Dual-band, dual-circularly polarised antenna element

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Published in IET Microwaves, Antennas & PropagationReceived on 24th October 2012Revised on 14th December 2012Accepted on 9th January 2013doi: 10.1049/iet-map.2012.0625

ISSN 1751-8725

Dual-band, dual-circularly polarised antenna elementWilliam Mark Dorsey1,2, Amir I. Zaghloul2,3

1Radar Division, U.S. Naval Research Laboratory, Washington, DC 20375, USA2Bradley Department of Electrical and Computer Engineering, Virginia Polytechnic Institute and State University,

Falls Church, VA 22043, USA3US Army Research Laboratory (ARL), Adelphi, MD 20783, USA

E-mail: wmdorsey@vt.edu

Abstract: A dual-band element that operates with dual-circular polarisation in two separate industrial, scientific and medicalbands is presented. The design that is based on concentric square slot/shorted-ring configuration is realisable in cost-effectiveprinted circuit board technology making it attractive for low-cost personal communications devices. The antenna elementprovides a flexible topology that allows for convenient, independent control of the polarisation and impedance bandwidth oftwo distinct frequency bands with no fundamental limitation on the ratio between the high- and low-band frequency ranges.The topology of this element is presented, and the antenna concept is validated via simulated and measured results.

1 Introduction

In multiband applications ranging from personal wirelesscommunication to space-based applications, the overallfootprint and weight of the system must be minimised. Strictweight restrictions, coupled with the increased complexity ofthe payload’s radio frequency (RF) system, have led tospace-based systems becoming an emerging area ofmultiband system design. For example, dual-band operationis now a necessity for systems employing frequencydiversity as a compensation mechanism for signal fading; inthis application, when the power margin for a signal iscompromised because of rain atmospheric attenuation,communication traffic is transferred to a lower-frequencyband that presents lower signal attenuation [1].Wireless access points and laptops are both trending

towards antennas capable of operating in multiplefrequency bands in order to support multiple protocol [2–3]. The 2.4 GHz industrial, scientific and medical (ISM)band is quickly growing in popularity for wirelesscommunications devices because of its use in Bluetoothtechnology and 802.11 b/g protocol. For higher data rates,the frequency band from 5.15 to 5.85 GHz is often used,and the 802.11a protocol operates within the 5.2 GHz ISMband [3]. Many of these applications utilise circularpolarisation (CP) owing to its ability to allow flexibleorientation in the plane of the transmitter and receiverantennas and to reduce multipath effects that can lead tosignal fading [4–5].A dual-band antenna element capable of operating with

dual-orthogonal polarisation in each frequency band wouldprovide invaluable flexibility to a system by reducing thenumber of antennas required to complete multiple tasks.Additionally, if the element is realised in printed circuit

IET Microw. Antennas Propag., 2013, Vol. 7, Iss. 4, pp. 283–290doi: 10.1049/iet-map.2012.0625

board technology, it would provide a low profile andlightweight design that could be easily integrated with theaccompanying electronics in the system. One example ofsuch a design is the antenna presented by Yang et al. in [6]that supports four polarisations x, y, u , w

( )in dual,

overlapping frequency bands. The flexibility in this designcomes from appropriately arranging shorting pins andL-shaped feeds in a single circular patch topology.This paper presents an element that uses two co-located

concentric radiators to achieve dual-band, dual-polarisationperformance. The low-band radiator is a shorted squareannular ring antenna, and the high-band radiator is a squarering slot. The operational frequency of each constituentelement is controlled by both its inner and outer perimeterlength. As a result, this design provides unique control tothe location of and ratio between the two frequency bandsin the dual-band design. This flexibility is difficult toachieve with many dual-band designs – specifically thoseachieving dual-band performance from a single aperture.The design presented in this paper operates at the 2.45 and

5.8 GHz ISM bands because of their significance to fieldsincluding wireless communications and airborne systems[7], but the design concept is not restricted to these bandsor frequency ratio. Prior work by Zaghloul et al. describedin [8] explored integrating a waveguide radiator at theshorted region of a shorted annular ring antenna. Thiselement was then modified to a planar geometry where thewaveguide radiator was replaced by a centrally locatedprinted element operating at a higher frequency [9]. Theevolutionary development of a dual-band, dual-circularlypolarised antenna element has been previously reported in[10], but this paper provides additional insight into theelement design and realisation, along with measured resultsof a prototype antenna.

