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Micromachined High Gain Wideband Antennas for Wireless Communications

Mobile and Wireless Communications: Key Technologies and Future Applicationsd the trend now is toward selectivena(usually a conducting patch) and theLower effective dielectric constant, hence wider circuit dimensionsEnhanced radiationEliminating surfaceMicromachining technology continues to develop, and it is being applied in new waysembedded antennasf selective lateral etching basedon micromachining techniques to enhance the performance of rectangular microstrip patchantennas printed on high-inafer such as siliin the past decade A novel polymer micromachining based method for achieving highperformance, cost effective antennas is described in this chapter2 Micromachined antennasOver the lastmicromachiningve been developed fore and millimeter wpatches printedatively high dielectric constant substrates, variomachining methods that habeen implemented recently are listed in the following21 Silicchining has been employed to fabricatet al 1998, Hou et alk, 2005) Examples with both equal and unequal thicknesses of air andsubstrate have been implemented

The micromachined antenna configuration consisted of actangular patch centred over the cavity, sized according to the effective index of the cavityregion, and fed by a microstrip line To produce the mixed substrate cavity region,icromachining was used to laterally remove the material from underneath the patchesulting in two separate dielectric regions ofon the amount ofaried from 50 to 80% of the original substrate thickness underneath themodel was used to estimate the effective refractive index value below the patch The walls ofthe hollowed cavity tend to be, slanted owing to the anisotropic nature of the chemching, and this has to be allowed for in the modelling This antenna has been shownefficiency having been increased by as much as 64% and 28%, respectivelng pd to fabricate an air suspended patchantenna either with supporting metallic posts or polymer pona structuresntechopen

Micromachined high gain wideband antennas for wireless communications323 Directivityof directivity, gain and efficiency for thenna device with reto frequency from 10 GHz to 155 GHz The leftythe magnitude of antenna directivity and gain while the right y axis gives thegnitude of absolute efficiency from 09 to 1 It can be easily seen from the plot the gaincurve follows the directivity curve suggesting almost 100% radiation efficiency within thees from about 5 d bi at 10 GHz to around 83 dBi atithin the radiation bandwidth(12-143 GHz)are below 1 dBi while it varies veryf this frge The radiation efficiencclose to 1 at 155 GHz It should be noted that the radiation efficiency is a meloss, including dielectric and conductor losses, within thened antenna structured it improves dramatically with the introduction of the micromachined air cavityrectivityFig

5 Simulated gain, directivity and radiation efficiency for the optimized micromachinedantenna device33 Stacked patch antenna devicesesign and modelling of the microstrip and CPW fed, stacked apertudevices is presented in the following sections These stacked antenna devices are desttern The bandwidth ised by utilizing the multiple closely resonant structuresng with thfiguration The stackeelements are fabricated on polyimide substrates forstrip fed devices and on LCP (liquid crystal polymer) film substrates for the CPWtions, supported, in all331 Microstrip fed aperture coupled stacked patch antenna3311 Antenna DesignStackedbacked, aperture coupled antenna geometries have been modelledAnsoft HFSS and optimizeddeig- bad pr tides a cross sectional view of a microstrip fedthis section while theperture on the lower microste surface is depicted in Fig 6(b) The arrangementconsists of a double claddedPTFE substrate andpended patches to forntechopen

Mobile and Wireless Communications: Key Technologies and Future Applicationsstacked antenna device, The microstrip feed line on the bottom surface of the microwavepatch through a rectangular coupling aperture in the grourlane which forms the top surface of the microwave substrate The patch elements areprinted on thin film(polyimide)substrates They are supported by micromachined polymerthe antenna loss and hence to improve its gain The cavitiesmer rings thus providing protection against the incursion of moisture andtribute unwanted loLmrr由sU Paymerantenna using micromachined polymer spacers, (b) The top view of the apand feedline on the substrate surfacesParameterMicrostrip fedCPW fed device(mm)tch lengthUpper patch lengthper patch width165Slot widthmicrostrip lengthIvityInner width of cavity18

2Thickness of polymer06Table 2 Summary of the design parameters for the suspended stacked patch antennasThe stacked antenna has been ding a similar approach to that describedreviously for the development of multi-layer stacked wideband antenna devices(Pavuluet al, 2008, Wang and Pavuluri, 2008, Croq and Pozar, 1991) It is designed for operation atntechopen

