A COMPACT, LOW-PROFILE, ULTRA-WIDEBAND AN- TENNA UTILIZING DUAL-MODE COUPLED RADIA- TORS

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1 Prgress In Electrmagnetics Research B, Vl. 50, , 2013 A COMPACT, LOW-PROFILE, ULTRA-WIDEBAND AN- TENNA UTILIZING DUAL-MODE COUPLED RADIA- TORS Meng Li *, Yazid Yusuf, and Nader Behdad Department f Electrical and Cmputer Engineering, University f Wiscnsin-Madisn, 1415 Engineering Drive, Madisn, WI 53706, USA Abstract In this paper, we present a lw-prfile, cmpact, ultrawideband antenna that uses a set f clsely cupled radiatrs. The system f tw cupled radiatrs has tw different linearly independent mdes f peratin with cmplementary frequency bands f peratin. These include the differential mde and the cmmn mde f peratin. When the antenna is excited in the cmmn mde f peratin, it acts as an ultra-wideband (UWB) antenna cvering a brad frequency band. When excited in the differential mde, the antenna perates as a wideband diple in a frequency range belw that f the cmmn mde. Thus, by apprpriately cmbining the tw mdes using a suitably designed feed netwrk, the bandwidth f the antenna can be extended and its lwest frequency f peratin is reduced. Mde cmbining is achieved with a feed netwrk that emplys a frequency-dependent phase shifter. Using this feed netwrk, the tw mdes f the antenna are cmbined and a single-prt bradband device is achieved that has a bandwidth larger than that f either the cmmn r the differential mde individually. A prttype f the antenna is fabricated and experimentally characterized. 1. INTRODUCTION Ultra-wideband (UWB)antennas have been widely used in many areas such as high data-rate wireless cmmunicatin systems, sensing, radar systems, and micrwave imaging [1]. In mst cases, UWB antennas are required t be impedance matched and radiate efficiently ver a large frequency span. In many applicatins (e.g., military cmmunicatins at HF, VHF, and UHF frequency bands), ultra-wideband antennas Received 7 March 2013, Accepted 8 April 2013, Scheduled 10 April 2013 * Crrespnding authr: Meng Li (mli42@wisc.edu).

2 236 Li, Yusuf, and Behdad that can efficiently radiate vertically-plarized waves are required. Mnple whips are the mst widely used types f antennas fr such applicatins. Hwever, mnples are very high-prfile structures that prtrude significantly frm the surfaces they are munted n. In many applicatins (particularly military systems), this is highly undesirable. Additinally, in such cmmunicatins applicatins, the antennas are usually munted n cmplex platfrms in the presence f ther antennas and large scatterers. In such a rich scattering envirnment, the radiatin patterns f the antenna are significantly affected by the platfrm it is munted n and by the interactin between the antenna and the platfrm. Cnsequently, the radiatin patterns f mst cnventinal antennas will change significantly when they are munted n the platfrm (e.g., a mnple antenna will n lnger be an mni-directinal radiatr when munted n a vehicle in clse prximity t ther scatterers). Thus, in develping UWB antennas fr such applicatins, achieving high radiatin efficiency and bradband impedance matching has a higher pririty than achieving cnsistent radiatin patterns acrss the entire band f peratin f the antenna. Develping cmpact and lw-prfile UWB antennas has been the subject f intense study recently. The size f a UWB antenna is mainly determined by its lwest frequency f peratin. In general, the maximum linear dimensin f a UWB antenna is inversely prprtinal t its lwest frequency f peratin. Therefre, fr a UWB antenna that is required t cver very lw frequencies, such as VHF and UHF bands, the size f the antenna can be prhibitively large. Therefre, the demand fr cmpact UWB antennas with high radiatin efficiency and bradband impedance matching is mre then ever felt. Varius UWB antenna miniaturizatin techniques have been prpsed in the past. Many f the existing UWB antennas are in the frm a mnplelike radiating structures with tp-laded capacitive plates and internal matching elements (e.g., shrting pins r series inductrs). One f the earliest antenna designs that falls int this categry is the Gubau antenna [2], which perates ver a bandwidth f abut an ctave (a VSWR f 3 : 1 is cnsidered as the criteria f frequency f peratin) within a cylindrical vlume with the diameter f 0.18λ min and the height f 0.065λ min (λ min is the free-space wavelength at the lwest frequency f peratin). Later n, antennas with similar perfrmance levels were presented by Friedman [3] and Nakan et al. [4]. All f these antennas share similar tplgies where tp-laded hat and shrting pins are included within the antenna structure. Despite the impressive perfrmance f these antennas, extending their bandwidths at the lwer ends f their perating bands (withut increasing their ccupied

