researcharticle an x-band dual-polarized vivaldi antenna with high...

Research ArticleAn X-Band Dual-Polarized Vivaldi Antenna with High Isolation

Denghui Huang,1 Hu Yang,1 YuqingWu,2 and Fei Zhao3

1College of Electronic Science and Technology, National University of Defense Technology, Hunan 410073, China2Cavendish Laboratory, University of Cambridge, London, UK3The Southwest Electronics and Telecommunication Technology Research Institute, Chengdu 610041, China

Correspondence should be addressed to Denghui Huang; [email protected]

Received 14 May 2017; Revised 14 July 2017; Accepted 20 July 2017; Published 24 August 2017

Academic Editor: Luciano Tarricone

Copyright © 2017 Denghui Huang et al.This is an open access article distributed under theCreative CommonsAttribution License,which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

An X-band dual-polarized Vivaldi antenna with high isolation is proposed. The procedure of this antenna design includes theconventional Vivaldi antenna with regular slot edge (RSE), the dual-polarized Vivaldi antenna with two Vivaldi antennas whichhave different feeding point positions in a cross-shaped form, and the two Vivaldi antennas with a galvanic contact in solderingpoint. By applying the RSE, it reduced the dimensions of the Vivaldi antenna and improved its radiation performance.Themodifiedantenna is fabricated andmeasured.Themeasured results show that 𝑆11 < −10 dB at the entire X-band for the twoVivaldi antennas.The isolation (𝑆21) between the two feeding ports, which has been improved by applying the different feeding point positions andthe galvanic contact in soldering point, is better than 34 dB at X-band. In addition, the cross-polarized discrimination is better than21 dB for the two Vivaldi antennas, and the measured results also include the gain of two Vivaldi antennas.

1. Introduction

The Vivaldi antenna was firstly described by Gibson in 1979[1] and then developed in an antipodal shape [2].The Vivaldiantenna has been considered for many radar or communi-cation system applications because it is an ultrawidebanddevice, it has a planar structure, and it is lightweight, ofhigh efficiency, and of high gain. For conventional Vivaldiantennas, the study results show that the electric field vectorof the Vivaldi antenna is parallel to the substrate, which issingle-linear-polarized on its two major radiation planes.

In some modern high-data or very high resolution imag-ing radar systems, the application of polarization diversitycan enhance the efficiency of such systems because the polar-ization information delivers further information about thetarget characteristics. So in order to improve the performanceof such systems, the antennas are required to have the char-acteristics of both ultrawideband and dual polarization. Thedual-polarized Vivaldi antenna is a good choice which can beused in modern imaging radar systems at X-band. In recentyears, many methods have been presented, to implement twoorthogonal cases of polarization with Vivaldi antennas. As

in [3], the dual-polarized antenna consists of four antipodalVivaldi antennas and places them along the outer edge. Andin [4–6], the dual-polarized Vivaldi antenna array has beenpresented, and the conventional Vivaldi antenna or all-metalVivaldi antenna was used as the array element.

In [7–11], the dual-polarizedVivaldi antenna is composedof two conventional Vivaldi antennas in a cross-shaped formwithout galvanic contact, and the isolation (𝑆21) between thetwo feeding ports is 23 dB in [9] and 25 dB in [7, 10]. Thebest result is presented in [12], the isolation is better than30 dB at the entire function frequency band, and for the twoVivaldi antennas, one is ahead of the other, which avoids theantenna feeding lines overlapping each other. Additionally,the published antennas in [7–11] cannot be used as thedual-polarized antenna array element because the width ofantenna is larger than a wavelength corresponding to thehigher frequency of the operating band, which will lead toantenna array with grating lobe in higher frequency band.

