26th EuMC * 9-12 September 1996 * Hotel Hilton Atrium * Prague Czech Republic

Single-fed dual band circularly polarisedmicrostrip patch antennas

David S4nchez-Hernaindez*Georgios Passiopoulos,Ian D Robertson*** Department of CommunicationsPolytechnic University of ValenciaCamino de Vera S/N E-46022 Valencia, Spain

Tel: +34(9)63877191Fax: + 34 (9) 6 387 7199e-mail: dsanchez@dcom.upv.es

** Communications Research GroupDepartment of Electronic &Electrical EngineeringKing's College LondonStrand, London WC2R 2LS, UK

Tel: + 44 (0) 171 873.2523Fax: + 44 (0) 171 873 2081e-mail: ianr@orion.eee.kcl.ac.uke-mail: georgep @ orion.eee.kcl.ac.uk

AbstractOver the past years several attempts have been made to

widen the bandwidth ofsingle-fed circularly polarised patchantennas. This paper presents a novel structure toaccomplish circularly polarised dual frequency operationby using two spur-line band-stop filters. This circularlypolarised dual frequency operation can be obtained withoutincreasing either the size or the thickness of the patch.Furthermore, through several experiments this paper alsodemonstrates that the location of the feed point establishesand controls a trade-off between axial ratio and impedancebandwidth.

IntroductionMicrostrip antennas are finding numerous applications

due to their attractive features of low profile, light weight,low cost and conformable structure. In many applications,however, a microstrip antenna operating as a circularlypolarised (CP) element is desired. A circularly polarisedmicrostrip antenna can be classified into two categories;single- and dual-fed type. This classification is based uponthe number of feeding points that are required for thegeneration of CP waves.A single-fed antenna is attractive because there is no need

for an external polariser or coupler. In these antennas, thegenerated mode is separated into two orthogonal modes bythe effect of a perturbation segment such as a slot or other

truncated segment. Circular polarisation in then obtainedwhen the two orthogonal modes of equal amplitude excitedin the patch have a differential phase of 900. In spite of this,one of the principal problems ofsuch antennas is the extremenarrowness of their bandwidth (generally less than 1%)when compared to ordinary antennas as well as the generalrestriction to a particular feed point.The bandwidth can be widened by using thick substrates,

where an improved cavity model can be employed to predictthe response [1]. Yet, there is a limit on the maximum usablethickness of the substrate so that surface waves will not beradiated. Single band circularly polarised microstripantennas have been studied extensively. The annular ring[2], although exhibiting wider circularly polarisedbandwidth (CPBW) due to the lower amount of energystored under metallisation, was found to have a good axialration in the broadside direction, which, however,deteriorates thereafter. The pentagonal antenna [3] wasfound to radiate CP waves at two different frequencies, butwith a CPBW of 1%, which is applicable where very narrow-band circularly polarised operation is needed. Besides, itsanalysis is not an easy task due to the fact that the boundaryconditions cannot be imposed in a simple manner. Similarly,the single-fed equilateral-triangular patch [4] can alsoexhibit two kinds of good CP waves at two differentfrequencies. One of its most attractive features is that thearea necessary for CP operation becomes half as large asthat of a nearly square patch. The CPBW for the twofrequencies, however, was found to be around 0.5 to 0.6 %for a 3 dB axial ratio.Previous research on single-fed circularly polarised dual-

band patches [5) has shown that there is a trade-offbetweenwider bandwidth and good axial ratio. In [5], the truncated-corners square patch was used along with a single spur-linefilter and a perturbation segment. Yet, no meticulous studywas given to this important trade-off, and no explanationswere provided as to how to counteract such compromisefrom a practical point of view.

In this paper a new structure to accomplish circularlypolarised dual frequency operation is presented. Thetechnique uses a single-fed nearly square patch and twospur-line filters. Through several experiments, importantremarks are concluded on how to control the trade-offbetween good axial ratio and impedance bandwidth. Thecalculations for the axial ratio, impedance bandwidth andradiation patterns are also compared with the measurements.

