An Ultra-Wideband MIMO Bowl-Shaped Monopole Antenna with Sturdy and Simple Construction

. Anultra-wideband (UWB) bowl-shaped monopole antenna with a sturdy, simple, and lightweight structure is proposed, and then is used to compose the 3 × 3 multiple input multiple output (MIMO) antenna. Te wide bandwidth is determined by the outline of the monopole, which has a quarter wavelength and high-order modes. Te inner part of the bowl-shaped monopole is removed for a light weight. Te simulated and measured results show that an ultra-wide band of 2.3–8.1 GHz (5.8GHz, 111.5%) and a high isolation of greater than 20dB between the antenna elements of the MIMO antenna can be achieved.


Introduction
In modern vehicle-to-X (V2X) communication systems, vehicles are required to provide the in-car and out-car wireless information exchange for diferent systems, such as the train control and management system (TCMS), sub-6 GHz base station, and WiFi [1]. To support a higher transmission rate, the base station communication has developed from 2G to 5G, and the WiFi-6E band (5925-7125 MHz) has been introduced from the existing WLAN bands of 2400-2484 MHz and 5150-5925 MHz [2,3]. Tus, an ultra-wideband (UWB) antenna backing up diferent frequency bands is preferred than the multiple antennas to save space and cost [4]. Since the vehicle is moving fast, the unavoidable vibration may cause the unreliable wireless link. To mitigate the efects of mobility and obtain an enhanced channel capacity, integrating the multiple input multiple output (MIMO) technique and omni-directional radiation performance with UWB can further facilitate the robustness of the communication system [5,6]. For the terminal antenna, a sturdy, lightweight, and simple structure is also indispensable when the vehicle is moving fast.
With their wideband and omni-directional radiation properties, monopole antennas have been widely used to achieve UWB-MIMO antennas [7][8][9][10][11][12][13]. In [7], a top-hat monopole antenna was conceived to achieve a conical radiation pattern, but it had a narrow bandwidth. By introducing four metallic parasitic columns and a capacitive disk to a conical monopole, a 2.9 : 1 impedance bandwidth (IMBW) was obtained [8] with a complex structure. In [9], only a conical monopole antenna was designed to simplify the structure, but the weight was heavy due to the solid brass cone's high profle. Perforated alloy surfaces were adopted to realize lightweight structure in [10]. However, a 3D printing technique was required, resulting in a high cost. In [13], novel wideband roll-monopole antennas were devised, but the structures were not sturdy. Among the monopole antennas [7][8][9][10][11][12][13], the structures are either complicated or heavyweight. To achieve UWB-MIMO antennas, decoupling structures, such as ground stubs, defected ground slots, and refectors, are mostly used to improve the isolation [14][15][16][17][18], which also complicates the design. Tus, designing an UWB-MIMO antenna with sturdy and simple construction is necessary for vehicle applications.
In this paper, a 3 × 3 MIMO antenna based on a UWB bowl-shaped monopole antenna is presented for vehicle applications. First, the bowl-shaped monopole with and without the inner part is studied for its light weight. Te outline of the monopole is optimized to achieve a broad band with a sturdy structure. Ten, the 3 × 3 MIMO antenna is made with a wide band and high isolation. Finally, the prototype is fabricated and measured for verifcation, showing a good radiation performance.

Antenna Structure.
Te confguration of the proposed stepped monopole antenna element is depicted in Figure 1, with a simple and robust structure. Te whole structure is like a bowl with an extended edge. Te outline of the bowl antenna consists of a series of oblique lines to enhance the depth of the impedance matching. Te coordinate system is established with the bottom point of the radiator as the coordinate origin. Te antenna profle z � f(x) equation is as follows: 18 ≤ x < 23, Four nylon posts are placed between the bowl edge and the ground plane to support the structure's stability. Te antenna is excited with the help of a SMA connector, with an extended inner pin inserting into the bottom of the bowl monopole. Finally, the antenna element is used to construct a 3 × 3 MIMO antenna, arranging it in line, as shown in Figure 2. Te fnal optimized parameters are included in Table 1 for fabricating the prototype.

Antenna Analysis.
To achieve a light weight, the inner part of the bowl-shaped monopole is removed. Te refection coefcients of the antenna with and without the inner part are compared in Figure 3, indicating that the inner part has no efect on the antenna performance. Tus, an empty bowl-shaped monopole is used to reduce the weight, exhibiting an impedance bandwidth of 2.3-8.1 GHz (5.8 GHz, 111.5%).
To further analyze the operating mechanism of the proposed antenna, current distributions at 2.4, 4, 6, and 7.5 GHz are presented in Figure 4. It can be seen that the current is mainly distributed outside the bowl monopole; thus, the inner part has little efect on the refection coefcient. Since the currents are uniformly distributed along the same altitude, omni-directional radiation patterns can be acquired. Te simulated results show that from 6.5 GHz to above, high-order mode can be observed from the current distribution, while the currents below 6.5 GHz exhibit a quarter wavelength along the monopole.
Te outline of the bowl-shaped monopole dominantly afects the impedance matching. Two parameter studies are presented, for example, the refection coefcients of diferent R 0 and R 7 are shown in Figure 5, afecting the higher and lower resonant frequencies, respectively. To have a wide bandwidth and good impedance matching, R 0 � 2 and R 7 � 5 mm are selected.

