Haochuan Qiuab,
Houfang Liu*ab,
Xiufeng Jiaab,
Xiao Liuab,
Yuxing Liab,
Tianyu Jiangab,
Benkuan Xiongab,
Yi Yangab and
Tian-ling Ren
*ab
aInstitute of Microelectronics, Tsinghua University, Beijing 100084, China. E-mail: hfliu@tsinghua.edu.cn; RenTL@tsinghua.edu.cn
bTsinghua National Laboratory for Information Science and Technology, Tsinghua University, Beijing 100084, China
First published on 7th November 2018
The transition from the current 4th generation mobile networks (4G) to the next generation, known as 5th generation mobile networks (5G), is expected to occur within the next decade. To provide greater network speed, capacity and better coverage, the wireless broadband technologies need to update traditional antennas for high frequency and millimeter wavelengths. In this study, meander line dipole antennas produced by direct ink-injecting technology have been successfully designed, fabricated and characterized, where the ink-injecting technology may open new routes to the fabrication of wireless antenna applications. An accurate electromagnetic numerical analysis model for the proposed meander line antenna is also developed. The designed dual-band antenna based on graphene flakes and Ag nanowires can operate from 1.2 GHz up to the 1.5 GHz band and from 3.2 GHz up to the 3.8 GHz band with |S11| > 10 dB for wireless communications applications. Different mixtures by mass ratio of aqueous dispersions of CNTs and Ag nanowires (1:
1, 5
:
1, 10
:
1, 20
:
1) are also prepared to investigate the influence of the network structure on the performance of the meander line antennas.
In this paper, dual-band meander line antennas based on GFs, CNTs and NWs-Ag are achieved for radiolocation at the 1.3 GHz band and 5G at the 3.4 GHz band. The fabrications of antennas are done by a direct ink-injecting method on PDMS substrates with ξr = 2.67 and a thickness d = 1.5 mm, which are molded with a 3D printed acrylonitrile-butadiene-styrene copolymer (ABS). The return loss and radiation patterns are experimentally characterized and numerically simulated by a Vector Network Analyzer (VNA), Ansoft HFSS and Advanced Design System (ADS), respectively. The validated excellent performance of the prototype dual-band meander line antennas suggests that antennas based on GFs, CNTs and NWs-Ag can be manufactured by a direct ink-injected process.
GFs, CNTs and NWs-Ag have emerged as good candidates for lightweight, durable, 5G and radiolocation antennas, due to their unique physical and electrical properties, such as corrosion resistance, weight saving, reliability and enhanced electrical characteristics.14–16 The microstructures of Me-CNTs, NWs-Ag, GFs, and a semi-CNTs/NWs-Ag complex were investigated using Raith e-line plus SEM, as presented in Fig. 2(a)–(d). The graphene has its typical multilayer sheet structure and the typical sheet size is shown in Fig. 2(c). The MR of semi-CNTs:
NWs-Ag = 5
:
1; it is well mixed and has a typical length, as shown in Fig. 2(d).
Fig. 2(e) and (f) show X-ray photoelectron spectra (XPS) for GFs and Me-CNTs. The charge correction has been used and the XPS spectra were also calibrated relative to C 1s at 284.8 eV. The C–C peak was set to 284.9 eV all over the spectra. It is clear that the sums (pink dashed lines) of four peaks at 284.3 eV (CC), 284.9 eV (C–C), 285.5 eV (C–O) and 287.0 eV (C–O–C) are accurately fitted with the deconvoluted C 1s spectra of graphene flakes, which are in good agreement with previous reports.17–19 Based on the XPS survey spectra, the atomic ratio between C and O can be estimated from
and
where area (N)i is the normalized area of different specific spectra, area (P) is the area of different specific spectra, SF is the sensitivity factor, TXFN is the transmission function and Ek is the kinetic energy19 (see concrete parameters in ESI, Table S2†). Hence, the C/O ratio for GFs is calculated as C/O = 8.1
:
1, which is almost equal to that for reduced GO (C/O ∼ 8–14).20 These results indicate that the GFs and Me-CNTs both have high conductivities, due to a lower content of oxygen and a high number of sp2 carbon bonds. In addition, the XPS spectra of GFs and CNTs did not contain any elements except C and O, indicating the absence of impurities; the survey spectra have been provided in Fig. S2† as proof (see ESI, Fig. S2†).