283& The Institution of Engineering and Technology 2013

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Fig. 1 Topology of the dual-band, dual-polarised antenna element showing

a Top section viewb Front viewc Right section viewd Perspective semi-transparent view

2 Constituent elements

The basic building blocks for the coplanar, dual-band,dual-polarised antenna element are the square ring slotsegment and the shorted square annular ring segment. Theformer is used as the high-band radiator, whereas the latteris selected as the low-band radiator. The overall topology ofthe element is illustrated in Fig. 1. The high-band and thelow-band radiators are arranged concentrically, where eachsegment has the same physical centre as depicted in thefront view of Fig. 1b. The critical parameter values for thisantenna topology are indicated in Fig. 1b.

Figs. 1a and c show section views of the antenna element’sdielectric profile, which is detailed in the exploded view ofFig. 2. The element contains four dielectric layers includingan upper substrate, feed substrate, lower substrate andconnector substrate. The radiating elements are printed atthe top of the vertical profile and shorted to the antennaground plane – which is located between the lowersubstrate and connector substrate – with the shorting vias.The feed substrate is sandwiched between the upper andlower substrates, and it is used to provide a vertical offsetbetween orthogonal high-band feed lines that are printed onopposing sides of this thin (10 mm) substrate later. The RF

Fig. 2 Exploded side view detailing the vertical location of dielectric and conductor layers in the dual-band, dual-CP antenna element

In the prototype design, all dielectric layers had an ɛr of 2.33

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element for low band of the dual-band element. This segmentconsists of a square ring patch shorted to the ground plane atits inner perimeter. By shorting the element at its innerperimeter, excitation of the dominant mode is restricted, thussuppressing surface waves [11]. For the printed circuit designdiscussed in this paper, the inner perimeter will be shorted tothe ground plane with a series of plated through holes in lieuof a continuous conducting region. This type of element iscommonly realised with a circular aperture shape, but it canalso be realised with the square aperture [8]. The modes inthe circular shorted annular ring antenna are governed by thetranscendental equation

Yn kn1b( )

J ′n kn1a( ) = Jn kn1b

( )Y ′n kn1a( )

where a and b are the outer and inner radii, respectively, whereYn and Jn are Bessel functions of the first and second kind,respectively, and where kn1 is the wavenumber of the TMn1

mode resonating in the cavity contained within the shortedannular ring. The geometry of the shorted square annular ringantenna does not lend itself to a convenient closed-formexpression, but the performance of the element is analogousto the circular design in that the resonant frequencies aredriven by both the inner and outer perimeters. The low-bandelement in the prototype antenna is excited by orthogonalcoaxial probes. However, this design is not restricted tocoaxial probe feeds. In practice, this element would mostlikely be excited by microwave transmission lines couplingthe antenna element to appropriate T/R circuitry.Subsequently, the coaxial probe could be replaced by a via tothe appropriate transmission design. Moreover, the designcould be modified to incorporate an aperture coupled design.The high-band radiator is a notched square ring slot

antenna. In this design, the slot is excited with orthogonalstripline feeds for the two CPs. These transmission linespass underneath of the square ring slot where they areterminated in open circuited stubs. In many instances, theideal stub length for achieving the best axial ratio andimpedance match requires a feed that extends beyond thephysical centre of the element. If the orthogonal feed lineswere present in the vertical same plane, they wouldphysically intersect as they extend beyond the same centrepoint. In order to eliminate this problem, a thin substrate(referred to as the feed substrate) is placed at the centre of

the dielectric profile. The two feed lines are printed onopposing sides of the feed substrate. The feed substrate isthen sandwiched between two other substrate layers andconductors are present on the top and bottom of thesandwiched dielectric profile to complete the striplinedesign as detailed in Fig. 2.The overlapping region of the feeding striplines generates a