3 Prgress In Electrmagnetics Research B, Vl. 50, vlumes) has prven t be extremely difficult. Mre recently, a number f research grups have tackled this prblem by using resistive lading r lading the antenna with lssy ferrite materials [5, 6]. While this can be used t reduce the lwest frequency f peratin f the antenna and achieve gd impedance matching at the lwer end f the band, this cmes at the expense f sacrificing the antenna gain and radiatin efficiency. Thus, develping techniques that culd be used t reduce the lwest frequency f peratin f a UWB antenna, withut increasing its ccupied size and using lssy materials, is f great practical interest. In this paper, a new technique fr reducing the lwest frequency f peratin f a UWB antenna is intrduced that des nt use lssy materials r increase the ccupied vlume f the structure. The prpsed technique is based n using tw linearly independent mdes f a radiating structure that have cmplementary frequency bands f peratin. The antenna is cmpsed f tw clsely cupled radiatrs. When the radiatrs are excited in phase, they act as a cupled lp antenna, which perates ver an extremely bradband frequency range extending frm 600 MHz t at least 4 GHz. When the radiatrs are excited with a 180 phase shift, hwever, the antenna acts as a bradband diple cvering the frequency range f 400 MHz 600 MHz. Thus, in its differential mde f peratin, the antenna perates ver a frequency band that is cmplementary and (mre imprtantly) belw that f the cmmn mde. Thus, by apprpriately cmbining these tw mdes, the lwest frequency f peratin f a cupled lp antenna can be reduced significantly, withut increasing its ccupied vlume. The antenna shape is ptimized t ensure that the tw mdes f peratin will radiate vertically plarized waves. The tw radiatrs are fed using a pwer divider/phase shifter feed netwrk that is designed t feed the antenna in the apprpriate mde f peratin based n the frequency f the excitatin signal. The cncept f such dual-mde antenna was first prpsed in [7, 8]. Hwever, in these prir wrk, n attentin has been paid t the radiatin perfrmances f the prpsed antenna with dual-mde peratin. In what fllws, we will carefully examine each f these tw radiatin mdes and describe the design f the feed netwrk. The resulting antenna is demnstrated t have a larger verall bandwidth cmpared t what can be achieved by using each mde individually. Because, the tw mdes f the antenna have different radiatin patterns and the feed netwrk is nt ideal, the radiatin patterns f the prpsed antenna change acrss its entire band f peratin. Hwever, the antenna maintains high radiatin efficiency, brad impedance match, and cnsistent plarizatin acrss the entire band. The fact that the radiatin patterns f the antenna change as a functin f frequency must be taken int accunt in determining

