IEEE ANTENNAS AND WIRELESS PROPAGATION LETTERS, VOL. 0, Tx-Rx Lens-Based Satellite-on-the-Move Ka-Band Antenna

Transcription

1 IEEE ANTENNAS AND WIRELESS PROPAGATION LETTERS, VOL. 0, Tx-Rx Lens-Based Satellite-on-the-Move Ka-Band Antenna Joana S. Silva, Member, IEEE, EduardoB.Lima, Member, IEEE, JorgeR.Costa, Senior Member, IEEE, Carlos A. Fernandes, Senior Member, IEEE, and Juan R. Mosig, Fellow, IEEE Abstract A simple low-cost mobile ground terminal antenna for Ka-band satellite communications is presented. The antenna is composed of a shaped dielectric lens which tilts and rotates in front of a feed to direct the beam. The wideband feed system is composed of a quad-ridged horn with a slant aperture and a coaxial-to-quad-ridged circular waveguide transition. The lens allows simple mechanical beam steering from 0 to 65 in relation to the zenith and 360 in azimuth by rotating both the lens and the feed slanted aperture. Performance measurements of a manufactured prototype prove the beam tilting capabilities in elevation with a maximum gain of 22 dbi and scan loss below 3 db at 20 GHz and 24 dbi gain with less than 4.5 db scan loss at 30 GHz. Index Terms Ka-band, low-cost, mechanical beam-steering, satellite communications, shaped dielectric lens, wideband feed. I. INTRODUCTION SATELLITE systems are progressively extending their well-recognized benefits to user mass-market application. Some of the strongest forces behind this large progress are the growing demand for higher speed links, smaller end-user terminals, and increased mobility in many relevant and emergent markets. Ka-band satellites can provide these attractive capabilities allowing cost-effective networks [1]. The characteristics of this new generation of satellites have greatly increased the interest in satellite-on-the-move (SOTM) applications. Furthermore, high-altitude platforms (HAPs) are also expected to provide services in the Ka-band [2]. The most attractive ground-segment antenna systems for SOTM reported in the literature are mechanical beam steering solutions due to their low cost. However, these systems have a Manuscript received January 16, 2015; accepted February 22, Date of publication March 06, 2015; date of current version nulldate. This work was supported by the Fundação para a Ciência e Tecnologia (Portugal) under Grant SFRH/BD/51925/2012 in the frame of IST-EPFL joint doctoral program and project mm-satcom PTDC/EEI-TEL/0805/2012 as well as by project PEst-OE/ EEI/LA/0008/2013. J. S. Silva is with Instituto de Telecomunicações, Instituto Superior Técnico Universidade de Lisboa, Lisbon P , Portugal and also with Laboratory of Electromagnetics and Acoustics, École Polytechnique Fédérale de Lausanne, Lausanne 1015, Switzerland ( joana.silva@epfl.ch). E. B. Lima and C. A. Fernandes are with the Instituto de Telecomunicações, Instituto Superior Técnico Universidade de Lisboa, Lisbon P , Portugal. J. R. Costa is with the Instituto de Telecomunicações, Instituto Superior Técnico Universidade de Lisboa, Lisbon P , Portugal and also with the Departamento de Ciências e Tecnologias da Informação, ISCTE - Instituto Universitário de Lisboa, Lisbon P , Portugal. J. R. Mosig is with Laboratory of Electromagnetics and Acoustics, École Polytechnique Fédérale de Lausanne, Lausanne 1015, Switzerland. Color versions of one or more of the figures in this letter are available online at Digital Object Identifier /LAWP considerable size which limits their use only to large transports or military vehicles [3], [4]. The goal of this work is to counter this trend exploiting the Ka-band characteristics. Hence the antenna presented in this letter conciliates performance with three essential attributes low-cost, simplicity, and small dimensions targeting not only at commercial moving platforms (high-speed trains, buses, or airplanes) but also for personal use (small boats or all-terrain vehicles) and favoring low-cost production for the mass market. The antenna concept is based upon a simple mechanical approach developed by some of the authors for 60 GHz [5]. The working principle is simple: the feed remains stationary and the beam steering is achieved by moving only a shaped lens in front of the feed in such a way that the lens focal point remains always coincident with the feed phase center. Together with an appropriate lens design, it becomes possible to obtain a wide beam tilt angle that is nearly the same as the lens tilt angle. A preliminary simulation study applied only for the downlink Ka-band was performed by some of the authors in [6] where the original [, ] scanning interval was extended to [, ] in relation to the zenith. The greatest new challenge solved in this letter is the modification of the previous single-band antenna system to work simultaneously in the downlink ( GHz) and uplink ( GHz) Ka-bands with circular polarization, keeping the extended scanning interval [, ] and also using just a single feed for compactness. This implies that the feed phase center in the widely separated bands should remain stable enough ensuring that, in combination with a proper lens, the output beam points at the same angle for both frequency Ka-bands. The shaped polyethylene lens is supported by a mechanically simple structure that allows the beam scanning. The feed is composed of two components: a quad-ridged horn with a -slant aperture that is essential to reach the extended scanning interval mentioned above and a dedicated wideband coaxial-to-waveguide transition that was pre-evaluated by simulation in [7]. The performance of the new solution is evaluated both by simulation and measurements on two fabricated prototypes using different techniques. II. ANTENNA DESIGN The geometry of the complete antenna system is shown in Fig. 1. The following subsections explain in detail each of the antenna components designed using commercial software. Web Version A. Shaped Dielectric Lens The lens shape was designed to collimate the radiated beam while widening as much as possible the scanning angle interval. The distance from the lens focal point to its bottom surface is IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission. See for more information.

