A novel polarization-reconfigurable conical helical antenna based on the liquid metal is presented. The antenna is implemented by using truncated structure, variable pitch angle, a matching stub, and a mechanical autorotation device. The experimental results show that a good agreement between simulations and measurements is obtained. The gain of the antenna achieves higher than 8 dBi in the work band (1525–1660.5 MHz), and the 3 dB axial ratio (AR) bandwidth reaches 410 MHz. The polarization mode of the antenna can be switched between right-hand and left-hand circular polarization.
1. Introduction
Antennas have played crucial roles in wireless communication systems. With rapid increase of the number of antennas in communication system, it is a great challenge to improve the electromagnetic compatibility of communication systems. Compared to traditional antennas, a reconfigurable antenna can act as several antennas by changing the antenna’s physical structure or incorporating switches. Since the concept of reconfigurable antenna was proposed by Schaubert in 1983 [1], it has attracted more and more attentions. Reconfigurable antennas include frequency-reconfigurable antenna, polarization-reconfigurable antenna, and pattern-reconfigurable antenna. A frequency-reconfigurable antenna using liquid metal as switching mechanism was proposed by Kelley et al. [2]. With the feature of easy reconstruction, liquid metal antennas attracted a growing number of scholars. In 2009, So et al. found that the liquid eutectic gallium indium alloy (EGaIn) has the ability to fabricate reconfigurable antenna because of its self-healing, which provides a new path for the realization of reconfigurable antenna. In the same year, a bendable frequency-reconfigurable monopole antenna was designed by embedding a liquid metal alloy into a polydimethylsiloxane (PDMS) substrate [3]. It is shown that the resonant frequency of the liquid metal antenna could be tuned by stretching the substrate and then altering the effective length of the antenna. In 2009, Cheng et al. proposed a foldable and stretchable liquid metal planar inverted cone antenna [4]. In 2011, a reconfigurable patch antenna was proposed by Mazlouman et al. which was fabricated by embedding liquid metal (eutectic gallium indium tin alloy, Galinstan) in a silicone substrate [5]. Khan et al. presented a frequency-reconfigurable liquid metal antenna, which was in response to the pressure to adjust the electric length [6]. In 2012, Hayes et al. studied a flexible liquid metal alloy (EGaIn) microstrip patch antenna [7]. A tunable frequency liquid metal monopole antenna has been introduced by several research groups [8, 9]. In 2013 Morishita et al. designed a liquid metal monopole array with tunable frequency, gain, and beam steering [10]. A circular beam-steering reconfigurable antenna with liquid metal parasitic has been demonstrated by Rodrigo et al. [11]. However, the research on helical antenna based on liquid metal has not been found yet.
Circularly polarized antennas are widely used for radar, navigation, and mobile systems [12]. An axial-mode helical antenna, firstly presented by Kraus [13], is an attractive candidate for circularly polarized (CP) applications. A conical helical antenna is a kind of deformation of cylindrical helical antenna, which not only has the advantages of high gain and wide band, but also has the characteristic of sidelobe suppression. Conical helical reconfigurable antennas based on liquid metal are very important value for applications.
In this letter, a polarization-reconfigurable conical helical antenna using liquid metal enclosed in a polydimethylsiloxane (PDMS) elastomer is introduced. In order to verify the proposed method, an antenna operating in the band (1525–1660.5 MHz) was designed as an example, and the polarization mode of the antenna can be switched between right-hand and left-hand circular polarization.
2. Antenna Design
The proposed polarization-reconfigurable conical helical antenna using liquid metal is enclosed in a tubular PDMS elastomer. PDMS as an elastomer has been used to design the reconfigurable antenna [3, 7]. The relative dielectric constant of the PDMS is about 2.67. The outer diameter and inner diameter of the PDMS elastomer are, respectively, 6 mm and 4 mm. Under normal temperature, liquid metal indium gallium alloy (EGaIn) of low melting point is liquid. If exposed to air, EGaIn forms oxide surface and cannot flow. The liquid metal antenna combines the fluidity and ductility of liquid metal with the flexibility of the tubular PDMS, which makes the shape of the antenna easy to adjust. Therefore it has the characteristics of reconfigurability.
