Low-Profile Compact Circularly Polarized Slot-Etched PIFA ...

IEEE TRANSACTIONS ON ANTENNAS AND PROPAGATION, VOL. 67, NO. 6, JUNE 2019 4189

CommunicationLow-Profile Compact Circularly Polarized Slot-Etched

PIFA Using Even and Odd ModesLibin Sun , Yue Li , Zhijun Zhang , and Zhenghe Feng

Abstract— In this communication, a novel dual-fed slot-etched planarinverted-F antenna (PIFA) is proposed for the miniaturization of wide-band circularly polarized (CP) antenna. By applying in-phase feedingand out-of-phase feeding at two ports of the antenna, the PIFA (evenmode) and open-slot (odd mode) modes can be excited, respectively,with orthogonal polarizations. A simple approach is used to deduce therequired phases at the two ports to generating a 90° phase shift betweeneven and odd modes. The λ/4 resonant property and shared-apertureconfiguration of even and odd modes fulfill the proposed size-reductionscheme for a CP radiation. A prototype was simulated, fabricated, andmeasured to verify the performance. The measured results show that anoverlapping fractional bandwidth of 14.7% with S11 < −10 dB, axialratio < 3 dB, and gain variation < 3 dB is achieved within a compactvolume of 0.3 × 0.23 × 0.032λ3

0. The excellent features of the proposeddesign, such as its simple feeding strategy, low profile, compact size,low cost, and wide bandwidth, make it applicable for universal radiofrequency identification readers.

Index Terms— Circularly polarized (CP), even and odd modes, planarinverted-F antenna (PIFA), radio frequency identification (RFID).

I. INTRODUCTION

Circularly polarized (CP) antennas have widely been investigateddue to their unique properties, such as insensitivity to the orienta-tion of transmitting and receiving antennas, resistance to multipathfading, and reduction in Faraday rotation effect [1], [2]. In ultrahighfrequency (UHF) radio frequency identification (RFID) systems, CPreaders are required to avoid the loss caused by polarization mis-matching. The operating frequency of the CP RFID systems is variedin different countries, thus the overlapping fractional bandwidthfor the universal RFID application is 12.7% (840–954 MHz) [3].Therefore, it is necessary to design a wideband CP antenna within acompact footprint for size-limited handheld devices.

In [4] and [5], simple single-fed dual-layer planar inverted-Fantennas (PIFAs) for CP radiation are proposed. In [6]–[9], someminiaturized single-fed CP patch antennas are investigated. Theabove-mentioned single-fed CP antennas have simple structures butthey cannot meet the demands of wideband within a limited size andprofile. Hence, multi-fed CP antennas are demonstrated to effectivelybroaden the antenna bandwidth. Dual-fed [11]–[13] and integrated-fed [14] patch antennas have been reported to achieve widebandCP radiation. However, the large size and high profile of the anten-nas reported in [11]–[14] make them inappropriate for size-limitedapplication.

Manuscript received November 12, 2018; revised February 19, 2019;accepted February 28, 2019. Date of publication March 19, 2019; date ofcurrent version May 31, 2019. This work was supported in part by theNational Natural Science Foundation of China under Contract 61525104 andContract 61771280 and in part by the Beijing Natural Science Foundationunder Contract 4182029. (Corresponding author: Zhijun Zhang.)

The authors are with the Beijing National Research Center for InformationScience and Technology (BNRist), Tsinghua University, Beijing 100084,China (e-mail: [email protected]).

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.2019.2905988

Sequential rotation arrays (SRAs), first proposed in 1986 [15],are widely used to achieve a wide axial ratio (AR) beamwidthand bandwidth [16], [17]. Recently, several tightly coupled SRAs[18]–[25] have been investigated toward reducing the footprint ofSRAs. In [18], a wide beam spiral antenna with three folded armsis presented; however, the bandwidth is limited to 3.5%. In [19],an improved triple-fed PIFA SRA is proposed to further increasethe bandwidth. An ultracompact PIFA SRA with four λ0/16 folded-shorted patch elements [26] is investigated in [20] and [21], with avolume of 0.2 λ0 × 0.2λ0 × 0.048λ0. In [22]–[24], 2 four-elementshorted patch SRAs [22], [23] and an inverted-F antenna (IFA)SRA [24] are presented, respectively, with a miniaturized size andwide bandwidth. The aforementioned CP SRAs [18]–[25] can meetthe miniaturization and broadband requirements; however, they needcomplex feed networks and extra workloads to squeeze the feednetworks into a compact footprint.

