omnidirectional microstrip patch antenna with reconfigurable pattern and polarisation

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Published in IET Microwaves, Antennas & PropagationReceived on 28th November 2013Revised on 21st February 2014Accepted on 14th May 2014doi: 10.1049/iet-map.2013.0665

Special Issue on Emerging Integrated ReconfigurableAntenna Technologies

72The Institution of Engineering and Technology 2014

ISSN 1751-8725

Omnidirectional microstrip patch antenna withreconfigurable pattern and polarisationAdam Narbudowicz1, Xiulong Bao2, Max James Ammann2

1CTVR – The Telecommunication Research Centre, Trinity College Dublin, Dunlop Oriel House, Dublin 2, Ireland2Antenna & High Frequency Research Centre, Dublin Institute of Technology, Kevin Street, Dublin 8, Ireland

E-mail: [email protected]

Abstract: An omnidirectional microstrip patch antenna, capable of reconfiguring both polarisation and radiation pattern isproposed. It operates with two orthogonal ±45° slanted linear polarisations and can produce two dipole-like radiationpatterns, providing 360° coverage in either the horizontal or elevation plane. With appropriate steering, this enables a singleantenna to provide full spherical coverage for any polarisation. The reconfiguration is realised by phase shifting, thus does notrequire switching elements, such as microelectromechanical system or pin diodes embedded into antenna. The basic principlesof operation are discussed and validated by numerical and measured data.

1 Introduction

The capability of an antenna to receive or transmit signals forany arbitrary angle over a full sphere is highly desirable formany radio applications, such as point-to-multipointtransmission, radiofrequency identification detection orsensing. Isotropic antennas, as proved by Mathis [1] areimpossible to realise because of the mathematical propertiesof a continuous vector field tangential to a sphere (as seenin far-field radiation pattern). To overcome this difficulty,some antennas offer full spherical coverage usingreconfigurable antennas [2–5] or integrated multiportantennas with different radiation patterns [5–8].In the first category, Zhang et al. [2] proposed a

reconfigurable antenna consisting of two symmetricalelements, which can swap the role of radiator and reflector.The resultant antenna offers two oppositely directed modes.The antenna operates over a large bandwidth, however theimpact of the switches and its steering on antennaperformance is not reported. Reconfigurable antennasemploying ‘pin’ diodes to switch the beam either in elevationand azimuth [3] or sweeping it over 360° in the azimuthplane [4] are reported. Both approaches focused on selectivebeams, and do not provide full spherical coverage. In [5], apattern and frequency reconfigurable antenna offers twodifferent radiation patterns, steered by two switching elements.The multiport antenna approach can provide better spherical

coverage, however at the price of expensive additionaltransceivers needed to process the signal. In [6], a compactantenna is proposed, which integrates a dual-polarised patch, amonopole and a quasi-loop antenna. This approach allowscoverage of most signals within a full sphere (with theexception of direction obstructed by the ground plane) andwith two orthogonal polarisations. Another approach byMartens and Manteuffel [7] uses characteristic modes on a

ground plane to provide a two port multiple-inputmultiple-output antenna covering all angles around a sphere.Since the ground plane acts here as a radiator, the antennacovers a very wide range of angles around a full sphere,however the polarisation issues are not investigated. In [8] afour-feed antenna is proposed, capable of producing either alinearly polarised conical beam or a circularly polarised beamon boresight. Finally, Narbudowicz et al. [9] proposes acircularly polarised antenna with a dipole-like radiation patternthat can be rotated. The solution also offers full sphericalcoverage, however covers only a single polarisation (righthand circular polarisation (RHCP)).In this paper we propose an antenna, capable of radiating two