Micromachined high gain wideband antennas for wireless communicationsX-band (8-12 GHz) with a 40% of bandwidth The height of the air cavities focated between each stacked patch and the substratedjusted for optimum bandwidth while at the same time maintaining a low profile for theoverall antenna structure For this microstrip fed device, the top patch and the aperturedimensionally adjusted tosimilar frequencies, while the dimensions ofthe microstrip line and the lower patch are tuned to secure the best possible impedancematch In the df the patchetermined by theantionalabout 10% of its lengThe heights of the air cavities aained earliera trade-off between the desire forbandwidth and the limitations set by the fabrication process The length of the feedlinethen varied to obtain sufficient bandwidth In order to obtain fixed band performance, theengths of the top patch and the aperture are modified to tune the band of operation Fineions of the lower patch, The shagood antennaeral iterations of the above steps toobtain an acceptable antenna delived wide bandwidth The optimizeddesign parameters are given in Table 2331

2 s parameters and study of the bandwidthof the optimtenna, the return loss and VSWR are plotted as a function of frequency The10 dB returnuantifies the bandwidth, Fivalue of 2 over this range Therefore the theoretical bandwidth of this de428 This is an improvement of a factor of 25 over that of the single patch deviceFig 7 Simulated insertion loss and vSwR parameters for the optimized micromachinedmicrostrip fed stacked aperture coupled antenna3313 3D antenna radiation patternsolution and is plotted as a function of frequency using the far field plotter interface inAnsoft HFSS, Figure 8 shows the 2Dntechopen

Mobile and Wireless Communications: Key Technologies and Future Applicationstenna device at 982 GHz near to the centre frequency (99 GHz) of the operating band Ithat the backFig 8, 2D(a) and 3D(b)radiation patterns at 982 GHz331 4 Directivity and galfrom 7 GHz to 12 GHz, for the microstrip fed stackeda The resultssimilar to those of the single patch antenna( Fig 4), bivariation over the optimum operating band The gain and direcry from about 6 dBat 8 Hz to around 78 dBi at 9

7 GHz and falls back below 6 dBi after 125 GHz Theity and gain are constant to within 2 dBi over the previously defined -10dBbandwidth Outside this bandwidth gain diminishes significantly The radiation efficiency isgreater than 095 from 8 GHz to 12 GHz-EfficiencFig 9 Simulated gain, directivity and radiation efficiency of the optimized micromachined2 CPW-fed aperture couned patchso been studied Fig 10 show the schematics of the cpw fed stacked patch antennangle cladded PTFE substrate(Tacon3-0200-CH/CH) was used to support the CPwline and the coupling aperture The stacked patches are suspended symmetrically above thentechopen

Micromachined high gain wideband antennas for wireless communicationsaperture using micromachined Sus polymer rims As the air gap between the baseest patch is smaller than that of the microstripfed stacked device(Fig 6(a))for ease of fabrication, an additional patch is required to yield asimilar bandwidth of -40% The upper pair of patches has the same dimenm that further increases the bandwidth of the antennaand the top two patches were designed to be in closea/4 stub are optimized for a wTables 2 and 3 show the physical dimensions of the structure layers for the antenna deviceg 10(a)Cross-sectional view of the stacked CPw fed antenna using micromachinedpolymer spacers, (b)topof the corresponding aperture and feed line on the substratebstrate ThicknessDielectric constaLossiconic ptFe00009Table 3 The thickness and mice property of Taconic PTFE and LCP substratesed insertion loss and vSwr parameters for the optimized mCPW fed stacked aperture coupled antenna

332 1 s parameters and study of thereturn loss and VSwR parameters are plotted as a function of frequency from 6 to 105GHz in Fig 11 It can be seen from the plot that the refntechopen

Mobile and Wireless Communications: Key Technologies and Future Applicationsaround 65 GHz to about 10 2 GHz, and the vswr is lower than a value of 2 over this rangending theoretical bandwidth is 443322 RadiatiAs with earlier examples, the radiation pattern for the antenna device is obtained from theelectromagnetic field solutionthe far-field plottnterface in Ansoft HFSS Fig 12the 2D and 3D far field patterns for the aperturene radiation characteristics The backwardn be attributed to the effect ofW based feeding method Thus theCPW feeding approach is redEFig 12 2D(a) and 3D(b) radiation patterns for the CPW fed stacked antenna at 8

GH333 The effect of polymer rim design on the performance of the CPw fed stackedofThe effect of the dimensions of the polymer rim on antennrformance has been studied using the CPW fed antenna design Two rim designs of23mmx23mnmmx18mm are usedantennas are shown in Fig 13 The other design parameters remain thedsTable 2 Fig, 14 shows the efficiency of the CPw fed stacked antenna device for the tw4 the efficiency of ther the larger rim is grethan 095GHz to 105 GHz but itthan 09 with the smalleralsothe roll-off rate of the antenna efficiency above 95 GHz is much faster for the smaller rimdicating rapid decrease of the antenna performancentechopen