2 2 IEEE ANTENNAS AND WIRELESS PROPAGATION LETTERS, VOL. 0, 2015 Fig. 1. Antenna and feed geometry. (a) Lens and horn. (b) Transition. denoted by and the lens total height by in Fig. 1. The lens design based on Geometrical Optics assumes that the feed is represented by its phase center. The design involves the Snell law at bottom lens surface and the usual path length condition for beam collimation. Two refracting surfaces allow imposing an additional design condition which is used to favor wide beam scanning. For that, the otherwise arbitrary bottom lens surface is written as a polynomial function where the coefficients are obtained by integrating the above lens design equations with Genetic Algorithm optimization [5]. The lens is designed for the lowest frequency band, which ensures its operation at the highest frequency band as well. The selected material is polyethylene with and. The obtained maximum lens diameter is 87.5 mm, is 15 mm, and is 60 mm. The final profile is presented in Fig. 1. B. Feed System The feed must fulfill the following challenging specifications: 1) wideband operation using one single antenna, complying with downlink and uplink Ka-bands; 2) about 12 dbi gain illumination with circular polarization and reasonably well defined stable phase center, coincident for both desired frequency bands; and 3) beam tilt contribution to help the lens to ensure high elevation scanning [, ]. Waveguide technology is adopted following earlier versions of this antenna [5], [6], but ridge waveguide technology is considered now to enlarge the bandwidth and accommodate simultaneously the downlink and uplink Ka-bands. Four ridges are used instead of two to enable circular polarization. 1) Quad-Ridged Horn: The 12 dbi gain calls for a horn aperture developing out from the waveguide, so the conical horn inherits its four-ridge configuration. The horn output beam tilting requirement toward the lens is not trivial, considering that the lens is allowed to rotate in azimuth while the feed is supposed to remain stationary and in the upright position to conserve symmetry. This is solved by splitting the horn into two parts: one part is fixed to the radio-frequency frontend and is fully symmetric with respect to the axis; the other part is asymmetrical ( -slant aperture) to produce the tilted beam, but it rotates around the feed axis, synchronously with the lens, to achieve full azimuth beam scan [see Fig. 1(a)]. The conical horn antenna is developed froman8 mmcircular waveguide where the ridges cross-section is mm mm. The air gap between the fixed and movable parts of the waveguide was optimized to 0.75 mm. It has no practical effect in the radiation pattern. The horn flare angle and slant angle were optimized together. We checked by simulations that a smooth transition between the fixed and movable parts along with a small and a slanted aperture would increase the beam tilt. On the other hand, a greater and a conventional horn aperture would benefit the gain and the phase center stability, respectively. The compromise was to reach the maximum beam tilt at both bands ensuring a minimum gain of 10 db, a cross-polarization level below db, and also a well-defined stable phase center. For and, the slant aperture dimensions are mm mm. In the far field the output beam is tilted by at 20 GHz and by at 30 GHz. The gain corresponds to 9.5 dbi at 20 GHz and to 12.5 dbi at 30 GHz. 2) Coaxial-to-Quad-Ridged Circular Waveguide Transition: A wideband coaxial-to-quad-ridged waveguide transition was designed to feed the above horn for laboratory tests. It comprises two different sections as presented in Fig. 1(b). The mode launcher section, probe fed, has double-ridged configuration and excites a linearly polarized wave. In the second section, the double-ridged cross section is gradually transformed into a quad-ridged one, in order to obtain an interface with two-symmetry planes to match the horn waveguide part. This symmetry allows feeding the horn with two orthogonal linear polarizations (mounting the transition in rotated positions with respect to the horn) and obtaining the circular polarization antenna performance by postprocessing. Refer to [7] for further details. The design of a circular polarization version of this launcher is out of the scope of this letter. The ridges in the launcher section are larger at the feeding region ( mm mm) to accommodate the coaxial cable diameter. The ridges taper into mm mm toward the horn end and into a shallow triangular prism of height at the opposite direction. A shallow conical shaped back-cavity closes the device in the feeding region. Its length is optimized to improve the return loss. A tuning pin is included Web Version

