Miniaturization of Quasi-Yagi Antenna Array with High Gain Using Split-Ring Resonators

In order to realize the miniaturization of quasi-Yagi antenna array, an antenna array with split-ring resonators (SRRs) based on two 7 units quasi-Yagi elements is designed in this paper. )e radiation performance of the quasi-Yagi antenna array degrades significantly when array element spacing is reduced. After embedding SRRs on both sides of the miniaturized dielectric substrate surface, the S parameters and gain of array are significantly better than the array without SRRs, and the adjustable wave beam energy is also enhanced effectively. It indicates that the proposed antenna array with SRRs has good directional radiation performance under the miniaturize process at the operation frequency of 2.45GHz, which could be widely applied in the fields of smart rail transportation and wireless power transfer.


Introduction
With the development of the intelligent transportation networks and the new energy Internet of ings (IOT), the antenna array for transmitting wireless communication signals needs to achieve miniaturized appearance and high gain radiation. e quasi-Yagi antenna is a typical representative of end-fire antennas. Due to good directionality and high gain, the quasi-Yagi antenna was most widely used in wireless communication as vehicle-mounted antenna or access points (AP) in metro tunnels [1][2][3]. In order to overcome the frequency selective fading caused by the multipath effect, the stacked quasi-Yagi array for 2.45 GHz has been used in wireless charging systems of REID chips [4]. e quasi-Yagi antenna by using the metamaterial concept has been utilized in wireless power transfer (WPT) to medical implants [5]. e beam-forming was also very important for multi-input-output (MIMO) communication [6]. A quasi-Yagi array of four radiating elements [7] and a wideband sectoral quasi-Yagi antenna has been presented for the MIMO system [8]. e mutual coupling [9] effect of quasi-Yagi antenna arrays was a ubiquitous problem in wireless communication [10]. e large reflection of the radiation port could cause the pattern distortion and gain reduction [11]. erefore, the research on decoupling of the quasi-Yagi antenna array has practical application significance. e electromagnetic band gap (EBG) [12,13], defected ground structure (DGS) [14,15], and metamaterials [16,17] could be applied to suppress mutual coupling of antennas. As the reflector of quasi-Yagi has been printed on the back of the dielectric substrate, the high-surface required for the EBG structure and the DGS gap could not be inscribed, which would affect the induction field on the reflector. Due to the feature of suppressing electromagnetic energy transmission, the single-negative magnetic metamaterials have been widely used to reduce mutual coupling between high-profile antennas [18][19][20] and have adopted different SRRs structures [21][22][23], but few were used for quasi-Yagi antenna arrays. e proposed antenna array based on two 7-units quasi-Yagi antenna elements adopt the single-negative magnetic metamaterial concept to suppress the mutual coupling of elements, and its different balun compound mode could form an adjustable beam for directional radiation through embedding the SRRs on both sides of the dielectric substrate surface. is article established different models to elaborate the variation of radiation performance of the quasi-Yagi array under its miniaturized process and carried out detailed quantizing analyses for the decoupling effect.

Quasi-Yagi Antenna Array Design
e operation frequency band of the communication based train control system is 2.4 ∼ 2.48 GHz in rail traffic, and the band is also applied in the wireless local area network (WLAN).
is paper designs a quasi-Yagi antenna array with operation frequency (f) of 2.45 GHz. e array is fabricated on the substrate with relative permittivity (ε r ) of 3.3, and the equivalent wavelength of electromagnetic waves in the substrate (λ e ) satisfies the following: where ε e is the effective permittivity of ε r . e geometry of the quasi-Yagi array structure of different balun compound modes is shown in Figure 1. e wilkinson power divider is used to split the input signal power into two equal power outputs and connected two quasi-Yagi elements as a dualistic antenna array. e thickness of the dielectric substrate is 1 mm, and the exciter, director, balun, and power divider are printed on the front of the substrate, while reflector is printed on the back.
Based on the finite element method, the proposed antenna array is calculated. e array is excited by a side signal through the port as shown in Figure 1. e S11 of the reference antenna array is simulated as shown in Figure 2. At 2.45 GHz, the S11 of the inverse balun array is −11.76 dB, and that of the directional balun array is −11.45 dB. e 3D gain simulated results of the antenna array are shown in Figure 3. e inverse balun array forms two symmetrical beams with a maximum gain of 11.15 dBi, and the directional balun array forms a single beam with a maximum gain of 13.08 dBi. e above simulation results indicate that the reference antenna array could ensure good radiation performance in the frequency band of 2.4 ∼ 2.48 GHz.