One great limitation of the antenna is that GFs and CNTs have a relatively higher surface resistance than that of metal. Therefore, it was designed with a relatively large strip width to reduce the resistance of the antenna. The radiation efficiency (ηr) of the antenna can be maximized by an increase in radiation resistance (Rr) or a decrease in loss resistance (Rl), following the basics of antenna theory: ηr = Rr/(Rr + Rl).21 The radiations always occur at the right-angle bends and the straight segments. The overall equivalent circuit is shown in Fig. 3(a). The designed antenna consists of straight elements and bends, which are considered as distributed transmission lines. The lumped parameter equivalent to straight elements can be represented by a T-equivalent network, as shown in Fig. 3(a). The equivalent lumped inductance Ln and capacitance Cn can be also described as and
.22
Numerical analysis based on the finite element method is used to design and optimize the design structure of the antennas. Here, for example, in simulations the conductivity of the GFs is calculated as σ = 1/(dRsquare) = 2.5 × 104 S m−1 with a thickness of d = 10 μm (see ESI, Table S1†). The current distributions are evaluated to understand the performance of the antennas. Fig. 3(b) shows the surface current and electric field distribution at resonant frequencies of 1.3 GHz and 3.4 GHz for the designed meander line dipole antenna. The electric field distributions are almost symmetric on the antenna and then cause symmetric distributions of the surface current. The currents are concentrated at the corners of the antenna, which cause a discontinuity in current and extra discontinuity inductance. At 1.3 GHz, the current distribution is along the +x direction, as shown in Fig. 3(b). On the other hand, the current distribution at 3.4 GHz is quite different (see ESI, Fig. S1†). The current direction in the middle part (+x) is opposite to that of the left and right edges (−x), which results in a reduction in the total electrical length.
As an important aspect of antenna characterization, the return loss |S11|, characterized by an Agilent PNA-X series N5242A network analyzer, represents the input power acceptance level of the antenna.23 Normally, an optimized antenna with return loss |S11| > 10 dB is acceptable for use at a desired frequency.24,25 Antennas were bonded to SMAs with a typical impedance of 50 Ω and connected with the network analyzer. Fig. 4(a) shows the measured return loss |S11| of the antennas based on GFs, CNTs and NWs-Ag from 100 MHz to 5 GHz. Dual-band modes are obtained for all of the antennas. The frequency band for |S11| > 10 dB is from 1.1 GHz to 1.4 GHz, and from 2.6 GHz to 3.3 GHz for the Me-CNTs antenna, from 1.2 GHz to 1.5 GHz for the NWs-AG antenna and from 3.2 GHz to 3.8 GHz for the GFs antenna, which means a wideband performance.
As shown in Table 1, the −10 dB bandwidths for GFs antenna are 316 MHz (1.3 GHz) and 773 MHz (3.4 GHz), 316 MHz (1.3 GHz) and 750 MHz (3.4 GHz) for the NWs-Ag antenna, and 314 MHz (1.3 GHz) and 640 MHz (3.4 GHz) for the Me-CNTs antenna. As shown in Table S4 (see ESI†), compared with other dipole antennas, our proposed GFs antenna has good performance with dual band and bandwidths of 316 MHz/773 MHz at 1.3 GHz/3.4 GHz, which can be used in radiolocation and 5G communication. Further details of the radiation properties of the antennas, including the resonant frequency and radiation efficiency, are summarized in Table 1. The efficiency can also be calculated from ηr = G/D, where G and D are the gain and directivity of the antenna, respectively.26 The measured efficiency of the graphene flakes antenna can reach 69% at 1.3 GHz and 52% at 3.4 GHz. Due to the increment in input resistance induced by the skin effect, the efficiency at 1.3 GHz is higher than that at 3.4 GHz, or for the GFs and Me-CNTs antennas. In comparison to NWs-Ag and Me-CNTs, the cost of graphene flakes is much lower, and the manufacturing process is relatively simple and scalable. These results indicate that the graphene-flakes dual-band antenna has great potential for low-cost and high-performance wireless communication.
