IEEE TRANSACTIONS ON ANTENNAS AND PROPAGATION, VOL. 62, NO. 4, APRIL 2014 1903

A Novel Null Scanning Antenna Using Even and OddModes of a Shorted Patch

Xiaolei Jiang, Student Member, IEEE, Zhijun Zhang, Senior Member, IEEE, Yue Li, Member, IEEE, andZhenghe Feng, Fellow, IEEE

Abstract—A novel null scanning patch antenna operating at 2.4GHz is presented. Unlike the existing reconfigurable techniques,the even and odd modes of a shorted patch are adopted to firstachieve a broadside pattern and pattern null on broadside. Contin-uous null scanning is achieved by combining these two modes withdifferent exciting powers. The mechanism of null scanning is dis-cussed from both theoretical analysis and simulated/measured re-sults. A rat-race switching network is used to excite the two modesand simultaneously obtain high port isolation. A prototype is fab-ricated and tested. The measured results verified the null scanningcapability of the proposed design.

Index Terms—Even/odd mode, null scanning, pattern reconfig-urable, shorted patch.

I. INTRODUCTION

P ATTERN reconfigurable antenna has drawn significantattention over the past few years because of its capa-bility to enhance the wireless system performance [1]. Varioustechniques have been proposed to realize multiple patterns ina single antenna. Mechanical methods are applied in [2], [3]which use actuators to change the position of the radiators.Tunable materials are used in [4] to change the electrical phaseconstant of the propagation wave and finally change the beamdirection. Switches or varactors are commonly adopted in mostliteratures on radiators [5]–[8], parasitic elements [9]–[11] orfeeding networks [12], [14].Recently, a special kind of reconfigurable antenna, null

scanning antenna, has become of research interest and receivedmuch attention [10], [13], [14]. Diodes are used on an an-nular slot [13] or on the arms of Wilkinson power divider ina feeding network [14]. Different states of the diodes couldmake the specific direction a null on radiated pattern. However,use of diodes can only achieve discrete null scanning andcannot satisfy the application requirements. In [10], a patternreconfigurable null scanning antenna is proposed to achieve

Manuscript received October 07, 2013; revised November 22, 2013; acceptedDecember 30, 2013. Date of publication January 09, 2014; date of current ver-sion April 03, 2014. This work was supported in part by the National Basic Re-search Program of China under Contract 2013CB329002, by the National HighTechnology Research and Development Program of China (863 Program) underContract 2011AA010202, the National Natural Science Foundation of Chinaunder Contract 61371012, the National Science and Technology Major Projectof theMinistry of Science and Technology of China 2013ZX03003008-002, andthe China Postdoctoral Science Foundation funded project 2013M530046.The authors are with State Key Lab of Microwave and Communications, Ts-

inghua National Laboratory for Information Science and Technology, TsinghuaUniversity, Beijing, 100084, China (e-mail: zjzh@tsinghua.edu.cn).Color versions of one or more of the figures in this paper are available online

at http://ieeexplore.ieee.org.Digital Object Identifier 10.1109/TAP.2014.2298884

continuous scanning. Four parasitic patches and 12 varactorsare employed in the design. A pattern with a null on broadsideis first excited by the center driven patch. Then by controllingthe varactors the broadside pattern can be achieved from theparasitic patch, which finally affects the null titling. However,its dc control circuit needs other components, which makes thedesign complex and increases the fabrication cost. In addition,its size (almost one wave length) is still the obstacle for arraydesign.Dual-mode or multimode antenna have been investigated and

used for pattern diversity applications [15]–[18]. In [15], theauthor excited the TM and TM modes of a short-circuitedcircular patch and obtained a broadside pattern and a conicalpattern, respectively. The possibility of reconfigurable antennawas also discussed but with only short description and no ex-perimental results. In addition, its large size would also be theobstacle in array design.The proposed null scanning antenna in this paper uses the