capacitance between the two feeds. A parametric study wasperformed in CST Microwave Studio to investigate theimpact of this capacitance on overall element performanceby varying the vertical separation between the orthogonalfeeds. The results showed that the capacitance between thefeeds increased as the vertical separation between theelements decreased. However, this capacitance did notimpact the radiation pattern, polarisation characteristics orantenna efficiency.Slotted stripline designs are subject to power loss,

low-efficiency and degraded pattern shape as a result of theparallel plate mode [12]. Work by Bhattacharyya et al. [13]has shown that vias can be used to suppress the parallelplate mode in slot-coupled patch antennas fed by striplinefeed networks. Their design contains vias surrounding theslot and concludes that the presence of the vias improvesthe antenna gain by increasing the available power forradiation. In the design presented in this paper, the shortingvias of the low-band element are also used as amode-suppresing via fence for the high-band feed lines.Each of the constituent segments achieves CP performance

through the introduction of triangular perturbations intoopposing corners of the radiating element. This techniquehas been shown to introduce two, near-degenerate modes inthe structure that – when excited in phase quadrature –combine to form CP [14–17] with a single feed point. Thesense of the CP is determined by the location of the feedpoint with respect to the perturbations. Both senses of CPare excited by the introduction of orthogonal feeds for eachof the two radiating elements. The area of the truncations inthis design can be calculated from

DAREA = 1

where Q is the quality factor of the unperturbed antenna [16].An orthogonal feed can be added for dual-CP performance.However, it has been shown that the orthogonal modes ofelliptical polarisation couple strongly to each other inmicrostrip patch structures, thus presenting a tradeoffbetween port-to-port isolation and axial ratio [18].

Fig. 3 Illustration of microstrip-to-stripline transition for the high-band feeding

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feeding transmission lines beneath the low-band radiatordegraded the axial ratio of that low-band shorted squareannular ring antenna operating in CP. In order to minimisethis problem, a microstrip layer was added beneath theantenna’s ground plane. A microstrip line with the samecharacteristic impedance as the stripline feeds was printedon this new layer, and a microstrip-to-stripline transitionwas present just beyond the outer perimeter of the squarering slot as described in Fig. 3. This figure provides insetviews detailing the microstrip-to-stripline transition as wellas the locations of the vias in proximity to the feedlines.The two vias closest to the feedlines were spaced 228 mmcentre-to-centre – 114 from the centre line of the 102 mmwide high-band stripline feed. The remaining vias have acentre-to-centre separation of 353 and 518 mm as indicatedin Fig. 3b. This figure shows that the vias closest to thefeedline have the closest spacing. It was seen throughsimulations that the vias in immediate proximity to the feedhad the greatest impact on performance, and the separationbetween the remaining vias was allowed to grow larger inoptimisations in order to also minimise the number of viasrequired in manufacturing. In these optimisations, the costfunction combined radiation efficiency and return loss overhigh band of the antenna element.

3 Prototype antenna topology

This antenna element was simulated, built and tested. Thesimulations were performed using CST Microwave Studio,a computational electromagnetic (CEM) software packageemploying the finite integration technique (FIT). Theassembly of the element involves a complex processbecause of the multilayered design of the antenna element.Each of the layers must be etched, and selected layers mustbe bonded together with precise alignment. The layers arebonded together with a dielectric-matched epoxy having anoverall thickness of approximately 0.003 cm (0.001″). Thelocation, approximate thickness and approximate materialproperties of the epoxy are included in the simulations. Aphotograph of the constructed element is provided in Fig. 4,and the corresponding values used in the prototype antennaare defined in Table 1. The photograph shows the top viewof the antenna element on the left and the bottom view onthe right. The bottom view shows the location where thefour connectors are installed. Fig. 5 shows the bottom viewwith connectors installed. The high-band ports are fed with

Fig. 4 Photograph of the dual-band, dual-CP antenna elementoperating at the 2.45 and 5.8 GHz ISM bands (ruler shown infigure has units of inches)

surface mount SMA connectors, and the low-band ports arefed with a four-hole panel mount SMA connector. Thepanel mount connectors have a 1.27 cm × 1.27 cm base. Allfour connectors have a centre conductor with a 127 mmdiameter centre conductor.