Characterization of SRR
To illustrate the decoupling principle of split-ring resonators (SRRs) considered in this work, a model of the SRR unit cell is established as shown in Figure 4. e SRR consists of a concentric circle ring with an outer radius (R) of 7 mm and a split gap (g) of 0.5 mm, and the strip width (w) of the ring is 1 mm. e material of SRR is copper with a thickness of 0.035 mm. e relative permittivity of the dielectric substrate is 3.3 with the thickness of 1 mm. e period of the SRR unit cell is 21 mm.
When an x-polarized incident wave (the magnetic field direction is along the y-axis direction) propagates along the −z-axis and passes through the SRR unit cell, a resonance point could be generated by the inductance and capacitance of SRR. At this resonance frequency point, the SRR unit cell would display a single-negative magnetic feature to suppress the propagation of the x-polarized incident wave. e scattering parameters and the equivalent electromagnetic parameters of SRR are simulated as shown in Figures 5 and  6, respectively. Figure 5 shows the x-polarized incident wave is almost completely reflected at the resonance close to 2.5 GHz. Figure 6 shows the equivalent magnetic permeability is negative and the equivalent permittivity is positive at the resonance point. e simulated results mean SRR could be regarded as the negative magnetic metamaterials. Due to the fact that the coupling magnetic field of the proposed quasi-Yagi antenna array passes through the SRR array perpendicularly, its mutual coupling could be suppressed. e electromagnetic performance of SRR with different parameters is discussed as follows.

Different Radius (R).
e equivalent permittivity and permeability of SRR with different R are simulated, respectively, with the values of 7 mm, 7.7 mm, and 8.4 mm and are shown in Figure 6. Figure 6 shows that with the increase of R, the resonance point with a single-negative magnetic feature moves to low frequency.

Different Split Gap (g).
e equivalent permittivity and permeability of SRR with different g are simulated, respectively, g with the values of 0.5 mm, 1.0 mm, and 1.5 mm and are shown in Figure 7. Figure 7 shows that with the increase of g, the resonance point with a single-negative magnetic feature moves to high frequency.

Different Strip Width (w).
e equivalent permittivity and permeability of SRR with different w are simulated, w with the values of 0.6 mm, 1.0 mm, and 1.4 mm and are shown in Figure 8. Figure 8 shows that with the increase of w, the resonance point with a single-negative magnetic feature moves to high frequency.
It can be seen from Figures 6-8 that the resonant frequency with a single-negative magnetic feature of SRR can be adjusted by changing its parameters.

Miniaturized Quasi-Yagi Array Design
In order to obtain a better performance, the size and arrangement of SRRs are constructed by optimizing design, and the proposed quasi-Yagi antenna arrays with different balun modes are established as shown in Figure 9. e SRRs are embedded on both sides of the dielectric substrate surface with the antenna element spacing of 30 mm and 40 mm, respectively.

2
International Journal of Antennas and Propagation e variation of antenna array radiation performance under the process of reducing the elements spacing to 40 mm (decrease of 0.2λ e ) and 30 mm (decrease of 0.34λ e ) is compared, respectively.

e Simulation of Quasi-Yagi Antenna Array with Ele
e S11 comparison of the antenna array with SRRs and without SRRs is shown in Figure 10.
At 2.45 GHz, it can be seen from Figure 11(a) that the S11 of the inverse balun quasi-Yagi antenna array with SRRs is −14.46 dB, and the array without SRRs is −9.14 dB. It can be seen from Figure 11(b) that the S11 of the directional balun quasi-Yagi antenna array with SRRs is −12.65 dB, and the array without SRRs is −2.71 dB. erefore, the SRRs could further reduce S11 of the quasi-Yagi antenna array. e 3D gain of the antenna array with SRRs and the array without SRRs is compared, respectively, as shown in Figure 11.
It can be seen from Figures 11(a) and 11(b) that the 3D gain of the inverse balun quasi-Yagi antenna array with SRRs is 13.32 dBi and 2 dBi more than that of the array without SRRs. It can be seen from Figures 11(c) and 11(d) that the 3D gain of the directional balun quasi-Yagi antenna array with SRRs is 12.96 dBi, but the array without SRRs appeared with the serious distortion.
As the spacing of the elements decreases, the antenna array without SRRs begins to deteriorate, and the degree of wave beam separation is worsened. Adding SRRs not only  International Journal of Antennas and Propagation effectively overcomes the distortion of the antenna array but also separates beam energy effectively and enhances gain by 1.05 ∼ 1.13 times.
In order to further analyse the decoupling effect, the electric field intensity distribution of the antenna array with SRRs and the array without SRRs is compared, as shown in Figure 12.
e induced current generated on the five directors would intensify electric field superposition, which increases the mutual coupling effected. Because of the resonance effect, adding SRRs could effectively balance the electric field strength on the two elements.