Material | NWs-Ag | Graphene flakes | Me-CNTs |
---|---|---|---|
Size mm2 | 75 × 20 | 75 × 20 | 75 × 20 |
1.3 GHz bandwidth | 316 MHz | 316 MHz | 314 MHz |
3.4 GHz bandwidth | 750 MHz | 773 MHz | 640 MHz |
1.3 GHz efficiency | 75% (Simu: 82%) | 69% (Simu: 75%) | 71% (Simu: 78%) |
3.4 GHz efficiency | 62% (Simu: 72%) | 52% (Simu: 64%) | 58% (Simu: 67%) |
It is reported that the lumped inductance of strips consists of magnetic inductance (LM) and kinetic inductance (LK). LM is the sum of self-inductance and mutual inductance, which are determined by the structure of the device, while LK is related to material alone.27 As shown in Fig. 4(a), the resonant frequency, which is related to lumped impedance, of Me-CNTs and NWs-Ag antennas shifted to slightly lower values compared with GFs. The simulation, based on the conductivity of graphene, of the Z parameter is shown in Fig. 4(f). At resonant frequency f0, the imaginary part of the Z parameter Im(Z) = 0 with the real part of the Z parameter Re(Z) = 35.3 Ω at 1.3 GHz and Re(Z) = 42 Ω at 3.4 GHz. This is probably because of the difference in carrier density and kinetic inductance, which may cause the change in Im(Z) of the antenna and further influence resonant frequencies. The measured and simulated normalized radiation patterns of the graphene flakes antenna in the E-plane and H-plane are shown in Fig. 4(b) and (c). Both at 1.3 GHz and at 3.4 GHz, the radiation patterns are almost omnidirectional in the H-plane, which is of most interest for wireless communication systems. The radiation pattern of the E-plane at 1.3 GHz is a typical dipole-like radiation pattern and at 3.4 GHz, influenced by the meander line structure, it has a mushroom-like radiation pattern.
Different MR of semi-CNTs/NWs-Ag complex (MRCNT:Ag = 1:
1, 5
:
1, 10
:
1 and 20
:
1) antennas are also prepared on the PDMS substrate with hs = 1.5 mm. The test results of |S11| are presented in Fig. 4(d). The performance of the semi-CNTs antenna causes a mismatch in impedance due to the low conductivity in Table S3 (see ESI†), as shown in Fig. 4(d). With the increment in MRCNT:Ag, the resonant frequency of the antennas increases. For example, when MRCNT:Ag = 10
:
1, the |S11| becomes less than 5 dB at 1.4 GHz, and the resonant frequency shifts from 2.8 GHz, compared with MRCNT:Ag = 1
:
1, to 3.55 GHz. When MRCNT:Ag increases to 20
:
1, the resonant frequency shifts to a higher value with the |S11| > 10 dB band from 4.5 GHz to 8 GHz (not shown here). Based on the tested square resistance in Table S3,† the simulation results are presented in Fig. 4(e). Note that the change in the ohmic resistance has little influence on the resonant frequency. NWs-Ag in the complex plays an important part in current transportation, due to the higher conductivity than that of semi-CNTs. The simulation result, as shown in Fig. 4(e), is obtained by using the same geometry but different measured conductivity of MRCNT:Ag, which means only the conductivity is changed while the current pass is the same as for pure materials (GFs, NWs-Ag and Me-CNTs) in the simulation. The simulation results indicate that the change in conductivity could not explain the shift in resonant frequency and the change in bandwidth observed in the experiment. To explain the shift, input impedance is shown in Fig. 4(f). At resonant frequency f0, the imaginary part of the Z parameter Im(Z) = 0. The discontinuous distribution of NWs-Ag can cause unevenly distributed currents, which contribute to extra parasitic inductance and capacitance; meanwhile, the NWs-Ag could also introduce extra kinetic inductance and further change the input impedance. With the increment in MRCNT:Ag, the distribution of the Z parameter is tuned, and hence the resonant frequency is changed. That is, the higher the MRCNT:Ag, the higher the resonant frequency measured. Therefore, the resonant frequency of the antennas can be monitored via the change in the MR of semi-CNTs and NWs-Ag.
Footnote |
† Electronic supplementary information (ESI) available. See DOI: 10.1039/c8ra08018f |
This journal is © The Royal Society of Chemistry 2018 |