4 Simulated and measured results

The dual-band, dual-CP antenna element can be viewed as afour-port microwave network. The high-band RHCP andLHCP are defined as ports 1 and 2, respectively, whereasthe low-band left hand circular polarization and right handcircular polarization are ports 3 and 4, respectively. Thissection will report the measured and simulated results. Allsimulated results were obtained through CST MicrowaveStudio, a CEM software package employing the FIT.

4.1 Scattering matrix parameters

The measured S-parameters of the dual-band, dual-CPantenna element are compared with the simulatedS-parameters as shown in Figs. 6 and 7. The results showgood agreement between the measurements and simulations.Some differences, especially in the return loss results, arepartially attributed to differences between the ideal feedused in the simulations and the SMA coaxial feed that isintegrated in the design. Additionally, the assembly processof this element involves a complex series of steps involvingetching of copper layers, alignment and bonding of multiplelayers, and drilling and plating a series of holes to maintainelectrical continuity between several layers. The tolerancesof the alignment and bonding process is essential to theoverall performance of the element, and careful attentionmust be placed on the locations and depths of the plated

Fig. 5 Photograph of the dual-band, dual-CP antenna elementshowing the installed SMA connectors

Table 1 Dimensions for dual-band, dual-polarised antennaelement operating at the 2.45 and 5.8 GHz ISM bands

Parameter Value

L0 5.131 cm (2.020″)L1 1.870 cm (0.735″)L2 1.461 cm (0.575″)ΔLB 0.724 cm (0.285″)ΔHB 0.147 cm (0.058″)

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Fig. 6 Measured (solid lines) and simulated (dashed lines) S-parameters for the high-band antenna ports of the dual-band, dual-CP antennaprototype

a Sii for High Band Portsb isolation between high band portsc isolation between bands (Cx-Pol)d isolation between bands (Cx-Pol)

Fig. 7 Measured (solid lines) and simulated (dashed lines) S-parameters for the low-band antenna ports of the dual-band, dual-CP antennaprototype

a Sii for Low Band Portsb isolation between low band portsc isolation between bands (Cx-Pol)d isolation between bands (Co-Pol)

IET Microw. Antennas Propag., 2013, Vol. 7, Iss. 4, pp. 283–290 287doi: 10.1049/iet-map.2012.0625 & The Institution of Engineering and Technology 2013

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antenna, where each port excites a single sense of CP withinthe desired band. It should be noted that the low isolationnumbers between the two low-band ports (S43 and S34) areinherent in dual-CP printed microstrip designs as noted byOwens and Smith in [18]. In these structures, orthogonalelliptical polarisations are strongly coupled, thus presentinga trade-off between axial ratio and port-to-port isolation.The low isolation is reflected in the overall efficiency of thelow-band polarisations (42.1% for LHCP and 42.3% forRHCP). Conversely, the high-band polarisations show highefficiencies of 89.0 and 89.4% for LHCP and RHCP,respectively.An alternative approach for achieving CP is to excite

dual-linear polarisations in phase-quadrature. In thisstructure, the low band can be modified to achievedual-linear polarisation by removing the triangularperturbations (i.e. DLB � 0). An antenna with this topologywas designed and simulated in CST Microwave Studio. Thesimulated S-parameters, as shown in Fig. 8, indicate that theport-to-port isolation was greatly increased while stillmaintaining a good impedance match. By increasing theport-to-port isolation, the radiation efficiency of the antennawas also increased to 85% compared with just over 42% forthe dual-CP antenna. However, this limits the ability toachieve simultaneous LHCP and RHCP because both portsmust now be excited in order to obtain CP. As a result, thedual-CP version – with low port-to-port isolation – wasselected as the prototype. Changing the low band to

Fig. 8 Simulated S-parameters for a dual-band, dual-polarisedantenna where the low band is dual-linearly polarised and thehigh band is dual-circularly polarised

dual-linear polarisation had minimal impact on theperformance of the dual-CP high-band ports.