TheoryOne approach to provlde circular polarisation is to attach

a single point at a location so as to excite two equalamplitude degenerate orthogonal modes, and then introducesome asymmetry into the cavity so that the degeneracy ofthe modes is removed. The radiated fields excited by thesetwo modes are perpendicular to each other and orthogonallypolarised in the boresight direction. This technique has beenextensively used, but some other perturbation elements mustbe introduced to reduce the unloaded quality factor of theantenna and hence obtain a wider impedance bandwidth.

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Bates [6] designed a microstrip spur-line band-stop filtersuitable for bandwidth of the order of 10% within the widthof a microstrip transmission line. The filter can be designedfollowing the procedure described in [5]. The polarisationof a rectangular patch antenna is normally linear. However,Kaloi [7] and Carver [8] have demonstrated that a cornerfed patch with L/W=1.029 should produce circularpolarisation in the broadside direction. Due to fringingfields, the physical dimensions for a and b are slightlysmaller, approximately by h or 1.5 h, depending on thematerial used. Sharma [9] reported a 0.17 dB axial ratio atboresight using this technique. However, the CPBW wasfound to be extremely narrow (0.8 %) and although as thethickness of the substrate increases there is an improvementon VSWR, the axial ratio limited bandwidth also decreases,producing a trade-off in the selection of substrate thickness.By a combination of the spur-line filter technique [10]

and a nearly square patch with optimum aspect ratio andfeed point location, a dual-band CP patch antenna can beobtained as depicted in figure 1. The position of the feedpoint is crucial to achieve a good compromise betweenimpedance and axial ratio bandwidth. The optimum feedlocation for impedance bandwidth was 0.3522 times thediagonal as reported by Carver (8). The input return loss ofthe novel dual-band patch antenna designed using thiscriteria can be observed in figure 2.For optimum CPBW operation, however, the feed point

must be displaced from the diagonal to create someasymmetry and hence 900 phase difference between thefields components, as also described by Lo [1]. Thus, a trade-off is encountered, where the position of the feed pointclearly determines the characteristics of the dualband CPmicrostrip antenna. To clarify this, the feed loci in a nearlysquare patch for left-hand circular polarisation (LHCP) andright-hand circular polarisation (RHCP) generation isreproduced in figure 3. It can be observed from this figurethat the feed loci for RHCP are the reflections of those forLHCP with respect to the lines x=a/2 and y=b/2. Theformulas given in [1] were based on the cavity model, whichhas a limitation to thin substrates. Yet, the results are validfor substrates as thick as 0.084 X, with a CPBW of over 3 %.

Simulation and Experimental ResultsThe eMTM package by Sonnet Software which calculates

S-parameters for arbitrary geometries using the Method ofMoments was selected for simulation purposes. Severalelements based on the proposed new structure, shown inthe picture of figure 4, were built on a 5880 RT/Duroidsubstrate of relative permittivity S,=2.2 and thicknessh=1.575 mm.The different elements were then measured with a HP8510

network analyser in an anechoic chamber with a reflectivitylevel of -25 dB. The bandwidth predicted by eMTM s 3.6 %in the case of a normal patch of the same dimensions thanthat of figure 1 (380 MHz, centre frequency fo= 10.6 GHz).This can be compared to figure 2 in which the measuredbandwidth of the CP dual-band patch designed followingthe maximum impedance bandwidth criteria is 6.33 %(598.16 MHz, centre frequency fo = 8.675 GHz) and 4.76 %(548.76 MHz, centre frequency 11.5 GHz). Yet, the axialratio at boresight was found to be 2.5 and 6.5 dB for thefirst and second resonant frequencies respectively.Another patch was designed to achieve better axial ratios

but less impedance bandwidth as can be observed from

figures 5 and 6, where the measured VSWR and axial ratiobandwidth are shown for the improved CP patch. EMTMpredicted an axial ratio at boresight of 2 and 1.02 dB, whilethe measured axial ratio was 0.458 and 0.0032 dB for thefirst and second resonant frequencies respectively. Themeasured bandwidth was 3.32 % (300 MHz at fol = 9.05GHz) and 3.43 % (400 MHz at 11.6 GHz). The concentrationof current in the edges of the coupled lines of the spur-linefilters can be observed in the simulated current distributionprovided by emvuTM and depicted in figure 7. The measuredradiation patterns, depicted in figures 8 and 9, agreed wellwith the simulated ones and with those typical of patchantennas.