orthogonal linear ±45° slanted polarisations, each with twodipole-like patterns: one covering 360° in horizontal planeand the other in the elevation. This, when combined withproper steering, allows coverage of any polarisation from anyarbitrary angle in a sphere. The switching between antennaconfigurations is realised by different phase excitations at thefour antenna ports. There are no switching componentsintegrated into antenna. To steer the antenna beam andpolarisation, any state-of-the-art method providing a sufficientphase shift can be used, such as tunable capacitors,reconfigurable couplers [10], liquid crystal delay line etc. Formeasurements conducted in this paper the reconfigurableelement was emulated by a rat-race coupler, which provided 0and 180° phase shifts. In addition, with this coupler, multipleantenna modes can be used at the same time. This is animportant advantage for many point-to-multiport radio systems.This paper is organised as follows: Section 2 provides a

description of the proposed antenna; Section 3 explains theprinciples of operation; Section 4 investigates the proposedantenna with idealised feeds; Section 5 presents simulatedand measured results; and finally conclusions are providedin Section 6.

IET Microw. Antennas Propag., 2014, Vol. 8, Iss. 11, pp. 872–877doi: 10.1049/iet-map.2013.0665

Fig. 1 Geometry of the proposed antenna: side view (left) and front view with important dimensions (right)

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2 Antenna design

The antenna comprises two back-to-back coupled patches[11], which share a common ground plane as seen in Fig. 1.It has three metallisation layers, milled on a Taconic RF-35substrate. Each substrate layer is 1.5 mm high, with arelative permittivity εr = 3.47. In the prototyping process thetwo layers are milled separately, each with its own groundplane. Next four SMA connectors are soldered, two to eachlayer. The layers are then joined together, providing goodelectrical contact between both ground planes andeffectively merging them into single metallisation layer.The patches are rectangular and are fed from microstrip lines

connected to the two lower corners. They operate at a basicresonance frequency of 2.56 GHz. Since the antenna is basedon a simple rectangular patch resonator, it can be easilytuned in frequency by varying parameters Lp and Lq. Twofeeds for each patch are required in order to achieve duallinear ±45° slanted polarisation. For pattern reconfigurability,the electric field must be excited along all four edges of eachpatch. To achieve this, each patch is fed at a corner. Althoughthis technique is usually applied for circularly polarisedantennas, here it realises a linear slanted polarisation as boththe length and width of the patch are equal. A significantproblem is very high input impedance at the corner.Considering this, two measures were undertaken to provide agood match: the corners, at which each patch is fed, weretruncated by Δs = 5 mm to decrease the input impedance; alsoquarter-wavelength transformers (which are 0.3 mm wide andhave impedance Z = 142 Ω) were implemented. The 50 Ωmicrostrip lines were traced into the edge of the substrate,

Table 1 Four antenna configurations and the corresponding portradiation pattern is produced

Configurations Polarisations Plane of radiation

A +45° xzB +45° yzC −45° xzD −45° yz

X means there is no excitation at the given port.


where SMA connectors were soldered. The antennadimensions are: Ws = 42 mm; Ls = 5 mm; Wg = 5.5 mm; Lg =5.5 mm; Lp = 31 mm; Lq = 16 mm; and Δs = 5 mm.The antenna has four input ports (two for each of the

back-to-back coupled patches). The pattern reconfigurabilityis implemented by a phase difference between the ports andpolarisation is determined by port selection. Theinvestigated configurations are listed in Table 1. It can beseen that each configuration excites only two of four ports.The change of the excited ports determines the polarisation,that is, if ports 1 and 3 are excited, the antenna produces+45° slanted polarisation, whereas for ports 2 and 4 it is−45° slanted polarisation. The excited ports are eitherin-phase (i.e. 0° phase difference) or out-of-phase (i.e. 180°phase difference). This enables rotation of the dipole-likepattern by 90° around z-axis. Fig. 2 shows a photo of theprototyped antenna, with the rat-race coupler [12] attachedto achieve proper configuration.