even and odd modes of a shorted patch, for the first time tomake the antenna radiate in two different ways. The radiatedpatterns could be broadside for the even mode and pattern nullon broadside for the odd mode. As the two modes need to be fedin-phase and out-of-phase, a rat-race coupler is quite consistentwith such a feeding requirement and the two input ports couldalso ensure good isolation in the same time because the isolationis very important when combining these two modes for patternreconfigurability.After the two patterns have been obtained from the even and

oddmodes, null scanning could be achieved by combining thesetwo modes with different exciting powers and a specific phasedifference. Theoretical analysis, simulated results and measuredresults will be given in this paper.This paper is organized as follows. The operating principle

along with the antenna design is described in Section II.Section III presents the simulated and measured results in-cluding S parameters and radiated patterns for the even and oddmodes. Null scanning mechanism and results are discussed inSection IV and conclusions are drawn in Section V.

II. OPERATING PRINCIPLE AND ANTENNA DESIGN

A. Operating Principle

Fig. 1 plots the electric field distribution in a shorted patch fortwo different scenarios. For Fig. 1(a), it is essentially the TMmode of a traditional patch, from which results a broadside radi-ated pattern. Although the field distribution is in odd symmetry,

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1904 IEEE TRANSACTIONS ON ANTENNAS AND PROPAGATION, VOL. 62, NO. 4, APRIL 2014

Fig. 1. Two modes of a shorted patch. (a) Even mode, (b) Odd mode.

Fig. 2. Geometry of the proposed antenna;, unit all in

mm. (a) Top View, (b) Side Cutting View.

we define it as an even mode, as the equivalent magnetic cur-rents and are in phase along the left and right sideslots, respectively. The shorting plane at the center has little ef-fect on the field distribution in this scenario as the field in thecenter plane is inherently zero. Owing to the introduction of theshorting plane, the field in the center plane is forced to be zeroso that the scenario in Fig. 1(b) could be of reasonable exis-tence where the field has a symmetrical distribution centered onthe shorting plane. We define it as an odd mode as the mag-netic equivalent currents and are out of phase alongthe two side slots. This mode makes the broadside a null in theradiated pattern as the two equivalent currents cancel out witheach other in this direction.

B. Antenna Design

Based on the model in Fig. 1, we proposed the reconfig-urable antenna as shown in Fig. 2. The metal patch with an areaof 50 45 mm suspends mm over the ground planewith the support of a shorting plane soldered in the center. Itis worth mentioning that the length and width of the patch areboth shorter than half wave, which can be used in array de-sign. The ground plane and the feeding network are on the twosides of an FR-4 substrate with aheight of mm. Slot-coupled feed technique is used to ex-cite the patch antenna. There are two identical U-shaped slotsetched on the ground plane. Impedance matching is realized by

Fig. 3. Rat-race feeding network;, unit all in mm.

tuning the parameters of the slots. When these two slots are ex-cited out of phase with the same amplitude, an odd symmetricalfield distribution will be generated between the patch and theground as shown in Fig. 1(a). When they are excited, in turn,in phase but still equally in amplitude, the field like what is inFig. 1(b) will be generated. Therefore, a feeding network withboth out-of-phase and in-phase excitations is required to achievethe even and odd modes.

C. Feeding Network

A 180 hybrid coupler is a four-port network with a180 phase shift between the two output ports and it can alsobe operated so that the outputs are in phase [19]. Such char-acteristics are exactly what it needs for the feeding networkdescribed above. So a 180 hybrid in rat-race form is designedas shown in Fig. 3. The lines from two input ports (1 and 2) andtwo output ports (3 and 4) below the two U-shaped slots havea characteristic impedance of 50 and the rat-race ring has acharacteristic impedance of .It’s worth noting that there is strong coupling between output