4 238 Li, Yusuf, and Behdad whether r nt this design wuld be suitable fr a given applicatin. It is expected that in certain applicatins (e.g., antennas perating in rich scattering envirnments r fast changing channels), this issue will nt be a majr surce fr cncern. The benefit f the prpsed UWB antenna with cmbined mdes f peratin is that the lwest frequency f peratin f the UWB antenna can be significantly reduced withut increasing its ccupied vlume. Such benefit ffered by the cmbined mdes f peratin can be best illustrated by cmparing the size f prpsed antenna with that f the previus existing antenna. This cmparisn is cnducted by evaluating the electrical height f the antennas at their respective lwest frequency f peratin. The electrical height is 0.065λ min fr the well-knwn Gubau antenna [2], 0.071λ min fr the Nakan antenna [4], and 0.053λ min fr the cupled sectrial lp antenna in [9]. In cntrast, the prpsed antenna has an electrical height f nly 0.036λ min and a lateral dimensin f 0.24λ min 0.24λ min at its lwest frequency f peratin, and achieves a VSWR lwer than 3 : 1 ver an extremely brad frequency band. A prttype f the prpsed antenna is fabricated and measured, and its radiatin perfrmances, including radiatin pattern, realized gain and efficiency, are experimentally characterized. The cnstraints and challenges assciated with the prpsed design are discussed thrughly in the fllwing sectins. 2. ANTENNA DESIGN AND PRINCIPLES OF OPERATION Figure 1 shws the three-dimensinal tplgy f the prpsed antenna. The antenna is cmpsed f tw clsely-cupled lps, each f which is in the frm f a bent diamnd-shaped cnductr munted n tp f a finite grund plane. Each lp antenna is shrt circuited at its end t the grund plane and fed at the center. The tw lp radiatrs are laded with pentagn-shaped tp hats which prvide capacitive lading that reduces the lwest frequency f peratin f the lps. In general, these tw cupled lp radiatrs d nt have t be identical t each ther. As can be seen frm the side view f the antenna shwn in Figure 1(b), the tw lp radiatrs are individually fed using tw caxial cnnectrs. Depending n the relative phase f the excitatin between the tw prts, the antenna can be perated in either the cmmn mde r the differential mde. In an ideal case, such mde selectin can be dne by using a feed netwrk cmpsed f a pwer divider and an ideal phase shifter as shwn in Figure 1(c). In an ideal perating scenari, the phase shifter prvides a 180 phase shift ver the frequency range where the antenna is expected t wrk in

5 Prgress In Electrmagnetics Research B, Vl. 50, L 4 W 3 W 1 W 2 L 3 h L 2 W 3 W2 L1 W 1 x L 4 φ z L 3 P 2 P 1 L 2 L G L 1 Antenna 2 y Antenna 1 (a) x z h Z 1 P 1 Antenna Prt 2 Prt 1 Z 0 Z 2 φ(f) P 2 (b) (c) Figure 1. (a) Three-dimensinal tplgy f the prpsed UWB antenna. (b) Side view f the antenna demnstrating the tw radiatrs and feeding arrangement. (c) The feeding netwrk architecture is cmpsed f a pwer divider and a phase shifter that can be used t cmbine the tw mdes f peratin f the antenna. the differential mde and a 0 phase shift in the frequency range where the antenna is suppsed t wrk in the cmmn mde. This way, the structure will be fed in the apprpriate mde f peratin based n the frequency f the excitatin signal. The cmmn mde is selected when φ = 0, and the differential mde is selected when φ = 180. In what fllws, bth f these tw mdes will be examined individually Cmmn Mde f Operatin In the cmmn mde, the tw lp radiatrs are fed in phase, and the resulting structure acts as a clsely-cupled lp antenna with ultrabradband bandwidth [10]. Ultra-wideband cupled lp antennas with different shapes and tplgies have been examined befre. In [10], it has been demnstrated that when tw lp antennas are clsely cupled t each ther, the variatins f their mutual impedance

6 240 Li, Yusuf, and Behdad can cancel thse f each lp s self impedance. Therefre, when the antennas are fed in phase, a relatively cnstant input impedance (vs. frequency) can be achieved. In this paper, the antenna tplgy shwn in Figure 1(a) is indeed evlved by replacing the simple lp radiatrs in [10] with bent-diamnd shaped radiatrs. Such evlutin in the shape f the lp radiatr intrduces mre mutual cupling between the cupled radiatrs, and is fund t be mre suitable in designing cmpact antennas. In [11], it was demnstrated that such cupled lp antennas can be further miniaturized by lading them with capacitive tp hat plates. The tp-laded hats f the prpsed antenna ffer a large capacitive lading [11, 12] t reduce the lwest frequency f peratin f a UWB antenna. The current distributin f the prpsed antenna in its cmmn and differential mdes f peratin are examined using full-wave EM simulatins in CST Micrwave Studi. In the cmmn mde, the current maxima ccur near the shrt circuit tips and the electrical length f the antenna frm the sht circuit lcatin t the center f the antenna (where the feed is lcated) is apprximately a quarter f a wavelength. Therefre, the majrity f the radiatin in this mde cmes frm tw effective vertical electric currents that are separated frm ne anther by a distance, which is smaller than the maximum linear dimensin f the antenna. Therefre, in the cmmn mde f peratin, the antenna behaves similar t a twelement mnple antenna array with small spacing between the tw elements and radiates a vertically-plarized electrmagnetic wave with an mni-directinal radiatin pattern. In the differential mde, hwever, the antenna acts as a diple antenna placed in clse prximity t the grund plane. Therefre, the current flwing in ne arm f the antenna will be ppsitely directed t the ne flwing in the ther arm. Additinally, because f the bent diamnd arm tplgy f the structure, the effective radiating currents will have bth a hrizntal cmpnent and a vertical cmpnent. Thus, in this mde f peratin, the antenna can be thught f as tw hrizntally riented electric currents placed in clse prximity t a grund plane and tw vertically-riented electric currents placed vertical t the grund plane. Since the antenna height is electrically small, the radiatin cming frm the hrizntal electric currents is suppressed but the verticallyplarized electric currents can radiate efficiently. Due t the antisymmetric nature f the excitatin, hwever, these vertical electric currents are ppsitely directed. Therefre, the radiatin pattern f the antenna (fr the differential mde) is expected t exhibit a null in the azimuth plane. Figure 2 shws the input vltage standing wave rati (VSWR) f the prpsed UWB antenna with gemetrical