To simplify the design architecture and feeding strategy of thecompact and wideband CP antennas, a new dual-fed slot-etched PIFAscheme is proposed in this communication. Although PIFA is wellknown for its quarter-wavelength resonant property, the absence ofdegenerate modes, however, makes it more difficult to achieve CPradiation compared with the conventional half-wavelength microstripantenna. Here, we offer a new orthogonal-polarized mode in thePIFA by simply etching an open slot in the center position. Then,by exciting these two modes with a tailored dual-fed strategy, a CPantenna with a simple structure, compact size, wide bandwidth, andhigh efficiency can be acquired within a simple PIFA architecture.

II. ANTENNA DESIGN AND OPERATING MECHANISM

A. Antenna Design

The proposed antenna is derived from a conventional PIFA thatenables x-polarized radiation, as shown in Fig. 1(a). With a throughslot etched in the center of the PIFA, the proposed slot-etchedPIFA with two symmetrical feeding ports is achieved as presentedin Fig. 1(b). The top and side views of the proposed antenna areshown in Fig. 1(c) and (d), respectively. As can be seen, one side ofthe center slot is open circuit, whereas another side is short circuitwith the ground plane, forming a standard quarter-wavelength open-slot layout. The total size of the antenna is 115 × 95 mm2, with alow profile of 10.8 mm; detail dimensions are listed in Table I.

B. Mode Analysis

The operating modes of the proposed antenna are illustratedin Fig. 2. When Ports 1 and 2 are fed in-phase with equal amplitude

Ee1 = Ee2 = A0e jωt (1)

the even mode is excited, as illustrated in Fig. 2(a). The E-fieldsof the left and right cavities are in the same direction, enablingan x-polarized PIFA mode with two equivalent in-phase magneticcurrents. The center open slot does not work under the condition of

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4190 IEEE TRANSACTIONS ON ANTENNAS AND PROPAGATION, VOL. 67, NO. 6, JUNE 2019

Fig. 1. Evolution and geometry of the proposed antenna. (a) ConventionalPIFA. (b) Proposed slot-etched PIFA. (c) Top view. (d) Side view.

TABLE I

DETAILED DIMENSIONS (UNIT: mm)

Fig. 2. Vector electric field distributions and equivalent magnetic cur-rents of the proposed slot-etched PIFA. (a) In-phase excited (even mode).(b) Out-of-phase excited (odd mode).

in-phase excitation. In contrast, when Ports 1 and 2 are fed equalamplitude and out-of-phase

Eo1 = A0e jωt and Eo2 = A0e j (ωt+π) (2)

the odd mode is excited, as shown in Fig. 2(b). The E-fields of theleft and right cavities are in the inverse direction, thus suppressingthe radiation of the PIFA mode. However, in this case, the centeropen slot works and generates a y-polarized radiation owing to thedifferential excitation. Therefore, a pair of λ/4 orthogonal modes isobtained; the next step is to achieve a 90° phase shift between thesetwo modes to yielding CP radiation.

C. Phase Analysis

To excite even and odd modes simultaneously with a 90° phaseshift, a simple feeding scheme is derived by utilizing a vector fieldsynthesis approach, as illustrated in Fig. 3. Fig. 3(a) presents theinitial feeding signals of the even (Ee1 and Ee2) and odd modes (Eo1and Eo2), plotted in a phase coordinate diagram. To generate CPradiation, the 90° phase shift should be supplied between these twomodes. With the 90° phase shift supplied in the even mode, the

Fig. 3. Vector field synthesis approach to deduce the required feeding signalsat Ports 1 and 2 to achieve a CP radiation. (a) Initial feeding signals for even(in blue) and odd (in red) modes. (b) Revised feeding signals for even (in blue)and odd (in red) modes with 90° phase shift considered and the final syntheticfeeding signals (in purple).