Micromachined high gain wideband antennas for wireless communicationsFig 14 Results of antenna efficiency of the CPW fed antenna for different SU8 polymer rim33, 4 Effect of the substrate lossAs the losses in the substrate layers inaffectsignificantly, for comparison, FR4 and PTFE based stacked CPWsimilar dimensions as that of the CPw antenna shein fig 10 are also dptimized for impedance matched performance These two devices consist of 4 layers of FR4PTFE material with 3 three stacked patches, the dielectric constant and thefor the FR4 material are taken as 42 and 0

020 respectively (Aguilarhows the efficiency and gainanction of frequency for the three CPW fed stackedantenna configurations Thedecrease of gain above g ghz of the frd based device isto the increased insertion loss as the frequency is out of the band of operaof the antennas was obtained from the simulation results of the radiabased on the multilayer PTFE structure and thewithd patch elementslarger bandwidth The performance of the FR4 based mullantenna is much poorer%°since the dielectric loss is minimal in both cases However, the micromachined device hasto the well known lossy behaviour of the FR4 material beyond the microwavemmary of the performance parameters The micromachinedantenna device with suspended patches showed the best bandwidth of about 38% close tohat required for ultra-wide band applicationsntechopen

Micromachined high gain wideband antennas for wireless communicationsdifferent frequency bands require different air cavity thickness to achieve optimum antennace and better impedance matching Photoresist based polymers such as SU8N can be used to obtain ultra thick supporting posts analso be used as mouldsfor electroplating metcromachining methods have beenplemented in the past ( Ryo-ji and Kuroki, 2007) A CPW fed post supported patchantenna has been fabricated on a Corning 7740 glass substrate which had a thickness of 800m and a dielectric constant of 4,6 Copper was used for metallization The feed line of theantenna was patterned with the thick photoresistas performed to form the posts of thena with a thick photoresist of THB15IN Aa nulated antenna gain in the range of 56 dBi to 90 d Bi and the radiation efficiency varyingemonstrated for single patch antennas In the case of aarray patch antenna, the simulated antenna gain and the radiation efficiency were from

58dBi to 112 dBi and from 936% to 953 %, respectivelySU8, a widely used negative tone photoresist, has been used to fabricateelevated patchantenna with micromachined posts of around 800 um of height(Pan et al, 2006; Bo et al2005) have successfully demonstrated an air-lifted patch antenna fabricated usingicromachining technology Both metal posts and polymer posts were used to providGHzd The proposed stentional patchbandwidth, efficiency and lower side lobe level While the traditional patch antenna directprinted on substrate usually gives a 3%-5% bandwidth and 70%0-80% radiationhe proposed elevated patch will double the fractional bandwidth and gives a theoretical7% radiation efficiency

This is achieved by eliminating the substrate loss Lrmittivitypin-on dielectric substrates are efficient for guiding microwaves and millimetre wavesWang et al, 2005) and they have been used for micrefilters to impIthe insertionloss of devices fabricated on silicon substrates Leung et al, 2002)3 Millimeter wave antennas using low permittivity dielectric substratessing low permittivity dielectric substrate have widerbandwidth andhigher gain whennic dielectric substrates Tong et al hayk dielectric substrate (Tong et al, 1995) The antennthe bottom, two layers of BCB dielectric substrate (Er=27 andfan6-0002 20GHz) in the middle and a cPa pattern on the top the total thickness of theBCB layer is 30 um, Fluid state BCB is spun onto a 3-inch ground plane coated silicon waftground plane and the CPa pattern are both about 15 um Theated andbandaidctively The measured resonantfrequency of the antenna is 383 GHz Micromachining techniques employing closely spacedholes have been used underneath a microstrip antenna on a high dielectric-constantize a localized low dielectric-constantal, 1997) The holes are drillednumerically controlled machine(NCM) and extend atleast 35 mm from the edge of the antenna in all directions andy the full substratentechopen

Mobile and Wireless Communications: Key Technologies and Future Applicationsheight The measured radiation efficiency of a microstrip antenna on a micromachinedsubstrate(Duroid 6018)increased from 483%o to 73 3% at 128-130 GHincluding the loss with a 33 cm long feed line2 4 Integrated chip-size antennas using laser micromachiningpactze and owing to the relatively high dielectric constant Mendes et alnetal sheets that are electrically connected bertical metal walls All this isembedded in a glass substrate having defined electrical permittivityThe antenna was designed to operate at 51 GHz, a frequency chosen to be inside the 5-6GHz ISM band

