Conformal Double Exponentally Tapered Slot Antennas (DETSA) for UWB Communcatons Systems Front-Ends Symeon Nkolaou, Luca Marcaccol, George E. Ponchak, Senor Member, IEEE, John Papapolymerou, Senor Member, IEEE and Manos M. Tentzers, Senor Member, IEEE Abstract Ths paper dscusses the use of a Double Exponentally Slot Antenna (DETSA) fabrcated on flexble Lqud Crystal Polymer (LCP) as a canddate for Ultra Wde Band (UWB) communcatons systems. The features of the antenna and the effect of the antenna on a transmtted pulse are nvestgated. Return loss and E and H plane radaton pattern measurements are presented. The return loss remans below -1 db and the shape of the radaton pattern remans farly constant n the whole UWB range (3.1-1.6 GHz). The man lobe characterstc of the radaton pattern remans stable even when the antenna s sgnfcantly conformed. The major effect of the conformaton s an ncrease n the cross polarzaton component ampltude. The system: transmtter DETSA - channel - recever DETSA s measured n frequency doman and shows that the antenna adds very lttle dstorton on a transmtted pulse. The dstorton remans small even when both transmtter and recever antennas are folded, although t ncreases slghtly. Index Terms Double Exponentally Slot Antenna (DETSA), Lqud Crystal Polymer (LCP), conformal antenna, UWB, ultrashort pulse, dstorton I. INTRODUCTION The rapd developments n broadband wreless communcatons and the great number of commercal and mltary applcatons necesstate new types of antennas whch can support hgher bt rates. The Ultra Wde Band (UWB) [1] protocol usng the spectrum from 3.1 GHz to 1.6 GHz s a new promsng technology sutable for hgh rate communcatons n small dstances. In ths paper, a double exponentally tapered slot antenna (DETSA) on flexble LCP organc materal sutable for packagng and ntegraton wth other components s ntroduced and proposed for the UWB range wth gan above 7 db and return loss below -1 db for the whole frequency range. The DETSA s a varaton of the Vvald antenna, wth the outer edge exponentally tapered, and t was ntroduced for the frst tme n [2] wth desgn and performance characterstcs dscussed n [3] and [4]. A coplanar wavegude fed verson of DETSA s explored n [5] for a UWB sub-band, whle n [6] a Vvald antenna s proposed for the ultra wde band. For the qualty of the receved pulse tme doman measurements can be used [7]-[8] but n the present work we use a frequency doman measurement to estmate the dstorton caused by the DETSA. II. ANTENNA DESIGN The proposed antenna was fabrcated on a 2 µm thck Lqud Crystal Polymer (LCP) substrate wth an 18 µm thck copper layer. The DETSA schematc s presented n Fg. 1. The length of the board s L=13.62 cm, the wdth s D=6.64 cm, and the slot gap at the feedng pont s 1 µm wde. LCP was preferred because of a number of desrable features. The delectrc constant ε r =3.1 s low enough to be used for an endfre antenna, t has low loss (tanδ=.2) whle beng conformal, and t s easy to fabrcate wth an engneered Coeffcent of Thermal Expanson (CTE) [9]. Standard photolthography was used for the fabrcaton. The desgn dmensons have been optmzed for the antenna to be matched over a frequency range of 3 GHz to 11 GHz. z x y Manuscrpt receved March 31, 25. Ths work was partally supported by the Georga Electronc Desgn Center and partally by the US Army Research Offce. S. Nkolaou, J. Papapolymerou and M. Tentzers are wth the Georga Insttute of Technology, Atlanta, GA 332 (phone:44-385-641, fax:44-894-222, e-mal:smos@ece.gatech.edu). G. E. Ponchak s wth NASA Glenn Research Center, Cleveland, OH 44135. Luca Marcaccol s wth the Department of Electronc and Informaton Engneerng, Unversty of Peruga, 6125, ITALY Fg. 1. DETSA schematc. E and H planes are defned
