A MIMO mobile phone antenna decoupled with an improved neutralization method

Summary:

A multi-input and multiple-output (MIMO) mobile antenna suitable for mobile terminals is proposed. The MIMO antenna is composed of two centrally symmetrical antenna elements and adopts a coupling feeding method to expand the antenna bandwidth and ensure the miniaturization of the antenna. Through the middle of the floor, a T-shaped branch is introduced, and the antenna units are connected with a neutral line to achieve the purpose of improving the isolation between the antenna units. Simulation results show that the antenna can cover two important operating frequency bands from 824 MHz to 960 MHz and from 2 300 MHz to 2 600 MHz. The lumped inductance component loaded on the neutral line can effectively reduce the physical length of the neutralization line. The physical processing of the antenna was tested, and the physical measurement results were in good agreement with the simulation results.

0 Preface

With the rapid development of communication technologies, users have put forward higher requirements for the performance of mobile communication systems and the utilization of spectrum, and are continuously searching for the miniaturization and portability of mobile devices (Multiple Input Multiple Output, MIMO). The antenna is suitable for multi-band wireless communication, but due to the small mobile phone space, there is an inevitable coupling between antenna elements, making the correlation between MIMO antenna elements larger [1], reducing the MIMO antenna in improving channel capacity On the advantage.

In recent years, a large number of experts and scholars at home and abroad have studied MIMO antennas. The main research direction is decoupling between MIMO antenna elements. The introduction of floor gaps is the simplest and most efficient method of decoupling, and is easy to fabricate. The principle of decoupling is mainly to reduce the correlation between elements by reducing the wavelength of the propagation signal between antenna elements [2-3] ]. In [4], a U-shaped wideband MIMO antenna is proposed. Using the method of introducing a floor gap, the MIMO antenna isolation in the 2.4 GHz to 6.35 GHz operating band can reach −20 dB. The ultra-wideband band-rejection MIMO antenna proposed in [5] introduces a T-shaped stub structure. The current coupled to antenna element 2 and the current of antenna element 1 coupled to antenna element 2 are opposite in phase and cancel each other, weakening the antenna element. Inter-relative [6-7]. Similarly, the decoupling principle of introducing parasitic elements [8] into MIMO antennas and loading T-shaped floor nodes is similar.

The proposed antenna is decoupled by introducing a parasitic element. The operating frequency band covers the 2.4 GHz to 2.5 GHz and 5.15 GHz to 5.825 GHz frequency bands, and has good isolation. In [10], a miniaturized ultra-wideband MIMO antenna based on a slot antenna is proposed. The method uses slotting and pattern diversity on the floor. The antenna has a S12<-10 dB in the frequency band from 3.1 GHz to 7 GHz, and is at 7 GHz. The S12 <-25 dB in the ~11 GHz band enables high antenna isolation. The neutralization line technology is a decoupling technique proposed in 2006. The decoupling of the MIMO antenna is realized by introducing a current path to cancel the surface wave coupling on the adjacent units. The UWB MIMO antenna proposed in [11] reduces the antenna element coupling by introducing a broadband neutralization line. The designed MIMO antenna achieves an isolation of -22 dB at 3.1 GHz to 5 GHz. The length and deformation of the neutralization line also have a great influence on the isolation of the antenna.

The proposed antenna uses two neutralizing lines of different lengths. Through a combined decoupling approach, decoupling is achieved over a wide range of different broadband frequencies with a standard bandwidth of 6 dB. The neutralization line designed in [13] adopts a multiple folding structure. The neutral line at the bottom of the dielectric substrate is folded 14 times, and the top is folded 20 times to achieve the decoupling of the antenna. However, the total physical lengths of the two proposed neutralization lines are relatively long, and the decoupling structure in the literature [13] is more complicated, which increases the complexity of antenna design and the cost of physical processing to some extent.

In this paper, two antenna element structures are placed on the floor, and the elements are connected by a metal neutral line, and a lumped inductance element is loaded on the neutral line, which is improved, and at the same time, the T-type floor is loaded on the floor. Branch nodes implement decoupling of MIMO antennas. Finally, on the basis of simulation and optimization, the antenna is processed and tested in kind, and the physical test results are basically consistent with the simulation results.