ports 3 and 4 as the currents generated by one output port willbe easily coupled to the other through the patch and groundplane. Due to the in-phase and out-of-phase excitations, the ac-tive impedance on the two output ports is different for the evenand odd modes. So we first design the rat-race structure whenthe antenna is in even mode and realize the impedance matchingon Port 1. Than a shorting line with length and distancefrom rat-race ring is used to further match the impedance of Port2, it serves as a shunt inductance and by tuning impedancematching can be obtained [21]. The path length between the fourpoints on the rat-race ring is labeled in the figure. The signal fedto Port 1 will be equally split into two components with a 180phase difference at the two output ports and the even mode ofthe shorted patch is obtained. If the signal is fed to Port 2, it willbe evenly split into two in-phase components at the two outputports and the odd mode of the shorted patch is then generated.The two input ports are always isolated as two signal paths fromone port to another on the ring have a difference of , whereis the wave length in the substrate.

JIANG et al.: A NOVEL NULL SCANNING ANTENNA USING EVEN AND ODD MODES OF A SHORTED PATCH 1905

Fig. 4. Pictures of fabricated antenna: (a) Top view without shorted patch;(b) Back view; (c) Overall look.

Fig. 5. Simulated electric field distribution by HFSS at 2.45 GHz. (a) Evenmode, (b) Odd mode.

III. SIMULATED AND MEASURED RESULTSTo verify the pattern reconfigurable characteristics, the pro-

posed antenna is first simulated by High Frequency StructureSimulator (HFSS), then fabricated as shown in Fig. 4, andfinally measured in anechoic chamber. Fig. 4(a) shows theground plane of the antenna where two identical U-shaped slotsare etched on. Fig. 4(b) shows the feeding network, which hasa little difference when compared with Fig. 3. An additionalopening strip is manually attached with a sticky copper foil afterthe substrate has been made up by photolithography technique.It is used to correct the impedance mismatch on Port 2 due tothe fabricated error of the shorted patch. Fig. 4(c) shows theoverall look of the antenna and a shorting plane is welded inthe center of the patch.

A. Simulated Field DistributionWith the help of HFSS, the electric field distribution between

patch and ground can be observed intuitively. Fig. 5 shows thefield plot of the two mode excitations around the proposed an-tenna in x-z cutting plane (20 mm offset the origin to avoid theinfluence of the excited U-shaped slots). It is clear to see theodd symmetry and even symmetry in the field distribution for

Fig. 6. Results of S parameters: (a) comparison of simulated and measuredones before correction; (b) measured ones after correction.

the even and odd modes, respectively, which verify the oper-ating principle of the antenna described in Section II.

B. S ParametersFig. 6(a) shows the simulated and measured S parameters be-

fore the impedance matching correction on Port 2. It can beseen that the measured results for both S11 and S22 have a fre-quency shift due to the fabricated errors. These errors are mainlycaused by the position error of the shorting plane and heighterror of the patch over the ground plane, as these two structuresare fabricated manually while the U-shaped slots and feedingnetwork on the substrate are fabricated by precise photolithog-raphy technique. The frequency shift for Port 1 is still within therequired operating bands. However, it moves out of the oper-ating bands for Port 2. This is because we designed the rat-racestructure by giving the priority to the impedance matching ofPort 1 which leading to the impedance matching on port 2 ismuch more sensitive to the manual fabrication errors. Thus, theimpedance matching correction is only done on Port 2.Fig. 6(b) shows the measured S parameters after the

impedance matching correction using the additional openingstrip. The opening strip here serves as a shunt capacitor [21] tocompensate the impedance matching caused by the short-cir-cuited strip . The final measured results show that both Port

1906 IEEE TRANSACTIONS ON ANTENNAS AND PROPAGATION, VOL. 62, NO. 4, APRIL 2014

Fig. 7. Radiated patterns for even mode: (a) E-plane; (b) H-plane.

Fig. 8. Radiated patterns for odd mode: (a) E-plane; (b) H-plane.