3 Principles of operation

To explain the principles of operation, the electric field foreach edge is investigated. Fig. 3 depicts the electric fieldson the top and bottom patches for the four configurationsmentioned in Table 1. The method relies on thesuperposition of the electric fields, produced by four edgesof each patch. This requires two orthogonal modes of thepatch (TM100 and TM010) to be excited, hence the use of acorner feed. Configurations A and B excite feeds located inthe left lower corner (i.e. ports 1 and 3), therefore thesuperposition of the four edges produce +45° slanted

phases, polarisation and plane in which the omnidirectional

Phase at port

1 2 3 4

0° X 0° X0° X 180° XX 0° X 0°X 0° X 180°

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Fig. 2 Photograph of the prototyped antenna with the rat-race coupler used in measurement and simulation

Coupler as seen here realises configurations A and BUnused ports are terminated with a 50 Ω match

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polarisation (please note, that bottom patch is shown from the+z-direction, however it produces main radiation in−z-direction). Similarly, the configurations C and D excitefeeds located in the right lower corner (i.e. ports 2 and 4),producing −45° slanted polarisation. To demonstrate how toswitch the plane in which the radiation is produced, thephase difference between the corresponding edges of thetop and bottom patches are investigated. Generally, sincethe patches are in a back-to-back orientation, a phasedifference of 0° means the electric field vectors are orientedin the opposing directions, therefore producing a null in theradiation pattern. However a phase difference of 180°means the electric field vectors are aligned in the samedirection and radiation is produced.Feeding the antenna ports with different phase shifts results

in a change of the phase shift between the edges of the patches

Fig. 3 Electric field for each patch, shown for the four configurations d

Non-excited ports are removed for clarity and the bottom patch is viewed from the


(i.e. between the corresponding edges of the top and bottompatches). This relationship is demonstrated in Fig. 3. Forinstance, in configuration A the two patches are fedin-phase. This causes the edges in ±y directions to be alsoin-phase, resulting in a null for this direction. However theedges in the ±x directions are out-of-phase, interferingconstructively and producing radiation. As a consequenceconfiguration A will produce a + 45° polarisation with anomnidirectional radiation pattern in the horizontal plane (i.e.xz-plane) and nulls in ±y directions. Configuration C willproduce the same radiation pattern with −45° polarisation.If the top and bottom patches are fed out-of-phase (as in

configurations B and D), the edges in the ±x directions arein-phase, producing nulls, and the edges in ±y directionsare out-of-phase, radiating in the yz-plane. It should benoted that this mode is much more challenging as the

escribed in Table 1

top through the substrates


Fig. 4 Simulated realised gains for antenna with idealised rat-racecoupler

a xz-planeb yz-plane

Fig. 5 Measured and simulated reflection coefficients of theproposed antenna

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radiation is produced in the plane where the feeds are located.This causes ripples in the radiation pattern and degrades thepolarisation purity in the −y directions (θ = 90°) where thefeeds and steering circuitry are located.

Fig. 6 Realised gains for the proposed antenna in configuration A


4 Idealised model

The antenna was first simulated with idealised excitations,that is, the signals with desired phase shift were applieddirectly at the antenna ports (ignoring the impact of feednetwork). For this purpose, a time-domain solver of CSTMicrowave Studio was used [13]. Fig. 4 presents simulatedrealised gains for configurations A and B. It can be seenthat the plots exhibit good dipole-like radiation pattern withone plane being omnidirectional (i.e. plane xz forconfiguration A and plane yz for configuration B) and theother being figure-of-eight shape (i.e. plane yz forconfiguration A and plane xz for configuration B) with twodistinctive nulls. The squint visible in the yz-plane forconfiguration A is most likely because of the microstriplines and quarter-wavelength transformers. The results forconfigurations C and D are similar (with the change ofdominant polarisation), and hence not shown for brevity.