Fig. 4. Vector current distributions of the proposed slot-etched PIFA at900 MHz when fed with equal amplitude and 90° phase shift. (a) t = 0.(b) t = T/4. (c) t = T/2. (d) t = 3 T/4.

feeding signals of the even mode are revised as

Ee1 = Ee2 = A0e j (ωt+π /2) (3)

whereas those of the odd mode remain unchanged. Fig. 3(b) showsthe revised plot with the 90° phase shift considered. When the feedingsignals of even and odd modes are superposed at Port 1, the totalfeeding signal at Port 1 is

Et1 = Ee1 + Eo1 = √2A0e j (ωt+π/4). (4)

Similarly, the total feeding signal at Port 2 is obtained as

Et2 = Ee2 + Eo2 = √2A0e j (ωt+3π/4). (5)

From (4) and (5), the final feeding signals at Ports 1 and 2 could beconcluded to have equal amplitude and 90° phase difference

Et2/Et1 = e jπ /2. (6)

Note that the ultimate feeding amplitudes and phases at Ports 1 and 2are coincident with those between the two modes, but they areessentially two different concepts.

Fig. 4 depicts the vector current distributions of the proposedantenna excited with equal amplitude and 90° phase shift. Whent = 0, the open-slot mode is excited with inverse currents between theleft and right cavities. When t = T/4, the PIFA mode is excited withthe same currents between the left and right cavities. Accordingly,broadside CP radiation is achieved within a simple slot-etched PIFAlayout.

D. Antenna Tuning and Parameter AnalysisThere are two vital conditions for a good CP performance: 1) equal

amplitude for two orthogonal polarizations and 2) a 90° phase shiftbetween two orthogonal polarizations. Thus, it is significant to ensure

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Fig. 5. Active S11 of the even and odd modes with the feeding position Lfvaried. The active S11 for even and odd modes are extracted when Ports 1and 2 fed in-phase and out-of-phase, respectively.

Fig. 6. Broadside AR of the proposed antenna with different sizes of theground plane Wg.

the consistency of the radiation field intensity for the PIFA andopen-slot modes in the proposed design, whereas the phase shift isdependent mainly on the performance of the feed network. That is,the two modes should have identical active impedance characteristicsand radiation gains to ensure the consistent field intensity. Accordingto the above-mentioned targets, the antenna tuning process is listedas following: 1) adjusting the resonant lengths of the PIFA and open-slot modes to achieve equal resonant frequency; 2) adjusting thefeeding position Lf to achieve similar active impedance performancefor the two modes; and 3) adjusting the radiation aperture of thePIFA (i.e., L1) and the sizes of the ground plane (i.e., Lg and Wg)to achieve similar radiation gain for the two modes.

Fig. 5 presents the active S11 of the even and odd modes with thefeeding position Lf varied. As shown Fig. 5, the feeding position Lfhas an optimal value to achieve similar active impedance matchingbetween the two modes.

The sizes of the ground plane (i.e., Lg and Wg) are also ofgreat significance to the radiation performance. In this design, Wg isalong the resonant edge of the PIFA and thus has a larger impacton the radiation performance compared with Lg. The impacts onthe AR bandwidth and beamwidth with different Wg are shownin Figs. 6 and 7, respectively. Note that the influence of the feednetwork is not considered in this section. As indicated in Fig. 6, theAR at low frequency can be improved by increasing Wg. Fig. 7 showsthe angle response of AR at 900 MHz; the optimal angle of AR isdecreased from positive to negative angles with the increasing of Wg.Therefore, Wg is optimized to 95 mm with both the frequency andangle responses considered.

The 3 dB gain bandwidth, which is narrower than the AR andimpedance bandwidth, is the major limitation of multi-fed size-limited antennas. The gain bandwidth with different aperture sizes(i.e., L1) is analyzed in Fig. 8. The realized gain at low frequencycan be improved by increasing the size of the radiation aperture.

E. Feed Network

To feed signals with equal amplitude and 90° phase shift at twoports, a classical Wilkinson power divider [27] with a 90° phase

Fig. 7. AR beamwidth of the proposed antenna at 900 MHz with differentsize of the ground plane Wg.

