IEEE ANTENNAS AND WIRELESS PROPAGATION LETTERS, VOL. 18, NO. 5, MAY 2019 941
A Wideband Circularly Polarized Crossed-SlotAntenna With Stable Phase Center
Hongliang Zhang, Yuanyue Guo , and Gang Wang , Member, IEEE
Abstract—A wideband circularly polarized crossed-slot antennawith a stable phase center is investigated in this letter. The pro-posed antenna consists of a crossed-slot radiator and a Γ-shapedfeeding structure. Orthogonal modes can be excited by perturbingthe crossed slot to realize circular polarization. Both simulationand measurement of the designed antenna demonstrate a –10 dBimpedance bandwidth of 43.8% and a 3 dB axial-ratio bandwidthof 42.6%, covering all the global navigation satellite system (GNSS)bands. Due to the overall symmetry of the radiation crossed slot, aphase center of the proposed antenna remains stable within a widebeam of 100° covering the whole GNSS bands, with phase centervariation retained below 5°.
Index Terms—Circularly polarized (CP) antenna, stable phasecenter, wideband.
I. INTRODUCTION
NOWADAYS, global navigation satellite systems (GNSSs)are intensively applied in various fields. Since integration
of multiple bands with a single antenna meets the requirement oflimited space on receivers, as well as avoiding coupling, a wide-band circularly polarized (CP) antenna covering all the GNSSsystems, ranging from 1.164 to 1.612 GHz (32.2%), has grad-ually become a trend. In high-precision positioning, a rangingerror is closely related to the phase center stability of antennabecause antenna phase center is actually used as the referencepoint of receiving or transmitting, and it varies in different ob-serving directions [1], [2]. Therefore, for a wideband GNSSantenna, phase center should remain stable for all the directionsin a broad beam in high-precision GNSS applications.
In recent years, printed-slot antennas have attracted a grow-ing interest because of wide operating bandwidth and easy in-tegration. In [3], a rotated slot was utilized to realize widebandradiation, with –10 dB impedance bandwidth (IBW) reachingup to 48.9%. In [4], a square slot CP antenna with stub protrud-ing from one side of the slot was reported for C-band operation,which yields a –10 dB IBW of 90.2% and 3 dB axial-ratio band-width (ARBW) of 40%. To further improve CP performance, aU-shaped slot was proposed in [5]. By adjusting the position of
Manuscript received November 19, 2018; revised January 7, 2019, February22, 2019, and March 16, 2019; accepted March 16, 2019. Date of publicationMarch 27, 2019; date of current version May 3, 2019. This work was supportedby the National Natural Science Foundation of China under Grant 61431016and Grant 61771446. (Corresponding author: Yuanyue Guo.)
The authors are with the Key Laboratory of Electromagnetic Space Informa-tion, Chinese Academy of Sciences, University of Science and Technology ofChina, Hefei 230027, China (e-mail:, [email protected]; [email protected]; [email protected]).
Digital Object Identifier 10.1109/LAWP.2019.2906363
feeding structure and portion of the slot, both –10 dB IBW and3 dB ARBW exhibit larger than 110%. Generally, for printed-slot antennas, a bandwidth of more than 40% can be realized.
For high-precision positioning service in GNSS, a stablephase center is indispensable within a wide beam for antennadesign. Generally, most antennas do not hold a unique phasecenter, but a mean phase center (MPC) over the interesting ob-serving directions. In practice, a phase center variation (PCV)can be used to evaluate the stability of MPC over the observ-ing directions, which is the deviation between the real phasefront and the ideal one when the antenna reference point (ARP)corrected to the MPC [6]. As suggested in [1], the stable phasecenter tends to require rotationally symmetric antenna config-urations. A four-arm spiral traveling-wave antenna with stablephase center was reported for GPS bands [1], with a maximalPCV of 7° for GPS L1 in the beam of 100°. In [7], a center-fed microstrip antenna with perfect stable phase center for BDSB1 band has been proposed, where the phase center deviationof no more than 1.19 mm over frequency is realized, whereasno study was carried out for MPC stability over observing di-rections. By adopting capacitive coupled feeding structure [8],ring slot [9], crossed dipole [10], cross-type meander line [11],and claw-shaped parasitic structure [12], wideband CP antennasworking at GNSS bands have been proposed. However, no stresswas laid on phase center stability. Therefore, achieving stablephase center in a wide beam covering all the GNSS bands is stillchallenging.