III. RETURN LOSS AND RADIATION PATTERN MEASUREMENTS A. Return loss measurement For return loss and radaton pattern measurements, an SMA connector was soldered drectly on the slot wth no tunng components. The contour that descrbes the projecton of the folded antenna surface on the H plane s gven by (1) where x s measured n cm. The antenna was attached on a pece of styrofoam wth the same curvature to provde mechancal support and stablty. 2 3 C( x) =.48 + 1.1x.121x +.4x (1) x 12.5 The smulated and measured return loss s presented n Fg. 2, where t can be seen that good agreement s acheved between smulated and measured return loss, that there s no dfference between flat and folded, conformal, antenna return loss, and the return loss remans below -1 db n the whole UWB range. The observed saw pattern s expected because DETSA s a travelng wave antenna, not a resonance radaton element. For the frequences n the 8 GHz to 1.6 GHz range the return loss s below -15 db. In those frequences relatvely hgher gan s measured. The planar and folded antennas are shown n Fgs 3 and 4 respectvely. -5-1 dstorton n the radaton patterns. Generally the ncrease n frequency n end-fre antennas causes the man beam to become narrower (reduced beamwdth) and the drectvty to ncrease. The y axs n Fg.1 corresponds to φ=9 o for the E plane cuts and to θ=9 o for the H plane cuts. x axs corresponds to φ= o and z axs to θ= o. For the folded antenna, the x-z plane s located at the SMA connector and the y axs s along the drecton of the SMA adapter. Thus, whle E and H plane cut defntons do not change, the DETSA antenna s not n the x-y plane as shown n Fg. 1. Normalzed radaton patterns at 6 GHz, close to the band center are presented. Co and cross polarzaton for planar and folded antennas are presented n Fgs 5 and 6 respectvely. The H plane beam s slghtly wder and wth fewer sde lobes compared to the E plane beam at ths frequency. There s a small asymmetry n the folded antenna pattern whch s due to the cable drecton whch was not algned wth the drectvty drecton as t was for the planar antenna measurement. Generally though, the shape of the beam does not present great varatons. The cross-polarzaton level for the planar antenna s below -2 db, and t s below -1 db for the folded antenna. Ths 1 db ncrease n cross-polarzaton E feld magntude s the major effect caused by foldng. The man beam characterstc of the radaton pattern remans unchanged as the frequency ncreases. Ths s a major advantage of ths antenna, wth respect to compettve UWB desgns [1]-[11] for whch the shape of the beam changes sgnfcantly whle the frequency ncreases. The gan at 6 GHz s measured at 9.8 db. -15 S11 (db) -2-25 -3-35 Smulaton Measurement Measurement folded -4 2 4 6 8 1 12 Frequency (GHz) Fg. 2. Return loss smulaton and measurement for planar and folded antenna. B. Radaton pattern measurements The far feld radaton pattern measurements were performed n the E and H planes defned n Fg. 1 wth the drectvty drecton beng the ntersecton of the two planes. To expermentally characterze expermentally the DETSA antenna, t s sandwched between two ½ nch thck styrofoam peces to gve t enough mechancal stablty for the measurements. The antenna s tested n a far-feld range wth the DETSA as the receve antenna, and a 2 to 18 GHz, rdgedrectangular horn antenna wth a gan of 6 db at 2 GHz and 12 db at 18 GHz s used as the transmttng antenna. After expermentally determnng the transmtted power to maxmze the detector senstvty wthout saturatng t, the system s calbrated. The rotary stage and the detector voltage recordng from the lock-n amplfer are automated. The man problem wth wdeband antennas s the electrcal length varaton wth frequency, whch causes sgnfcant Fg. 3. Planar DETSA antenna connected to the HP 853A Network Analyzer Fg. 4. Folded DETSA antenna connected to the HP 853A Network Analyzer
