“Dual-broadband multistandard

Dual-broadband multistandard printed slot antenna with a composite back-patchJ.-Y.Sze,C.-I.G.Hsu and S.-C.HsuAbstract:A square slot antenna fed by a coplanar waveguide for dual-band multistandard appli-cations is presented.Printed on the back of the square slot is a composite patch formed by anannular metal ring inscribed by a circular patch.Appropriate design of the composite back-patchresulted infive resonant bands,thefirst(last)two of which were combined to form a lower(upper)operating band.The ratio of the two operating band centre frequencies can be tuned larger than2.5.A pair of notches was embedded in the annular metal ring to further broaden the lower(upper)operating band up to a fractional bandwidth of37%(20%).The measured results indicatethat the proposed antenna can cover the frequency bands of the following wireless communicationstandards:digital communication system,personal communication services,universal mobiletelecommunications system and2.4/5-GHz wireless local-area networks.1IntroductionDual-broadband antennas have become very important in the applications of mobile communications and wireless local-area networks(WLANs)that employ multiple stan-dards.Among these standards,digital communication system(DCS)requires a band of1710–1880MHz,personal communication services(PCS)a band of1850–1990MHz, universal mobile telecommunications system(UMTS)a band of1920–2170MHz[1–6],IEEE802.11b/g a band of 2.4–2.484GHz,IEEE802.11a a band of 5.15–5.35GHz and an additional band of 5.725–5.825GHz and HiperLAN2a band of5.47–5.725GHz in addition to the band of 5.15–5.35GHz[7].Since these standards may be used simultaneously in many systems,there is a need for designing a single antenna that can cover all these bands.This need can be fulfilled if the designed antenna is capable of supporting two wide frequency bands,one being1720–2500MHz and the other5150–5850MHz(whose fractional bandwidths are about37.5% and12.7%,respectively).In recent years,printed slot anten-nas(PSAs)fed by a coplanar waveguide(CPW)have drawn much attention because of their wide impedance band-widths,low profile,easy integration with active devices or MMICs and higher antenna gains than those of printed monopole antennas.Some CPW-fed PSAs employed mul-tiple ring slots to obtain two or multiple operating bands [8–11],whereas others used only one ring slot or circular slot in conjunction with a back-patch[12,13],a slit back-patch[14]or double T-match stubs[15]to generate two operating bands.Although these antennas can support dual-band operations,they suffer from a relatively small bandwidth for their lower operating bands.Hence,the two operating bands of each of these antennas cannot simul-taneously cover all the ones needed by the multiple stan-dards that include DCS,PCS,UMTS and 2.4/5-GHz WLANs.For achieving two separate frequency bands broad enough to cover completely all the frequency ranges of the abovementioned wireless communication standards, the authors propose a novel CPW-fed PSA with a properly designed composite patch printed on the backside of the square slot in this paper.The presence of the back-patch can lead tofive resonant bands,thefirst(last)two of which are sufficiently closer to each other to form a single wide lower(upper)operating band and the middle of which is not well matched in input impedance.Even though wide enough,these two operating bands may not be located at proper frequency positions to enclose the entire multiple-standard frequency bands of interest.By carefully examining the simulated current distributions on the back-patch and the ground plane of the proposed antenna for the four centre frequencies,one can create notches in the back-patch in a clever way to shift the two operating bands to appropriate frequency positions so that the design goal can be achieved.2Antenna designThe structure of the proposed dual-broadband antenna is shown in Fig.1.The antenna is designed on a square micro-wave substrate with a side length of G,a thickness of h,a dielectric constant of1r,and a loss tangent of tan d. Etched at the centre of the top-side ground plane is a square slot with a side length of L s,which is fed by a 50-V CPW consisting of two identical g wide slots and, in between,a signal line having a width of w f.The signal line of the CPW is protruded into the slot by a length of t, which can be adjusted for optimum impedance matching in the designed operating bands.Printed on the other side of the substrate(opposite to the square slot)is a composite back-patch,comprised of an annular ring and,inscribed to it in theÀy direction,a circular patch.The outer and inner#The Institution of Engineering and Technology2008doi:10.1049/iet-map:20070102Paperfirst received30th April and in revised form12th November2007J.-Y.Sze is with the Department of Electrical and Electronic Engineering, Chung Cheng Institute of Technology,National Defense University,Ta-shi, Tao-yuan335,Taiwan,Republic of ChinaC.-I.G.Hsu and S.