In this paper, a modified dual-polarized Vivaldi antennawith good radiation performance and high isolation isdesigned and measured. On the one hand, the low cut-offfrequency of the Vivaldi antenna was extended by RSE which

HindawiInternational Journal of Antennas and PropagationVolume 2017, Article ID 3281095, 9 pageshttps://doi.org/10.1155/2017/3281095

https://doi.org/10.1155/2017/3281095

2 International Journal of Antennas and Propagation

9

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Vivaldi 1

Vivaldi 2

L

W

x

y

L2

P2(x2, y2)

P1(x1, y1)

Figure 1: Geometry of the designed antenna elements: Vivaldi 1 andVivaldi 2. Dimensions are in mm.

indicates that it can be used as the dual-polarized antennaarray element at X-band, and the radiation performance wasalso improved. On the other hand, we used the differentfeeding point positions for two Vivaldi antennas insteadof the one Vivaldi antenna being ahead of another Vivaldiantenna, and there is a galvanic contact in soldering pointbetween the twoVivaldi antennas.Theoverall effect is that thehigh isolation (𝑆21) is better than 34 dB. Additionally, for thetwo Vivaldi antennas, the measured gain is between 6.4 dBand 12.2 dB at X-band.

2. Antenna Design

Figure 1 demonstrates the geometry of the dual-polarizedVivaldi antenna elements: Vivaldi 1 and Vivaldi 2. Thoseantennas with the same RSE are fed by a 2.74mmmicrostripline, and the characteristic impedance of microstrip linecan be calculated as 50Ω in order to match with SMAconnector. The thickness of the substrate is 1mm and itsrelative permittivity is 2.65. The substrate dimension (𝐿SUB ×𝑊SUB) is 60 × 25mm2.

The inside of the radiation flares for Vivaldi 1 and Vivaldi2 fits the following exponential curves:

𝑦 = 𝐶1 × 𝑒(𝛼1×𝑥) + 𝐶2,𝐶1 = 𝑦1 − 𝑦2𝑒(𝛼1×𝑥1) − 𝑒(𝛼1×𝑥2) ,

𝐶2 = 𝑦2𝑒(𝛼

1×𝑥

1) − 𝑦1𝑒(𝛼1×𝑥2)𝑒(𝛼1×𝑥1) − 𝑒(𝛼1×𝑥2) .

(1)

𝛼1 = 0.052 stands for exponential rate. 𝑃1 = (0, 0.6) and𝑃2 = (40, 7.6) stand for the start and end point of exponentialcurves.

According to [12], for the conventional Vivaldi antenna,the radius of the stub limits the upper working frequencyand in order to get the best impedance matching, the cavityshould keep the same radius as the stub. In [12], it offers a fastway to design wideband Vivaldi antenna except that it takes

Port 1

Port 2

Port 3 Port 4

Microstrip lineSlot line

Couplingpoint

ZＭＮＯ＜

Z＝；ＰＣＮＳ Z；ＨＮ？ＨＨ；

ZＭＮＯ＜ Z＝；ＰＣＮＳ Z；ＨＮ？ＨＨ；

Z0

Figure 2:The network for the equivalent circuit of Vivaldi antenna.

the position of the feeding or coupling point into account.In this paper, based on the theory of the equivalent circuitof Vivaldi antenna [13], the position of the feeding point hasbeen studied.

The network for the equivalent circuit model of theVivaldi antenna is shown in Figure 2. From the coupling pointof microstrip to slot-line transmission, the Vivaldi antennacan be equivalent to a microwave network with four ports.As shown in Figure 2, 𝑍0 is the character impedance of themicrostrip line and 𝑍stub, 𝑍cavity denote the input impedanceof the microstrip open-end and slot short-end, respectively.𝑍antenna expresses the input impedance of the isolated Vivaldiantenna fed by a slot transmission line from the couplingpoint.