e Simulation of Quasi-Yagi Antenna Array with Element Spacing Decrease (0.34λ e ).
e S11 comparison of the antenna array with SRRs and without SRRs is shown in Figure 13. At 2.45 GHz, it can be seen from Figure 13(a) that the S11 of the inverse balun quasi-Yagi antenna array with SRRs is −13.11 dB, and that of the array without SRRs is −8.81 dB. It can be seen from Figure 13(b) that the S11 of the directional balun quasi-Yagi antenna array with SRRs is −15.91 dB, and that of the array without SRRs is −13.52 dB. e 3D gain of the antenna array with SRRs and the array without SRRs is compared, respectively, as shown in Figure 14.
It can be seen from Figure 14(a) that the 3D gains of the inverse balun quasi-Yagi antenna array with SRRs is 11.77 dBi and 2.02 dBi more than that of the array without SRRs. It can be seen from Figure 14    directional balun quasi-Yagi antenna array with SRRs is 12.14 dBi and 5.04 dBi more than that of the array without SRRs. e electric field intensity distribution of the antenna array with SRRs and the array without SRRs is compared, as shown in Figure 15.
As the spacing of the elements is further reduced, the mutual coupling effect is further aggravated correspondingly. e SRRs could form a more symmetric distribution of the antenna array to reduce the mutual coupling and ensure that the radiation performance of the antenna array is not affected. e radiation performance of the antenna array with decreased element spacing to the reference antenna array at 2.45 GHz is compared, as shown in Table 1. Table 1 shows that in the process of reducing the array element spacing, the S11 of the antenna array with SRRs is much lower than −10 dB, which achieves the engineering requirements. It indicates that the antenna array with SRRs could reduce the mutual coupling and get better matching characteristics.

Antenna Array Fabrication and Its
Measurement Results e manufactured antenna array prototypes with different element spacing are shown in Figure 16. e fabricated dielectric substrate has same relative permittivity and thickness as the simulation model. e reference array is shown in Figures 16(a) and 16(b), and the proposed antenna array with SRRs is observed in Figures 16(c) and  16(d).  International Journal of Antennas and Propagation e fabricated design is measured by using a Fieldfox N9917A microwave analyzer, and the measurement results of S11 are shown in Figure 17.
It can be seen that the error between the simulation value and the measured value is less than 1.2%. At the 2.45 GHz, the S11 measurement value of the array with SRRs is −13.49 dB, which is −2.5 dB better than the reference array. e measurement results of the radiation pattern are shown in Figure 13.   International Journal of Antennas and Propagation It can be observed from Figure 18(a) that the gain of the antenna array at −22°is 11.1 dBi and at 20°is 12 dBi; from Figure 18(b), the gain of the antenna array at −22°is 10 dBi and at 22°is 12 dBi. e compared results indicate a very decent maximum gain matching achieved in the direction of the main lobe. e slight variation in the side lobe is attributed to inevitable error in the manufacturing process and different tangent loss of the substrate.

Conclusion
In this paper, the mutual coupling of the quasi-Yagi antenna array is investigated at 2.45 GHz. e effect of SRRs on the radiation performance of the quasi-Yagi antenna array during the element spacing reduced process is simulated. e simulation results indicate that the SRRs could suppress mutual coupling effectively and guarantee high gain. e measurement results of    e work is useful for quasi-Yagi antenna miniaturization and could obtain better application in transportation and WPT. e proposed SRR structure needs to be further optimized to adapt different antenna arrays.

Data Availability
e data used to support the findings of this study are included within the article.

Conflicts of Interest
e authors declare that they have no conflicts of interest.