A reconfigurable microstrip antenna is proposed for low-cost adaptive beam-switching applications. A small patch-slot-ring structure is used as the radiating element where an asymmetrical arrangement of p-i-n diodes is employed to switch the pattern in four directions. The antenna provides pattern switching of 65° and 45° in its fundamental mode for the elevation and azimuth planes, respectively. By maintaining the resonant frequency and beamwidth as relatively constant, beam switching is realized using a single feed point

Ammann, Max

Shynu, S.

English

Arrow@TUDublin

Technological University Dublin ARROW@TU Dublin Articles Antenna & High Frequency Research Centre 2010-04-01 Reconfigurable Antenna with Elevation and Azimuth Beam Switching S. Shynu Technological University Dublin, sshynu@tudublin.ie Max Ammann Technological University Dublin, max.ammann@tudublin.ie Follow this and additional works at: https://arrow.tudublin.ie/ahfrcart  Part of the Systems and Communications Commons Recommended Citation Nair,S., Ammann, M. (2010) Reconfigurable Antenna with Elevation and Azimuth Beam Switching. IEEE Antennas and Wireless Propagation Letters, Vol.9, 2010, pp367-370. doi:10.1109/LAWP.2010.2049332 This Article is brought to you for free and open access by the Antenna & High Frequency Research Centre at ARROW@TU Dublin. It has been accepted for inclusion in Articles by an authorized administrator of ARROW@TU Dublin. For more information, please contact yvonne.desmond@tudublin.ie, arrow.admin@tudublin.ie, brian.widdis@tudublin.ie. This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 3.0 License IEEE ANTENNAS AND WIRELESS PROPAGATION LETTERS, VOL. 9, 2010 367Reconfigurable Antenna With Elevationand Azimuth Beam SwitchingS. V. Shynu Nair, Member, IEEE, and Max J. Ammann, Senior Member, IEEEAbstract—A reconfigurable microstrip antenna is proposed forlow-cost adaptive beam-switching applications. A small patch-slot-ring structure is used as the radiating element where an asymmet-rical arrangement of p-i-n diodes is employed to switch the patternin four directions. The antenna provides pattern switching of 65and 45 in its fundamental mode for the elevation and azimuthplanes, respectively. By maintaining the resonant frequency andbeamwidth as relatively constant, beam switching is realized usinga single feed point.Index Terms—Beam steering, p-i-n diodes, reconfigurableantenna.I. INTRODUCTIONP ATTERN switchable antenna arrays are used in theemerging personal and in-building communication sys-tems. Electronically steerable antenna arrays are preferredbecause of low maintenance, fast orientation speed, lowpower requirements, and small physical size. Electronic beamswitching can be used in mobile applications to enhance spec-tral efficiency as well as reduce the problems associated withmultipath propagation. Switched parasitic smart antenna arrayscan adjust their pattern so that their main beam always pointstoward the mobile receiver or transmitter [1], [2]. The phasedarray is the most common type of electronically steerable array.With multiple radiating elements and a sophisticated controlalgorithm, it can achieve virtually unconstrained control overazimuth and elevation scan angles at reasonably high speeds.Conversely, many applications do not require such complexity,and thus an alternative, simpler system is desirable [3]–[5].Other wireless technologies such as vehicle-to-vehicle com-munications have attracted attention due to their potential toprevent traffic accidents by gathering information from othervehicles. In vehicle-to-vehicle communications, pattern controlimproves the system performance [6]–[8]. Digital beamformingantennas have typical architectures for steering the beam, whereRF chains are connected to individual radiating elements. Aflat array antenna for beam switching using a 3-dB couplerManuscript received March 05, 2010; accepted April 12, 2010. Date of pub-lication April 29, 2010; date of current version May 10, 2010. This work wassupported by Science Foundation Ireland under Grant 08/CE/I1523.S. V. S. Nair is with the CTVR, The Telecommunications Research Centre,School of Electronic and Communications Engineering, Dublin Institute ofTechnology, Dublin 2, Ireland. (e-mail: shynunair@gmail.com).M. J. Ammann is with the Antenna & High Frequency Research Centre,School of Electronic and Communications Engineering, Dublin Institute ofTechnology, Dublin 8, Ireland (e-mail: max.ammann@dit.ie).Color versions of one or more of the figures in this letter are available onlineat http://ieeexplore.ieee.org.Digital Object Identifier 10.1109/LAWP.2010.2049332and p-i-n diodes was recently developed for low-gain modernwireless communication systems. In this array, four-beams areswitched in the H-plane