3 SILVA et al.: TX-RX LENS-BASED SATELLITE-ON-THE-MOVE KA-BAND ANTENNA 3 Fig. 2. Basic prototype. (a) Quad-ridged horn. (b) Half-part of the coax-towaveguide transition with H-ridges. Fig D-printed prototype. (a) Complete feed system. (b) Close top view. in the design for laboratory bench fine tuning. The final parameter values of the device are presented in Fig. 1(b). The overall length of the structure is 48.1 mm. III. ANTENNA MEASUREMENTS A. Feed System The basic feed prototype was fabricated in brass using computer numerical control (CNC) milling technology with m precision. The complexity of the structure does not allow its fabrication as a single block what will affect the mentioned accuracy. The manufactured quad-ridged horn [Fig. 2(a)] comprises four pieces: two-half parts of the horn which include the E-plane ridges and two independent H-plane ridges. In this prototype the movable part of the horn is actually fixed in order to simplify the manufacturing. For the same reason the external part of structure has parallelepiped form instead of being conformal with the interior. Neither of these simplifications affects the horn performance or elevation scan demonstration. The same approach was used for prototyping the coaxial-to-quad-ridge waveguide [Fig. 2(b)], but in this case one more piece was required to form the back cavity. All parts are assembled with screws and alignment pins. The simulated and measured reflection coefficients are presented in Fig. 4(a). As shown, the reflection coefficient for the frequency bands of interest is acceptable with respect to the requirements. However, there is a disagreement between the overall shape of the return losses for the complete frequency range between simulation and measurements. Due to the use of split-block technology, the most probable causes for the disagreement are the assembly and fabrication tolerances. In order to verify this, a new 3-D-printed prototype (see Fig. 3) working only in a single linear polarization has been manufactured using a novel additive technique, as describedin[8]and[9].sincethis technique offers a resolution of m, and the overall volume of the feed system is sufficiently small, it can be manufactured as a single piece, thus avoiding any assembling issues. The measured results of this prototype are shown in Fig. 4(b), and clearly show much better agreement with the simulations. This confirms that the origin of the disagreement for the basic prototype is due to some inaccuracies in the fabrication and assembling procedures. Nevertheless, as previously explained the matching in the frequency bands of interest is acceptable, and for this Fig. 4. Measured and simulated reflection coefficient curves of the full feed subsystem. (a) Basic prototype. (b) 3-D-printed prototype. Fig. 5. (Dashed lines) Simulated and (solid lines) measured linear polarized radiation patterns of both prototypes at (a) 20 GHz and (b) 30 GHz. Fig. 6. (Dashed lines) Simulated and (solide lines) measured circular polarized radiation patterns of the basic prototype at (a) 20 GHz and (b) 30 GHz. reason the measured fields of this prototype can be trusted. This is confirmed by the reasonable agreement in terms of gain and beam tilt shown in Fig. 5 that compares simulated and measured co-polar components of both prototypes for the same linear polarization at 20 and 30 GHz. As such we will continue with the characterization of the circularly polarized antenna using the basic feed prototype. Fig. 6 compares simulated and measured circularly polarized radiation patterns obtained at 20 and 30 GHz. Apart from some ripple due to lower measurement dynamic range at 30 GHz, the agreement is very reasonable. Note that the results shown in Fig. 5 and Fig. 6 are in the asymmetric plane. B. Shaped Dielectric Lens The polyethylene lens was fabricated using a CNC milling machine. A simple mechanical support was fabricated to allow the lens tilt movements maintaining proper feed alignment. The complete prototype is shown in Fig. 7. It was simulated and measured for different lens tilt angles in the interval with steps. Fig. 8 compares the simulated and measured radiation patterns with synthesized circular polarization at 20 and 30 GHz for. An excellent agreement is observed between measured and simulated radiation patterns. A Web Version

