Wheel-of-Time Array Devoted to Near-Field Imaging Applications K. Alkhalifeh (1) , R. Sarkis (2) , C. Craeye (1) (1) Université catholique de Louvain, (2) AGC company ICTEAM Institute, Place du Levant, 2, 1348 Louvain-la-Neuve {khaldoun.alkhalifeh, christophe.craeye}@uclouvain.be, remi.sarkis@eu.agc.com Abstract— The design and fabrication of an ultra- wideband (UWB) imaging radar system for near-field applications is presented. First, the geometric parameters related to the elliptical curve of Vivaldi antenna, i.e. major and minor radii and center, are introduced as additional design parameters. Second, the complete circular UWB radar imaging system is fabricated. Finally, time-domain responses of a target under test are processed. An initial near-field image of the target under test in the area of radiation exposure is depicted, and the target is localized. I. INTRODUCTION Recent years have seen a growing interest in exploiting ultra-wideband (UWB) technology for imaging applications. The wide bandwidth provides the range resolution, while the array interferometry provides the azimuthal discrimination. UWB systems are used in medical applications to obtain internal images of various objects [1]. Large bandwidth is very important for this type of applications, where high frequencies provide high resolutions and low frequencies penetrate considerably deeper into the object. Vivaldi antennas printed on a substrate have been used in several imaging systems [2]. Recent studies focused on metal antennas to reduce loss in the dielectric material [3]. In the current work, 10 connected metal elements were arranged in a circular array to form the UWB imaging system, which we called the “Wheel-of-Time” (name proposed by Eloy de Lera Acedo from University of Cambridge) [8]. New design solutions for Vivaldi antennas were applied to improve the antenna efficiency over the bandwidth of interest. The new method is based on the formulation of the design curves, which are main elements of the antenna construction. One approach to imaging a target is radar-based. A short-time pulse is sent toward the target under test (TUT) using one or more antennas. In the presence of a target, reflections can be measured and inform us about the composition of the object. Recently, time- domain systems have been successfully used for breast cancer detection [4]. The remainder of this paper is organized as follows. Section II presents the design of a traditional 3D Vivaldi antenna. Then, the new design solution is applied on the antenna and the improved 3D Vivaldi antenna is discussed. In Section III, the “Wheel-of- Time” array is presented. The experimental setup in the presence of a reference target (RT) is illustrated. In Section IV, time-domain responses are processed in the free space, reference target and TUT by performing an Inverse Fourier Transform. An imaging algorithm is implemented to give an initial localization of targets. The image map has been located successfully the target. Section V concludes the paper. II. DESIGN OF THE ISOLATED 3D VIVALDI ANTENNA A. Traditional 3D Vivaldi antenna Notch-antenna feeds are usually comprised of a stripline-to-slotline or microstrip transitions [5]. This complex feeding adds significant losses, and affects at the sensitivity of the receiving arrays. One of the best solutions to overcome this problem consists of considering the feed as one of the built-in 3D metal antenna component like 3D Vivaldi circular array [6]. The shape of the traditional Vivaldi slot has been modified to fit the design of the circular antennas array as shown in Fig.1. The type of taper profile is elliptical. An electromagnetic simulation software (MOM3D) developed at UCL and based on the Method of Moments is used to simulate the different structures. The width of the ground plane is chosen to 10 cm, and the thickness to 5 mm for the optimized behavior. Figure 1: Mesh view of the 3D Vivaldi antenna.