In this letter, a single-fed wideband CP antenna is proposedto realize a stable phase center for GNSS applications. A newfeeding structure is designed, and wideband performance canbe achieved through tuning the offset between feeding line andcrossed-slot and matching stubs. Owing to the quasi-symmetriccrossed slot, an antenna phase center remains stable in a widebeam. In addition, by deploying substrate on the metallic reflec-tor [13], antenna profile can be reduced to 0.12 λ0 (wavelengthof the lowest passband).
II. ANTENNA DESIGN AND ANALYSIS
A. Antenna Configuration
An architecture of the proposed crossed-slot antenna is shownin Fig. 1. This antenna consists of a quasi-symmetric crossedslot, a feeding network, and a substrate slab on the reflector. Thecrossed slots function as a pair of dipoles to generate the orthog-onal electric field modes for circular polarization, which can be
942 IEEE ANTENNAS AND WIRELESS PROPAGATION LETTERS, VOL. 18, NO. 5, MAY 2019
Fig. 1. Configurations of the proposed antenna on printed circuit board (PCB).(a) Top view. (b) Side view.
TABLE IDIMENSIONS OF THE PROPOSED ANTENNA (IN MILLIMETERS)
realized by adjusting the width and length of the slots. For con-venience of welding, a metallic plate is added at the bottom ofthe vertical slot, with a size of 4.0 mm × 2.8 mm. The feedingnetwork is a Γ-shaped strip, which is typically asymmetric andwith certain offset from the symmetry axis of the antenna. In or-der to enhance the –10 dB IBW and 3 dB ARBW, two matchingstubs, with dimensions ofLs1 ×Ws1 andLs2 ×Ws2, are addedon the back of the Γ-shaped strip. A substrate slab is loaded onthe reflector, and low profile can be realized by carefully choos-ing the relative dielectric constant and thickness. After imple-menting optimization in ANSYS electronics desktop, antennageometrical parameters are obtained as listed in Table I.
B. Evaluation of Phase Center Stability
The MPC is determined within the total region of 100°beamwidth (i.e., θ from –50° to 50°, ϕ from 0° to 360°), whichis conducted in the ANSYS electronics desktop software. Asthe GNSS requires right-hand circular polarization for receiv-ing antennas, phase patterns are derived from right-hand CP
component→E RHCP(θ, ϕ). We need to note that the MPC loca-
tions are relative to the global coordinate system in Fig. 1 withits origin located at the ARP, which is the geometry center ofan antenna. Then, we can determine PCV from the phase pat-tern by correcting ARP to the MPC. For an ideal point sourceantenna, a phase pattern is constant when the ARP corrected tothe unique phase center, that is, the PCV is zero. However, formost antennas, there exists a deviation on the actual phase frontfrom the ideal phase front at various angles of wave incidence,and the deviation is defined as PCV, which will cause rangingerror for GNSS applications. Therefore, MPC stability can be
Fig. 2. Effects of various D on (a) axial ratio and (b) PCV.
evaluated by PCV, where a low PCV means a stable phase cen-ter over the observing directions. Our goal aims at designing anantenna with low PCV in a wide beam covering all the GNSSbands.