The fabricated antenna has dimensions of 4mmx4mmxlmasuresg frelulated radiation efficiency of 60% A method of applying lasertechnologies to fabricate a compact, high performance and low-cost 3D monopole antennaas proposed by Huang et al(Huang et al, 2005) The coplanar waveguide(CPw) fedconfiguration was used owing to its simple structure, wide bandwidth, and the ability of5 LTCC micromachiLTCC multilayer technology can be used to build up antenna arrays because it provides thedration for the high-density microwavet andPackage)(Wolff, 2007, Baras and Jacob,al, 2007) To optimise the material properties by reducing theLTCC, a material modulation procedure based on punching air holes into the substrate isperformed Thereby, the relative permittivity of the material is replaced by the effectivelative permittivity er of the modulated material Schuler et al 2003)3 Antenna design aelectronic systems is the patch antenna shown in Fig 1 This type of antenna can be excitedin one of four ways(Pozar, 1992, James and Hall, 1989, Bahl and Bhartia, 1980):(a)directlimplementation of multi-layered formats and consequently the section is directed towardassessing this geometrelation to the micromachining of such structuresa)Microstrip feedntechopen

Micromachined high gain wideband antennas for wireless communications(c) Coaxial feeFig,1Ⅲ lustratef feeding methods for microstrip antennasdescribo np and aperture fed stacked patchdevices have been mng an electromagnetic simulatisses will be described These aperture coupled devices are impedancetched for wideband operation RFperation for the devices and the resultsod agreement with that of simulation Thegain and bandwidth are determined to be 78 dBi and% for a microstrip fed antennadevice while they are 7

6 d Bi and 38% for a CPW fed device31 Introduction to aperture coupled patch antennafeed dimensions, soldering of probes associated with the classical feeding(Fig 1 (c)) or edge feeds(Fig 1 (a)and (b) Thentechopen

Mobile and Wireless Communications: Key Technologies and Future Applicationsore important for widebandns,which require thicker substrates On the otherand, the aperture coupled feeding technique( Figmake it an attractive feature for millimeter wave applications Wide-band operation of thistype of microstrip fed antenna has been demonstratedfrequencies using either single or stacked patch configurations Although all of the couplingmethods depicted in Fig I have been shown to give excellent bandwidth characteristics, thedirect methods(Fig 1(b)and (d)) give rise to a high back-radiation level However thonly true for aperture coupling(Fig 1(d)) if the apealso be fed with a cpw feeddeal at millimeteraL, 2004) It has been found that theseantennas canly be impedance-matched by tuning the dimensions of the excitationaperture and adding a small tuning stubrue for an aperture coupled configuration including thoseummarized pr985)

They are listed beleThe configuration is suited fowhere active devicintegrated on, for example, a gallium arsenide substrate with the feed network, andpled to the feed network through apertures in the ground plane separating therates Thebstrates avoids the deleterious effect of a higldielectric-constant substrate onbandwidth andperff a printedthe ground plane separates the two mechanismsiii) No direct connection is made to the antenna elements, so problems such as larprobe self reactances or wide microstripline(relative to patavoidedv) Ideal for micromachined antennas The fabrication of a directly coupled feed probe(v) The aperturech make it)Wide-band operation of this type of microstrip antenna has been detratedked patchconfigurations(vii) A simple aperture coupled antenna structure gives rise to a high back-radiation leveldopting a stackedantenna configurationconsists of a radiating patch on one substrate coupled tostripline feed on anothersubstrate, through an aperture in the intervening ground plane It should be notedthat the aperture coupled microstripna can be used for both linear and circularlarizations It requires two co-located orthogonal apertures each one excited by a differenorthogonal linearly polarized resonances under the normallyquare patch The polarf the radiation from the patch is then dependent on theive phase of the signals entering the independent feed linesntechopen

Micromachined high gain wideband antennas for wireless communicationsCircular polarisation is obtained when the signals are equal in magnitude and in quadratureentionally fed patch antennas(Fig 1(a)to(c)) it is well known that tpproximation the patch resonant frequency is dictated largely by the size and shape of thepatch This is not the case in aperture coupled patches The aperture also hasby the aperture and the patchfrequency determined by simple filter theory In thisdied in depth We thencromachined aperturenna devices fed both from microstrip and CPw line Antennamances are assessed for various antenna configurations Reflection coefficienVSWR, normalise radiation pattern, gain, directivity and efficiency parameters are presentedth the results plottea function of frequency asnecessary The effects of these different design parameters on the antenna performance a3

