Self-Decoupled MIMO Antenna Realized by Adjusting the Feeding Positions

Abstract

This paper proposes a novel decoupling technique achieved by adjusting the position of feeding probes of antennas. Two inherent radiation modes (patch mode and monopole mode), with different patterns and polarizations, are simultaneously excited by the same feeding probe. High isolation is realized based on manipulating the relationship of two-mode couplings by moving the feeding positions. Since the two radiation modes are generated by the same antenna element, the proposed MIMO antenna features a simple structure and compact size. For verification, a two-element array with center-to-center spacing of 0.404 ${\lambda }_0$ (${\lambda }_0$ is the wavelength in the air) is prototyped and characterized. Simulation and experimental results show that the proposed novel technique can offer higher port isolation (>18.1 dB), increased efficiency (>70%), and a lower envelope correlation coefficient (ECC < 0.1) in the operating frequency band (11.61–12.49 GHz).

1. Introduction

It is known that the MIMO antenna plays an important role in the 5G communication system due to its high channel capacity, low delay, and outstanding transmission rate [1]. However, the inherent coupling between different radiating elements can decrease the performance of the MIMO system [2]. In recent years, much effort has been put into reducing the mutual couplings between antennas [3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20,21,22,23], including the use of decoupling networks [3,4], electromagnetic band gap (EBG) structure [5], meta-surface [6], defected ground structure (DGS) [7], neutralization line [8], parasitic element [9], and self-decoupled elements [10]. In [11], an inductance-based decoupling structure is presented to decrease the mutual coupling between extremely closely spaced microstrip antennas. A cascaded power dividing decoupling network (C-PDDN) is proposed in [12], and it can reduce the mutual coupling in adjacent or even contiguous frequency bands of the two antennas. The EBG structure is also adopted to enhance the isolation of antenna arrays [13,14]. In [14], a novel slot–array DGS is presented. Additionally, based on the analysis of equivalent circuit model, the DGS can contribute to reducing the mutual coupling with stop-band characteristics. Another well-known decoupling method is by using a neutralization line (NL) [15]. An additional coupling path can be generated by NL to cancel out the original coupling in MIMO antenna arrays [15]. Parasitic elements are used in [16] to decrease the mutual coupling of antennas, and the performance of MIMO system is then enhanced. However, most above-mentioned proposals adopt additional structures (circuit or scatters) for decoupling purpose, which often occupy an excessive volume of space and increase the complexity of the systems. Besides this, the extra structures usually have negative impact on antenna performances, such as reducing the operating bandwidth and decreasing the radiation efficiency. A high isolation air-patch antenna array loaded with vertical resonators was proposed in [17]. However, three-dimensional structure is necessarily required to construct the loaded resonators, which significantly increase the manufacturing complexity. So, the antenna array in [17] cannot be produced (on a large scale) using the traditional printed board technology.

To overcome this problem, a novel and simple self-decoupled design method is proposed in this paper. Two radiation modes create two types of mutual couplings in the patch-monopole antenna array. The feeding position can adjust the strength and phase relationship of two coupling modes. When the two couplings satisfy equal magnitude and out-of-phase relationship, the total coupling can be canceled out, and high isolation is realized. Moreover, the proposed self-decoupled antenna array has the advantages of size miniaturization and simple design, thanks to the elimination of any extra decoupling structure. For demonstration, a compact two-port patch–monopole antenna is designed, fabricated, and characterized. Experimental results reveal that the proposed technique can provide enhanced port isolation (>18.1 dB), increased antenna efficiency (>70%), and improved pattern diversity (ECC < 0.1) in the operating frequency band, with simple and compact construction.

2. Proposed Design

Figure 1 shows the structure of the two-port patch–monopole antenna, which is composed of two patch radiators on the top layer of the substrate, a ground plane, and two 50 Ω coaxial feeds. Figure 1b depicts the configuration of the MIMO antenna. The left and right elements are symmetrical about the x axis. The two-port MIMO antenna has a center-to-center spacing of d. The single element has a size of $W_C\times L_C$, and the whole PCB size is $W_G\times L_G$. Changing the position of the feed port can adjust the components of patch and monopole modes. High isolation is realized when two-mode couplings are canceled out. Next, a detailed explanation on this decoupling method is illustrated as follows.

2.1. A. Radiation Modes and Decoupling Mechanism

Figure 2 shows two different operating modes of the proposed patch–monopole antenna. For patch mode (Figure 2a), the current flows from one edge of the patch to the other edge. Note that the position of the feeding probe can also change the specific horizontal polarization of the patch mode (for example, the specific polarization could be along the x-direction, y-direction, or other directions). For the monopole mode (Figure 2b), the current starts from the end of feeding probe (near the ground), flowing upwards, and then going to the edges of the patch. In principle, these two modes co-exist (more or less) for every feeding position. However, their strength and phase relationships are quite different (for different positions). For better demonstration, as shown in Figure 1b, the center of the dielectric substrate is defined as the coordinate origin (O). Therefore, the location (taking right element as reference) of the feed port can be represented by coordinate points ($X_i,Y_i$). The solid line in Figure 3 shows two special cases (unit: mm) when the patch mode and monopole mode are dominated, respectively. The middle pattern between two specific radiation modes is shown by the dotted line in Figure 3. At location (1.275, 5), the feeding probe is in the middle line of the patch (similar to the feeding approach of a traditional patch). The patch mode is then dominated, which resulted in broadside radiation. However, at location (0, 5), the feeding probe is at the center of the patch. Since the center position is a virtual-ground (0 voltage) condition for patch mode, this position can only excite monopole mode, which resulted in end-fire radiation. It is worth mentioning that, by adjusting the position of the feed point $X_i$ from 0 to 0.125, it can be observed that the radiation mode is similar to the monopole mode, but there is already a trend of broadside radiation. Continuing to adjust $X_i$ from 0.125 to 0.425, broadside radiation is further increased. The simulated results illustrate the existence of two radiation modes of the proposed patch–monopole antenna, and some intermediate modes can be obtained by adjusting the feed point position $X_i$.