4 4 IEEE ANTENNAS AND WIRELESS PROPAGATION LETTERS, VOL. 0, 2015 TABLE I MEASURED PERFORMANCE INDICATOR VALUES OF THE ANTENNA PROTOTYPE Fig. 7. Complete antenna prototype with basic feed prototype. Fig. 8. (Dashed lines) Simulated and (solid lines) measured radiation pattern of the complete prototype for at: (a) 20 GHz and (b) 30 GHz. Fig. 9. Measured radiation pattern of the complete prototype for all lens tilt angles at (a) 20 GHz and (b) 30 GHz. similar agreement is observed for the other.fig.9showsonly the measured results for all at both frequencies. The radiation patterns are well defined with low side lobes. Table I summarizes the corresponding performance indicators for both bands, where is the beam tilt angle, is half of the corresponding beam pointing difference at 20 and 30 GHz, is the gain, and its scan loss. The direction of the beam maximum is almost coincident with the corresponding lens tilt angle. The beam pointing error is zero for low and stays below for increasing, which is less than one tenth of the 3 db beam width. Gain scan loss is below 3 db for all while cross-polarization level is below db at 20 GHz and db at 30 GHz. These characteristics demonstrate the viability of the concept. IV. CONCLUSIONS The letter presents a simple and potentially low-cost lensbased antenna system for satellite Ka dual-band user terminals, with volume about dm. The simulations and measurements prove that the system is able to mechanically steer a circular polarized beam between and in relation to the zenith. This is a much wider scan angle than the best performance obtained with traditional designs [10]. The maximum gain ranging is from dbi at 20 GHz and from dbi at 30 GHz. The radiation pattern is reasonably stable with good cross-polarization. The beam pointing is a linear function of the lens tilt with 0.93 slope and the beam pointing difference between the two bands is negligible. This is a clear advancement regarding previous studies of this antenna concept. The compactness and simplicity of the system are additional relevant characteristics that make it appropriate for personal use not only for satellite services but also for HAPs. ACKNOWLEDGMENT The authors acknowledge the collaboration of Swissto12, A. Almeida, T. Debogovic, and S. Capdevila for prototype manufacture and measurements and also M. García Vigueras and P. Robustillo for the useful technical discussions. REFERENCES [1] Ka-band, Sep [Online]. Available: Downloads/whitepapers.html, Oct [2] J. Gavan, S. Tapuchi, and D. Grace, Concepts and main applications of high altitude platform relays, URSI Radio Sci. Bull., no. 330, pp , Sep [3] H. Bayer, A. Krauss, R. Stephan, and M. A. Hein, A dual-band multimode monopulse tracking antenna for land-mobile satellite communications in Ka-band, in Proc. 6th Eur. Conf. Antennas and Propag. (EuCAP), Prague, Czech Republic, Mar. 2012, pp [4] Ka-band on-the-move antenna system DRS Technologies, Arlington, VA, USA, Jun. 2013, Oct [Online]. Available: [5] J. R. Costa, E. B. Lima, and C. A. Fernandes, Compact beam-steerable lens antenna for 60-Ghz wireless communications, IEEE Trans. Antennas Propag., vol. 57, no. 10, pp , Oct [6] J.S.Silva,E.B.Lima,J.R.Costa,andC.A.Fernandes, Ground terminal antenna for Ka-band satellite communications, in Proc. 7th Eur. Conf. Antennas and Propag. (EuCAP), Gothenburg, Sweden, Apr. 2013, pp [7] J.S.Silva,E.B.Lima,J.R.Costa,C.A.Fernandes,andJ.R.Mosig, Design and analysis of a Ka-band coaxial-to-quad-ridged circular waveguide transition, in Proc. 8th Eur. Conf. Antennas and Propag. (EuCAP), The Hague, The Netherlands, Apr. 2014, pp [8] Swissto12. Lausanne, Switzerland [Online]. Available: www. swissto12.com, Oct [9] A.Macor,E.deRijk,S.Alberti,T.Goodman,andJ-P.H.Ansermet, Three-dimensional stereolithography for millimeter wave and terahertz application, Rev. Sci. Instrum., vol. 83, no. 4, p , [10] A. Peebles, A dielectric bifocal lens for multibeam antenna applications, IEEE Trans. Antennas Propag., vol. 36, no. 5, pp , May Web Version

5 IEEE ANTENNAS AND WIRELESS PROPAGATION LETTERS, VOL. 0, Tx-Rx Lens-Based Satellite-on-the-Move Ka-Band Antenna Joana S. Silva, Member, IEEE, EduardoB.Lima, Member, IEEE, JorgeR.Costa, Senior Member, IEEE, Carlos A. Fernandes, Senior Member, IEEE, and Juan R. Mosig, Fellow, IEEE Abstract A simple low-cost mobile ground terminal antenna for Ka-band satellite communications is presented. The antenna is composed of a shaped dielectric lens which tilts and rotates in front of a feed to direct the beam. The wideband feed system is composed of a quad-ridged horn with a slant aperture and a coaxial-to-quad-ridged circular waveguide transition. The lens allows simple mechanical beam steering from 0 to 65 in relation to the zenith and 360 in azimuth by rotating both the lens and the feed slanted aperture. Performance measurements of a manufactured prototype prove the beam tilting capabilities in elevation with a maximum gain of 22 dbi and scan loss below 3 db at 20 GHz and 24 dbi gain with less than 4.5 db scan loss at 30 GHz. Index Terms Ka-band, low-cost, mechanical beam-steering, satellite communications, shaped dielectric lens, wideband feed. I. INTRODUCTION SATELLITE systems are progressively extending their well-recognized benefits to user mass-market application. Some