7 Prgress In Electrmagnetics Research B, Vl. 50, parameters listed in Table 1. In this case, symmetric pairs f cupled lp radiatrs are assumed. The radiatin patterns f the antenna in bth mdes f peratin are calculated using full-wave EM simulatins. Figure 3 shws the radiatin patterns f the antenna in the cmmn mde in the azimuth plane (fr vertical plarizatin). As can be seen the antenna shws a relatively mnidirectinal radiatin pattern in the azimuth plane, especially at lw frequencies. As frequency increases, hwever, the azimuthal radiatin pattern starts t lse its mnidirectinality. This is mainly due t the fact that the antenna becmes electrically large at these high frequencies and the same phenmenn can be bserved in almst all nn bdy-f-revlutin, cmpact UWB antennas [13]. Nevertheless, the simulated radiatin patterns in Figure 3 demnstrate that the prpsed antenna behaves as a mnple-like radiating structure in the cmmn mde f peratin. As can be seen frm Figure 2, the prpsed antenna exhibits an ultra-bradband range f peratin in the cmmn mde with the lwest frequency f peratin at 622 MHz. This lwest frequency f peratin is determined by the first resnance f each bent-diamnd lp radiatr that is laded with a tp hat. Althugh increasing the resnant length f the lp structure can decrease this first resnant frequency, it dse nt necessarily mean that the lwest frequency f peratin f the prpsed antenna in its cmmn mde can be easily reduced. In mst cases, increasing the resnant length f the lp cmes at the expense f increasing the quality factr f this resnance, which can severely deterirate the bradband matching cnditin f 6 5 Cmmn mde, φ=0 Differential mde, φ=180 VSWR Frequency [GHz] Figure 2. Simulated VSWR f the prpsed antenna in Figure 1 with physical parameters listed in Table 1 btained by feeding its tw prts using ideal excitatin cefficients.

8 242 Li, Yusuf, and Behdad Table 1. Physical parameters f the prpsed antenna with symmetrical cupled lp radiatrs. All units are in cm. L 1 = L 4 L 2 = L 3 L G W 1 W 2 W 3 h the antenna. Therefre, given a fixed antenna vlume size, it becmes extremely difficult t further lwer the lwest frequency f peratin f the prpsed antenna in its cmmn mde f peratin Differential Mde f Operatin As discussed in Sectin 2.1, the main challenge in the miniaturizatin f the prpsed UWB antenna in cmmn mde is t reduce its lwest frequency f peratin while preserving the bradband impedance matching f the antenna. This can be dne effectively by intrducing the differential mde f peratin. This differential mde can be realized by feeding the tw antenna prts with a phase difference f 180 (φ = 180 in Figure 1(c)). In this mde, the tw cupled lp radiatrs nw act as ne single large lp with rughly twice the circumference f each individual lps. Alternatively, the antenna in this mde can be thugh f as a wideband diple which is shrt circuited t the grund plane at its bth ends. As a result, the lwest frequency f peratin in the differential mde is expected t be significantly lwer than that f the cmmn mde. This is shwn in Figure 2, where the VSWRs f bth cmmn and differential mdes f peratin are shwn fr the same antenna with the physical parameters listed in Table 1. It is bserved that the lwest frequency f peratin f the differential mde is at 430 MHz, which is significantly lwer than the lwest frequency f peratin f the cmmn mde (622 MHz). Therefre, in situatins where achieving cnsistent radiatin patterns is nt f a very high pririty, it can be advantageus t use the differential mde f peratin t reduce the lwest frequency f peratin f the antenna, since this can be dne withut increasing the antenna vlume. A passive feed netwrk can be designed t autmatically feed the antenna in its crrect mde f peratin based n the input signal s frequency, as will be shwn later in Sectin 2.3. As we have already seen frm Figure 3, in the cmmn mde, the prpsed antenna behaves like a mnple with mnidirectinal radiatin patterns in the azimuth plane. In the differential mde, hwever, the radiatin patterns f the antenna will be different. As shwn in Figure 4, the effective current distributin in the differential mde can be decmpsed int bth hrizntal and vertical cmpnents.

