IEEE TRANSACTIONS ON ANTENNAS AND PROPAGATION, VOL. 65, NO. 6, JUNE 2017 3217

CommunicationA Design of U-Shaped Slot Antenna With Broadband

Dual Circularly Polarized RadiationRui Xu, Jian-Ying Li, Jiang-Jun Yang, Kun Wei, and Yang-Xiao Qi

Abstract— In this communication, a novel broadband dual circularlypolarized (CP) antenna is presented. This antenna consists of anU-shaped slot to achieve broadband CP radiation and two 50-�microstrip-fed ports to realize both right-hand circular polarization andleft-hand circular polarization at the same frequency band. The axialratio bandwidth (ARBW) is greatly broadened by moving the feedingports to upper portion of the U-shaped slot. To verify this design, theproposed antenna is fabricated and measured. The measured −10 dBreflection coefficient bandwidth is about 114.4% (1.80–6.61 GHz) andthe 3-dB ARBW is approximately 110.5% (1.83–6.35 GHz). The isolationbetween two ports is better than 14.8 dB within the ARBW.

Index Terms— Broadband, dual circularly polarized (CP), U-shapedslot antenna.

I. INTRODUCTION

With the rapid development of modern wireless communicationsystems, circularly polarized (CP) antennas have been paid moreattentions because of their advantage on reducing multipath interfer-ences. CP antennas have been widely used in many systems, such asradio frequency identification devices (RFIDs) and wireless local areanetworks. Traditional microstrip CP antennas have very narrow band-width. A communication system, which works at several frequencybands, needs more than two microstrip CP antennas. To realize thesimultaneous operation of multiple communication systems in thesame module, broadband CP antenna has become a hot research topic.There exist some basic types to achieve broadband CP antennas, suchas microstrip slot antenna [1], substrate integrated waveguide (SIW)slot antenna [2], loop antenna [3], monopole antenna [4], [5], slot-monopole antenna [6], [7], and dipole antenna [8]. A planar squareslot CP antenna is proposed for RFID reader with an arc-shapedstrip [1]. The operating band can reach about 142 MHz and the 3-dBaxial ratio bandwidth (ARBW) is about 166 MHz. A wideband CPantenna element and antenna array are achieved by positioning a slot-coupled rotated strip above a slot cut onto the broad wall of an SIWfor 60-GHz wireless personal area networks applications [2] withlow-temperature cofired ceramic technology. The ARBW is about10.7% (60.2–67 GHz). A CP antenna array using shorted loop fedby a four-port structure with sequential phase of 0°, 90°, 180°, and270° is presented in [3]. The ARBW is improved from 2% ofsingle CP patch to 7% of the CP array. A monopole antenna withasymmetric feed and a chifre-shaped structure is designed for broad-band CP radiation [4]. By adjusting the location of the monopolemicrostrip feed, a CP antenna with 72% impedance bandwidth (IBW)

Manuscript received October 20, 2016; revised February 22, 2017; acceptedMarch 21, 2017. Date of publication March 29, 2017; date of cur-rent version May 31, 2017. This work was supported in part by theNational Natural Science Foundation of China under Grant 61271416,Grant 61301093, and Grant 61401365, and in part by the Fundamen-tal Research Funds for the Central Universities under Grant GEKY8002.(Corresponding Author: Rui Xu.)

The authors are with the School of Electronics and Information, Northwest-ern Polytechnical University, Xi’an 710072, China (e-mail: rxuilj@163.com;jianyingli@nwpu.edu.cn).

Color versions of one or more of the figures in this communication areavailable online at http://ieeexplore.ieee.org.

Digital Object Identifier 10.1109/TAP.2017.2689069

and 41.6% 3-dB ARBW is achieved. Another antenna uses aC-shaped monopole and an open loop to realize a wider CP band-width, the ARBW is extended to 65.2% [5]. A combination of aslot and a monopole antenna is used to produce CP wave with 30%ARBW in [6]. Another slot-monopole structure antenna applies atriangular monopole and a slot etched on the ground plane to produceCP wave with 46.8% ARBW [7]. A new broadband rotationalsymmetry CP antenna is designed based on dipole antenna, whichARBW is 96.6% [8]. The CP antenna with dual polarization canmeet the requirement of polarization diversity in many differentcommunication systems. Many articles have made study of dual-CPantennas [9]–[14]. A patch antenna with two cross slots, which isfed by two L-strips, produces dual-CP radiation [9]. It exhibits anIBW and ARBW of about 10% and 4.8%, and the isolation is betterthan 15 dB. A square slot antenna loaded with a T-shaped strip andtwo microstrip T-junctions is produced for dual-CP radiation [10].The ARBW is 19.6% and the isolation is better than 20 dB. A dual-CP antenna fed by two fed ports connected with a circular split-ring microstrip line underneath the ground plane is designed forRFID reader [11]. It provides IBW of 18% and ARBW of 11%, andthe isolation within the RFID operating band is better than 25 dB.Another RFID reader uses a branch-line coupler with H-shapedground slots to excite dual-CP radiation with wideband isolation [12].The IBW is 29.1%, and ARBW is 18.7%. A broadband dual-CP patchantenna is fed by four cross slots [13]. The IBW, 10-dB isolationbandwidth, and 3-dB ARBW are 24%, 19%, and 16%, respectively.Two asymmetric T-shaped feed lines and inverted-L grounded stripswith three straight strips are used to achieve dual-CP radiation [14].The ARBW can reach up to 60%. A new monopole structure isdesigned for dual-CP in [15]. The ARBW is expanded to 80.7%.