of the strongest forces behind this large progress are the growing demand for higher speed links, smaller end-user terminals, and increased mobility in many relevant and emergent markets. Ka-band satellites can provide these attractive capabilities allowing cost-effective networks [1]. The characteristics of this new generation of satellites have greatly increased the interest in satellite-on-the-move (SOTM) applications. Furthermore, high-altitude platforms (HAPs) are also expected to provide services in the Ka-band [2]. The most attractive ground-segment antenna systems for SOTM reported in the literature are mechanical beam steering solutions due to their low cost. However, these systems have a Manuscript received January 16, 2015; accepted February 22, Date of publication March 06, 2015; date of current version nulldate. This work was supported by the Fundação para a Ciência e Tecnologia (Portugal) under Grant SFRH/BD/51925/2012 in the frame of IST-EPFL joint doctoral program and project mm-satcom PTDC/EEI-TEL/0805/2012 as well as by project PEst-OE/ EEI/LA/0008/2013. J. S. Silva is with Instituto de Telecomunicações, Instituto Superior Técnico Universidade de Lisboa, Lisbon P , Portugal and also with Laboratory of Electromagnetics and Acoustics, École Polytechnique Fédérale de Lausanne, Lausanne 1015, Switzerland ( joana.silva@epfl.ch). E. B. Lima and C. A. Fernandes are with the Instituto de Telecomunicações, Instituto Superior Técnico Universidade de Lisboa, Lisbon P , Portugal. J. R. Costa is with the Instituto de Telecomunicações, Instituto Superior Técnico Universidade de Lisboa, Lisbon P , Portugal and also with the Departamento de Ciências e Tecnologias da Informação, ISCTE - Instituto Universitário de Lisboa, Lisbon P , Portugal. J. R. Mosig is with Laboratory of Electromagnetics and Acoustics, École Polytechnique Fédérale de Lausanne, Lausanne 1015, Switzerland. Color versions of one or more of the figures in this letter are available online at Digital Object Identifier /LAWP considerable size which limits their use only to large transports or military vehicles [3], [4]. The goal of this work is to counter this trend exploiting the Ka-band characteristics. Hence the antenna presented in this letter conciliates performance with three essential attributes low-cost, simplicity, and small dimensions targeting not only at commercial moving platforms (high-speed trains, buses, or airplanes) but also for personal use (small boats or all-terrain vehicles) and favoring low-cost production for the mass market. The antenna concept is based upon a simple mechanical approach developed by some of the authors for 60 GHz [5]. The working principle is simple: the feed remains stationary and the beam steering is achieved by moving only a shaped lens in front of the feed in such a way that the lens focal point remains always coincident with the feed phase center. Together with an appropriate lens design, it becomes possible to obtain a wide beam tilt angle that is nearly the same as the lens tilt angle. A preliminary simulation study applied only for the downlink Ka-band was performed by some of the authors in [6] where the original [, ] scanning interval was extended to [, ] in relation to the zenith. The greatest new challenge solved in this letter is the modification of the previous single-band antenna system to work simultaneously in the downlink ( GHz) and uplink ( GHz) Ka-bands with circular polarization, keeping the extended scanning interval [, ] and also using just a single feed for compactness. This implies that the feed phase center in the widely separated bands should remain stable enough ensuring that, in combination with a proper lens, the output beam points at the same angle for both frequency Ka-bands. The shaped polyethylene lens is supported by a mechanically simple structure that allows the beam scanning. The feed is composed of two components: a quad-ridged horn with a -slant aperture that is essential to reach the extended scanning interval mentioned above and a dedicated wideband coaxial-to-waveguide transition that was pre-evaluated by simulation in [7]. The performance of the new solution is evaluated both by simulation and measurements on two fabricated prototypes using different techniques. II. ANTENNA DESIGN The geometry of the complete antenna system is shown in Fig. 1. The following subsections explain in detail each of the antenna components designed using commercial software. Print Version A. Shaped Dielectric Lens The lens shape was designed to collimate the radiated beam while widening as much as possible the scanning angle interval. The distance from the lens focal point to its bottom surface is IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission. See for more information.