9 Prgress In Electrmagnetics Research B, Vl. 50, db -20 db -30 db f=0.6 GHz f=1.5 GHz f=2 GHz Figure 3. Simulated radiatin patterns f the antenna shwn Figure 1 with physical parameters listed in Table 1. The results shw the nrmalized radiatin patterns fr the vertical plarizatin alng the azimuth plane in the cmmn mde f peratin. The hrizntal cmpnents f the current distributin can be perceived as a hrizntal diple ver perfect electric cnductr (PEC) and the vertical cmpnents can be cnceptually regarded as tw-element mnple array with ppsite excitatin cefficients. The hrizntal cmpnents f this effective electric current radiate in the presence f an infinite PEC grund plane and are effectively shrt circuited due t the small verall antenna height. Therefre, the radiatin patterns f the antenna in the azimuth plane are mstly determined by the radiatin patterns f the tw-element mnple array that is cmpsed f the tw vertical effective radiating currents. In the azimuth plane, this array prduces a vertically plarized radiated field. When perfectly symmetric cupled lp radiatrs are used in the prpsed antenna, the crrespnding excitatin cefficients fr these tw elements will be exactly ppsite with respect t each ther. This, accrding t the cannical antenna array thery [14], will prduce nulls alng the axis f symmetry (φ = ±90 in Figure 1(a)) f the antenna. Figure 4(b) shws the radiatin patterns f the antenna in the differential mde f peratin btained using full-wave EM simulatins in CST Micrwave Studi at 500 MHz. Clearly bserved, deep nulls are created alng the plane f symmetry f the antenna. The radiatin pattern f the differential mde is cntrlled by the excitatin cefficients f the virtual tw-element array depicted in Figure 4(a). While the tw mnples in this array carry currents in ppsite directins, the magnitudes f their excitatin cefficients can be changed by intrducing an asymmetry in the tplgy f the

10 244 Li, Yusuf, and Behdad Destructive Interference null directin (φ = 90 ) db -20 db 60 PEC Grund db Effective Vertical Currents (a) Vertical Pl (b) 120 Figure 4. (a) The current distributin f the differential mde f a symmetric versin f the prpsed antenna (e.g., similar t the ne having physical parameters prvided in Table 1). (b) Simulated radiatin patterns f the antenna with physical parameters listed in Table 1 in the azimuth plane fr vertical plarizatin. prpsed antenna. This will reduce the depth f the null bserved in the radiatin patterns f the antenna in the azimuth plane. As shwn in Table 2, ne f the lps is made deliberately larger than the ther ne. The effect f such asymmetric cnfiguratin n the radiatin mechanism can be best understd using the cnceptual current distributin depicted in Figure 5(a). Due t the asymmetry intrduced in the antenna, the excitatin cefficients f the tw-element array will nt have the same magnitude as each ther (e.g., in Figure 5(a) they are cnceptually depicted as +1 and 0.7). Figure 5(b) shws the vertically plarized radiatin pattern f the asymmetric antenna in the azimuth plane at 500 MHz. Clearly bserved, the depth f the null is significantly reduced cmpared t its cunterpart in Figure 4(b). The VSWRs f bth cmmn and differential mdes f peratin are als shwn in Figure 6, where the lwest frequency f peratin in differential mde (410 MHz) is bserved t be rughly 1.5 times lwer than that f cmmn mde (600 MHz). Table 2. Physical parameters f the prpsed antenna with asymmetrical cupled lp radiatrs. All units are in cm. L 1 L 2 L 3 L 4 L G W 1 W 2 W 3 h