Considering portability, the size of the antenna is designed as small as possible. Variable pitch angle and truncated structure are used together, not only to reduce the profile but also to improve the antenna performance [11]. The geometry of the proposed antenna is depicted in Figure 1(a). The parameter equation of conical helical antenna is [14](1)x=R′+R-R′2πNtcost,y=R′+R-R′2πNtsint,z=R′tanα0+α1-α0t2πNπ180t,where the variables R′, R, N, α0, and α1 denote, respectively, the basal radius of the antenna, the top radius of the antenna, the helical turns, the start pitch angle, and the end pitch angle. The variable t represents the radian of the spiral tube, and its scope is 0~2πN.
Geometry of the proposed antenna. (a) Panoramic view of the proposed antenna. (b) Side view of the proposed antenna.
The ratio D/λ is an important parameter of the helical antenna. According to antenna theory, when the ratio ranges from 0.25 to 0.42, the helical antenna will work in axial radiation mode [15]. Beyond this range the helical antenna will no longer exhibit circular polarization characteristics, and lobe pattern may appear distorted. Under the demand of axial radiation pattern, the basal radius of the antenna is chosen as R′=33 mm, the start pitch angle is α0=14°, and the helical turns are N=4. In order to reduce the profile and not to destroy the current distribution on the spiral line [16], the end value of t is selected as 2π×2. The truncated structure is fabricated by cutting off two circles of the antenna, and the remaining part is taken as the body of the conical helical antenna.
The proposed antenna was composed of the ground plate, the matching stub, the supported foam, and the liquid metal EGaIn enclosed in a tubular PDMS elastomer. The tubular PDMS elastomer is wound on conical polyethylene foam (dielectric constant of 1.05) which mounted on a copper ground plate. The feed network is designed by using a matching stub to achieve the excitation. A short vertical feed line penetrates the ground plate through a hole and is connected to the matching stub, as shown in Figure 1(a).
In order to design a polarization-reconfigurable helical antenna, both left and right spiral cylindrical grooves on the conical polyethylene foam were dug. And the radii of the grooves are both 6 mm, which could fit the tubular PDMS appropriately. One end of the tubular PDMS elastomer is fastened on the upper plate which fixes together with the axis and the subplate; the other end is fixed on the matching stub which closes to the ground plate and links the feed line for matching, as shown in Figure 1, while the gear, the ground plate, and the polyethylene foam are fixed together. Motor drives the gear through a belt, and the gear would drive the ground plate and polyethylene foam rotating with the PDMS elastomer. In the process of spinning, the matching stub connected with the antenna could rotate clockwise or anticlockwise. The axis and the subplate which is fastened to one end of the tubular PDMS elastomer are fixed. By starting the motor, the rotation direction of circular polarization of the helical antenna can be changed.
According to the simulation, and based on the value of R′ and the start pitch angle α0, it is found that the end pitch angle α1 plays an important role in the current distribution, so it can affect the AR of the proposed antenna. As shown in Figure 2, the 3 dB AR bandwidth is 24.8% at α1=5°; it can be enhanced by increasing the value of α1. When α1=7° and α1=9°, the AR bandwidths are both around 33.5%. Considering designing a low profile helical antenna, we choose α1=7° in the proposed antenna, which makes the height of the antenna 76 mm.
Effects of the end pitch angle α1 on the axial ratio of the proposed helical antenna (α0=14°, R=3 mm, and R′=33 mm).
The significance of the top radius R on the antenna’s 3 dB AR performance is shown in Figure 3. It is seen that the 3 dB AR bandwidth is 33.5% at R=3. When R=5 and R=7, the AR bandwidths are, respectively, 31.6% and 25.2%. By decreasing the value of R, the 3 dB AR bandwidth can be enhanced.
Effects of the top radius R on the axial ratio of the proposed helical antenna (α0=14°, α1=7°, and R′=33 mm).
According to the value given above, the simulation based on HFSS is done. It is found that the imaginary part of the antenna impedance can be controlled by changing the stub width pw, as shown in Figure 4(a). And the real part can be regulated by mainly changing the stub length pl, as shown in Figure 4(b). Table 1 shows the detailed dimensions of the proposed antenna.
Detailed dimensions of the proposed antenna.
Parameters
R1
R1′
RGND
h
pl
Value
14.7 mm
30 mm
150 mm
9 mm
24.2 mm
Parameters
R
R′
α1
α0
pw
Value
3 mm
33 mm
7°
14°
28 mm
Input impedance as a function of the matching stub length and width. (a) Changing pw. (b) Changing pl.
3. Experimental Results
To demonstrate the validity of the presented design strategy, a prototype of the antenna has been fabricated and measured, as shown in Figure 5.