Fig. 8. Realized gain of the proposed antenna with the radiation apertureL1 varied.

Fig. 9. Layout of the Wilkinson feed network.

delay line is designed, as shown in Fig. 9. The 90° phase differenceof output Ports 2 and 3 is achieved by the delay line, which satisfies

β(d1 − d2) = π/4 (7)

where β = 2π /λg. A 100 � chip resistor is soldered between twofeed lines to achieve a high isolation.

The S-parameters of the proposed Wilkinson feed network arereported in Fig. 10. The S11 is better than −25 dB across thedesired band of 0.8–1.0 GHz. Good amplitude consistency with| S21–S31| < 0.2 dB is obtained. Besides, the isolation between theoutput ports are better than 19 dB with the help of the isolated resistor.The phase shift between the output ports is 90° ± 10° across thedesired band.

III. ANTENNA FABRICATION AND MEASUREMENT RESULTS

A. Antenna Fabrication

In order to validate the performance of the proposed antenna,a prototype was fabricated, as shown in Fig. 11. The antenna isconstructed by using two folded 0.3 mm-thick brass plates, cutby a laser cutting machine with a high precision of ±0.1 mm.The strength of the brass plates enables the proposed structure tobe self-supporting. Under the brass plates is a 0.8 mm-thick FR-4(εr = 4.4, tanδ = 0.02) substrate: the front is a metal ground plane,

4192 IEEE TRANSACTIONS ON ANTENNAS AND PROPAGATION, VOL. 67, NO. 6, JUNE 2019

Fig. 10. Simulated S-parameters of the proposed Wilkinson feed network.

Fig. 11. Photographs of the proposed antenna. (a) Prototype. (b) Feednetwork on the back side of the FR-4 substrate.

Fig. 12. Simulated and measured S11 and AR of the proposed antenna.

and the back is a Wilkinson feed network, as shown in Fig. 11(b).A 50 � semirigid cable is soldered at the input port of the Wilkinsonfeed network for testing. The ground plane is soldered with thevertical walls of the folded brass plates to form a good electricalconnection. The simulated and measured broadside AR bandwidthsare also presented in Fig. 12. The simulated AR is less than 3 dBacross the band of 0.8–1 GHz, whereas the measured 3 dB ARbandwidth is from 0.825 to 1 GHz. Fig. 13 shows the angle responsesof AR in the xz and yz planes at 0.84, 0.9, and 0.95 GHz. At thecenter frequency of 0.9 GHz, the 3 dB AR beamwidth is from −35°to 19° (54°) in the xz plane and from −31° to +29° (60°) in the yzplane. The asymmetric angle response in the xz plane is caused bythe asymmetric boundary of the PIFA.

The simulated and measured normalized radiation patterns in the xzand yz planes at 0.84, 0.9, and 0.95 GHz are depicted in Fig. 14.Small angle tilts are observed in both planes, with a small gaindrop in the broadside direction. In the xz plane, the maximum gaindeviates to θ = −18° due to the asymmetric boundary of the PIFA.In the yz plane, the maximum gain deviates to θ = 25° due to

Fig. 13. Simulated angle responses of AR in the xz and yz planes at 0.84,0.9, and 0.95 GHz.

Fig. 14. Simulated and measured normalized radiation patterns of theproposed antenna. (a) xz plane at 0.84 GHz. (b) yz plane at 0.84 GHz. (c) xzplane at 0.9 GHz. (d) yz plane at 0.9 GHz. (e) xz plane at 0.95 GHz. (f) yzplane at 0.95 GHz.

the asymmetric feed phase at the two ports. Fig. 15 presents thesimulated and measured broadside CP gains and total efficienciesversus the frequency. In contrast to the wide impedance and ARbandwidths, the 3 dB gain bandwidth is not that wide due to theintrinsic narrow-band nature of the low-profile PIFA. The measuredmaximum CP gain in the broadside direction is 2.1 dBic at 0.91 GHz.A measured stable gain is realized with the variation less than3 dB from 0.838 to 0.97 GHz, which indicates a fractional 3 dBgain bandwidth of 14.7%. The proposed antenna has simulated andmeasured maximum efficiencies of 86.6% and 83.2%, respectively.The air medium contributes to the high efficiency of the proposedantenna due to the absence of dielectric loss.