11 Prgress In Electrmagnetics Research B, Vl. 50, Destructive Interference Significantly Reduced db -20 db db Effective Vertical Currents (a) (b) 150 Figure 5. (a) The current distributin f the differential mde f an asymmetric versin f the prpsed antenna (e.g., similar t the ne with physical parameters given in Table 2). (b) Simulated radiatin patterns f the antenna with physical parameters listed Table 2 in the azimuth plane fr vertical plarizatin. 6 5 Cmmn mde, φ=0 Differential mde, φ=180 VSWR Frequency [GHz] Figure 6. Simulated VSWR f the prpsed antenna in Figure 1 with physical parameters listed in Table 2 btained by feeding its tw prts using the ideal excitatin cefficients (i.e., +1 and +1 fr the cmmn mde and +1 and 1 fr the differential mde) Design f the Pwer Divider/Phase Shifter Feed Netwrk As mentined previusly, a key cmpnent f the prpsed cncept f dual-mde radiatrs is a feed netwrk that can autmatically feed the antenna in its crrect mde f peratin depending n the excitatin

12 246 Li, Yusuf, and Behdad Z 0 L s Z 1 L s C C Z 2 L φ (f) (a) L P 1 P 2 Antenna S 21 [db] Z h C C L L Frequency [GHz] (b) Z h S 21 Phase [degrees] Figure 7. (a) The feed netwrk used t excite the antenna in Figure 1(a). (b) The respnse f the lumped element frequencydependent phase shifter in (a). Here, Z = 50 Ω, Z h = 115 Ω, L s = 50 mm, L = 23 nh, and C = 3.3 pf. signal s frequency. This way, a seamless transitin between these tw mdes can be realized and frm a user s perspective, the antenna wrks within an ultra-bradband frequency range that is larger than that the bandwidth f each mde. Such a feed netwrk can be realized using the tplgy shwn in Figure 7(a). The netwrk cnsists f a pwer splitter and a frequency-dependent phase shifter. The frequency-dependent phase shifting is achieved by a lumped element high-pass netwrk cmpsed f capacitrs and inductrs. This phase shifter is designed t prvide a transmissin phase clse t 180 in the frequency range f 400 MHz 600 MHz and a phase difference f 0 in the frequency range abve 600 MHz as shwn frm Figure 7(b). As can be seen frm Figure 7(b), the prpsed phase shifter is capable f prviding a desired high transmissin cefficient acrss all mdes f peratin, and an apprximate 180 and 0 phase shift in differential and cmmn mde. A sharp transitin between the 180 and 0 phase shift requires a feed netwrk with a large number f lumped elements. In this case, t simplify the design, nly fur lumped elements are chsen with their values listed in Figure 7(b). This frequency dependent phase shifter netwrk will be used t cmbine the differential and cmmn mde f the antenna shwn in Figure 1. The micrstrip lines f the feeding netwrk are designed n a 1.6-mm-thick RT/Durid 5880 (ε r = 2.2, tan δ = ) substrate. Once the high-pass phase shifter is designed, the feed netwrk parameters are ptimized using a circuit simulatr (Agilent s Advanced Design System (ADS)) in rder t achieve a VSWR less than 3 ver the frequency range frm 360 MHz t 4 GHz. In ding this, the S-parameters f the antenna btained frm full-wave EM simulatins are used in the ADS simulatins and ptimizatins.

