Lightweight Portable Planar Slot Array Antenna for ... - IEEE Xplore

IEEE ANTENNAS AND WIRELESS PROPAGATION LETTERS, VOL. 10, 2011

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Lightweight Portable Planar Slot Array Antenna for Satellite Communications in X-Band J. M. Fernández González, P. Padilla, G. Expósito-Domínguez, and M. Sierra-Castañer, Member, IEEE

Abstract—A lightweight portable planar slot array antenna at X-band for fixed satellite communications is presented in this letter. The terminal is composed of two radial-line slot antennas (RLSAs) with the slots placed on the upper layer in spiral: one for reception (RX) band with left-handed circular polarization (LHCP), and the other one for transmission (TX) band with right-handed circular polarization (RHCP). A lightweight two-layer dielectric structure is used to achieve and optimize the RLSA weight. The radiating element is a slot pair, designed to provide circular polarization. The feeding network consists of a 50SMA connector for each RLSA. A radiation efficiency of more than 70% is achieved due to the low dielectric constant substrate. Finally, measurements of the lightweight portable planar RX/TX antenna prototypes are presented and compared to simulations, where very good agreement is observed.

Fig. 1. Details of the antenna structure. TABLE I RX/TX ANTENNA SPECIFICATIONS

Index Terms—Lightweight antenna, planar antenna, portable antenna, radial-line slot antenna (RLSA), satellite communications, slot array antenna, X-band.

I. INTRODUCTION

T

HE REQUIREMENTS of satellite communication systems are more and more demanding. In particular, they are expected to provide a variety of services using new systems such as small-size ground antennas [1], [2]. The introduction of more powerful satellite transponders along with the emergence of new commercial applications of satellite communications have been the major driving force behind the development of this kind of small-size antennas. In addition, low profile, weight, and cost requirements lead to printed antenna technology rather than parabolic reflectors [3]. Along with microstrip patch technology [4], slot antenna technology using narrow slots in planar conductors is used to achieve a compact and planar antenna design. Due to the low conduction and dielectric losses in such structures, the resulting gain of a slot array antenna is usually higher than the equivalent-size microstrip patch array. Different pattern configurations and feeding/combining methods can be applied to develop slot arrays and are described in the literature [5], [6].

In this letter, a lightweight portable planar slot array antenna with dual circular polarization for fixed-position portable satellite communication systems is provided. The objective of this design is a significant weight reduction of the antenna with respect to conventional RLSA [7], [8]. The letter is organized as follows. Section II presents the main characteristics of antenna structure and the design process. Section III is devoted to prototypes and their measured results. Finally, conclusions are drawn in Section IV. II. ANTENNA DESIGN

Manuscript received September 14, 2011; revised October 29, 2011; accepted November 28, 2011. Date of publication December 07, 2011; date of current version December 22, 2011. This work was supported by the Spanish Ministry of Education and Science and the Spanish Government (Comisión Interministerial de Ciencia y Tecnologia) under Ref. TEC2008-06736-C03-01/TEC. J. M. Fernández González, G. Expósito-Domínguez, and M. Sierra-Castañer are with the Radiation Group, Signals, Systems and Radiocommunications Department, Technical University of Madrid, 28040 Madrid, Spain (e-mail: [email protected]). P. Padilla is with the Department of Signal, Theory Telematics and Communications, Universidad de Granada, 18071 Granada, Spain (e-mail: [email protected]). Color versions of one or more of the figures in this letter are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/LAWP.2011.2178584

In this section, the antenna structure and the design of the radiating plate are provided. The system requirements are provided in Table I. The dual circular polarization is obtained using two RLSAs: one for reception (RX) band with left-hand circular polarization (LHCP), and the other one for transmission (TX) band with right-hand circular polarization (RHCP). Fig. 1 shows the proposed portable RX/TX antenna structure. The designed antenna consists of two metal plates spaced a . Thus, the antenna is composed of the distance following: • a metallic ground plane;

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Fig. 2. Radiating element in RX antenna for LHCP.

• a low density lightweight polypropylene substrate of dielectric constant , loss tangent of , density of 0.6 g/cm , and thickness of 10 mm. This material has half the density of conventional commercial polyethylene materials. Also, because of its dielectric constant between 1.4 and 2.5, it allows to avoid grating lobes in the radiation pattern of the antenna design; • an FR4 fiberglass NELCO N4000-6 FC substrate of dielectric constant of , loss tangent of , and thickness of 0.127 mm with a top copper foil of 35 m on which the slot radiating element is printed; • no conductive silicone ELASTOSIL E41 glue for the junction between substrates. The feeding structure of the portable antenna is formed by two 50- SMA connectors: one for RX antenna and the other one for TX antenna. The 50- SMA connectors are centrally located in the phase center of the antenna in order to generate a radially outward traveling-wave mode. The slots are placed on the upper copper plate of an FR4 NELCO substrate, designed to obtain uniform amplitude and phase. Two slots form a slot pair as a radiating element of length , width of 1 mm, and separated measured along the radial direction and tilted 90 one to the other at their center arranged to radiate the circular polarization in the broadside direction when it is excited [7], as it is shown in Fig. 2. The pairs are arranged spirally. If the slot pairs are excited with a relative phase shift of , they work as a radiating element of the RHCP, and if they are excited with a relative phase shift of , they work as LHCP. Furthermore, the slot pairs are inclined by the equal angle of from the feeding point (radial direction) to couple the same field intensity. With this configuration, the polarization purity is optimized. The design steps to calculate the position and the length of the slots to maximize the directivity are explained in [7]. The distance between radiating elements must be shorter than a