IV. DISCUSSION

A. Comparison With State-of-the-Art CP Antennas

To highlight its merits, the proposed design is compared with thestate-of-the-art CP antennas in Table II. In [12]–[14], broadband CP

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TABLE II

COMPARISONS OF THE PROPOSED ANTENNA WITH DIFFERENT COMPACT OR BROADBAND CP ANTENNAS

Fig. 15. Simulated and measured broadside CP gains and total efficiencies.

patch antennas with dual-fed and integrated-fed schemes are reportedwith large sizes and high profiles. In [28], a compact dual-fed CPstacked patch antenna with a high-permittivity substrate (εr = 6.5)is presented, but its bandwidth is narrow. In [18], [19], [21], and[22], some compact triple- and quadrature-fed SRAs are proposedto achieve CP radiations; however, their narrow bandwidths arenot suitable for the universal RFID application. In [24], a compactquadrature-fed SRA is presented, with a similar size and bandwidthto that in the present work; however, it is complex to squeeze the feednetwork into a small footprint, and more than 15 parameters shouldbe finely adjusted for the squeezed feed network. Thus, comparedwith the above-mentioned works, the present work offers a novel andsimple scheme to achieve a compact, low-profile, low-cost, wideband,and high-efficiency CP antenna. However, the compact ground planeand tilting beam in the proposed design lead to a low gain.

B. T-Shaped Slot-Etched PIFA Scheme

In the previous design, the center slot is cut thoroughly with the leftand right cavities separated, which is similar to the layout of a two-element array. However, the operating mechanism of the proposeddesign is absolutely different from that of a two-element array.

Fig. 16. Discussion of the T-shaped slot-etched PIFA scheme. (a) Geometry.(b) Simulated S11 and AR versus frequency.

Fig. 17. Geometry of the conventional dual-fed sequential rotation PIFA.

The through slot utilized here is only to ensure the same resonantlength for the PIFA and open-slot modes. The design scheme stillworks when the left and right cavities are connected as a continuouselement with a T-shaped slot etched, as indicated in Fig. 16(a).The corresponding impedance and AR bandwidths are reported inFig. 16(b), which are similar to those of the previous through slot-etched scheme.

C. Comparison With the Conventional Dual-Fed SRA

The conventional dual-fed sequential rotation PIFA can alsoachieve CP radiation with a simple feeding strategy, compact size, lowprofile and high efficiency. Fig. 17 shows the geometry of an

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TABLE III

COMPARISON WITH THE CONVENTIONAL DUAL-FEDSEQUENTIAL ROTATION PIFA

optimized dual-fed sequential rotation PIFA with a same profile.The performances of the proposed antenna and the conventional dual-fed SRA are compared in Table III, which indicates that the proposeddesign has a wider overlapping bandwidth and 3 dB AR beamwidthwithin a smaller footprint.

V. CONCLUSION

This communication proposes a dual-fed compact and widebandCP antenna within a simple slot-etched PIFA architecture. Good CPperformance is achieved by integrating two orthogonal λ/4 resonantmodes, i.e., PIFA and open-slot modes, in an elaborately designedlayout. Both the full-wave simulation and measurement results showthat the proposed antenna can offer an overlapping fractional band-width of 14.7% within a low profile of 0.032 λ0 and a compact sizeof 0.3×0.23λ2

0. A robust CP angle response is obtained at the centerfrequency, with 3 dB AR beamwidths of 54° and 60° in the xz and yzplanes, respectively. We envision that the proposed CP antenna, withadvantages over the conventional tightly coupled SRAs like simplefeeding strategy, low cost, low profile, compact size, wide bandwidth,and high efficiency, has potential application in universal UHF RFIDreaders.

ACKNOWLEDGMENT

The authors would like to thank Z. Shao from Shanghai Jiao TongUniversity, Shanghai, China, for his help in the literature survey. Theauthors would also like to thank the anonymous reviewers for theirvaluable comments to improve the quality of this communication.

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