1 and 2 cover the WLAN band from 2.4 to 2.48 GHz. Theisolation between the two input ports maintains higher than30 dB through the whole band which provides the essentialconditions for the null scanning mechanism.

C. Radiated Patterns for Even and Odd Modes

Figs. 7 and 8 show the simulated and measured normalizedradiated patterns for even and odd modes at 2.45 GHz. Whenone port is under measurement, the other is in open circuit asthe high port isolation already ensures that the two ports workwithout interference. A broadside pattern obtained from evenmode is shown in Fig. 7. The measured and simulated gainsare 8.93 dB and 9.30 dB, respectively. A pattern with a null inbroadside obtained from odd mode is shown in Fig. 8. The mea-sured and simulated gains for this mode are 4.34 dB and 4.34dB, respectively. The beam direction of the odd mode is about40 in elevation plane. Fig. 9 shows the E-plane of the total mea-sured gain patterns of even and odd modes for comparison andthese two measured patterns are the basis patterns for null scan-ning that will be discussed in the following Section 2.

IV. DISCUSSION AND RESULTS OF NULL SCANNING

As was mentioned in Section I, null scanning could beachieved by combining the two patterns we have obtained inSection III with different exciting powers and a specific phasedifference. Thus, in this section, we will first demonstrate themechanism from theoretical analysis, then give the simulatedand measured results for null scanning and finally discuss theeffect of the phase difference on the pattern null.

Fig. 9. Total gain patterns of even and odd modes for comparison.

Fig. 10. Illustration for two magnetic dipole array.

A. Theoretical Analysis

A patch antenna can be treated as two magnetic currents sep-arated by distance from the view point of radiation, as shownin Fig. 10, and these two magnetic currents make up a two-el-ement array. For brevity, we consider the null scanning mecha-nism on the E-plane (xz-plane or ).It is known that the pattern of the element magnetic current

in xz-plane is omnidirectional. Thus, we define the field patternas

(1)

The array factor for the two elements with the same magni-tude and phase difference is [20]

(2)

where is the wave number in the air andfor a half-wave patch. For the even and odd modes, equals 0and , respectively. Thus, the array factors for these two modeson xz-plane are

(3)

JIANG et al.: A NOVEL NULL SCANNING ANTENNA USING EVEN AND ODD MODES OF A SHORTED PATCH 1907

and

(4)

respectively.Then, the total pattern for the twomodes would be the product

of the element pattern and the corresponding array factor, i.e.,

(5)

(6)

We assume that in (1), (5) and (6) is the value when theexciting power is unity. If the even and oddmodes are excited si-multaneously and the exciting power of the two modes is and, respectively, considering the relationship between power

and field intensity

(7)

the synthetic pattern then can be derived as

(8)

For a pattern null, it satisfies the equation

(9)

The angle of pattern null thus can be derived by substituting (9)into (8) as

(10)

From (10), we can see that by changing the ratio of the twoexciting powers, the pattern null will move on the xz-plane.Specifically, when the ratio is zero, which means only the oddmode is excited, will be zero indicating the null on broadside.And when the ratio is infinite, which means only the even modeis excited, will be 90 and the pattern null is endfire. In addi-tion, the range of the ratio is , where the neg-ative value means the two modes have a 180 phase differenceand the positive value implies that the two modes are excited inphase. Therefore, the null angle has the range from to90 in E-plane, which means that the null scanning could coverthe half-plane in elevation plane.It is worth noting that such the equivalence of the two mag-

netic currents in Fig. 10 isn’t reasonable for the patter of the oddmode in yz-plane. In (2), if is set to be 90 and equals , AFof the odd mode in yz-plane is zero which means the pattern inyz-plane is zero in every place and it contradicts to the resultsin Fig. 8. The fact is that, in contrast to the traditional patch,

Fig. 11. Pattern null at : (a) two ports excited in phase; (b) two portsexcited out of phase.

Fig. 12. Pattern null at : (a) two ports excited in phase; (b) two portsexcited out of phase.