4.2 Axial ratio

In addition to showing reasonably good impedance match andisolation performance, this element also shows good CPpurity (axial ratio) for all polarisation states. If the elementis used in an antenna array or in an application requiring abroad beamwidth, it is important that the axial ratio remainslow over a wide range of angles. The contour plots inFigs. 9a and b provide a look at the simulated axial ratioagainst frequency against theta for the high-band RHCPand low-band LHCP, respectively, where theta is the anglemeasured from the z-axis (broadside) in the element’sprincipal planes. The high-band plot shows that the axialratio is <3 dB over a theta region of ±60° within the 5.8GHz ISM frequency band. At 5.85 GHz, the axial ratio isbelow 1 dB over for |θ| < 30°. The axial ratio holds up wellfor frequencies outside of the 5.8 GHz ISM band. Thecontour plot of Fig. 9a shows that the axial ratio is < 3 dBover |θ| < 30° from 5.65 to 6.05 GHz, with a fractionalbandwidth of 6.8%. The low-band plot shown in Fig. 9bindicates excellent axial ratio over the 2.45 GHz ISMfrequency band as well. The axial ratio is below 1 dB for|θ| < 40° between the frequencies of 2.43 and 2.45 GHz.Over the remainder of the 2.45 GHz ISM band, thesimulated axial ratio is <3 dB for |θ| < 60°. These resultswere simulated in the j = 0 (x–z) plane. For the high-bandRHCP port of Fig. 9a, this is the plane perpendicular to thatcontaining the feeding port. Conversely for the low-bandLHCP port of Fig. 9b, this plane contains the feed port. Inboth cases, the similar results were seen in the orthogonalplane. Comparisons of the simulated and measuredbroadside axial ratios at the two bands are shown inFig. 10. The high-band element exhibits similar, but slightlyshifted in frequency, simulated and measured results. Theshifted results are consistent with the slight shift inS-parameters for ports 1 and 2 as shown in Figs. 6 and 7,

Fig. 9 Simulated axial ratio against frequency against theta for

a High-band RHCPb Low-band LHCP port of a dual-band, dual-CP antenna element

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respectively, and it is attributed to discrepancies between theeffective and reported dielectric constants of the substratematerial and dielectric adhesive. Good agreement betweenmeasurements and simulations is also achieved at the lowband, with some discrepancy at the higher end. This is aresult of the slight de-tuning of the near degenerate modesthat was shown in the S-parameters of Figs. 6 and 7 and isattributed to achievable tolerances on the size of thetriangular perturbations.

4.3 Radiation patterns

The radiation performance of the dual-band, dual-polarisedantenna element was characterised in a compact rangefacility. The characterisation involved measurement of thelinearly polarised patterns in the principal planes of the

Fig. 11 Measured radiation patterns for

a HB RHCPb LB LHCP ports of a dual-band, dual-circularly polarised antenna element asa function of frequency

Fig. 10 Measured and simulated axial ratio for

a HB RHCPb LB LHCP ports of a dual-band, dual-circularly polarised antenna element asa function of frequency

antenna element. Fig. 11 shows the measured patterns for(a) the high-band RHCP port and (b) the low-band LHCPports, respectively. The high-band results show linearorthogonal polarisation components that are nearly equal inmagnitude for a wide angular region – a result consistentwith the good axial ratio performance shown in Figs. 9aand 10a. The low-band results show a greater discrepancybetween the two orthogonal polarisations – which isconsistent with the results shown previously in Figs. 9b and10b.

5 Conclusions

A dual-band element has been designed. It provides dual-CPperformance in the two distinct ISM frequency bands. Theelement, consisting of two concentric radiators, was builtand tested to verify the concept and performance shown insimulations. The measured results show good agreementwith the simulations, thus establishing a high confidencelevel in the validity of the simulation models used.Moreover, the measured results confirm the ability of thiselement to generate dual-CP at two distinct frequency bandswhile providing a lightweight, low-profile, printed circuitdesign capable of facilitating system integration. The sensesof CP are generated using the well-known cornertruncations of the square radiator and a single-point feedingfor each polarisation. Linear polarisations can also beachieved by feeding the two CP inputs using theappropriate power divider.