Fourer transform S(f). The pulse detected at the recever has a Fourer transform R(f)=S(f)H(f). The transmtted pulse s(t) and the receved pulse r(t)=f -1 {R(f)} are correlated to get an estmaton of the dstorton added by the DETSA UWB communcatons system. The dstance between the two antennas s long enough to guarantee a far feld measurement and at the same tme, small enough that s reasonable to assume that dstorton of the early response by channel can be neglected. Fg. 5. E and H plane far feld measurements for the planar DETSA. Both co and cross polarzaton are presented. Fg. 6. E and H plane far feld measurements for the folded DETSA. Both co and cross polarzaton are presented. IV. FREQUENCY DOMAIN MEASUREMENT FOR DISTORTION ESTIMATION A. Setup To get an estmate of the effect of the antenna on a transmtted pulse, the setup n Fg. 7 was used. Two antennas are used, one as a transmtter and the second as a recever. The two DETSA antennas are algned for optmum recepton and the dstance between them s set at 1.5 m. The mnmum far feld dstance gven from R=D 2 /λ s calculated at.65 m when D=13.6 cm and λ for 1.6 GHz are used. Both antennas are connected to an HP 853A network analyzer and S 21 measurement s taken. The antennas planes are kept perpendcular to the ground as they appear n Fg. 3 to avod the use of mechancal support wth styrofoam. The measurement s repeated when both the antennas are dentcally conformed. The whole process takes place n a laboratory envronment that resembles an actual communcatons envronment more realstcally compared to a potental measurement n an anechoc chamber. The measurements are presented n Fg. 8 for both planar and folded antennas. The msalgnment, as a result of the foldng, causes hgher fadng for the hgher frequences (8-1.6 GHz) due to the respectve hgher drectvty. B. Dstorton estmaton The S 21 measurement represents the transfer functon H(f) of a black box that conssts of the transmtter DETSA the 1.5 m laboratory envronment channel and the recever DETSA. Any tme doman pulse s(t) to be transmtted has a Y Transmtter Antenna Network Analyzer H(f) Y Recever Antenna Fg. 7. Setup for the system: transmtter - channel recever, frequency doman measurement. S21 (db) -2-4 -6-8 -1-12 Planar Antennas Folded Antennas -14 2 4 6 8 1 12 Frequency (GHz) Fg. 8. S 21 measurements for the system: transmtter - channel recever. We nvestgate the effect on an deal rectangular pulse of duraton of 1 nanosecond. The spectrum for a rectangular pulse s nfnte. We compare the receved pulse wth the equvalent tme doman pulse that s created when the nfnte baseband spectrum s modulated at (6.65=(1.6-3.1)/2) GHz and fltered from an deal passband flter wth passband range at 3.1-1.6 GHz. The supermposed pulses are presented n Fg. 9 and very good agreement s observed. The folded antennas cause stronger dstorton than the planer antennas but stll the maxmum correlaton value, estmated wth functon (2), results n.96 for the folded antennas and.99 for the planar antennas. ( x () mx)( y ( d) my) ) Corr( d ) = (2) ( x ( ) m) ( y ( ) m) 2 2 x y
