1288 IEEE TRANSACTIONS ON ANTENNAS AND PROPAGATION, VOL. 63, NO. 4, APRIL 2015 Modulated Metasurface Antennas for Space: Synthesis, Analysis and Realizations Gabriele Minatti, Marco Faenzi, Enrica Martini, Senior Member, IEEE, Francesco Caminita, Paolo De Vita, David González-Ovejero, Member, IEEE, Marco Sabbadini, and Stefano Maci, Fellow, IEEE Abstract—This paper presents design and analysis methods for planar antennas based on modulated metasurfaces (MTSs). These antennas operate on an interaction between a cylindrical surface-wave (SW) excited by an isotropic TM radiator, and an MTS having a spatially modulated equivalent impedance. The MTS is realized by using sub-wavelength patches printed on a grounded slab, thus resulting in a structure with light weight and compact volume. Both features are appealing characteristics for space applications. This paper introduces for the first time an impedance-based amplitude synthesis of the aperture field distribution and shows several new examples of antennas for space applications obtained in recent research projects financed by the European Space Agency. Index Terms—Leaky-waves, metasurface antennas, modulated surface impedance, space antennas, surface-waves. I. INTRODUCTION A NTENNAS are key components in all satellite systems. Reflector antennas are the most common solution for high and very-high gain antennas, whereas arrays are used to a minor extent owing to their higher complexity. Several types of an- tennas may be used for low-gain and medium-gain needs. De- spite the high degree of flexibility and the technological ma- turity reached, new solutions are actively sought to improve upon existing ones in response to the continuous demand for increased performances. In this framework modulated metasur- face antennas (MoMetAs) appear to be a promising alternative. Holloway et al. presented in [1] a review on metasurfaces (MTSs), where they showed the wide range of their potential applications in electromagnetics. However, the emphasis in [1] Manuscript received May 30, 2014; accepted August 03, 2014. Date of pub- lication December 05, 2014; date of current version April 03, 2015. This work was supported by the European Space Agency under contracts 22808/09/NL/JD and AO/1-7069/12/NL/MH. G. Minatti and M. Sabbadini are with the European Space Agency, 2201 AZ Noordwijk, The Netherlands (e-mail: gabriele.minatti@esa.int; marco.sabbadini@esa.int). M. Faenzi, D. González-Ovejero, and S. Maci are with the Department of Information Engineering and Mathematics, University of Siena, 53100 Siena, Italy (e-mail: macis@dii.unisi.it). E. Martini and F. Caminita are with the Department of Information Engi- neering and Mathematics, University of Siena, 53100 Siena, Italy and also with Wave Up S.r.l., 50129 Firenze, Italy. P. De Vita is with the Ingegneria dei Sistemi, Pisa, Italy (e-mail: p.devita@ idscorporation.com). Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/TAP.2014.2377718 and in the great part of the papers cited therein is on non-mod- ulated MTSs. The applications of modulated MTSs are in fact important and challenging, and can be divided in three classes: a) MTSs for aperture antennas [2]–[4], b) MTSs for the con- trol of surface-waves (SWs) wave-front [5]–[8], c) MTS screens for the control of field transmission [9]–[11]. In this paper, we focus the attention on the first class of applications. In [2] both linearly and circularly polarized (CP) tilted-beams have been obtained using an elliptically modulated MTS. There, the inter- action between local SW and impedance is described by means of a holographic principle. In [3] the antenna is constituted by a spiral-shape modulated surface impedance, excited at the centre by a TM source feed. CP broadside radiation is obtained by a leaky-wave (LW) pseudo-mode induced by a cylindrical SW excitation. In [4] anisotropic impedance is used to realize a CP isoflux shaped beam for satellite applications. The basic concept can be applied as well to MTS-covered shaped reflectors illuminated by an external field. Therefore, the class of MoMetAs is to be considered much broader than the planar solutions presented here. In fact, a modulated MTS boundary can be used to enhance several kinds of antennas, the most notable exceptions being low-gain antennas, due to their small size. Although MoMetAs are based on a LW mechanism, their behaviour offers an enormous flexibility with respect to (w.r.t.) those of conventional LW antennas. The key features making MoMetAs very appealing for space applications can be summarized as follows: • Unique decoupling of electrical and thermo-mechanical design. • Unique capability of low-complexity (dynamic) beam con- trol (shaping, pointing and scanning). • Unique support for late electrical performance refinement and design re-usability. • Complete and simple on-surface control of medium-to- large size apertures. • Low power losses. • Low mass and low envelope. • Implementation amenable to very different technologies. • Simple low-cost manufacturing. Consequently, MoMetAs are in principle suitable for all space applications and for a very wide range of ground applications. The research projects funded by the European Space Agency have been concentrated mainly on Earth Observation, Space Ex- ploration and Telecommunications. The most mature solutions so far are those developed for Instrument Data Transmission 0018-926X © 2014 IEEE. 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