6 2 IEEE ANTENNAS AND WIRELESS PROPAGATION LETTERS, VOL. 0, 2015 Fig. 1. Antenna and feed geometry. (a) Lens and horn. (b) Transition. denoted by and the lens total height by in Fig. 1. The lens design based on Geometrical Optics assumes that the feed is represented by its phase center. The design involves the Snell law at bottom lens surface and the usual path length condition for beam collimation. Two refracting surfaces allow imposing an additional design condition which is used to favor wide beam scanning. For that, the otherwise arbitrary bottom lens surface is written as a polynomial function where the coefficients are obtained by integrating the above lens design equations with Genetic Algorithm optimization [5]. The lens is designed for the lowest frequency band, which ensures its operation at the highest frequency band as well. The selected material is polyethylene with and. The obtained maximum lens diameter is 87.5 mm, is 15 mm, and is 60 mm. The final profile is presented in Fig. 1. B. Feed System The feed must fulfill the following challenging specifications: 1) wideband operation using one single antenna, complying with downlink and uplink Ka-bands; 2) about 12 dbi gain illumination with circular polarization and reasonably well defined stable phase center, coincident for both desired frequency bands; and 3) beam tilt contribution to help the lens to ensure high elevation scanning [, ]. Waveguide technology is adopted following earlier versions of this antenna [5], [6], but ridge waveguide technology is considered now to enlarge the bandwidth and accommodate simultaneously the downlink and uplink Ka-bands. Four ridges are used instead of two to enable circular polarization. 1) Quad-Ridged Horn: The 12 dbi gain calls for a horn aperture developing out from the waveguide, so the conical horn inherits its four-ridge configuration. The horn output beam tilting requirement toward the lens is not trivial, considering that the lens is allowed to rotate in azimuth while the feed is supposed to remain stationary and in the upright position to conserve symmetry. This is solved by splitting the horn into two parts: one part is fixed to the radio-frequency frontend and is fully symmetric with respect to the axis; the other part is asymmetrical ( -slant aperture) to produce the tilted beam, but it rotates around the feed axis, synchronously with the lens, to achieve full azimuth beam scan [see Fig. 1(a)]. The conical horn antenna is developed froman8 mmcircular waveguide where the ridges cross-section is mm mm. The air gap between the fixed and movable parts of the waveguide was optimized to 0.75 mm. It has no practical effect in the radiation pattern. The horn flare angle and slant angle were optimized together. We checked by simulations that a smooth transition between the fixed and movable parts along with a small and a slanted aperture would increase the beam tilt. On the other hand, a greater and a conventional horn aperture would benefit the gain and the phase center stability, respectively. The compromise was to reach the maximum beam tilt at both bands ensuring a minimum gain of 10 db, a cross-polarization level below db, and also a well-defined stable phase center. For and, the slant aperture dimensions are mm mm. In the far field the output beam is tilted by at 20 GHz and by at 30 GHz. The gain corresponds to 9.5 dbi at 20 GHz and to 12.5 dbi at 30 GHz. 2) Coaxial-to-Quad-Ridged Circular Waveguide Transition: A wideband coaxial-to-quad-ridged waveguide transition was designed to feed the above horn for laboratory tests. It comprises two different sections as presented in Fig. 1(b). The mode launcher section, probe fed, has double-ridged configuration and excites a linearly polarized wave. In the second section, the double-ridged cross section is gradually transformed into a quad-ridged one, in order to obtain an interface with two-symmetry planes to match the horn waveguide part. This symmetry allows feeding the horn with two orthogonal linear polarizations (mounting the transition in rotated positions with respect to the horn) and obtaining the circular polarization antenna performance by postprocessing. Refer to [7] for further details. The design of a circular polarization version of this launcher is out of the scope of this letter. The ridges in the launcher section are larger at the feeding region ( mm mm) to accommodate the coaxial cable diameter. The ridges taper into mm mm toward the horn end and into a shallow triangular prism of height at the opposite direction. A shallow conical shaped back-cavity closes the device in the feeding region. Its length is optimized to improve the return loss. A tuning pin is included Print Version