by selecting the input port or p-i-ndiode state [9]. A reactively steerable antenna that consists ofone driven element and many parasitic elements terminatedby variable reactance circuits provides a simpler architecture[10]. A reactively steerable antenna for cars has also beenreported [11]. In the UHF band, single antenna elements willbe required to have pattern control. A frequency-tunable dipoleantenna with variable reactance circuits has been reported.This dipole with silicon photo switches was demonstrated toavoid unwanted radiation caused by leakage current on dcbias circuit. The dipole also provided elevation steering of 12for the direction of the maximum radiation [12]. However,single-element methods are limited by small steering anglesand cannot switch the beam in two planes using a single feedoperating at a fixed frequency.In this letter, a novel single-feed microstrip antenna capableof beam switching in four different directions in the - and-planes is proposed. A conical radiation pattern is realizedin a patch-slot-ring combination, which can be shaped intoa single lobe pattern, switchable in different directions. Theconical pattern is produced by a constant current loop formedin the microstrip square-ring structure [13]–[15]. The funda-mental mode of the microstrip ring is used rather thanthe higher order modes. The microstrip ring is excited usinga small patch at its center and fed by a coaxial probe. Beamshaping is achieved by modifying the current distribution inthe microstrip ring using a combination of shorts and shortingposts. For electronic beam switching, these shorts and shortingposts are replaced by p-i-n diode switches. This arrangementis less complex than an array, and the asymmetrical shortingof the slot-patch-ring structure can realize symmetrical beamswitching in four different directions. Another advantage of thisconfiguration is that it only has components on two sides of thering and thus uses a smaller number of nonlinear components,reducing the cost and also the intermodulation generated dueto the p-i-n diodes. The design procedure with integrated p-i-ndiode switches and biasing circuitry is presented.II. RECONFIGURABLE ANTENNA DESIGNThe antenna consists of a small patch of dimension8 8 mm on an FR4 substrate of thickness 1.52 mm( ), fed using a coaxial probe, 2.5 mm offset frompatch center. The patch is surrounded by a square microstripring of inner dimension 10 mm and outer dimension 15 mm.The slot separation between the patch and the ring is 2 mm.This patch-slot-ring structure is embedded in a metallic patch1536-1225/$26.00 © 2010 IEEE368 IEEE ANTENNAS AND WIRELESS PROPAGATION LETTERS, VOL. 9, 2010Fig. 1. The geometry of the proposed pattern reconfigurable antenna with in-tegrated p-i-n diode switches (     mm,      mm,   	 mm,    mm).of dimension 72 68 mm as shown in Fig. 1. The struc-ture is placed 6 mm above the ground plane of dimension17.5 17 cm .The current distribution in the ring can be changed either byshorting it to the central patch or to the outer patch. The conicalbeam is shaped to become more directive in the elevation andazimuth planes by modifying the current distribution in the ringusing a combination of shorts and shorting posts. The shorts be-tween the ring and the inner patch can provide a directive beamtilting toward the direction of the short. When the outer patch isconnected to the ring using another short, the pattern switchestoward the opposite direction. The ring itself is connected to theground plane using a shorting post. To demonstrate the four op-erating modes and the corresponding beam switching capability,an equivalent passive model is shown in Fig. 2. The feed positionin the inner patch is fixed for the four modes. This property sig-nificantly simplifies the conversion of the passive design to anelectronically reconfigurable antenna with a switchable pattern.In general, complex multilayered capacitive feeding is neces-sary to obtain good matching of the high input impedance fun-damental mode of the ring. In this case, the feed mecha-nism is simple with a small embedded patch at the center of thering, which can maintain a 6-dB return loss even with differentshort positions.For electronic beam switching, the shorts and shorting postsare replaced by p-i-n diode switches. The four p-i-n diodes in-tegrated between the inner patch and the square rings are ar-ranged so that only one diode is forward biased at a time. Hence,a single dc supply of 1.1 V is sufficient to bias the reconfig-urable antenna. In general, to switch the beam in the elevationand azimuth planes, there must be a symmetrical placement ofswitches in the four respective directions of the radiating ele-ment. In the proposed design, p-i-n diode switches are only usedin two directions ( and ) in order to switch the beam infour directions. The bias circuitry and battery are located be-hind the ground plane. The bias is applied across the outer andFig. 