Based on [13], the input reflection coefficient Γin for port1 can be given:

Γin = 𝑍𝑙 − 𝑍0𝑍𝑙 + 𝑍0 , (2)

where

𝑍𝑙 = 𝑛2 × Re(𝑍antenna × 𝑍cavity𝑍antenna + 𝑍cavity) + 𝑗 × 𝑛

× Im(𝑍antenna × 𝑍cavity𝑍antenna + 𝑍cavity) + 𝑍stub(3)

and the coupling coefficient 𝑛 can be found in [14].For the different feeding or coupling point position 𝐿2,

shown in Figure 1, the real and imaginary parts of 𝑍𝑙 can becalculated with the simulated input impedance for open-endstub, 𝑍stub, short-end cavity, 𝑍cavity, and the isolated Vivaldiantenna fed by a slot transmission line, 𝑍antenna, shown inFigure 2, and those simulations were also presented in [12].

International Journal of Antennas and Propagation 3

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Figure 3: The real and imaginary parts of the 𝑍𝑙.

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S11

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L2 = 4ＧＧ

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Figure 4: Simulated 𝑆11 for the different feeding point positions.

Figure 3 shows the real and imaginary parts of 𝑍𝑙 withdifferent 𝐿2. As the feeding point is closer to the cavity, thesmaller variation of the imaginary and real parts of the 𝑍𝑙has been gotten, which indicates that the wider bandwidthfor Vivaldi antenna will be obtained. Based on the full-waveelectromagnetic model simulation of Vivaldi antenna withdifferent 𝐿2, the magnitude of the reflection coefficient withdifferent feeding point position 𝐿2 is shown in Figure 4;the simulated results show that the feeding point position iscloser to the cavity; thewider frequency bandhas been gotten,which is in good agreement with the conclusion shown inFigure 3.

Based on the study of the feeding point position, theVivaldi antenna element for dual-polarized Vivaldi antennahas been designed. The coupling point 𝐿2 is greater than

3mm for 𝑆11 < −10 dB at X-band and the optimal cavityradius is 4mm, and the radius of the stub is 2.5mm, with astub angle of 120∘. Moreover, the simulated 𝑆11 and gain ofVivaldi antenna with and without RSE are shown in Figure 5.In Figure 5(a), the low cut-off frequency is extended by RSEfrom 9.6GHz to 7GHz without increasing the volume of theantenna, and the gain of Vivaldi antenna is also improved byRSE at X-band in Figure 5(b).

For the most common dual-polarized Vivaldi antennashown in Figure 6(a), in order to avoid the feeding linesoverlapping each other, Vivaldi 2 is ahead of Vivaldi 1with 𝑑mm. Based on the parameter study of 𝑑, which isthe distance between Vivaldi 2 and Vivaldi 1, the isolationbetween two antenna ports is shown in Figure 7(a). Thesimulated results indicate that the isolation between twoports


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Figure 5: The simulated 𝑆11 and gain for Vivaldi antenna with RSE and Vivaldi antenna without RSE.

(a)

(b)

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d

Figure 6: The structure of dual-polarized Vivaldi antenna: (a) conventional dual-polarized Vivaldi antenna; (b) modified dual-polarizedVivaldi antenna.

is better than 24 dB for 𝑑 = 2mm, while it is better than29 dB for 𝑑 = 1mm. As shown in Figure 6(b), we used thedifferent feeding or coupling point position 𝐿2 for the twoVivaldi antenna elements instead of Vivaldi 1 being ahead ofVivaldi 2, which avoids the feeding lines overlapping eachother.𝐿2 forVivaldi 1 is 4.1mm,while𝐿2 is 3.5mm forVivaldi2. The isolation between the two feeding ports is better than32 dB with 𝑑 = 0mm.