6 References

1 Dossi, L., Tartara, G., Matricciani, E.: ‘Frequency diversity in millimeterwave satellite communications’, IEEE Trans. Aerosp. Electron. Syst.,1992, 28, (2), pp. 567–573

2 Bialkowski, K.S., Zagriatski, S.: ‘Investigations into a dual band 2.4/5.2GHz antenna for WLAN applications’. 15th Int. Conf. Microwaves,Radar and Wireless Communications, 2004. MIKON-2004, 17–19May 2004, vol. 2, pp. 660–663

3 Liu, D., Gaucher, B.: ‘A new multiband antenna for WLAN/cellularapplications’. 2004 IEEE 60th, Vehicular Technology Conf., 2004.VTC2004-Fall, 26–29 September 2004, vol. 1, pp. 243–246

4 Hsu, S.-H., Chang, K.: ‘A novel reconfigurable microstrip antenna withswitchable circular polarization’, IEEE Antennas Wirel. Propag. Lett.,2007, 6, pp. 160–162

5 Sung, Y.J., Jang, T.U., Kim, Y.-S.: ‘A reconfigurable microstrip antennafor switchable polarization’, IEEE Microw. Wirel. Compon. Lett., 2004,14, (11), pp. 534–536

6 Yang, S.-L.S., Luk, K.-M., Lai, H.-W., Kishk, A.-A., Lee, K.-F.: ‘Adual-polarized antenna with pattern diversity’, IEEE Antennas Propag.Mag., 2008, 50, (6), pp. 71–79

7 Ketcham, R., Frolik, J., Covell, J.: ‘Propagation measurement andstatistical modeling for wireless sensor systems aboard helicopters’,IEEE Trans. Aerosp. Electron. Syst., 2008, 44, (4), pp. 1609–1615

8 Zaghloul, A.I., Ravipati, C.B., Kawser, M.T.: ‘Modeling and analysis ofa dual-Band dual-Polarization radiator using FEKO’, Appl. Comput.Electromagn. J., 2007, 22, (1), pp. 125–133

9 Dorsey, W.M., Zaghloul, A.I.: ‘Dual-polarized dual-band antennaelement for ISM bands’. IEEE Antennas and Propagation Society Int.Symp., 2009. APSURSI’09. 1–5 June 2009, pp. 1–4

10 Zaghloul, A.I., Dorsey, W.M.: ‘Evolutionary development of adual-band, dual-polarization, low-profile printed circuit antenna’. Int.Conf. Electromagnetics in Advanced Applications, 2009. ICEAA’09,14–18 September 2009, pp. 994–997

11 Bhattacharyya, A.K., Jackson, D.R., Williams, J.T., Smith, R.:‘Microstrip patch designs which do not excite surface waves’.Antennas and Propagation Society Int. Symp. 1991. AP-S. Digest,24–28 June 1991, vol. 1, pp. 68–71

12 Itoh, K., Yamamoto, M.: ‘A slot-coupled microstrip array antenna with atriplate line feed where parallel-plate mode is suppressed efficiently’.IEEE Antennas and Propagation Society Int. Symp., 1997. 1997Digest, 13–18 July 1997, vol. 4, pp. 2135–2138

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slot-coupled patch antennas with vias for parallel-plate modesuppression’, IEEE Trans. Antennas Propag., 1998, 46, (4),pp. 538–545

14 Sorbello, R.M., Zaghloul, A.I.: ‘Wideband, high-efficiency, circularlypolarized slot elements’. Antennas and Propagation Society Int.Symp., 1989. AP-S. Digest, 26–30 June 1989, vol. 3, pp. 1473–1476

15 Sharma, P., Gupta, K.: ‘Analysis and optimized design of single feedcircularly polarized microstrip antennas’, IEEE Trans. AntennasPropag., 1983, 31, (6), pp. 949–955

16 Niroojazi, M., Azarmanesh, M.N.: ‘Practical design of single feedtruncated corner microstrip antenna’. Proc. Second Annual Conf.Communication Networks and Services Research, 2004., 19–21 May2004, pp. 25–29

17 Chen, W.-S., Wu, C.-K., Wong, K.-L.: ‘Single-feed square-ringmicrostrip antenna with truncated corners for compact circularpolarisation operation’, Electron. Lett., 1998, 34, (11), pp. 1045–1047

18 Owens, R.P., Smith, A.C.: ‘A low-profile dual band, dual polarised arrayantenna module’. IEE Colloquium on Flat Plate and Low Profile MobileAntennas, 10 January 1990, pp. 6/1–6/6

Dual-band, dual-circularly polarised antenna element

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