-C.Hsu are with the Department of Electrical Engineering, Da-Yeh University,112Shan-Jeau Rd,Da-Tsuen,Changhua515,Taiwan, Republic of ChinaE-mail:jeffszejy@radii of the annular metal ring and the radius of the circular patch are R 1,R 2and r ,respectively.The lowermost point of the composite back-patch,when projected onto the ground plane,is tangent to the lower edge of the square slot.In the antenna design,the first resonance of a convention-al slot antenna without any back-patch occurs when the slot size is approximately half a guided wavelength l g .That is to say,the resonant frequency (f r )of the fundamental res-onant band of a square slot antenna can be estimated byf r (GHz)’300l g ffiffiffiffiffiffiffi1eff p ¼300(2L s )ffiffiffiffiffiffiffi1effp (1)In (1),l g and L s are in the unit of millimetres and the effec-tive dielectric constant 1eff for the slot can be estimated by the so-called zero-order solution [16,17]1eff ¼1þ1r 2(2)which,although crude,is quite useful for practical design purposes.In fact,the above formulas have been confirmed to yield acceptably accurate fundamental resonant-band centre frequencies after extensive experiments.In this design,commercially available FR4substrates (1r ¼4:4,tan d ¼0.02and h ¼0.8mm)are adopted for antenna circuit implementation.Structural parameters,such as G ¼75mm,L s ¼40mm,w f ¼4.2mm and g ¼0.3mm,are fixed throughout the study.The side length L s of the square slot is appropriately chosen so as to excite a fundamental mode with its resonant frequency close to the desired lower operating frequency of 2.3GHz.The annular ring and circular patches that consti-tute the composite back-patch can be considered as parasitic elements of the square slot for exciting additional resonant modes.The annular metal ring can be appropriately designed to excite an additional resonant mode which is close to the desired upper operating band.The circular patch can be used to excite three more resonant modes by properly adjusting the radius (r ).Two of the extra resonant bands can be located close enough to the two exist-ing ones to form two wide operating bands,and,meanwhile,the other extra resonant band can be made poorly impedance-matched.On the basis of the current distri-butions on the back-patch and the ground plane of the pro-posed antenna at the resonant-band centre frequencies,notches can be etched in the annular metal ring at appropri-ate positions to make the two operating bands cover the frequency ranges of the desired wireless communication standards.3Experimental results and discussion3.1Effects of the composite back-patch on return-loss characteristicsIn this study,the return losses of the fabricated antenna pro-totypes were measured using an Agilent N5230A network analyser.The effect of the composite back-patch on return loss of the square PSA is studied in detail in this subsection.The measured return loss of the antenna with the absence of the back-patch (denoted by reference)is shown in Fig.2.The fundamental resonant band of the square PSA is around 2.3GHz.For the case of only an annular metal ring on the back (i.e.no circular patch in the com-posite back-patch)with R 1¼19.5mm (’0.49L s )and R 2¼16.5mm (’0.41L s ),the antenna (designated as Antenna 1)has not only a fundamental resonant band around 2.55GHz,which is very close to that of the refer-ence antenna,but also an additional resonant band around 5.3GHz.In the presence of the complete composite back-patch,formed by adding a circular patch with r ¼13mm (’0.325L s )to the annular metal ring of Antenna 1,the antenna (called Antenna 2)is observed to have three additional resonant bands.Hence for Antenna 2in the fre-quency range of interest,there are five resonant bands in total,which are termed the first,second,delta,third and fourth resonant bands in the sequence of the resonant-band centre frequencies from low to high.The reason why the middle one is called the delta resonant band is that the input impedance in this band is mismatched in the final design.In addition,the radiation patterns in the delta res-onant band were found to have relatively large cross-polarisation levels (not shown in the paper),when compared with those in the other resonant bands.Note that the centre frequency of the first resonant band is lower than that of the fundamental resonant band of the reference antenna,whereas the centre frequency of the fourth resonant band is higher than that of the second resonant band of Antenna 1.Since the first (last)two resonant bands are very close to each other,they combine into a broader oper-ating band with a centre frequency of around 2116MHz (5478MHz)and a fractional bandwidth of up to 30%(15%).These measured data are not only shown in Fig.2,but summarised in Table 1also.The simulated RL curve of Antenna 2shown in Fig.2was obtained by using Ansoft HFSS and appeared to have a trend similar to that of the measured one,except for small discrepancies in the centre frequencies of the two higher resonant bands.Fig.3shows on the ground plane and the composite back-patch of Antenna 2the current distributions re-sketched from the simulated results using Ansoft HFSS.ShowninFig.1Geometry of the proposed dual-broadband square slot antenna with a compositeback-patchFig.2Return losses of the reference antenna and Antennas 1and 2;the structural parameters of the antennas are shown in Table 1Fig.3a is the current distribution at the simulated lowest resonant-band centre frequency(1.96GHz).Each half-guided-wavelength current is distributed in a quarter of the edge of the circular patch and a quarter of the annular metal ring and has a maximum