterns to operate successfully under these harsh conditions. Publisher’s Note: MDPI stays neutral Several microstrip array antennas have been extensively employed in radar and other with regard to jurisdictional claims in wireless communications systems [7–17]. Although they have promising features, such as published maps and institutional affil- low profile, low cost, and fabrication ease, their low-power handling capacity and high iations. losses often make them less suitable for marine radar [18–20]. Consequently, the slotted waveguide array (SWA) antennas [21–25], based on the pioneering work published by Elliot [26–28], are widely employed in marine radar applications. This is because of their high-power handling capacity, low losses, and good phase stability. The conventional Copyright: © 2021 by the authors. designs of SWA arrays typically consist of a slotted waveguide structure, a polarizing filter, Licensee MDPI, Basel, Switzerland. and a large metal reflector to enhance directivity, especially in the vertical plane. By using This article is an open access article this reflector device, especially in the lower frequency bands, the conventional SWA arrays distributed under the terms and become weighty, geometrically large, and mechanically complex [29]. They are, therefore, conditions of the Creative Commons undesirable for marine radar systems of which the antenna rotates at high-angular speed. Attribution (CC BY) license (https:// A miniaturization technique is thus required to make the traditional SWA more suitable creativecommons.org/licenses/by/ for the marine environment without resorting to gain degradation. Nonetheless, reducing 4.0/). Sensors 2021, 21, 4745. https://doi.org/10.3390/s21144745 https://www.mdpi.com/journal/sensors Sensors 2021, 21, x FOR PEER REVIEW 2 of 15 Sensors 2021, 21, 4745 2 of 14 SWA more suitable for the marine environment without resorting to gain degradation. Nonetheless, reducing the reflector size as a simple miniaturization process results in con- siderable antenna gain reduction. the reflectorIn this paper, size asa 62% a simple size reduction miniaturization of the SWA process array results was obtained in considerable by incorporating antenna again tapered reduction. dielectric-inset guide structure in the conventional slot array design. At the same time, Indirectivity this paper, in athe62% verticalsize reduction plane was of improved the SWA array significantly was obtained without by the incorporating inclusion of a thetapered reflector dielectric-inset device. The guide function structure of the indielec the conventionaltric structure slotwith array the staircase design. Atmodel the same is to smoothlytime, directivity transform in the the vertical incident plane and wasstrongly improved bound significantly surface waves without from thethe inclusionwaveguide of intothe reflectorfree space device. characterized The function by minimum of the dielectric reflection structure and phase with disparity. the staircase Thus, model the results is to smoothly transform the incident and strongly bound surface waves from the waveguide indicated that the proposed SWA array produces antenna gain up to 30 dB comparable to into free space characterized by minimum reflection and phase disparity. Thus, the results the conventional SWA designs. This new configuration of the SWA array was compared indicated that the proposed SWA array produces antenna gain up to 30 dB comparable with its conventional counterpart to prove the practicality of the approach used in this to the conventional SWA designs. This new configuration of the SWA array was com- paper. pared with its conventional counterpart to prove the practicality of the approach used in this paper. 2. Antenna Configuration 2. AntennaFigure 1 Configuration compares the geometry of the conventional SWA array, set as a benchmark here, Figureto that1 of compares the proposed the geometry antenna based of the on conventional the design specifications SWA array, set in as Table a benchmark 1. In both cases,here, tothe that antenna of the consists proposed of a antennaslotted wave basedguide on the arrayed design with specifications ninety-four inslot Table radiators1. In toboth produce cases, a the fan-beam antenna radiation consists pattern of a slotted at the waveguide center frequency arrayed f with= 9.41 ninety-four GHz. The fan- slot beam,radiators due to to produce the long a waveguide fan-beam radiation axial length pattern (L) along at the the center y-axis, frequency has a narrowf = 9.41 beam- GHz. widthThe fan-beam, of 1.2° in due the tohorizontal the long waveguide() plane and axial a broader length (L)beamwidth along the ofy -axis,20° in has the a vertical narrow ◦ ◦ (beamwidth) plane to compensate of 1.2 in the for horizontal the roll of the (x) ship plane [30]. and The a broader design, performances, beamwidth of and 20 analy-in the sisvertical of the ( xantennas) plane to presented compensate in this for articl the rolle have of the been ship performed [30]. The using design, CST performances, Microwave Studioand analysis software. of the antennas presented in this article have been performed using CST Microwave Studio software. (a) (b) Figure 1. Full 3D views of slotted waveguide array antenna. ( a)) Conventional case; ( b) Proposed case. Table 1. Antenna design specifications for X-band marine radar. Table 1. Antenna design specifications for X-band marine radar. Parameters Value Parameters Value Parameters Value Parameters Value Frequency 9.4 GHz ± 30 MHz Polarization Horizontal Frequency 9.4 GHz ± 30 MHz Polarization Horizontal Horizontal: 1.2° or less Gain ≥28 dB Beamwidth Horizontal: 1.2◦ or less Gain ≥28 dB BeamwidthVertical: 20° or higher Vertical: 20◦ or higher Voltage standing wave Horizontal: −10 dB or less Voltage≤1.5 standing Sidelobelevel (SLL) Horizontal: −10 dB or less ratio (VSWR) ≤1.5 SidelobelevelVertical: (SLL) −10 dB or higher wave ratio (VSWR) Vertical: −10 dB or higher Sensors 2021, 21, 4745 3 of 14 Sensors 2021, 21, x FOR PEER REVIEW 3 of 15 2.1. Slotted Waveguide Design 2.1. Slotted Waveguide Design The slotted waveguide structure shown in Figure2 consists of a WR-90 standard The slotted waveguide structure shown in Figure 2 consists of a WR-90 standard waveguide (a = 22.86 mm, b = 10.16 mm) with aluminum material and a wall thickness, waveguide ( = 22.86 mm, = 10.16 mm) with aluminum material and a wall thick- t = 1.27 mm. An array of tilted slots is milled into the narrow wall of a rectangular ness, = 1.27 mm. An array of tilted slots is milled into the narrow wall of a rectangular waveguide in order to generate the desired horizontally polarized electric fields [30], waveguide in order to generate the desired horizontally polarized electric fields [30], as as specified in Table1. The slots introduce discontinuities in the waveguide’s conduct- specified in Table 1. The slots introduce discontinuities in the waveguide’s conducting ing walls to interrupt the flow of electric currents along the waveguide axial. Hence, walls to interrupteach the slot flow acts of electric as a dual currents electric along dipole, the accordingwaveguide to axial. Babinet’s Hence, principle, each slot to elicit radia- acts as a dual electric dipole, according to Babinet’s principle, to elicit radiations from the tions from the traveling waves propagating in the fundamental TE10 (transverse electric) traveling waves propagating in the fundamental TE (transverse electric) mode in the mode in the waveguide [31]. The cutoff frequency for the TE10 mode is computed as waveguide [31]. The cutoff frequency for the TE mode is computed as = /(2) = fc = c/(2a) = 6.56 GHz (c is the speed of light in a vacuum).