13 Prgress In Electrmagnetics Research B, Vl. 50, Phase Shifter C L C L (a) (b) Figure 8. (a) The phtgraph f the fabricated antenna with asymmetric cupled lp radiatrs. The detailed dimensins f the antenna is listed in Table 2. (b) The fabricated feed netwrk with frequency dependent phase shifter. The detailed values f lumped elements are listed in Figure 7(b). 6 5 Simulatin Measurement VSWR Frequency [GHz] Figure 9. Simulated and measured VSWR f the antenna cmbined with the feed netwrk. 3. RESULTS AND DISCUSSION One prttype f the antenna with the detailed dimensins listed in Table 2 is fabricated and shwn in Figure 8. The antenna has maximum dimensins f 20 cm 20 cm 3 cm and has finite grund plane dimensins f 20 cm 20 cm. The feed netwrk is patterned n a dielectric substrate lcated n the bttm side f the grund plane as shwn in Figure 8(b). The respnse f the antenna with the feed netwrk is measured using a vectr netwrk analyzer (VNA). Figure 9 shws the cmparisn between the measured and simulated input VSWR f the antenna. A gd agreement between

14 248 Li, Yusuf, and Behdad the measurement and simulatin is achieved, especially at frequencies belw 2 GHz. The measured lwest frequency f peratin is 360 MHz, which is reasnably clse t the simulated value f 390 MHz. The slight disagreement between the simulatin and measurement can be attributed t the variatin f the lumped element capacitr and inductr values frm their nminal values. Nevertheless, the measurement results demnstrate the bradband perfrmance f the antenna and the fact that the respnses f bth mdes can be cmbined tgether using the prpsed pwer divider/phase shifter feed netwrk. The radiatin characteristics f the antenna are measured using a multi-prbe near field system. Figure 10 shws the measured radiatin patterns f the antenna in the frequency range f GHz. Observe frm Figure 10(a) that the antenna s radiatin patterns change as frequency changes and the shape f the radiatin patterns d nt agree very well with the radiatin patterns predicted frm full-wave simulatin results. There are tw reasns fr this. The first cntributr t this is the finite size f the grund plane used in the measurement (simulatin results were btained fr infinite grund plane). The secnd, and perhaps mre imprtant reasn, is that the feed netwrk emplyed in this design des nt behave like an ideal feed netwrk. In particular, the feed netwrk perfrms perfectly well when the tw utputs f the netwrk (shwn in Figure 7(a)) are cnnected t fixed impedances. Hwever, the impedance f the antenna is nt fixed and changes with frequency. This changes the respnse f the feed netwrk frm the ideal respnse (e.g., the ne shwn in Figure 7). This results in variatin f the radiatin patterns f the antenna versus frequency db -20 db -30 db MHz 1 GHz 2 GHz 3 GHz db db -10 db -30 db -20 db db (a) (b) (c) 150 Figure 10. Measured nrmalized radiatin patterns (vertically plarized) f the antenna in the azimuth x-y and tw elevatin planes x-z and y-z. (a) Azimuth (x-y) plane. (b) Elevatin plane (x-z). (c) Elevatin plane (y-z).

15 Prgress In Electrmagnetics Research B, Vl. 50, The radiatin patterns in tw elevatin planes (x-z and y-z planes) are als shwn in Figure 10(b) and Figure 10(c). As can be seen, a significant amunt f radiatin exists in the lwer hemisphere (belw the grund plane). This is due t the fact that the antenna has finite grund plane size f 20 cm 20 cm, which is the same as the antenna s maximum lateral dimensins. Similar t any ther mnple type radiatr, as the grund plane size increases, the radiatin levels belw the grund plane decreases. Hwever, due t the limitatins f the sizes f antennas that we culd measure using the available setup, the radiatin patterns f this antenna with a larger grund plane were nt measured. The gain and radiatin efficiency f the antenna are als measured using the same near-field system. The measured realized gain f the antenna, which includes the impedance mismatch effects, is shwn in Figure 11. Ntice that the antenna gain can be increased if the antenna is munted n a large platfrm r if the grund plane size is increased. The radiatin efficiency f the antenna is als measured using the same multi-prbe near field system and the results are presented in Figure 11 as well. Over mst f the perating band, the radiatin efficiency remains abve 75%. Realized Gain [dbi] Radiatin Efficiency 25 Realized Gain Frequency [GHz] Figure 11. Measured realized gain and the radiatin efficiency f the antenna. The gain values reprted take int accunt the effect f the impedance mismatch Efficiency [%] 4. CONCLUSIONS In this paper, the cncept f clsely-cupled, dual-mde radiatrs is utilized t design a cmpact, lw-prfile, ultra-wideband antenna. Tw distinct mdes f peratin with cmplementary frequency bands f peratin are explited in the design. The prpsed antenna behaves as a tp-hat laded, cupled lp antenna with ultra-bradband frequency respnse in its cmmn mde f peratin and it acts as a wideband diple with bent-diamnd shaped arms in the differential mde f peratin. The frequency f peratin f the differential