magnitude near the junction of the circular patch and the annular metal ring.The half-guided-wavelength routes of the current dis-tributions are denoted by the dashed lines drawn aside. Figs.3b–3e illustrate the current distributions at the centre frequencies(2.39,5.35,5.93and3.07GHz)of the second,third,fourth and delta resonant bands,respectively. Although the input impedance in the delta resonant band is mismatched,still included here for reference is the associ-ated current distribution,whose half-guided-wavelength routes in the annular ring are shorter than those correspond-ing to second resonant band and longer than those pertain-ing to the third resonant band.It appears that the second and third resonant modes have weaker currents distributed in the circular patch than do thefirst and fourth resonant modes.Hence,the centre frequencies of the second and third resonant bands are,when compared with those of the first and fourth resonant bands,relatively insensitive to the variation of the radius(r)of the circular patch,a phenomenon which can be observed in Fig.4.As r increases,the route of the current distribution for thefirst resonant mode is lengthened,causing the corresponding res-onant band to decrease and separate further from the second resonant band(see Fig.4).The simulated current distri-butions for r¼14(’0.35L s)and15mm(’0.375L s)are not shown here for brevity.These current distributions in conjunction with the ones shown in Figs.3d and3e revealed that for a larger value of r,the routes of the current distri-butions for the fourth and delta resonant modes on the annular metal ring are slightly shorter.Hence,the centre fre-quency of the fourth resonant band increases and separates further from that of the third resonant band as r increases.In addition,the centre frequency of the delta resonant band also increases with r.With r¼14mm,although the antenna is impedance-matched in the delta resonant band, thefirst two resonant bands cannot be combined to form a wide operating band.Likewise,the antenna with r¼15mm havefive discrete impedance-matched resonant bands,none of which,however,can be combined with the adjacent one to constitute a wide operating band.On the other hand,Antenna2(with r¼13mm),although not pos-sessing an impedance-matched delta resonant band,does feature two wide operating bands.Note that the two operating bands of Antenna2cannot completely cover the operating bands of the desired wireless communication standards.To achieve the design goal,the second operating band(consisting of the last two resonant bands)should be greatly lowered,whereas thefirst one (formed by thefirst two resonant bands)can be lowered only slightly.Observe that the current distributions on the annular metal ring for the third and fourth resonant bands reach maxima around the rightmost and leftmost points of the annular metal ring,whereas those for thefirst and second resonant bands around the same points arerelativelyTable1:Characteristics of some antenna prototypes with1r54.4,tan d50.02,h50.8mm,G575mm,L s540mm, w f54.2mm and g50.3mmR1,mm R2,mm r,mm l n,mm w n,mm t,mm f,MHz BW1,MHz(%)f2,MHz BW2,MHz(%)reference—————20.02295622(27.1)——antenna119.516.5———36.32553137(5.4)5347417(7.8) antenna219.516.513——20.02116641(30.3)5748882(15.3) antenna319.516.51310224.42100792(37.7)53871126(20.9)f1and f2are the centre frequencies of thefirst and second operating bands;the impedance band is the frequency range where VSWR2Fig.3Electric current distributions on the ground plane and the back-patch of Antenna2at the simulated centre frequenciesa1960MHz(thefirst resonant band)b2390MHz(the second resonant band)c5350MHz(the third resonant band)d5390MHz(the fourth resonant band)e3070MHz(the delta resonant band)weak.This suggests that embedding notches around those two points in the annular ring will effectively lengthen the routes of the current distributions for the last two resonant bands,but only slightly affect those for the first two resonant bands.When an indent with a depth of w n ¼2mm and a width of ‘n ¼10mm is etched from the annular ring around each of its rightmost and leftmost points (namedAntenna 3),the first (second)operating band with a centre frequency of 2100MHz (5387MHz)and a fractional band-width of 37.7%(20.9%)is obtained,as demonstrated in Fig.5and Table 1.These two operating bands are seen broad enough to support the desired wireless communi-cation standards.3.2Radiation characteristicsThe far-field patterns of Antenna 3in both the yz plane (E -plane)and xz plane (H -plane)at its four resonant-band centre frequencies were measured and are shown in Fig.6.These four resonant modes have broadside radiation patterns with very low cross-polarisation levels and are pretty similar to those (not shown here)of the reference antenna at its fundamental resonant-band centre frequency.These measured radiation patterns were also validated by the ones simulated using Ansoft HFSS and good agreement is shown for the co-polarised radiation.The measured antenna gain shown in Fig.7in the lower operating band ranges from 4to 5dBi,which is very close to that (5dBi)of the reference antenna,whereas the higher operating band has an antenna gain of 5–6dBi.4ConclusionIn this paper,a CPW-fed square-slot antenna with a 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