In this communication, a simple wideband U-shaped slot CPantenna is proposed. It consists of an asymmetric-fed radiator and aframe structure lower ground with an U-shaped slot. By optimizingthe parameters, a wideband dual-CP slot antenna is achieved. Themeasured ARBW reaches to 110.5% (1.83–6.35 GHz). This com-munication is organized as follows. Some introductions about singleand dual-CP antennas are introduced in Section I. The structure andanalysis of proposed antenna are shown in Section II. The measuredresults are presented in Section III. The conclusion is given inSection IV.

II. ANTENNA DESIGN AND ANALYSIS

A. Antenna Structure

The top view and side view of proposed dual-CP antenna are shownin Fig. 1. This antenna is printed on a square FR4 substrate material(εr = 4.4, tanδ = 0.02). It consists of two microstrip-fed ports(Port 1, Port 2), an U-shaped slot, and a frame structure ground.This CP antenna uses a microstrip-fed rectangular radiator to excitean U-shaped slot. CP wave is generated by two orthogonal E vectorswith equal amplitude and 90° phase difference. The broadband CPwave of this antenna is mainly implemented by the U-shaped slot(mode1) and asymmetric feeding structure (mode2). The U-shaped

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3218 IEEE TRANSACTIONS ON ANTENNAS AND PROPAGATION, VOL. 65, NO. 6, JUNE 2017

Fig. 1. Top and side views of proposed antenna.

slot can provide mode1 with superimposed perturbation orthogonalcurrent distributions on the outer contour surface at lower frequency.And the asymmetric feeding structure provides another mode2 withcurrent distributions in x and y directions on the inner contoursurface at upper frequency. A broadband CP antenna is achieved bysuperimposing the two modes.

When Port 1 is excited and Port 2 is matched, it radiates right-hand circular polarization (RHCP) wave. On the contrary, it radiatesleft-hand circular polarization (LHCP) wave. Since the symmetrystructure, this work just gives the simulated and measured results(Simulated by HFSS 13.0) when Port 1 is excited and Port 2 ismatched.

As shown in Fig. 1, the length of the radiator(L M = L + G) is approximately quarter wavelength of theslot at the mid-frequency (FM ) of the ARBW. The L M is roughlyevaluated

L M ≈ L + G = 300

4 × FM ×√

1 + εr2

(mm) (1)

where εr is the relative permittivity of the loaded substrate FR4. Sizeunit is in millimeters. The effect of other geometric parameters on thefrequency range is not patency. This antenna is designed to work atthe center frequency of 4 GHz, the estimated L M is about 10.5 mm,which is very close to the simulation value (L + G = 8.5 + 1.5 =10 mm).

The length of the wide U-shaped slot (Length−slot) is about half-space wavelength (λ0/2) at lowest frequency (2 GHz) (Length−slot ≈λ0/2 = 300/2/2 = 75 mm). The estimated wide Length−slot isabout 75 mm (λ0/2 = 300/2/2 mm), which is also very close tothe simulation value. To cut an U-shaped slot with λ0/2, a squareFR4 substrate material with dimension of 48 × 48 mm2 is used.

B. Steps to Realize Proposed Antenna

To explaining the design process of the proposed antenna, six stepsdepicting the evolution structure are shown in Fig. 2. The simulatedresults of six antennas, including S-parameters (S11, S21), amplituderatio (EX/EY), phase difference of two orthogonal far fields EX andEY in +z direction, and AR, are shown in Fig. 3.

Ant. 1 includes only two straight microstrip-fed lines (0.8 mm ×16.5 mm) and a rectangular ring ground structure, and it is adual-linear-polarized antenna. The operating frequency band is from3.6 to 8 GHz, while the isolation at lower band is not good. Ant. 2adds a metal patch (13 mm × 15.5 mm) on the lower ground toproduce a concave-shaped slot. Compared to the rectangular ring

Fig. 2. Steps to realize proposed antenna (Ant. 1–6, unit: mm).

Fig. 3. Simulated results of Ant. 1–6. (a) S11. (b) S21. (c) Amplitude ratio(|Ex/Ey|). (d) Phase difference. (e) AR.

ground structure, concave-shaped slot extends the current path, andit makes the resonant frequency shift to lower frequency (3–8 GHz).The concave-shaped slot excites mode1 to generate CP wave at lowerfrequency (2.6–3.4 GHz). Ant. 3 moves the two microstrip-fed linesalong the −x direction (Sf = 8 mm) to motivate another mode2. It is adual-band CP antenna. In order to get a broadband CP antenna, Ant. 4fabricates two circular chamfers (R1, R2) at the lower ground. Thisapproach not only reduces the difference of two resonant frequency

IEEE TRANSACTIONS ON ANTENNAS AND PROPAGATION, VOL. 65, NO. 6, JUNE 2017 3219

Fig. 4. Surface current distributions of proposed antenna.