2Mchined aperture coupled patchproved by introducing a micromachinedaperture coupled antenna devices which apin ensuing sections are produceddesign method is provided in order to communicate the fundamental operating principl, e%PCB substrate Themicromachined polymer ringfull-wave simulation software package is employed to design the antenna devices anddjust them for optimum performance The effects of the design parameters, such asbstrate material, air gap thickness, polymer rim dimensions and conductor materials,etic behaviour are investigated The performances of severamachined antenna dare compared Microstrip fed, CPW fed, single and stackedtenna configuratAn aperture coupled antenna structure can be effectively modelled by means of a range ofTransmline model (TLMel (FEMinite difference time domain technique(FDTD)Method of moments technique(MOM)All of these techniques exist in commercial packages, The general purpose modellingckage, ANSOFT HFSS, is based on the finite element method, while CST Microstripased on the transmission -line matrix (TLM) method in time domain form The IE3Dploys the method of moments All are suitable for the kind of micromachinedantenna devices described in this chapter The antennas presented here are all modelled andoptimised in the ANSOFT HFSS design environmentcked patch antenna design, realizedspacer, is quite similar to the strip-slot-form- inverted patch(SSFIntechopen

Mobile and Wireless Communications: Key Technologies and Future ApplicationsFig 2 shows a schematic of the cross-sectional and the top views of this potentially highpolymer ring(SU8 rimivity between the substrate and the polyimide thin film The cavity-backed aperturethe antennadevice The configuration also improves the bandwid th of the antenna device owing to theproximity of the resonances of the coupling aperture and the patchPatchSiot h te grand plane(b)Fig 2

Geometry of the (a) cross-sectional viewingle patchmicromachined aperture coupled antenna deviceThe antenna centre frequency depends primarily on the dimensions of the resonant patchelement and is given b2where fo is the centre frequency of the antenna, c is the speed of light, Leff is the effectivelength of the patch element andthe effective dielectric constant, Thee at around 12 GHz for ease of characterization using in-housemeasurement facilities The design parameters such as the dimensiof the microstripeed, aperture and top patch were optimized using the Ansoft HFSS electromagneticsimulation package with the aim of achieving high antenna radiation efficiencyntechopen

Micromachined high gain wideband antennas for wireless communicationsUntch lengthatch widttch thicknesstyThickness of polyimide filmThickn15PCB substrateof the design parameters for the microstrip fed sugh gain In the HFSS simullateral dimensions of the polymer rim were chosen to achieve high efficiency andfor the antenna structure table 1he design parameters of thedevice a microwave pTFE materi00a, from arLon meD (httP: //wwwctsindcomsg/arlonhtmd as the base substrate while a polyimide thin filupporting sulfor the suspended patch

The dielectricstant and loss tangent of the PTFE substrate are respectively 3 and 0003 For the SU8 rim998)and 0042 (Lucyszyn, 2001)respectively, while theesponding values for polyimide substrate material are 35accurately model the performance oftenna device, the parasitic effects of the SMaonnector are simulated by introducing a short and widthe inpuhe extensionatch the length and diameter of the pin of the SMAIt has been observed thatnmodating the effect of the connector is vital in order to accurately model the reflectionharacteristics of the suspended patch antenna In the following sections the Sn parametersfor estimating the bandwidth, normalised radiation pattern and radiation efficiencyobtained from the HFSS designer environment are presented and discussed321 s parameters and study of thethe perfand VSWR(voltparameters are plotted for an optimFintechopen

Mobile and Wireless Communications: Key Technologies and Future Applications,0g 3 Simulated return loss and the vSwr parameters for the optimized micromachinedperture couplThe return loss and vswr parametersthan a value of 2 in this range Therefore therresponding theoretical bandwidth of the antenna is 23 GHz or 17%32

2 3D antenna radiation pattertenna device is obtained from the electred as a function of frequency from the far field plotter interface inAnsoft HFss4 shows the 2d and 3D far-fieldpatterns for the aperturepatterns show that there is high backward radiation and obvious side lobes in the e planeBut it will be shown in the later sections that the side lobes and back side radiation areked patch configuration惠2m(a)Fig 4 2D(a)and 3D(b)radiation patterns of a microstrip fed single patch antenna device a32 GHzntechopen