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.2016.2622919, IEEE Antennas and Wireless Propagation Letters Abstract— A conformal array antenna has been designed in waveguide technology in order to obtain an omnidirectional radiation pattern in the azimuth plane at Ku-band. The antenna shape is a regular prism where the single radiating elements, which consist on a rectangular waveguide ended on a cavity-slot, are placed on the planar faces of the prism. The physical dimensions as well as the number of faces of the regular prism have been optimized following the conformal array formulation in order to achieve a minimum ripple omnidirectional pattern. An eight-faced regular prism with an apothem of 20.6 mm has been selected as radiating structure. The single elements are fed by an eight-way radial waveguide power divider symmetrically excited by a 50Ω SMA input port. The use of waveguide technology to design the single elements and the feeding network makes the conformal array suitable for low losses and high power applications. Several prototypes have been manufactured in order to experimentally validate the antenna performance. An omnidirectional pattern with a maximum ripple of 1.6 dB in the azimuth plane, 95.3% efficiency, and 11.7% input matching bandwidth has been measured. Index Terms— Slot arrays, conformal antennas, omnidirectional antennas I. INTRODUCTION ONFORMAL antennas usually form a determined shape in order to accomplish certain aerodynamic or electromagnetic considerations such as special angular coverage [1]. Either omnidirectional or directional patterns with arbitrary direction can be obtained using cylindrical, conical or spherical radiating structures [2]-[5]. Typically, the omnidirectional pattern can also be accomplished by means of conventional wire antennas such as monopoles or dipoles. Nevertheless, an strict tolerance manufacturing process is required for these wire antennas in very high frequency applications [6]. In addition, the possibility to generate beam forming in conformal array antennas can provide more flexibility to the communication system [1, Ch. 9], [4]. On the other hand, the microstrip conformal antennas are widely used in satellite, aerospace or point-to-point Manuscript received Jun 3, 2016. This work was supported by Ministerio de Economía y Competitividad (Spanish Government) under project TEC2014-55735-C3-1-R (ENABLING5G) and Comunidad de Madrid under project S2013/ICE-3000 (SPADERADAR-CM). P. Sanchez-Olivares, P. P. Sanchez-Dancausa, J.L. Masa-Campos and J. A. Ruiz-Cruz are with the Department of Electronic and Communication Technology, Universidad Autónoma de Madrid, Escuela Politécnica Superior, C/ Francisco Tomás y Valiente, 11, 28049, Madrid Spain (e- mail:pablo.sanchezo@uam.es). R. V. Haro-Baez is with the Universidad de las Fuerzas Armadas, ESPE, Quito, Ecuador (e-mail: rvharo@espe.edu.ec). communications due to the many advantages such as low profile, low cost or easy fabrication [7]. Nevertheless, the power handling capacity of this kind of antennas is limited and the dielectric substrate losses as much as the bending of the structure can produce a significant efficiency reduction, especially for a large antenna array [8]. In this way, the conventional waveguide technology is suitable to resolve these issues due to their advantages of low losses, high power capacity and high efficiency [9]. In addition, a complex conformal structure can be approximated by using another shape with planar faces like a regular polyhedron [10], reducing the costs derived from the conformal structure manufactured using conventional waveguide technology. For all of these reasons, a conformal array antenna with an omnidirectional radiation pattern in the azimuth plane and linear polarization at Ku-band (17 GHz) is proposed [11], manufactured and measured in this letter. This antenna takes part of a user portable transponder for a radio-localization system. The radiating structure conforms a regular N-faced prism, where both, the number of radiating elements and the size of the prism, establish the omnidirectional behavior [11]. The single radiating elements consist of rectangular waveguides ended on a cavity-slot fed by a radial waveguide power divider. A remarkable advantage of this topology, presented as a proof of concept, is that allows a very compact unit, suitable for antenna applications requiring low losses and high power handling capabilities. Additionally, this conformal array antenna is proposed as a previous step of a future reconfigurable antenna design which incorporates sectorial coverage besides the omnidirectional radiation pattern. This work is divided as follows: Section II describes the conformal array design process. Section III shows the experimental results derived from the conformal array prototype. Finally, Section IV draws some conclusions. P. Sanchez-Olivares, R. V. Haro-Baez, P. P. Sanchez-Dancausa, J. L. Masa-Campos and J. A. Ruiz-Cruz Compact Omnidirectional Conformal Array Antenna in Waveguide Technology C Fig. 1. Schematic view of the proposed N-faced regular prism conformal array antenna fed by a radial power divider