Photograph of the proposed antenna.
The measurement was carried out with Agilent N5230A vector network analyzer. From Figures 6(a) and 6(b), it can be seen that the simulated impedance bandwidth for S11<-15 dB is from 1.43 GHz to 1.85 GHz and the measured impedance bandwidth for S11<-15 dB is from 1.41 GHz to 1.805 GHz for the LHCP, while for the RHCP the simulated impedance bandwidth is from 1.43 GHz to 1.85 GHz and the measured impedance bandwidth is from 1.41 GHz to 1.81 GHz, which show reasonable agreements between the simulated and measured results. There exists a little frequency offset between simulation and measurement due to the assembly error.
The simulated and measured reflection coefficients. (a) LHCP. (b) RHCP.
Figures 7(a) and 7(b) depict the simulated and measured axial ratio and the power gain of the proposed antenna against frequency for the LHCP and RHCP separately. The simulated 3 dB axial ratio bandwidth is found to be nearly 33.5% and 30.6% for the LHCP and RHCP, respectively. Within the whole working band, the measured results demonstrate that the peak gains are higher than 8 dB, and the AR bandwidth is nearly 410 MHz. The measured results are in good agreement with the simulated ones, while the little discrepancy between them can be mainly attributed to fabrication and measurement errors.
The simulated and measured axial ratio and gain. (a) LHCP. (b) RHCP.
The radiation patterns of both RHCP and LHCP at the center frequency 1593 MHz are shown in Figures 8(a), 8(b), 8(c), and 8(d). As can be seen, the measured 3 dB beam widths for LHCP are about 44° at xoz plane and 56.4° at yoz plane, while for RHCP 44° at xoz plane and 56.2° at yoz plane. There also exists a little offset between simulation and measurement due to the processing deviation in the antenna fabrication.
The simulated and measured radiation patterns. (a) LHCP at xoz plane. (b) LHCP at yoz plane. (c) RHCP at xoz plane. (d) RHCP at yoz plane.
Figure 9 depicts that the simulated efficiencies of the same structure antennas vary with the frequency. The conductors are separately the aluminum, the copper, and the EGaIn. As can be seen, the efficiency of the EGaIn antenna is above 90% within the whole working band.
Comparison of the efficiency among different conductors.
The simulated efficiencies of the antenna, respectively, by HFSS and CST are shown in Figure 10. It is shown that, within the whole working band, the results keep consistent by different simulator.
Comparison of the efficiency between HFSS and CST.
Table 2 summarizes the performance comparison of the state-of-the-art helical antennas. Compared to the other nonplanar helical antennas, our proposed antenna clearly exhibits the obvious advantages in terms of impedance and AR bandwidths. It is seen that although the antenna in [17] retains wider impedance and AR bandwidths and has an obvious advantage than others, its structure is planar. In addition, our antenna is a novel design that applied liquid metal to design a polarization-reconfigurable helical antenna. So far, the research on helical antenna based on liquid metal has not been found yet.
Performance comparison of the state-of-the-art helical antenna.
Ref.
Structure
Impedancebandwidth|S11| < −10 dB
AR bandwidth(AR < 3 dB)
Height (λ0 is the wavelength at the center frequency)
Gain at center frequency (dB)
[18]
Nonplanar
6.9% 3.4% 21.7%
4.2% 1.3% 15%
0.33λ00.297λ00.29λ0
4.5 dBi
[19]
Nonplanar
9.23%
3.2%
0.23λ0
4.77 dBi
[20]
Nonplanar
18.2%
—
2.96λ0
13 dBi
[21]
Nonplanar
12.8%
—
1.11λ0
6.5 dBi
[17]
Planar
54%
34%
0.11λ0
8 dBi
Our work
Nonplanar
35.6% (RHCP) 36% (LHCP) (|S11| < −15 dB)
26.5%
0.4λ0
8 dBi
4. Conclusion
A novel polarization-reconfigurable conical helical antenna with liquid metal is achieved. A truncated structure, a variable pitch angle, a matching stub, and a mechanical autorotation device are adopted in the proposed antenna. The circular polarization radiation mode of the antenna can be switched between the left hand and the right hand. In the entire work band (1525–1660.5 MHz), the gain of the antenna achieves higher than 8 dBi and the 3 dB axial ratio (AR) bandwidth reaches 410 MHz from 1340 MHz to 1750 MHz. Experimental results confirm that the proposed liquid metal conical helical antenna can be a good candidate for circular polarization-reconfigurable antenna.