7 SILVA et al.: TX-RX LENS-BASED SATELLITE-ON-THE-MOVE KA-BAND ANTENNA 3 Fig. 2. Basic prototype. (a) Quad-ridged horn. (b) Half-part of the coax-towaveguide transition with H-ridges. Fig D-printed prototype. (a) Complete feed system. (b) Close top view. in the design for laboratory bench fine tuning. The final parameter values of the device are presented in Fig. 1(b). The overall length of the structure is 48.1 mm. III. ANTENNA MEASUREMENTS A. Feed System The basic feed prototype was fabricated in brass using computer numerical control (CNC) milling technology with m precision. The complexity of the structure does not allow its fabrication as a single block what will affect the mentioned accuracy. The manufactured quad-ridged horn [Fig. 2(a)] comprises four pieces: two-half parts of the horn which include the E-plane ridges and two independent H-plane ridges. In this prototype the movable part of the horn is actually fixed in order to simplify the manufacturing. For the same reason the external part of structure has parallelepiped form instead of being conformal with the interior. Neither of these simplifications affects the horn performance or elevation scan demonstration. The same approach was used for prototyping the coaxial-to-quad-ridge waveguide [Fig. 2(b)], but in this case one more piece was required to form the back cavity. All parts are assembled with screws and alignment pins. The simulated and measured reflection coefficients are presented in Fig. 4(a). As shown, the reflection coefficient for the frequency bands of interest is acceptable with respect to the requirements. However, there is a disagreement between the overall shape of the return losses for the complete frequency range between simulation and measurements. Due to the use of split-block technology, the most probable causes for the disagreement are the assembly and fabrication tolerances. In order to verify this, a new 3-D-printed prototype (see Fig. 3) working only in a single linear polarization has been manufactured using a novel additive technique, as describedin[8]and[9].sincethis technique offers a resolution of m, and the overall volume of the feed system is sufficiently small, it can be manufactured as a single piece, thus avoiding any assembling issues. The measured results of this prototype are shown in Fig. 4(b), and clearly show much better agreement with the simulations. This confirms that the origin of the disagreement for the basic prototype is due to some inaccuracies in the fabrication and assembling procedures. Nevertheless, as previously explained the matching in the frequency bands of interest is acceptable, and for this Fig. 4. Measured and simulated reflection coefficient curves of the full feed subsystem. (a) Basic prototype. (b) 3-D-printed prototype. Fig. 5. (Dashed lines) Simulated and (solid lines) measured linear polarized radiation patterns of both prototypes at (a) 20 GHz and (b) 30 GHz. Fig. 6. (Dashed lines) Simulated and (solide lines) measured circular polarized radiation patterns of the basic prototype at (a) 20 GHz and (b) 30 GHz. reason the measured fields of this prototype can be trusted. This is confirmed by the reasonable agreement in terms of gain and beam tilt shown in Fig. 5 that compares simulated and measured co-polar components of both prototypes for the same linear polarization at 20 and 30 GHz. As such we will continue with the characterization of the circularly polarized antenna using the basic feed prototype. Fig. 6 compares simulated and measured circularly polarized radiation patterns obtained at 20 and 30 GHz. Apart from some ripple due to lower measurement dynamic range at 30 GHz, the agreement is very reasonable. Note that the results shown in Fig. 5 and Fig. 6 are in the asymmetric plane. B. Shaped Dielectric Lens The polyethylene lens was fabricated using a CNC milling machine. A simple mechanical support was fabricated to allow the lens tilt movements maintaining proper feed alignment. The complete prototype is shown in Fig. 7. It was simulated and measured for different lens tilt angles in the interval with steps. Fig. 8 compares the simulated and measured radiation patterns with synthesized circular polarization at 20 and 30 GHz for. An excellent agreement is observed between measured and simulated radiation patterns. A Print Version