Figure 7(b) plots the isolation between the two feedingports with galvanic contact in soldering port shown inFigure 6(b). The length of galvanic contact 𝐿1 has an effecton the return loss of Vivaldi 2, and the lower frequency limitis 8.3 GHz when 𝐿1 exceeds 3mm. In this antenna design, inorder to complement dual polarization, we place two Vivaldiantennas cross-shaped with different feeding point position,which avoids the feeding lines overlapping each other, and


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S21

(dB)

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d = 0ＧＧd = 1ＧＧ

d = 2ＧＧ

(a)

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S22

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−34

−108.3 ＇（Ｔ

L1 = 4ＧＧ

L1 = 3ＧＧ

L1 = 2ＧＧ

L1 = 1ＧＧ

(b)

Figure 7: (a) Effect of the different 𝑑 on the isolation of two ports. (b) Simulated return loss of Vivaldi 2 and isolation with the different lengthof galvanic contact.

also there is a galvanic contact in soldering point.The overalleffect is that the return loss of the two Vivaldi antennas islower than −10 dB at X-band, and the isolation between thetwo feeding ports is better than 34 dB with 𝐿1 = 2mmgalvanic contact.

3. Results and Discussion

The photograph of modified dual-polarized Vivaldi antennahas been fabricated and measured. Figure 8(a) shows the topand bottom view of the constructed Vivaldi 1 and Vivaldi2. By using the different feeding point positions, the lengthsof the slots in Vivaldi 1 and Vivaldi 2 are 16.2 and 43.8mm,and the summarized length of the slots is equal to thelength of antenna, which indicates the 0mm displacementbetween Vivaldi 1 and Vivaldi 2. The 0mm displacementbetween Vivaldi 1 and Vivaldi 2 also indicates that theseparate antennas almost have the same phase center in 𝑥-axis although the feeding point of Vivaldi 2 ahead of Vivaldi1 is 0.6mm. The width of the slots is 1.1mm and it is widerthan the thickness of the substrate (1mm), where Vivaldi 1can be inserted into Vivaldi 2 in a cross-shaped form. Thestructure of the modified dual-polarized Vivaldi antenna isshown in Figure 8(b). As shown in Figure 8(b), there is agalvanic contact in soldering point and the feeding point ofVivaldi 2 ahead of Vivaldi 1.

Figure 9 illustrates the measured 𝑆-parameters of thedual-polarized Vivaldi antenna.The 𝑆-parameters were mea-sured with Agilent Performance Network Analyzer N5230C.As can be seen from Figure 9, the frequency band of 𝑆11 <−10 dB is 7.4–12GHz for Vivaldi 1, while it is 7.8–12GHzfor Vivaldi 2, where the frequency band for 𝑆11 < −10 dBis covering the X-band. In addition, with the help of thedifferent feeding point positions for Vivaldi antennas and

the only galvanic contact in soldering point, the isolationbetween two ports is better than 34 dB at X-band. Comparedto the simulated 𝑆11 and 𝑆22, because the losses of thedielectric and the SMA connector are not taken into accountin simulations, the measured 𝑆11 and 𝑆22 are lower than thesimulated results.

Figure 10 plots the measured radiation patterns of Vivaldi1 and Vivaldi 2 at different frequencies for 𝐸-plane and 𝐻-plane. As shown in Figure 10, both antennas have endfirecharacteristics with the main lobe in the axial direction (𝑋-direction in Figure 1). And for the two Vivaldi antennas, theyhave a similar performance within the X-band. Additionally,based on the measured results in Figure 10, the 𝐸-plane 3 dBbeamwidth for Vivaldi 1 is 63∘ at 8GHz, 56∘ at 10GHz, and39∘ at 12GHz, while it is 67∘ at 8GHz, 51∘ at 10GHz, and 35∘at 12GHz for Vivaldi 2. All the results indicate that Vivaldi 1and Vivaldi 2 have a good radiation performance at the totalfrequency band.

According to the measured radiation patterns, the mea-sured cross-polarization discrimination versus frequency ofVivaldi 1 and Vivaldi 2 is shown in Figure 11. For Vivaldi 1,the cross-polarization discrimination is below 21.9 dB at theX-band and is below 21 dB for Vivaldi 2. In addition, the dual-polarized Vivaldi antenna with good polarization purity canbe applied to many radar systems at X-band.