ConclusionsIn a large array, dual-fed elements require many hybrids

or power dividers and, consequently, are more expensive,heavier and RE loss is greater. Thus, lightweight single-fedcircularly polarised microstrip antennas provide manyadvantages over linearly polarised and dual-fed CP elementsfor communication systems that require mobility or weatherpenetration, typically mobile and satellite communicationsystems. These antennas are not affected by thedepolarisation caused by rain and do not require accurateangular orientation of the receive antennas.

In single-fed microstrip antennas, however, the narrowCPBW arises mainly from the difficulty of maintaining theproper excitation conditions and hence polarisation purityover a wide bandwidth. In this paper, it has been made clearthat the trade-off between good axial ratio and impedancebandwidth usually limits the designer in single-fed CPschemes, where techniques for achieving widebandpolarisation techniques as well as wideband impedancefeatures are important.

In summary, a new CP single-fed dual-band microstripantenna has been described. The new geometry of themicrostrip patch antenna has proven to be a flexible andCP dual-band element. This is particularly important formobile communications systems where small size, lightweight, low profile and low cost are often demanded inportable or pocket-size equipment and vehicles. It isespecially relevant when a flush-mounted or built-in antennais required. It can also be applied to satellite systems wheretwo channels are needed to receive/transmit thetelecommand and telemetry signals (GPS receivers) or in ahand-held message communication terminal (HCMT).

AcknowledgementsThe authors wish to thank Alfred D. Lucas and Brian

Travel for their technical assistance. This work wassupported by the Comision Interministerial de Ciencia yTecnologfa (CICYT), Spain, under the project TIC950983-C03-02 (Universidad Politecnica de Valencia (UPV)) andby the Engineering and Physical Sciences Research Council,UK.

References[1] Lo, Y.T. et al.,'Simple Design Formulas for

Circularly Polarised Microstrip Antennas', IKEProceedings, Part H, 1988, vol. 135, no. 3, pp. 213-215.

[2] Bhattacharya, A.K. and Shafai, L.,'Annular Ring asa Circularly Polarized Antenna', 1987 IEEE Antennas &Propagat. Int. Symp. Digest, pp. 1020-1023.

[3] Weinschel, H.D.,'A Cylindrical Array of Circularly

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Polarized Microstrip Antenna', 1975 IEEE Antennas &Propagat. Int. Symp. Digest, pp. 177-180.

[4] Nakano, H. et al.,'A Spiral Antenna Backed by aConducting Plane Reflector', IEEE Trans. on Antennas &Propagat., 1986, vol. 34, no. 6, pp. 791-796.

[5] Sanchez-Hernandez, D. and Robertson,I.D.,'Analysis and Design of a Dual Band CircularlyPolarised Microstrip Patch Antenna', IEEE Trans. onAntennas and Prop., 1995. Vol 43. No. 2. pp. 201-205.

[6] Bates, R.N.,'Design of microstrip spur-line band-stop filters', Int. Journal Microwaves, Optics and Acoustics,Nov. 1977, vol. 1, no. 6, pp. 209-214.

[7] Kaloi, C.M.,'Corner Fed Electric Microstrip Dipole',

Naval Missile Center, Pt. Mugu, California, March 1978.[8] Carver, K.R. and Coffey, E.L.,'Theoretical

Investigation of the Microstrip Antenna', Tech Rept. PT-00929, Physical Science Laboratory, Las Cruces, 1979.

[9] Sharma, P.C. and Gupta, K.C.,'Analysis andOptimised Design of Single Feed Circularly PolarisedMicrostrip Antennas', IEEE Trans. on Antennas & Propag,1983, vol. 31, pp. 949-955.

[10] Sanchez-Hernandez, D. and Robertson, I.D.,'Dual-band Microstrip Rectangular Patch Antenna Using a Spur-line Filter Technique'. 23rd European Microwave Conf:Proceedings, Madrid 1993. pp. 357-360.

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26th EuMC * 9-12 September 1996 * Hotel Hilton Atrium ' Prague * Czech Republic

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Fig. 9. Measured H-plane radiation patterns for improved dual-band CP patch.

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