Obviously, since there are two radiation modes for every antenna, two types of mutual couplings co-exist in the proposed array. The relative strengths and phases of the two radiation modes are controlled by tuning the coordinate point of the feed probe. Correspondingly, the coupling paths are also modified when modifying the feeding position. It can be concluded that, if the amplitudes of two coupling paths are equal, and their phases are opposite, the mutual coupling can be totally cancelled out, and high port isolation is then achieved. The freedom of feeding position is mainly used for decoupling purposes. As for the impedance matching of antennas, it is simply realized by adjusting the width (W_C) of the patch.

2.2. B. Simulation Verification

To further verify the decoupling mechanism of the proposed technique, the patch–monopole antenna arrays with different feeding points are simulated by using an EM simulator. The antennas are fed by 50 Ω coaxial lines from the back of the ground. The center-to-center distance between two patches is 0.404 ${\lambda }_0$, where ${\lambda }_0$ is the wavelength in free space. Figure 4 shows the simulated scattering parameters of the patch–monopole antenna with different feeding positions. It is obvious that, in the frequency band (11.65–12.58 GHz) with return loss under −10 dB, the mutual coupling is reduced step by step with the altering of coordinate positions. Moreover, the port isolation is enhanced by about 3.5 dB~24.09 dB in the operating band.

Figure 5 gives the electric filed distribution for the reference and proposed antenna arrays. The feeding position of coupled antenna for the reference array exhibits high electric filed. The feeding position of the coupled antenna for the proposed array shows a null-filed-position (equivalent to short-circuited condition). Therefore, the isolation is enhanced after decoupling.

3. Experimental Validation

The proposed antenna array with center-to-center spacing of 0.404 ${\lambda }_0$ is designed, fabricated, and measured. A reference array based on traditional patch antennas is also designed and characterized for comparison. Rogers RO4003 is selected to fabricate the dielectric substrate, whose dielectric constant is 3.55, loss tangent is 0.0027, and thickness is 1.542 mm. The photographs of fabricated MIMO antennas are depicted in Figure 6, and the dimensions of antennas are shown in

Table 1. Dimensions of fabricated antennas. (Unit: mm).

	LG	WG	d	LC	WC	d1	d2
Reference	50	50	10	5.555	6.9	1.5	4.5
Proposed	50	50	10	5.700	5.3	1.98	2.0

. The S-parameters of MIMO antennas are recorded by the Vector Network Analyzer (VNA), with frequency ranging from 11 to 13 GHz. Figure 7 shows the measured S-parameters of both arrays. It can be seen that the frequency band with reflection coefficient under −10 dB is from 11.61 to 12.49 GHz, and the port-isolation of the proposed antenna is improved by about 6~32 dB in comparison to the reference one. It is observed that the proposed MIMO antenna has a smaller frequency deviation than the reference one. The difference is mainly due to the manufacturing errors of substrate, welding, and test environment. The total efficiency and radiation patterns are measured by a microwave chamber (Figure 6e). When one antenna port is excited for measurement, another port is terminated by a 50 Ω load. Figure 8 shows the measured efficiency of two antenna arrays. Obviously, in the operating band, more than 10% of the total efficiency is increased after using the proposed decoupling method. Figure 9 gives the measured ECC values (far-field) of both arrays. The proposed patch–monopole array has much lower ECC (<0.1) than the traditional patch array (>0.5). Figure 10 shows the radiation patterns of the arrays. In summary, the performance of the proposed array is much better than the reference array.

Table 2. Comparison with other decoupling methods.

Ref. No. (Year)	Additional Structure	Element Spacing	Frequency (GHz)	Isolation Enhancement	Peak Efficiency
[11] (2020)	Required (Connect-Strip)	0.440 λ0	2.39–2.53	10 dB	87%
[14] (2020)	Required (DGS)	0.504 λ0	1.25–1.27	5 dB/15 dB	N.A.
[18] (2019)	Required (Resonators)	0.260 λ0	2.20–2.23	12 dB	74%
[19] (2020)	Required (Natura Line)	N.A.	0.94–0.99	17 dB	N.A.
This work	Not Required	0.404 λ0	11.61–12.49	7 dB	73%

shows the comparison of the proposed antenna with recently published designs. The performance of the proposed antenna is competitive compared to other antenna designs. In addition, this method requires no additional decoupling structure, which makes the design simple and compact.

4. Conclusions

This paper proposes a novel decoupling technique to improve the port isolation of two-port MIMO antennas. In traditional patch arrays, there exist strong mutual couplings between two antenna elements. However, by adjusting the position of feeding probes, two types of mutual couplings are canceled out, and high isolation is then realized. The proposed MIMO antenna has huge superiority thanks to its compact structure, smaller size, and easier manufacturing. The proposed patch–monopole MIMO antenna is a good candidate for modern wireless communication systems.