C. Discussion on Effective Parameters for a StablePhase Center
In the viewpoint of antenna geometry, PCV with observingdirections (θ, ϕ) vanishes with increased antenna symmetry ver-sus angular coordinate ϕ, which means the radiator should berotationally symmetric [1], and the proposed crossed slot showsrotational symmetry as a whole. However, for this crossed-slotantenna, an asymmetric feeding structure is required to perturbthe orthogonal electric fields for CP radiation, thus a Γ-shapedstructure is proposed, which contradicts the requirement of sym-metry property for stable phase center. To realize stable MPC andwideband ARBW simultaneously, a tradeoff should be reached.Fig. 2 presents the effects of D on AR and PCV to study themechanism of phase center stability. We read from Fig. 2(a) thatAR is quite sensitive to the offset D because, by moving the feed-ing structure, the crossed slot can tune the orthogonal electricfields generated by the slots to realize CP wave [4]. It is seen thatincreasing and decreasing D with respect to 3.0 mm will deteri-orate the 3 dB ARBW. At the same time, antenna phase patternsin xoz plane at GPS L1 (1575.42 MHz) are shown in Fig. 2(b),where we find PCV is affected by the offset of a feeding structureand the most stable phase center is realized when D is 1.7 mmbecause the overall symmetry of the antenna is improved bycontrolling D. As D increases, phase center stability gets worse,with PCV reaching up to 8° when D is 4.3 mm. Therefore, bycontrolling D, a tradeoff can be reached to realize wideband 3 dBARBW and phase center stability simultaneously.
As we know, the phase quantity of a dipole antenna can be de-picted as e−jkr, independent with observing angle (θ, ϕ), whichmeans it owns very stable phase center. The proposed crossedslot consists of a pair of orthogonal dipoles, and their centers arebasically coincident, thus it is beneficial to realize stable phasecenter in a wide beamwidth. Based on the analysis above, a CPantenna has been designed as a reference to study the mechanismof low PCV, shown in Fig. 3. Both antennas are with the samesubstrate, ground plane, and height. The most significant dif-ference between the reference and the proposed antennas is theT-slot for radiation, which is not a rotationally symmetric struc-ture. The results show that the reference antenna has a PCV of
ZHANG et al.: WIDEBAND CIRCULARLY POLARIZED CROSSED-SLOT ANTENNA WITH STABLE PHASE CENTER 943
Fig. 3. PCV of the reference and the proposed antennas.
Fig. 4. Electric field distributions at GPS L1 with different phase angles.
22°, much higher than the crossed-slot antenna of 4.2°. From thiscomparison, it is demonstrated that antennas with rotationallysymmetric radiation structure can benefit phase center stability.
D. Electric Field Distribution
In order to verify the CP radiation of the proposed antenna,the simulated field distributions at GPS L1 with different phaseangles are shown in Fig. 4. It is observed that the electric fieldsare almost with equal magnitude in+z-direction. Fromϕ= 0° toϕ= 270°, the total electric field flows in anticlockwise directionin +z-direction, demonstrating that this antenna performs right-hand CP radiation.
III. ANTENNA PERFORMANCE
The prototype of the designed antenna is fabricated andmeasured. The antenna is printed on a PCB (εr = 6.15,tanδ = 0.001), with a size of 107 mm × 107 mm × 1.524 mm.The bottom substrate is FR-4 (εr = 4.4, tanδ = 0.02), with asize of 125 mm × 125 mm × 23 mm.
A. Bandwidth and Radiation Pattern
Fig. 5 shows the measured and simulated S11, as well as thefabricated antenna. We read from the curves that a reasonableagreement has been obtained between the simulated and mea-sured results, where –10 dB IBW can be acquired in frequencyranging from 1.07 to 1.67 GHz (43.8%), covering all the GNSSbands.
Fig. 5. Simulated and measured S11.
Fig. 6. Simulated and measured axial ratio and gain.
Fig. 7. Simulated and measured radiation patterns in xoz and yoz plane at threetypical GNSS bands. (a) GPS L5. (b) BDS B3. (c) GPS L1.
Fig. 6 shows the simulated and measured right-hand CP gain(GainRHCP) and axial ratio of the antenna. It is observed thatthe measured 3 dB ARBW is obtained ranging from 1.07 to1.65 GHz (viz., 42.6% relative bandwidth), covering the wholeGNSS bands as well. The antenna has a measured gain at bore-sight between 4.6 and 6.3 dBic over the 3 dB ARBW. The mea-sured gains at GPS L5, BDS B3, and GPS L1 are 5.3, 5.7, and5.3 dBic, respectively.