8 4 IEEE ANTENNAS AND WIRELESS PROPAGATION LETTERS, VOL. 0, 2015 TABLE I MEASURED PERFORMANCE INDICATOR VALUES OF THE ANTENNA PROTOTYPE Fig. 7. Complete antenna prototype with basic feed prototype. Fig. 8. (Dashed lines) Simulated and (solid lines) measured radiation pattern of the complete prototype for at: (a) 20 GHz and (b) 30 GHz. Fig. 9. Measured radiation pattern of the complete prototype for all lens tilt angles at (a) 20 GHz and (b) 30 GHz. similar agreement is observed for the other.fig.9showsonly the measured results for all at both frequencies. The radiation patterns are well defined with low side lobes. Table I summarizes the corresponding performance indicators for both bands, where is the beam tilt angle, is half of the corresponding beam pointing difference at 20 and 30 GHz, is the gain, and its scan loss. The direction of the beam maximum is almost coincident with the corresponding lens tilt angle. The beam pointing error is zero for low and stays below for increasing, which is less than one tenth of the 3 db beam width. Gain scan loss is below 3 db for all while cross-polarization level is below db at 20 GHz and db at 30 GHz. These characteristics demonstrate the viability of the concept. IV. CONCLUSIONS The letter presents a simple and potentially low-cost lensbased antenna system for satellite Ka dual-band user terminals, with volume about dm. The simulations and measurements prove that the system is able to mechanically steer a circular polarized beam between and in relation to the zenith. This is a much wider scan angle than the best performance obtained with traditional designs [10]. The maximum gain ranging is from dbi at 20 GHz and from dbi at 30 GHz. The radiation pattern is reasonably stable with good cross-polarization. The beam pointing is a linear function of the lens tilt with 0.93 slope and the beam pointing difference between the two bands is negligible. This is a clear advancement regarding previous studies of this antenna concept. The compactness and simplicity of the system are additional relevant characteristics that make it appropriate for personal use not only for satellite services but also for HAPs. ACKNOWLEDGMENT The authors acknowledge the collaboration of Swissto12, A. Almeida, T. Debogovic, and S. Capdevila for prototype manufacture and measurements and also M. García Vigueras and P. Robustillo for the useful technical discussions. REFERENCES [1] Ka-band, Sep [Online]. Available: Downloads/whitepapers.html, Oct [2] J. Gavan, S. Tapuchi, and D. Grace, Concepts and main applications of high altitude platform relays, URSI Radio Sci. Bull., no. 330, pp , Sep [3] H. Bayer, A. Krauss, R. Stephan, and M. A. Hein, A dual-band multimode monopulse tracking antenna for land-mobile satellite communications in Ka-band, in Proc. 6th Eur. Conf. Antennas and Propag. (EuCAP), Prague, Czech Republic, Mar. 2012, pp [4] Ka-band on-the-move antenna system DRS Technologies, Arlington, VA, USA, Jun. 2013, Oct [Online]. Available: [5] J. R. Costa, E. B. Lima, and C. A. Fernandes, Compact beam-steerable lens antenna for 60-Ghz wireless communications, IEEE Trans. Antennas Propag., vol. 57, no. 10, pp , Oct [6] J.S.Silva,E.B.Lima,J.R.Costa,andC.A.Fernandes, Ground terminal antenna for Ka-band satellite communications, in Proc. 7th Eur. Conf. Antennas and Propag. (EuCAP), Gothenburg, Sweden, Apr. 2013, pp [7] J.S.Silva,E.B.Lima,J.R.Costa,C.A.Fernandes,andJ.R.Mosig, Design and analysis of a Ka-band coaxial-to-quad-ridged circular waveguide transition, in Proc. 8th Eur. Conf. Antennas and Propag. (EuCAP), The Hague, The Netherlands, Apr. 2014, pp [8] Swissto12. Lausanne, Switzerland [Online]. Available: www. swissto12.com, Oct [9] A.Macor,E.deRijk,S.Alberti,T.Goodman,andJ-P.H.Ansermet, Three-dimensional stereolithography for millimeter wave and terahertz application, Rev. Sci. Instrum., vol. 83, no. 4, p , [10] A. Peebles, A dielectric bifocal lens for multibeam antenna applications, IEEE Trans. Antennas Propag., vol. 36, no. 5, pp , May Print Version