Conflict of Interests
The authors declare that there is no conflict of interests regarding the publication of this paper.
Acknowledgments
This work was supported jointly by the National Natural Science Foundation of China (no. 61401056 and no. 61571075), the Scientific Research Project of the Education Office of Liaoning Province (no. L2012171), and the Liaoning Normal University Youth Project (no. LS2014L003).
SchaubertD.Frequency-agile, polarization diverse microstrip antennas and frequency scanned arraysUS:US Patent 4367474, 1983KelleyM.KooC.McQuilkenH.LawrenceB.LiS.HanA.HuffG.Frequency reconfigurable patch antenna using liquid metal as switching mechanism201349221370137110.1049/el.2013.29302-s2.0-84887047547SoJ.-H.ThelenJ.QusbaA.HayesG. J.LazziG.DickeyM. D.Reversibly deformable and mechanically tunable fluidic antennas200919223632363710.1002/adfm.2009006042-s2.0-72849145431ChengS.WuZ.HallbjörnerP.HjortK.RydbergA.Foldable and stretchable liquid metal planar inverted cone antenna200957123765377110.1109/TAP.2009.20245602-s2.0-72649098369MazloumanS. J.JiangX. J.MahanfarA.MenonC.VaughanR. G.A reconfigurable patch antenna using liquid metal embedded in a silicone substrate201159124406441210.1109/TAP.2011.21655012-s2.0-82455221258KhanM. R.HayesG. J.SoJ.-H.LazziG.DickeyM. D.A frequency shifting liquid metal antenna with pressure responsiveness201199101350110.1063/1.36039612-s2.0-79960483855HayesG. J.SoJ.-H.QusbaA.DickeyM. D.LazziG.Flexible liquid metal alloy (EGaIn) microstrip patch antenna20126052151215610.1109/TAP.2012.21896982-s2.0-84860672336DeyA.GuldikenR.MumcuG.Wideband frequency tunable liquid metal monopole antennaProceedings of the IEEE Antennas and Propagation Society International Symposium (APSURSI '13)July 2013Orlando, Fla, USAIEEE39239310.1109/aps.2013.67108572-s2.0-84894127506MorishitaA. M.KitamuraC. K. Y.OhtaA. T.ShiromaW. A.Two-octave tunable liquid-metal monopole antenna2014501192010.1049/el.2013.29712-s2.0-84891359725MorishitaA. M.KitamuraC. K. Y.OhtaA. T.ShiromaW. A.A liquid-metal monopole array with tunable frequency, gain, and beam steering20131211388139110.1109/lawp.2013.22865442-s2.0-84887855820RodrigoD.JofreL.CetinerB. A.Circular beam-steering reconfigurable antenna with liquid metal parasitics20126041796180210.1109/TAP.2012.21862352-s2.0-84859792290WuZ.-H.LouY.BaoJ.YungE. K. N.A circular patch fed by a switch line balun with printed L-probes for broadband CP performanceProceedings of the IEEE Antennas and Propagation Society International SymposiumJuly 2008San Diego, Calif, USAIEEE1410.1109/aps.2008.46198732-s2.0-55649117106KrausJ. D.Helical beam antenna194720109111FuS.ZhouY.FangS.CaoY.Design of low profile and variable pitch angle helical antenna for maritime satellite communications201328163672-s2.0-84876017715BalanisC. A.20053rdHoboken, NJ, USAWiley-InterscienceNakanoH.TakedaH.HonmaT.MimakiH.YamauchiJ.Extremely low-profile helix radiating a circularly polarized wave199139675475710.1109/8.868722-s2.0-0026172088ChenZ.ShenZ.Planar helical antenna of circular polarization201563104315432310.1109/tap.2015.2463746BaiX.TangJ.LiangX.GengJ.JinR.Compact design of triple-band circularly polarized quadrifilar helix antennas20141338038310.1109/LAWP.2014.23055922-s2.0-84897900282GuoJ.YangY.HuangY.SunB.Slot multi-arm helix antenna with simple and efficient feeding network201551161224122610.1049/el.2015.1301LiuL.LiY.ZhangZ.FengZ.Compact helical antenna with small ground fed by spiral-shaped microstrip line201450533633810.1049/el.2013.40212-s2.0-84896928457ZhangT. L.YangX. Q.FeiD. L.YanZ. H.Single-arm helical antenna with width of arm varying periodically for tilted beam2015511073673810.1049/el.2015.0351