Fig. 13. Pattern null at : (a) two ports excited in phase; (b) two portsexcited out of phase.

the proposed shorted patch in odd mode also has two equiva-lent out-of-phase magnetic currents on other two sides (perpen-dicular to y axis), thus making the pattern in yz-plane similarto that in xz-plane as shown in Fig. 8. In addition, these twoout-of-phase magnetic currents have no contribution either tothe pattern in xz-plane and previous discussion is not affected.However, null scanning could not be achieved in yz-plane aspolarizations of two modes are not the same in this plane.

B. Simulated and Measured Results for Null Scanning

Figs. 11–13 show the comparison of simulated and measuredresults at several different pattern nulls for discussion. Thepower ratio between the two input ports is also given forcomparison in the figure. The simulated results are derivedfrom HFSS and the measured results are derived according tothe following steps:

1908 IEEE TRANSACTIONS ON ANTENNAS AND PROPAGATION, VOL. 62, NO. 4, APRIL 2014

Fig. 14. The effect of exciting power ratio on both null angle and null depth.

— the patterns with magnitude and phase for even and oddmodes are measured in anechoic chamber respectively,and when the port of one mode is being measured, theother is in open circuit; the pattern results and

obtained in this step are actually those in part Cof Section III;

— import the measured even and odd pattern results toMATLAB to calculate the synthetic pattern using

(11)

— find the in calculated where the concave spot lo-cates in upper half plane as the patternnull;

From the figure, we can see that the measured null patternsagree very well with the simulated ones. And the simulated andmeasured power ratio for the same null angle has only littledifference. From the figure we could also see that when the twoports were excited in phase and out of phase, the pattern nullscans in the plane of and , respectively, and the twopatterns with the same power ratio are in mirror symmetry withthe symmetry plane of .Fig. 14 illustrates the angle of null with the function of the

port exciting ratio. Theoretical, simulated, and measured curvesare all plotted for comparison. The ratio of the horizontal axis isdisplayed in logarithmic format. The theoretical curve is drawnfrom (10). It first indicates that three curves coincide with eachother pretty well except for large scanning null angles where thedifferencemay be caused by the effect of the finite ground plane.Both the measured and simulated results validate the theoreticalanalysis discussed at the beginning of this section and it is veryinstructive that we could follow this principle to design otherdifferent antennas with the capability of null scanning. Fig. 14also indicates that when the power ratio changes from (oddmode) to (even mode), the pattern null could scan in theupper half xz plane, i.e., the elevation angle changes from 0 to90 .The simulated effect of the exciting power ratio on the null

depth is also illustrated on Fig. 14 for demonstration. The resultsshow that for different power ratio, in other words, for differentangle nulls, the depth of the nulls is all below dB and this

Fig. 15. The effect of phase different between two input ports.

value is completely enough to define the pattern null. Thus wecan conclude that the power ratio has little effect on the nulldepth.

C. The Effect of Phase DifferenceFig. 15 demonstrates the effect of phase difference between

the two exciting ports for both the angle and depth of the patternnull. All the results are derived when port ratio equals 1(0 dB). In theory, the deepest null occurs at the phase differenceof 0 or 180 , which is applicable for all the angles of patternnulls; however, the results show that there is a shift. Thisphase shift is introduced by the unequal length of the feedinglines from the port to the rat-race ring. It can be seen from thefigure that the null depth changes with different phase differ-ences while the pattern null stays at 35 with no change.This result indicates that by setting a specific phase difference,the null depth could reach the desired level while the angle ofthe pattern null remains stable in a wide range around the phasedifference where the deepest null is achieved. And the phase dif-ference can be set in the design of the feeding network or tunedwhen the ports are excited. Meanwhile, the angle of pattern nullis only controlled by the power ratio as it is demonstrated in partB, that is to say, null angle and null depth can be controlled in-dependently.