3 × 3 MIMO Antenna.
To increase the channel capacity, a 3 × 3 MIMO antenna is constructed based on the antenna element. To simplify the structure, three antennas are placed in line at a distance of 115 mm for high isolation. Finally, a cover box made of polycarbonate with a dielectric constant of 2.9, which can be combined with the ground by screws, is used to protect the MIMO antenna. Since this material has the property of fre prevention, the antenna inside is more practical for vehicle applications. Te simulated refection and transmission coefcients of the MIMO antenna with and without the cover are shown in Figure 6. It can be seen that the cover has little efect on the results of the refection and transmission coefcients. Although a small variation in isolation can be observed, the overall isolation remains higher than 20 dB within 2.3-8.1 GHz. Te IMBWs of the three antenna elements remain about 2.3-8.1 GHz, and the isolations are higher than 20 dB within the operating band.
Te isolation with diferent values of interelement distance d is also investigated in Figure 7. It can be seen that when the three antenna elements are arranged in parallel, the isolation basically shows a downward trend with the increase in frequency. For WiFi/WLAN applications, the band of 2.4-2.4835 GHz is important; thus, the isolation between diferent antenna elements at 2.4 GHz should be kept larger. Considering the antenna fabrication tolerance and higher isolation, d � 115 mm is considered. When changing the ground width ub, the simulation found that s11, s22, and s33 did not change much, but the isolation s12 and s23 could not reach −20 dB, so ub � 92 mm is selected.

Simulated and Measured Results
Te prototype of the proposed MIMO antenna is fabricated with a total dimension of 290 × 92 × 23.5 mm 3 (2.22 × 0.71 × 0.18 λ, where λ is the free-space wavelength at the lowest frequency of 2.3 GHz), as shown in Figure 8. We use computer numerical control (CNC) to fabricate the antenna with aluminum, and 3D printing techniques are used to process the white cover plate. Finally, screws are employed to assemble the antenna prototype and the white cover together. Considering the polycarbonate cover, the overall height of the antenna is 28 mm (0.21λ). Te cover made of polycarbonate material is used to protect the MIMO antenna. Since this material has the characteristics of fre prevention, the antenna interior is more practical for vehicle applications. Compared with other works [9][10][11][12], whose structure is solid or requires several components, the proposed antenna, as a single component, is easy to fabricate by CNC, and the weight is light due to the hollow structure made of aluminum. Tus, the proposed antenna is more suitable for the use of vehicle antennas.
Te refection coefcients, isolation, and radiation patterns are measured. A comparison of the simulated and measured result of the S-parameters of the antenna is presented in Figure 9, showing a good agreement. Te simulated IMBWs of the three antennas are in the range of 2.3-8.1 GHz, while the measured IMBWs are in the range of 2.3-9 GHz. Both the simulated and measured isolations are higher than 20 dB within 2.3-8.1 GHz. Te discrepancy between the simulation and measurement can be accepted due to the fabrication and measurement errors.
Te simulated and measured radiation patterns at different frequencies for port 1 and port 2 are shown in Figures 10-12 and Figures 13-15, respectively. Both copolarized (E-theta) and cross-polarized (E-phi) radiation patterns in the three principal planes are given. Since antenna 1 and antenna 3 have symmetrical positions, similar radiation patterns are achieved. Only the radiation patterns of antenna 1 and antenna 2 are shown in brief. A good agreement between the simulation and measurement is achieved. It can be seen that the xoy plane shows a consistent circular shape, and the yoz and xoz planes exhibit a doughnut shape with low cross-polarization, indicating a good omni-directional radiation performance.

International Journal of Antennas and Propagation
Reflection coefficient (dB) S11 without cover S11 with cover S22 without cover S22 with cover S33 without cover S33 with cover Frequency (GHz) Frequency (GHz)

Conclusions
Te research on the 3 × 3 MIMO UWB monopole antenna has been designed and tested. A simple bowl-shaped monopole antenna with a sturdy structure is proposed for an ultra-wide band. A 3 × 3 MIMO antenna without any decoupling elements is constructed based on the bowlshaped monopole, showing high isolation and wide bandwidths. A cover box made of freproof material is used to protect the MIMO antenna, which has an insignifcant impact on the antenna performance. Te good performances and simple structure indicate the proposed antenna can be used for in-vehicle WiFi applications, covering 2400-2483.5 MHz, 5180-5825 MHz, and 5925-7125 MHz.

Data Availability
Te data used to support the fndings of this study are included within the article.

Conflicts of Interest
Te authors declare that they have no conficts of interest. International Journal of Antennas and Propagation 9