Fig. 3. Antenna simulation model: (a) RX antenna; (b) TX antenna.

wavelength to avoid grating lobes. The number of radiating elements in one spiral ring is selected in order to maintain the distance between two consecutive ones in the range of 0.74 to . Lower values produce an overlapping of the slots, and higher values produce grating lobes. All the elements are arranged equidistant. The termination for each RLSA is a short circuit in order to reradiate all the residual power, reducing the reflection and cross-polar radiation [9] by means of a short circuit at a distance of from the last ring of the spiral. The problem with the attenuation of outward traveling wave is solved by controlling the slot coupling. In order to validate initially the designs, CST Studio Suite simulation software has been used for both designed antennas (TX and RX). Fig. 3 shows the simulated radiation pattern in 3-D of the RX and TX RLSA antennas in the center frequency band, where the pencil beam of the antenna is depicted. The simulation results of both antenna models show that the copolar–cross-polar ratio is better than 30 dB in the broadside direction, and the sidelobe levels are lower than 16 dB. III. ANTENNA PROTOTYPE AND MEASUREMENT RESULTS Once a suitable arrangement of slots has been confirmed, the manufacture of the radiating surface is accomplished. For the slot surface, the designs use the copper foil of 35 m described in Section II in order to reduce weight. The slots made in the copper foil are manufactured by means of conventional copper photo etching process. Fig. 4 presents the zoom views in the

FERNÁNDEZ GONZÁLEZ et al.: LIGHTWEIGHT PORTABLE PLANAR SLOT ARRAY ANTENNA FOR X-BAND

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Fig. 4. Zoom view in the slot radiating elements of the antenna prototype: (a) RX antenna; (b) TX antenna.

Fig. 7. RX/TX antenna prototype return losses (RX: 7.25–7.75 GHz, and TX: 7.9–8.4 GHz): measurements.

Fig. 5. RX/TX antenna prototype assembled to the portable case: (a) RX antenna; (b) TX antenna.

Fig. 8. Radiation pattern of RX antenna prototype at 7.5 GHz.

Fig. 6. Measurement setup for TX antenna prototype in the anechoic chamber at Universidad Politécnica de Madrid.

slot array of the RX and TX antenna prototypes. As it can be observed, in the RX antenna the slot arrangement trace on which slot pairs are located follows the clockwise direction. On the contrary, the slot arrangement trace in TX antenna follows the counterclockwise direction. Fig. 5 shows the antenna prototypes with copper foil in the final portable case. The prototype measurements in compact range anechoic chamber (Fig. 6) include antenna patterns, antenna reflection return losses, and gain versus frequency. The reflection coefficients (Fig. 7) fulfill the specifications in Table I ( 15.6 dB). The antenna pattern measurements are carried out in the four principal planes in Cartesian plot at the central frequency (RX for LHCP: 7.5 GHz, and TX for RHCP: 8.15 GHz) as can be seen in Figs. 8 and 9. The copolar–cross-polar ratio is better than 25 dB in the broadside direction, and sidelobe

levels are around 16 dB. In Fig. 10, measurements for the axial ratio (AR) are provided. An AR better than 1 dB is measured for the RX antenna, and better than 1.4 dB for the TX antenna. These differences are due to the two different designs for TX and RX bands. In them, the AR is optimized for one of the bands to fulfill the specifications (see Table I). The gain measurements versus frequency are compared to simulations in Fig. 11. Good agreement is observed in all cases for the measurements and simulations. In Table II, the antenna measurement performances are compared to the simulation results. Very good agreement is observed, and the design process of the RX/TX antenna is validated. Only slight discrepancies between simulation and measurements are observed. These differences are due to the additional material losses of the real prototype. IV. CONCLUSION This letter presents and proposes a lightweight portable planar slot array RX/TX antenna at X-band for fixed-position portable satellite communication systems. The antenna is based on slot arrays arranged on the upper plate of the RLSA in a

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TABLE II RX/TX ANTENNA PERFORMANCES: MEASUREMENTS VERSUS SIMULATION

Fig. 9. Radiation pattern of TX antenna prototype at 8.15 GHz.

is built with a lightweight two-layer dielectric structure to optimize its weight and is finished by a short circuit. The main technical improvements are related to the possibility of the antenna portability and its weight optimization and a particular combination of substrates related to the weight reduction. The antenna characteristics are analyzed, and the promising performances are pointed out. Experimental results are also presented to examine the basic antenna performances. The possibility of a high radiation efficiency antenna ( more than 70%) has been verified by these experiments. The agreement of the experiment with the simulation indicates the validity of the analysis, design, and fabrication of this antenna. ACKNOWLEDGMENT

Fig. 10. RX/TX antenna prototype axial ratio: measurements.

The authors are deeply indebted to P. Caballero Almena and C. Martinez Portas for their assistance with measurements and to the engineers J. Sanmartin and F. Moyano for their fruitful discussions. The simulations done in this work have been carried out using CST Studio Suite 2011 under a cooperation agreement between Computer Simulation Technology (CST) and Universidad Politécnica de Madrid. Substrates used in the prototypes were kindly given by NELCO S.A., and free samples of silicone ELASTOSIL E41 glue were provided by IMCD España. REFERENCES

Fig. 11. RX/TX antenna prototype gain: measurements versus simulation.

spiral arrangement, separated one effective wavelength. Slots length and position are designed to get uniform amplitude and phase in the aperture and circular polarization. Each RLSA

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