Fig. 5. Simulated and measured S-parameters.

modes, but also improves the amplitude ratio and phase differenceof two orthogonal far fields at middle frequency. The U-shaped slotshortens the current path, and it makes the resonant frequency shift toupper frequency (2.7–6.3 GHz). To extend the IBW, Ant. 5 broadensthe end of microstrip-fed line with a wider metal patch (K × L).By adjusting the parameter K , the input resistance (Re) and inputreactance (Im ) can be controlled. A wide IBW (2.1–6.7 GHz) isrealized with increasing K to 3.9 mm. In addition, the amplituderatio is also improved at middle frequency, and the AR value isbelow 3 dB at 5 GHz. Ant. 6 (Prop.) inserts a rectangular strip(2 mm × 14 mm) along x-direction. It can slightly improve theamplitude ratio and phase difference of two orthogonal at upperfrequency, the ARBW (2–6.5 GHz) of Ant. 6 is further expanded.Because parts of electromagnetic wave of Ant. 1–4 are reflected atlower frequency band, the isolations between two ports are good. Theonly drawback of Ant. 5 and 6 is that the isolation between two portsbecomes poor at lower frequency band, since the electrical length isshorter with decreasing of working frequency. Thus, a broadbanddual-CP slot antenna is achieved.

C. Surface Current Distributions of Proposed Dual-CP Antenna

To verify the generation of dual-CP radiation, the simulated surfacecurrent distributions of proposed dual-CP antenna at 2, 4, and 6 GHz

Fig. 6. Simulated and measured AR and antenna gain.

Fig. 7. Simulated and measured antenna efficiency.

are illustrated in Fig. 4. The surface currents at 2 GHz are shownin Fig. 4(a). The CP wave is produced by the synthetic currents onthe outer contour of U-shaped slot. The 0° phase shows that thesuperimposed current is −x directed with ϕ is −45°. For 90° phase,the superimposed surface current is in +x directed with ϕ is 45°. Thesurface currents at 4 and 6 GHz are shown in Fig. 4(b) and (c). It canbe seen that the CP waves are generated by the currents on the innermetal patch on the lower ground at 4 and 6 GHz. When this antennaworks at 4 GHz, the 0° phase current is along +x direction. The90° phase surface current is along +y direction. When this antennaworks at 6 GHz, the 0° phase current is along −y direction. The 90°phase surface current is along +x direction.

III. EXPERIMENTAL RESULTS

The S-parameters and picture of proposed antenna are shownin Fig. 5. The S-parameters are tested by Agilent N5230A. Themeasured IBW for S11 < −10 dB is 114.4% (1.80–6.61 GHz), andthe measured S21 is below −14.7 dB within the whole operatingband. The tested results agree well with the simulated results.

Fig. 6 illustrates the simulated and measured antenna gainand AR curves against frequency. The measured 3-dB ARBWis 110.5% (1.83–6.35 GHz). The measured peak antenna gain is4.5 dBic at 3.6 GHz, comparing the measured and simulated date,the working frequency of the actual antenna shifts about 300 MHz

3220 IEEE TRANSACTIONS ON ANTENNAS AND PROPAGATION, VOL. 65, NO. 6, JUNE 2017

TABLE I

COMPARISON OF BROADBAND DUAL CIRCULARLY POLARIZED ANTENNAS

Fig. 8. Simulated and measured normalized radiation patterns at.

to lower frequency. The antenna efficiency is shown in Fig. 7, boththe measured and simulated efficiency are more than 85% within thewhole 3-dB ARBW.

The normalized radiation patterns of the proposed antenna at 2, 4,and 6 GHz in xz plane and yz plane are shown in Fig. 8. There areabout 16, 19, and 23.5 dB differences between the simulated RHCPand LHCP components at 2, 4, and 6 GHz, respectively. The radiationpatterns deviate from the +z direction, which is shown in Fig. 8(c),is the main reason why the peak gain at upper band is lower thanthe lower band.

A comparison between this work and previous works [9]–[15] isillustrated in Table I. It shows that this antenna has the advantages ofwider IBW, the smallest relative size, good isolation performancewithin the whole working frequency band, and the widest 3-dBARBW.

IV. CONCLUSION

A novel broadband dual-CP U-shaped slot antenna is proposed inthis communication. The U-shaped slot is excited by two asymmetric

microstrip-fed ports. By adjusting the position of fed ports and theproportion of U-shaped slot, a broadband dual-CP antenna with goodisolation is achieved. The 10-dB return-loss bandwidth is 114.4%(1.80–6.61 GHz), 3-dB ARBW is 110.5% (1.83–6.35 GHz), and theisolation between two ports is better than 14.8 dB. The antenna gainis stable and varies from 3.1 to 4.5 dBic. Therefore, this antenna isvery suitable for broadband dual-CP communication system.

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