This configuration is in fact similar to the intendedcommercial application, where the antenna will be fed froman integrated transceiver. However for the laboratorymeasurements, an external steering network was required toswitch the antenna configuration. For this, a rat-race coupler


Fig. 7 Realised gains for the proposed antenna in configuration B


Fig. 8 Realised gains for the proposed antenna in configuration C


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(as seen in Fig. 2) was used both in measurement andsimulation. In addition, a feed cable was attached toconnect the antenna under test with measurementequipment. The cable is attached in yz-plane at θ = 90°, thatis, at a location where a radiation is produced inconfigurations B and D. This perturbs the radiation patternmeasurement [14] and is visible as ripples in the radiationpatterns, as well as decreased polarisation purity (incomparison with simulated results) for some angles inconfigurations B and D.

5 Results

Fig. 5 depicts simulated and measured reflection coefficientsSnn for each of the four antenna ports. It can be seen that theresults are in a reasonable agreement, but with a minor shifttowards higher frequencies for the prototyped antenna. Inaddition in the simulated results, a very small shift can alsobe seen between the ports located on top layer (i.e. ports 1and 4) and bottom layer (i.e. ports 2 and 3). This is notseen in the measured data, as prototyping inaccuracies havea greater dominating effect. The isolation between ports is>15 dB, and hence is not shown for clarity. The radiationpatterns for various configurations are shown in thefollowing sections. All measured data are shown for2.56 GHz and simulated for 2.55 GHz.


5.1 Configuration A

Fig. 6 presents the realised gains for the proposed antenna inconfiguration A. It can be seen that the omnidirectionalcoverage was produced in the xz-plane. Unlike in Fig. 4, theripples can be seen in the yz-plane, which are the effect offeed cable and rat-race coupler. The simulation in CSTMicrowave Studio incorporated the coupler and a goodagreement is achieved between measurement andsimulation. The measured gain for +45° polarisation in thexz-plane varies between 2.7 and −0.7 dBi. In this plane, thecross-polarisation level (i.e. −45°) is better than 11 dB.

5.2 Configuration B

Fig. 7 presents the realised gains for the proposed antenna inconfiguration B. It can be seen that the 180° phase differencebetween ports 1 and 3 produced omnidirectional radiation inthe yz-plane and a shape-of-eight pattern with two distinctivenulls. This is similar radiation pattern, as expected from adipole antenna. The figure-of-eight is more apparent for thisconfiguration, as the xz-plane is more resilient to distortioncaused by reflection from the feed cables and coupler.A deterioration of the measured polarisation purity canhowever be seen in the yz-plane for angles θ = 90° andθ = 270°, compared with the simulated results. This is


Fig. 9 Realised gains for the proposed antenna in configuration D


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caused by reflection from the feed cable, which is mounted inthis plane at θ = 90°. The maximum measured realised gain is3.2 dBi and degrades to −5.7 dBi at θ = 90°, that is, locationwhere the feed cable is mounted. The simulated results, whichinclude the effect of the coupler but not the feed cable, are 2.2and −3.5 dBi, respectively. The measured cross-polarisationlevel in the yz-plane is better than 10 dB for angles θ from125 to 255° and from 295 to 60°.

5.3 Configuration C

Fig. 8 presents the realised gains for the proposed antenna inconfiguration C. Similar to configuration A, theomnidirectional radiation pattern is produced in thexz-plane, however the dominant polarisation is −45°.The measured realised gain in the xz-plane varies between2.1 and −2.4 dBi. The measured cross-polarisation level inthe whole xz-plane is better than 9.5 dB.

5.4 Configuration D

Fig. 9 presents the realised gains for the proposed antenna inconfiguration D. The dominant polarisation is −45° and the


omnidirectional pattern is produced in the yz-plane, with thetwo distinctive nulls and a figure-of-eight shape in thexz-plane. In addition here, the effect of the feed cablereflection is visible in the yz-plane as decreased polarisationpurity for θ = 90 and 270°. Measured realised gain in theyz-plane varies between 3.4 and −4 dBi, with simulatedgain of 2.1 and 4.3 dBi. The measured cross-polarisationlevel in the yz-plane is better than 10 dB for angles θ from115 to 255° and from 300 to 55°.

