3528 IEEE TRANSACTIONS ON ANTENNAS AND PROPAGATION, VOL. 65, NO. 7, JULY 2017 High Gain Dual-Band Beam-Steering Transmit Array for Satcom Terminals at Ka-Band Sérgio A. Matos, Member, IEEE , Eduardo B. Lima, Student Member, IEEE, Joana S. Silva, Jorge R. Costa, Senior Member, IEEE , Carlos A. Fernandes, Senior Member, IEEE, Nelson J. G. Fonseca, Senior Member, IEEE , and Juan R. Mosig, Fellow, IEEE Abstract—Transmit array design is more challenging for dual- band operation than for single band, due to the independent 360° phase wrapping jumps needed at each band when large electrical length compensation is involved. This happens when aiming at large gains, typically above 25 dBi with beam scanning and F/ D ≤ 1. No such designs have been reported in the literature. A general method is presented here to reduce the complexity of dual-band transmit array design, valid for arbitrarily large phase error compensation and any band ratio, using a finite number of different unit cells. The procedure is demonstrated for two offset transmit array implementations operating in circular polarization at 20 GHz(Rx) and 30 GHz(Tx) for Ka-band satellite-on-the-move terminals with mechanical beam-steering. An appropriate set of 30 dual-band unit cells is developed with transmission coefficient greater than -0.9 dB. The full-size transmit array is characterized by full-wave simulation enabling elevation beam scanning over 0°-50° with gains reaching 26 dBi at 20 GHz and 29 dBi at 30 GHz. A smaller prototype was fabricated and measured, showing a measured gain of 24 dBi at 20 GHz and 27 dBi at 30 GHz. In both cases, the beam pointing direction is coincident over the two frequency bands, and thus confirming the proposed design procedure. Index Terms— Dual band, flat lens, frequency selective surface, mechanical scanning, polarization-insensitive, satellite- on-the-move (SOTM), transmit arrays, wireless communication network. Manuscript received July 8, 2016; revised March 31, 2017; accepted April 30, 2017. Date of publication May 9, 2017; date of current version July 1, 2017. This work was supported in part by the European Space Agency under Contract 40009111/13/NL/AD and in part by the Fundação para a Ciência e a Tecnologia in the frame of IST-EPFL Joint Doctoral Program under Grant SFRH/BD/51925/2012. (Corresponding author: Sérgio A. Matos.) S. A. Matos and J. R. Costa are with the Instituto de Telecomunicações, Instituto Superior Técnico, Universidade de Lisboa, 1049-001 Lisbon, Portugal, and also with the Departamento de Ciências e Tecnologias da Informação, Instituto Universitário de Lisboa (ISCTE-IUL), 1649-026 Lisbon, Portugal (e-mail: sergio.matos@iscte.pt). E. B. Lima and C. A. Fernandes are with the Instituto de Telecomunicações, Instituto Superior Técnico, Universidade de Lisboa, 1049-001 Lisbon, Portugal. J. S. Silva is with the Instituto de Telecomunicações, Instituto Superior Técnico, Universidade de Lisboa, 1049-001 Lisbon, Portugal, and also with the Laboratory of Electromagnetics and Antennas, École Polytechnique Fédérale de Lausanne, 1015 Lausanne, Switzerland. N. J. G. Fonseca is with Moltek Consultants Ltd., TN12 8J Kent, U.K., and also with the Antenna and Sub-Millimetre Wave Section, European Space Agency, 2200 AG Noordwijk, The Netherlands (e-mail: nelson.fonseca@esa.int). J. R. Mosig is with the Laboratory of Electromagnetics and Antennas, École Polytechnique Fédérale de Lausanne, 1015 Lausanne, Switzerland. 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.2017.2702658 I. I NTRODUCTION T HERE is an intensive on-going research effort to find compact, lightweight, and cost-effective antenna solu- tions for satellite-on-the-move (SOTM) applications [1]–[10]. The challenge is to combine these characteristics with high gain, beam-steering capability, and multiband opera- tion. Mechanical beam-steering solutions [1] are cost-effective but tend to be bulkier than their phased array counter- parts [11], [12]. However, the feeding network of millimeter wave phased arrays, both analog and digital, is the main lim- itation in terms of RF performance and cost. Reflector-based antennas are the traditional solution for mechanical beam- steering [13], but the use of reflect arrays and transmit arrays is becoming a new trend as they can potentially reduce the antennas’ profile and weight while maintaining high RF per- formance [3], [7]–[9], [14]–[17]. In particular, frequency selec- tive surface theory is being applied for the design of a new breed of flat Fresnel lenses or transmit arrays [18]–[23]. Two basic types of cell phasing mechanisms are being used: the phase-delay (PD) and phase rotation (PR) types. In PD cells, the relative phase between adjacent cells is obtained through different equivalent transmission line lengths for each cell, usually associated with different effective per- mittivities [24], [25]; in the PR type of cells, all have exactly the same geometry, the relative phase being obtained through the relative angle of in-plane rotation of the cell elements [18]–[21]. It is worth mentioning that these two types of transmit arrays have different working principles. The PR case is exclusive for circular polarization (CP), whereas the PD counterpart can operate with an arbitrary incident polarization. To the best of our knowledge, the use of dual band high- gain beam-steerable antennas based on passive transmit arrays for SOTM, using either PD or PR cells, has not yet been reported in the state of the art. It may appear in a first thought that the design of a dual-band transmit array is not fundamentally different from the single-band case, but actually dual-band transmit arrays are more complex to design when the following conditions are verified simultaneously: 1) need to compensate several wavelengths of phase error resulting from a very high gain requirement or low F/ D (or both); 2) unfavorable ratio between the frequency bands as explained ahead. 0018-926X © 2017 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.