1 antenna design

This paper proposes and designs a MIMO cell-phone antenna with high isolation, which is mainly implemented by coupled feed technology and band-rejection matching circuit loading technology. It satisfies S11<-6 dB, S21<-10 dB, and the working frequency band is 824 MHz to 960 MHz and 2 300 MHz to 2 600 MHz.

1.1 Antenna Structure Design

The specific structure of the proposed MIMO antenna is shown in Fig. 1. It consists of two symmetrically-shaped antenna elements. The middle is connected by a U-shaped neutral line. The size of each antenna element is 10 mm by 22 mm. The dimensions are 10 mm x 60 mm.

The antenna was printed on an FR-4 dielectric substrate with a dielectric constant of 4.4. The substrate size was 120 mm x 60 mm x 0.8 mm. A T-junction is added in the middle of the antenna's floor to increase the isolation of the MIMO antenna. The antenna uses a 50 Ω coaxial feed excitation method.

1.2 Composition of MIMO Antennas

The designed MIMO antenna adopts a coupling feeding method, and a direct feeding portion is formed by combining a band-resistance circuit and a feeder line, and the distance between the two antenna units is only 16 mm. Figure 2 shows the specific geometry and dimensions of the antenna and each cell. The specific parameters of the dimensions of each part of the antenna structure are shown in Table 1. Which A1 ~ A4 is an inductor, C1 is a capacitor.

The coupling feed section is composed of a short-circuit line and a lumped element. The feed line of the antenna structure in Fig. 2(d) directly generates a high frequency (2 300 MHz to 2 600 MHz) and a low frequency (824 MHz to 960 MHz) from Fig. 2(b). The combination of the band-stop circuit and the coupling feed-in produces. The length of the U-shaped metal wire used to connect the two antenna elements is 50 mm, which is much less than the length of the low-frequency resonance point. The purpose of adding the microstrip metal wire is to achieve decoupling at a low frequency band. On the back of the antenna, a T-shaped floor node is added. As shown in Fig. 2(c), this structure is mainly used to reduce the coupling of the two antenna units at high frequencies. Due to the limitation of the antenna size requirements of mobile equipment, in order to cover the GSM850/900 MHz operating band, the antenna has an additional band-restriction matching network at the feed port. The structure is shown in Figure 2(b). The addition improves antenna impedance matching.

1.3 Decoupling of Antennas

The designed MIMO antenna requires less coupling between the antenna elements, so the U-shaped neutral line is used for decoupling. Decoupling using the neutralization line can effectively reduce the coupling at low frequencies on the basis of maintaining the original antenna size. However, when a neutralization line is placed between the antenna units, the position is difficult to determine, which is also a difficulty in the design of the MIMO antenna.

The antenna designed in this paper uses a U-shaped neutralization line structure with a size of 50 mm×0.5 mm and has been modified to load a lumped inductor with a size of 15 nH in the middle of the neutralization line. Increasing the isolation between the antenna units also shortens the physical length of the neutralization line.

2 Analysis of simulation results

2.1 MIMO antenna isolation effect analysis

Isolation is one of the important indicators to measure the performance of a MIMO antenna. The antenna uses a method of decoupling T-shaped floor nodes and neutralization lines to improve isolation between antenna elements. The role of the T-shaped floor branch is to reduce the isolation of the antenna at high frequencies. This structure enables the two opposite-phase currents between the antenna units to cancel each other, thereby improving the isolation of the antenna. The addition of the neutralization line is mainly used to reduce the coupling of the antenna at the low frequency band. Here we mainly examine the effect of the lumped inductor A2 loaded on the modified U-shaped neutral line on antenna isolation.