The gain variation of Vivaldi 1 and Vivaldi 2 at the entirefrequency range is shown in Figure 12. As shown in Figure 12,the gain of Vivaldi 1 is between 7.4 dB and 11.6 dB at theX-band, while is between 6.4 dB and 12.2 dB at the samefrequency band for Vivaldi 2. The gain of the antennas isimproved with the increase of the frequency. Compared toVivaldi 2, Vivaldi 1 has approximately the same gain from9.2GHz to 12GHz, and at 8GHz, the gain of Vivaldi 1 is 1 dBhigher than Vivaldi 2. The difference of the gain between


Vivaldi 1Vivaldi 2

The slots

Soldering point

16.20

43.80

(a)

Vivaldi 2

Vivaldi 1

feeding point of Vivaldi 2ahead of Vivaldi 1The galvanic contact in

soldering point

0ＧＧ displacement with the

(b)

Figure 8: Photograph of (a) top and bottom view for Vivaldi elements: Vivaldi 1 and Vivaldi 2; (b) modified dual-polarized Vivaldi antenna.

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S-pa

ram

eter

s (dB

)

Frequency (GHz)

S11, meas.S22, meas.S21, meas.

S11, sim.S22, sim.

−34

−10

Figure 9: Simulated and measured 𝑆-parameters of the dual-polarized Vivaldi antenna.


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H-plane at 12 ＇（ＴE-plane at 12 ＇（Ｔ

H-plane at 10 ＇（ＴE-plane at 10 ＇（Ｔ

H-plane at 8＇（ＴE-plane at 8＇（Ｔ

Figure 10: 𝐸- and 𝐻-plane radiation patterns of the antennas at different frequencies with black solid lines for Vivaldi 1 copolarization,red solid line for Vivaldi 2 copolarization, black short dash line for Vivaldi 1 cross-polarization, and red short dash line for Vivaldi 2 cross-polarization.


8.0 8.4 8.8 9.2 9.6 10.0 10.4 10.8 11.2 11.6 12.0−27−26−25−24−23−22−21−20−19−18−17−16−15

Cros

s-po

lariz

atio

n di

scrim

inat

ion

(dB)

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Vivaldi 1, meas.Vivaldi 2, meas.

Figure 11: Variation of the cross-polarization discrimination forVivaldi 1 and Vivaldi 2.

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10111213

Gai

n (d

B)

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Vivaldi 1, meas.Vivaldi 2, meas.

Vivaldi 1, sim.Vivaldi 2, sim.

Figure 12: Simulated and measured gain for Vivaldi 1 and Vivaldi 2.

Vivaldi 1 and Vivaldi 2 in lower frequency is mainly causedby the slot behind Vivaldi 2 and the manufacturing errorin the two Vivaldi antennas. In addition, those factors alsocaused the difference on the return loss for Vivaldi 1 andVivaldi 2 shown in Figure 9. For the two Vivaldi antennas,the measured gain is lower than the simulations, because ofthe losses of the dielectric.

4. Conclusion

Amodified X-band dual-polarized Vivaldi antenna has beendesigned and constructed. First, the dimensions of the dual-polarized Vivaldi antenna elements were reduced by RSE.And for the two Vivaldi antennas, the measurements showthat the frequency band for 𝑆11 < −10 dB is covering

the X-band. With the help of the different feeding pointpositions and the only galvanic contact in soldering point,the high isolation between two antenna ports is better than34 dB. Additionally, the cross-polarization discriminationfor Vivaldi 1 and Vivaldi 2 is below 21 dB, and they bothhave approximately the same gain at the entire frequencyband. Finally, all measurements which support our sim-ulations indicate that the dual-polarized Vivaldi antennahas miniaturized dimensions, high isolation, good radiationperformance, and good polarization purity, which can beapplied inmany imaging radar systemapplications atX-band.