16 250 Li, Yusuf, and Behdad mde is cmplementary t and belw that f the cmmn mde. These tw mdes are cmbined tgether using a feed netwrk that uses a frequency-dependent lumped element phase shifter. The resulting antenna has a larger verall bandwidth cmpared t what can be btained using each mde individually. Mre imprtantly, the cmbining f the tw radiating mdes allws fr reducing the lwest frequency f peratin f an ultra-wideband antenna while maintaining its ccupied vlume. The primary limitatin f the prpsed apprach, i.e., the variatin f radiatin patterns, was als discussed. In particular, the incnsistencies f the antenna s radiatin pattern as a functin f frequency is the main price that is paid fr using this apprach. This might be vercme t sme extent by using a rtatinally symmetric versin f this antenna and expliting techniques such as quadrature feeding. In its current frm, we envisin that this antenna will be mst suitable fr applicatins where having a high radiatin efficiency and bradband peratin is mre imprtant than having cnsistent radiatin patterns r a specific shape f radiatin patterns. ACKNOWLEDGMENT This material is based upn wrk supprted by the Office f Naval Research under ONR Award N. N REFERENCES 1. Schantz, H., The Art and Science f Ultrawideband Antennas, Artech Huse, Nrwd, MA, Gubau, G., N. N. Puri, and F. Schwering, Diakptic thery fr multielement antennas, IEEE Trans. Antennas and Prpag., Vl. 30, N. 1, 15 26, Friedman, C. H., Wide-band matching f a small disk-laded mnple, IEEE Trans. Antennas and Prpag., Vl. 33, N. 12, , Nakan, H., H. IWaka, K. Mrishita, and J. Yamauchi, A wideband lw-prfile antenna cmpsed f a cnducting bdy f revlutin and a shrted parasitic ring, IEEE Trans. Antennas and Prpag., Vl. 56, N. 4, , Mn, H., G.-Y. Lee, C.-C. Chen, and J. L. Vlakis, An extremely lwprfile ferrite-laded wideband VHF antenna design, IEEE Antennas Wirel. Prpag. Lett., Vl. 11, , 2012.

17 Prgress In Electrmagnetics Research B, Vl. 50, Palud, S., F. Clmbel, M. Himdi, and C. L. Meins, Wideband mnidirectinal and cmpact antenna fr VHF/UHF band, IEEE Antennas Wirel. Prpag. Lett., Vl. 10, 3 6, Yusuf, Y. and N. Behdad, Cmpact, lw-prfile UWB antennas expliting the cncept f clsely-cupled dual-mde radiatrs, IEEE Antennas and Prpagatin Sciety Internatinal Sympsium (APSURSI), 1 2, Jul. 8 14, Yusuf, Y. and N. Behdad, Miniaturizatin f a class f ultra-wideband antennas using dual-mde radiating structures, 2012 IEEE Internatinal Cnference n Wireless Infrmatin Technlgy and Systems (ICWITS), 1 4, Nv , Elsherbini, A. and K. Sarabandi, Very lw-prfile tp-laded UWB cupled sectrial lps antenna, IEEE Antennas Wirel. Prpag. Lett., Vl. 10, , Behdad, N. and K. Sarabandi, A cmpact antenna fr ultrawideband applicatins, IEEE Trans. Antennas and Prpag., Vl. 53, , Behdad, N., M. Al-Jumayly, and M. Salehi, Ultra-wideband lw prfile antenna, US Patent N. 8,228,251, Seeley, E. W., An experimental study f the disk laded flded mnple, IRE Trans. Antennas Prpag., Vl. 4, N. 1, 27 28, Jan Baum, C. E., A. P. Stne, and J. S. Ty, Ultra-wideband, Shrtpulse Electrmagnetics 8, Springer, New Yrk, Balanis, C. A., Antenna Thery: Analysis and Design, 3rd editin, Wiley-Interscience, 2005.