Fig. 7 shows the normalized radiation patterns at GPSL5 (1176.45 MHz), BDS B3 (1268.52 MHz), and GPS L1
944 IEEE ANTENNAS AND WIRELESS PROPAGATION LETTERS, VOL. 18, NO. 5, MAY 2019
Fig. 8. PCV relative to the MPC at GPS L1.
(1575.42 MHz). For the prototype antenna, the gain of right-hand circular polarization is larger than the left-hand in +z-direction, indicating the right-hand CP radiation is realized.Moreover, the 0 dB beamwidth of these bands is less than 100°.As the 0 dB minimal gain requirement recommended in [8] forGNSS applications to ensure the quality of receiving signal, an-tenna phase responses in this letter are studied within 100° (i.e.,from –50° to 50°, from 0° to 360°).
B. Antenna Phase Response
To demonstrate the phase center stability, the PCV is analyzedat GPS L1. The simulated and the measured PCV with respectto the same MPC at (–12.1 mm, 1.4 mm, –9.4 mm) is shown inFig. 8 in two orthogonal planes with the azimuth angles of 0° and90°. We find that the measured PCV curve is closely matchedwith the numerical simulation ranging its polar angles from –50°to +50°, respectively capable of its maxima within 5° and 4.2°,which indicates a stable phase center for GPS L1. It is observedthat the phase patterns show its asymmetry, mainly caused bythe asymmetric feeding structure, which affects the symmetryproperty of antenna phase response. The MPC locations for BDSB3 and GPS L5 are (19.1 mm, 3.0 mm, –10.0 mm) and (11.9 mm,21.2 mm, –18.3 mm), respectively. The PCVs for these twobands are below 5°, as well. Compared to the antenna PCVsreported in [1], the proposed antenna yields more stable phasecenter in the beamwidth of 100°.
To give an illustrative show of phase center stability relativeto the corresponding MPC, Fig. 9(a) shows the phase patternsevaluated with the geometry center of the antenna as ARP, whileFig. 9(b) shows the phase patterns evaluated with the MPC asARP. Three typical GNSS bands, with respect to the low, middle,and high frequency of GNSS bands, respectively, are chosen fordemonstration. We observe that the phase fluctuation decreasesgreatly after choosing the MPC as the ARP. In Fig. 9(a), phasefluctuation reaches up to 30° for GPS L5, whereas, in Fig. 9(b),it keeps below 5° for the three bands. Therefore, it demonstratesagain that the proposed antenna has realized stable phase over awide beam, covering the whole GNSS bands.
C. Comparison
A performance comparison between the designed antenna andother wideband GNSS antennas with unidirectional radiation are
Fig. 9. Phase fluctuation at three typical GNSS bands with respect to (a) thegeometry center of ground and (b) the MPC.
TABLE IICOMPARISON OF WIDEBAND GNSS ANTENNAS WITH
UNIDIRECTIONAL RADIATION
λ0 is the wavelength of the lowest passband.
listed in Table II. It is seen that the proposed antenna simulta-neously exhibits a wide band, a simple feeding network, anda compact size. Since no phase center stability was stressed inother designs, comparison of phase center stability is not in-cluded in Table II.
IV. CONCLUSION
This letter proposes a design of wideband CP antenna with astable phase center. Its simulation and measurement demonstratea –10 dB IBW of 43.8% and a 3 dB ARBW of 42.6%, as wellas a stable phase center in a broad beam of 100° with its PCVretained within 5° for high-precision positioning, which coverall the GNSS bands ranging from 1.164 to 1.612 GHz.
ZHANG et al.: WIDEBAND CIRCULARLY POLARIZED CROSSED-SLOT ANTENNA WITH STABLE PHASE CENTER 945
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