2. Four equivalent passive antenna models with shorts and shorting posts.TABLE IBIASING AND BEAM DIRECTION FOR THE FOUR MODESinner patches and to the square ring through three chip induc-tors to isolate the RF current. Biasing details for all the operatingmodes are illustrated in Table I.III. MEASURED RESULTSOptimization of the antenna was carried out using CST Mi-crowave Studio. p-i-n diodes in forward bias were modeled as1- resistors and as 0.1-pF capacitors in reverse bias. The mea-sured and simulated for the four modes are given in Figs. 3and 4, respectively. The 6-dB impedance bandwidth commonlyavailable to all modes is 2.6% centered at 2.055 GHz. The simu-lated resonant frequencies for the four modes are slightly lowercompared to the corresponding measured results mainly due tothe parasitic effects of the six integrated p-i-n diodes and thebias chip inductors. Since the position of the feed is fixed andthe position of the short and shorting post are changing (withdifferent bias conditions), the relatively small shift in the reso-nant frequency is acceptable.The pattern direction depends on the p-i-n diode biasing atthe terminals A, B, C, and D in Fig. 1. When terminals A, C,and D are positive with respect to B, the antenna operates in themode, switching the beam toward the -direction. Whenbias is removed from C, diode D2 is in an OFF state, and the beamturns toward the direction ( mode). Similarly, when A, C,and D are negative with respect to B, the antenna operates in theNAIR AND AMMANN: RECONFIGURABLE ANTENNA WITH ELEVATION AND AZIMUTH BEAM SWITCHING 369Fig. 3. Measured   for the four different operating modes.Fig. 4. Simulated   for the four different operating modes.mode, switching the beam toward the -direction. Whenbias is removed from C, diode D4 is OFF, and hence the beamturns toward the direction ( mode).The measured 3-dB beamwidth is 55 for both planes. Themeasured radiation patterns are shown in Fig. 5. The 3D simu-lated 3-dB patterns are shown in Fig. 6 for reference. The mea-sured peak gain for the , , , modes was 4.58, 2.71,1.11, and 3.2 dBi, respectively. The variation in gain for themodes is due to the asymmetry of the switching p-i-n diode ar-rangement. The lower gain for mode is mainly attributedto the dc lines and biasing circuitry, which are directed alongthe x-axis. These bias lines run from the radiating element andperturb the current distribution in the ring and patch structurefor the mode. Details of the beam direction and switchingsequence are tabulated in Table I. The smaller scanning anglein the elevation plane compared to the azimuth plane is also at-tributed to the -directed bias lines. For single-element an-tennas, larger beam switching angles can only be achieved byusing higher order leaky-wave modes or by complex multifeedarrangements. However, in this proposed design, switching an-gles of 65 and 45 are achieved using the fundamentalmode of the microstrip ring. The gain is relatively high for anantenna with a large number of lumped elements integrated intoFig. 5. Measured pattern for the four modes. (a) Azimuth plane (   and  modes). (b) Elevation plane (  and  modes). The cross-polar valuesare too small to be visible in the above scale.Fig. 6. Simulated 3D radiation pattern showing the beam direction in four op-erating modes. (a) Azimuth plane (   and  modes). (b) Elevation plane ( and  modes).the radiating element. Thus, by keeping the resonant frequencyand beamwidth relatively constant, this design can provide beamswitching in the - and -planes with a single feed point.IV. CONCLUSIONA pattern reconfigurable microstrip antenna using integratedp-i-n diodes for beam switching in the elevation and azimuthplanes is presented. The antenna has an asymmetrical switchingscheme using a minimum number of lumped components,reducing the complexity, cost, and intermodulation character-istics. The antenna provides beam switching of 65 and 45in the - and -planes, respectively. The proposed methodis appropriate for adaptive beam switching in indoor wirelessapplications.370 IEEE ANTENNAS AND WIRELESS PROPAGATION LETTERS, VOL. 9, 2010REFERENCES[1] A. T. Kolsrud, M. Y. Li, and K. Chang, “Dual-frequency electronicallytunable CPW-fed CPS dipole antenna,” Electron. Lett., vol. 34, no. 7,pp. 609–611, Feb. 1998.[2] S. L. 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Reconfigurable Antenna with Elevation and Azimuth Beam Switching

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Reconfigurable Antenna with Elevation and Azimuth Beam Switching

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