6 Conclusions

The proposed antenna offers a high degree of pattern andpolarisation reconfigurability. Two slanted linearpolarisations of ±45° are provided, each with twodipole-like radiation patterns. Using proper steering allowsreception or transmission of signals of any polarisation,from any arbitrary spherical angle. The antenna does notincorporate any switching components, avoidinginter-modulation issues and simplifying the design. It alsoallows two configurations to be used at the same time,which is beneficial for MIMO applications. The basicprinciples of operation were successfully validated. Theantenna is proposed for combating multipath fading forindoor wireless propagation scenarios.

7 References

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2 Zhang, G.-M., Hong, J.-S., Song, G., Wang, B.-Z.: ‘Design and analysisof a compact wideband pattern-reconfigurable antenna with alternatereflector and radiator’, IET Microw. Antennas Propag., 2012, 6, (15),pp. 1629–1635

3 Shynu, S.V., Ammann, M.J.: ‘Reconfigurable antenna with elevationand azimuth beam switching’, IEEE Antennas Wirel. Propag. Lett.,2010, 9, pp. 367–370

4 Bai, Y.-Y., Xiao, S., Tang, M.-C., Liu, C., Wang, B.-Z.: ‘Patternreconfigurable antenna with wide angle coverage’, Electron. Lett.,2011, 47, (21), pp. 1163–1164

5 Shoaib, I., Shoaib, S., Chen, X., Parini, C.: ‘A single-element frequencyand radiation pattern reconfigurable antenna’. Proc.: EuCAP – EuropeanConf. Antennas and Propagation, Gothenburg, Sweden, 2013,pp. 2057–2060

6 Heberling, D., Oikonomopoulos-Zachos, C.: ‘Multiport antennas forMIMO-systems’. Proc.: LAPC – Loughbourough Antennas andPropagation Conf., Loughborough, UK, November 2009, pp. 65–70

7 Martens, R., Manteuffel, D.: ‘2-port antenna based on the selectiveexcitation of characteristic modes’. Proc.: APSURSI – Antennas andPropagation Society Int. Symp., Chicago, USA, July 2012

8 Cao, W., Zhang, B., Liu, A., Yu, T., Guo, D., Pan, K.: ‘A reconfigurablemicrostrip antenna with radiation pattern selectivity and polarisationdiversity’, IEEE Antennas Wirel. Propag. Lett., 2012, 11, pp. 453–456

9 Narbudowicz, A., Bao, X.L., Ammann, M.J., Shakhtour, H., Heberling,D.: ‘Circularly polarized antenna with steerable dipole-like radiationpattern’, IEEE Trans. Antennas Propag., 2014, 62, (2), pp. 519–526

10 Ferrero, F., Luxey, C., Staraj, R., Jacquemod, G., Yedlin, M., Fusco, V.:‘Theory and design of a tunable quasi-lumped quadrature coupler’,Microw. Opt. Technol. Lett., 2009, 51, (9), pp. 2219–2222

11 Iwasaki, H.: ‘A back-to-back rectangular-patch antenna fed by a CPW’,IEEE Trans. Antennas Propag., 1998, 46, (10), pp. 1527–1530

12 Pozar, D.M.: ‘Microwave engineering’ (John Wiley & Sons, 1998, 2ndedn.). pp. 401–407

13 ‘CST Microwave Studio–Workflow & Solver Overview’, CST–Computer Simulation Technology AG, 2013

14 Manteuffel, D., Martens, R.: ‘Improved radiation pattern measurementsof MIMO handheld mobile terminals’. Proc.: Seventh European Conf.Antennas and Propagation, EuCAP 2013, Goteborg, Sweden, April2013


omnidirectional microstrip patch antenna with reconfigurable pattern and polarisation

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