Conflicts of Interest

The authors declare that they have no conflicts of interest.

References

[1] P. J. Gibson, “Vivaldi aerial,” Conference Proceedings - EuropeanMicrowave Conference, pp. 101–105, 1979.

[2] E. Gazit, “Improved design of theVivaldi antenna,” IEE Proceed-ings H: Microwaves, Antennas and Propagation, vol. 135, no. 2,pp. 89–92, 1988.

[3] H. Loui, J. P. Weem, and Z. Popovic, “A dual-band dual-polarized nested vivaldi slot array with multilevel groundplane,” IEEE Transactions on Antennas and Propagation, vol. 51,no. 9, pp. 2168–2175, 2003.

[4] S. Balling,M.Hein,M.Hennhofer, G. Sommerkorn, R. Stephan,and R. Thoma, “Broadband dual polarized antenna arrays formobile communication applications,” in Proceedings of the 33rdEuropean Microwave Conference, 2003, pp. 927–930, Munich,Germany, October 2003.

[5] J. Remez, A. Segal, and R. Shansi, “Dual-polarized widebandwidescan multibeam antenna system from tapered slotline ele-ments array,” IEEE Antennas and Wireless Propagation Letters,vol. 4, no. 1, pp. 293–296, 2005.

[6] J.-B. Yan, S. Gogineni, B. Camps-Raga, and J. Brozena, “A dual-polarized 2–18-GHz Vivaldi array for airborne radar measure-ments of snow,” Institute of Electrical and Electronics Engineers.Transactions on Antennas and Propagation, vol. 64, no. 2, pp.781–785, 2016.

[7] G. Adamiuk, R. T. Zwick, and W. Wiesbeck, “Dual-orthogonalpolarized vivaldi antenna for ultra wideband applications,” inProceedings of the 17th International Conference on Microwaves,Radar andWireless Communications (MIKON ’08), pp. 1–4,May2008.

[8] L. Song and Q. Fang, “Design and measurement of a kind ofdual polarized Vivaldi antenna,” in Proceedings of the 2011 CrossStrait Quad-Regional Radio Science and Wireless TechnologyConference, CSQRWC 2011, pp. 494–497, chn, July 2011.

[9] X. Han, L. Juan, C. J. Cui, and Y. Lin, “UWB dual-polarizedVivaldi antenna with high gain,” in Proceedings of the Interna-tional Conference on Microwave and Millimeter Wave Technol-ogy (ICMMT ’12), vol. 3, pp. 1–4, May 2012.

[10] L. Reichardt, J. Kowalewski, L. Zwirello, and T. Zwick, “Com-pact, Teflon embedded, dual-polarized ultra wideband (UWB)antenna,” in Proceedings of the Joint 2012 IEEE InternationalSymposium on Antennas and Propagation and USNC-URSINational Radio Science Meeting, APSURSI 2012, July 2012.


[11] Y. Li, M. Su, Y. Z. Sheng, and L. Dong, “Ultra-wideband dualpolarized probe for measurement application,” in Proceedings ofthe in Proceedings of the International Symposium on AntennasPropagation, pp. 1025–1028, 2013.

[12] M. Sonkki, D. Sanchez-Escuderos, V. Hovinen, E. T. Salonen,and M. Ferrando-Bataller, “Wideband dual-polarized cross-shaped Vivaldi antenna,” IEEE Transactions on Antennas andPropagation, vol. 63, no. 6, pp. 2813–2819, 2015.

[13] C. Deng and Y. J. Xie, “Design of resistive loading Vivaldiantenna,” IEEE Antennas and Wireless Propagation Letters, vol.8, pp. 240–243, 2009.

[14] J. B. Knorr, “Slot-line transitions (short papers),” IEEE Trans-actions on Microwave Theory Techniques, vol. 22, pp. 548–554,1974.

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