x() and y() are the compared waveforms, m x and m y are the respected mean values. The rectangular pulse s an deal pulse and s not used n practce for any communcaton systems because of the huge spectrum requred. Therefore we use a rased cosne pulse to nvestgate the same effect. The rased cosne pulse has a spectrum (3) that causes zero ntersymbol nterference (ISI) and s a popular pulse for several modulatons. The tme doman pulse as a result of the spectrum descrbed by (3) s gven by (4): ( ) Ts f 1 a /2Ts Ts πts Ts (3) Hrc = 1sn f (1 a)/2ts f ( 1 a) /2Ts 2 a 2 + ( 1 + a) /2Ts f 1 sn πt/ Τs cos aπ t/ Ts src = F { HRC ( f )} = (4) 2 2 2 πt/ Τs 1 4 a t / Ts For the pulse presented n Fg. 9 Ts = 1/BW was used, where BW=(1.6 3.1) GHz and the roll-off factor s a=.5. The pulse descrbed from the analytcal expresson (4) s plotted wth the normalzed nverse Fourer transform of the pulse s spectrum multpled by the measured H(f) and very good agreement s deduced (Fg. 1). The maxmum correlaton value s estmated at.98 for the planar antennas and.95 for the folded antennas. 1.8.6.4.2 transmtted tme doman receved tme doman reveved folded -2-1 1 2 Tme (nanoseconds) Fg. 9. Tme doman rectangular pulse. The sold lne pulse s a rectangular pulse wth 7.5 (1.6-3.1) GHz bandwdth that s used as a transmtted pulse. The duraton of the pulse s only 1 nanosecond. Small dstorton s added by the system: transmtter channel recever. The measurement and the mathematcal analyss of the data clearly ndcate that very lttle dstorton s ntroduced by the antennas, and the antennas practcally do not affect n a destructve way the transmtted pulses. Ths s manly due to ther broadband performance (3.1-1.6 GHz) and hgh drectvty. 1.8.6.4.2 transmtted tme doman receved tme doman reveved folded -.2-1 -.5.5 1 Tme (nanoseconds) Fg. 1. Tme doman pulse descrbed by (4) supermposed wth dstorted pulse from both planar and folded antennas. The pulse has theoretcally nfnte duraton because of the confned spectrum but t decays wth 1/t. V. CONCLUSION A DETSA on flexble organc materal (LCP), sutable for conformal packagng and ntegraton wth other components, s ntroduced and proposed for the UWB range. The measurements agree farly well wth the smulatons and ths antenna s proved to perform well n the whole UWB range. Return loss below -1 db s measured for the whole frequency range of operaton both when the antenna remans planar and when s folded. The antenna performs wth hgh gan startng from 7 db for low frequences and up to 12 db for the hgher frequences. Frequency doman measurements ndcate that the antenna, when s planar or even when s sgnfcantly folded causes mnmum dstorton to any transmtted pulse. The antenna on LCP s conformal, can be easly fabrcated wth relatvely low cost and s a good canddate for a number of UWB applcatons. REFERENCES [1] M. Z. Wn and R. A. Scholtz, Ultra-wde bandwdth (UWB) tme hoppng spread-spectrum mpulse rado for wreless multple access communcatons, IEEE Trans. Commun., vol. 48, no. 4, pp.679-689, Aprl 2. [2] J. J. Lee and S. Lvngston, Wde band bunny-ear radatng element, IEEE Antennas Propagaton Socety Internatonal. Symposum. pp. 164-167., July 1993, Ann Arbor, MI. [3] M.C. Greenberg, L.L. Vrga, Characterzaton and desgn methodology for the dual exponentally tapered slot antenna IEEE Antennas and Propagaton Socety Internatonal Symposum, Volume: 1, pp:88-91, July 1999, Atlanta, GA. [4] M.C. Greenberg, K.L. Vrga, C.L. Hammond, Performance characterstcs of the dual exponentally tapered slot antenna (DETSA) for wreless communcatons applcatons IEEE Transactons on Vehcular Technology, vol.52, no.2, pp.35 312, March 23. [5] Yo-Shen Ln, Tzyh-Ghuang Ma, Shyh-Kang Jeng, Chun Hsung Chen, Coplanar wavegude-fed dual exponentally tapered slot antennas for ultra-wdeband applcatons, Antennas and Propagaton Socety Symposum, Volume:3, pp. 2951 2954, June 24, Monterey, CA. [6] Sang-Gyu Km and K. Chang Ultra wdeband exponentally-tapered antpodal Vvald antennas IEEE Antennas and Propagaton Socety Symposum, Volume: 3, pp. 2273 2276, June 24, Monterey, CA.
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