As shown in Fig. 3, the increase in the value of A2 has basically no effect on the S parameters of the antenna at low frequencies. In the curve of Fig. 3(b) S21, when A2 increases, the isolation effect of the antenna in the high frequency portion gradually deteriorates. According to the simulation optimization result graph, when the value of A2 is 15 nH, the decoupling effect of the antenna in the coverage band is the best, and the isolation can basically meet the antenna design requirements.

The S-parameters of the designed MIMO antenna and two reference antennas are compared respectively to analyze whether the isolation performance is good or bad. Among them, the reference antenna 1 does not add a T-shaped floor branch and a U-shaped neutral line, and the reference antenna 2 does not add a U-shaped neutral line. The comparison of the S-parameter curves of the three antennas is shown in FIG. 4 .

The S-parameter of the reference antenna 1 has only one resonance point at a high frequency, and the frequency band is relatively narrow and the isolation is about -7 dB. The S-parameter band of the reference antenna 2 is shifted to the left by a certain distance, but the overall effect on the low frequency band is small, and the isolation of the antenna is still not ideal. The MIMO antenna designed can achieve -10 dB or more isolation in both low and high frequency bands. The isolation effect is improved compared to the two reference antennas, and the antenna can cover 824 MHz to 960 MHz and 2 300 MHz. Two frequency bands - 2 600 MHz.

The current distribution on the antenna structure can well reflect the decoupling effect of the neutralization line. When the antenna is operating at 850 MHz, the current distribution is shown in Figure 5.

After adding the U-shaped neutralization line, the current of the left antenna unit weakens and the current transmitted to the right antenna unit is also weakened. When the neutralization line structure is not added, the isolation of the antenna in the low frequency band is about 7.5 dB. After the U-shaped neutral line structure is added, the isolation between the two antenna elements is increased by about 3 dB. The correlation between them has decreased significantly. Due to the addition of a U-shaped neutral line, the MIMO antenna achieves a decoupling effect at a low frequency.

2.2 Antenna radiation pattern

The antenna radiation pattern is also one of the important indicators to measure the performance of the antenna. In this paper, the MIMO antenna designed has been simulated. Figure 6 shows the direction of the antenna on the xoz plane and the yoz plane. The vertical axis represents the antenna in different directions. The gain.

It can be seen from Fig. 6 that at the operating frequency of 850 MHz, the radiation pattern of the antenna in the xoz plane presents an ∞ type, which is similar to the radiation pattern of the monopole antenna. When the antenna is operating at 2 500 MHz, the radiation pattern of the antenna shows diversity. In the directional diagram, Eθ and Eφ are basically the same size, and the designed antenna has good stability.

3 antenna physical processing test results

This article uses the electromagnetic simulation software HFSS to design and simulate the MIMO antenna, and processes the antenna. Figure 7 shows the processing of the antenna.

The physical object of this MIMO antenna is tested with a vector network analyzer. The test results are shown in FIG. 8 . From the actual test results and simulation results, the S-parameters are basically the same in the frequency range covered by the antenna, satisfying S11<-6 dB in the low-frequency 824 MHz to 960 MHz and high-frequency 2 300 MHz to 2 600 MHz, respectively. S21<-10 dB. However, in other frequency bands, there is a certain gap between the actual measurement results and the simulation results. The main reason for the error is that the band-resistance matching circuit in the antenna structure, the capacitance and inductance of the neutralization line easily cause errors in the welding process. Second, there is a certain error in the size of the capacitive inductance itself, and the coaxial line will be lost during the processing. At the same time, in the welding process of capacitors and inductors, the amount of solder can also cause errors.

4 Conclusion

This paper proposes and designs a MIMO mobile phone antenna decoupled with an improved neutralization method. In terms of decoupling of the MIMO antenna, the antenna is improved by adding a lumped inductance element between the two antenna elements by adding a neutralization line between the two antenna elements, and the decoupling of the antenna is successfully achieved, and the antenna is made small Change. At the same time, the designed antenna has a prominent ground plane, and other components can be mounted on it to save the space of the mobile phone mainboard and make the structure of the physical antenna more compact. The result of the physical test of the antenna further proves that the antenna has a good working performance and basically meets the design requirements of the mobile phone antenna.