V. CONCLUSIONThis paper presents a pattern null reconfigurable antenna op-

erating at WLAN band of 2.4 GHz. The antenna consists of ashorted patch, and a slot coupled feed method is used to excitethe patch. The proposed antenna has a broadside pattern fromits even mode and a pattern with null in broadside from its oddmode. These two modes are excited by a rat-race feeding net-work and they are combined to achieve null scanning mecha-nism in the E-plane. The exciting power ratio of the two modescontrols the pattern null angle and the phase difference controlsthe null depth. Small size makes the antenna suitable for thearray design. Theoretical analysis provides a kind of new guid-ance to design null scanning antennas and with this other fea-tures such as wideband and dual polarization could be added inthe future.

JIANG et al.: A NOVEL NULL SCANNING ANTENNA USING EVEN AND ODD MODES OF A SHORTED PATCH 1909

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Xiaolei Jiang (S’13) was born in Suzhou, China, in1989. He received the B.S. degree in electronics andinformation engineering from Tsinghua University,Beijing, China, in 2011. He is currently working to-ward the Ph.D. degree in electrical engineering at Ts-inghua University.His current research interests include antenna

design and theory, particularly in reconfigurableantennas and base station antennas.

Zhijun Zhang (M’00–SM’04) received the B.S. andM.S. degrees from the University of Electronic Sci-ence and Technology of China, in 1992 and 1995, re-spectively, and the Ph.D. degree from Tsinghua Uni-versity, Beijing, China, in 1999.In 1999, he was a Postdoctoral Fellow with the

Department of Electrical Engineering, University ofUtah, Provo, UT, USA, where he was appointed a Re-search Assistant Professor in 2001. In May 2002, hewas an Assistant Researcher with the University ofHawaii at Manoa, Honolulu, HI, USA. In November

2002, he joined Amphenol T&M Antennas, Vernon Hills, IL, USA, as a Se-nior Staff Antenna Development Engineer and was then promoted to the posi-tion of Antenna Engineer Manager. In 2004, he joined Nokia Inc., San Diego,CA, USA, as a Senior Antenna Design Engineer. In 2006, he joined Apple Inc.,Cupertino, CA, USA, as a Senior Antenna Design Engineer and was then pro-moted to the position of Principal Antenna Engineer. Since August 2007, he hasbeen with Tsinghua University, where he is a Professor in the Department ofElectronic Engineering. He is the author of Antenna Design for Mobile Devices(Wiley, 2011). He is serving as Associate Editor of the IEEE TRANSACTIONS ONANTENNAS AND PROPAGATION and the IEEE Antennas and Wireless Propaga-tion Letters.

Yue Li (S’11–M’12) received the B.S. degree fromthe Zhejiang University, Zhejiang, China, in 2007,and the Ph.D. degree from Tsinghua University, Bei-jing, China, in 2012.Since June 2012, he has been with Tsinghua

University, where he is a Postdoctoral Fellow inthe Department of Electronic Engineering. He wasalso a Visiting Scholar in the Institute for InfocommResearch (I R), A*STAR, Singapore, and HawaiiCenter of Advanced Communication (HCAC),University of Hawaii, USA. He has authored and

coauthored over 30 journal papers, and holds over 10 granted and filed Chinesepatents. His current research interests include antenna design and theory,particularly in reconfigurable antennas, mobile/handset antennas, diversityantennas, and antennas in package. He is also serving as a reviewer of the IEEETRANSACTIONS ON ANTENNAS AND PROPAGATION and the IEEE Antennas andWireless Propagation Letters.

Zhenghe Feng (M’05–SM’08–F’12) received theB.S. degree in radio and electronics from TsinghuaUniversity, Beijing, China, in 1970.Since 1970, he has been with Tsinghua University,

as an Assistant, Lecture, Associate Professor, andFull Professor. His main research areas includenumerical techniques and computational electro-magnetics, RF and microwave circuits and antenna,wireless communications, smart antenna, and spatialtemporal signal processing.