# CPW Fed Compact UWB 4-Element MIMO Antenna with High Isolation

^{1}

^{2}

^{*}

## Abstract

**:**

## 1. Introduction

^{3}. The simulations have been conducted using the High Frequency Structure Simulator (HFSS). A prototype has been built and measured with close agreement between experimental and simulated results.

## 2. Antenna Design and Structure

#### 2.1. CPW Feed

_{r}= 4.4, loss tangent of 0.02 and thickness of 1.6 mm. The effective permittivity of the coplanar waveguide configuration is given by [18]:

#### 2.2. Antenna Configuration

#### 2.2.1. Single Element

#### 2.2.2. MIMO Antenna

_{1}and A

_{2}represent the areas of the ground plane and radiating patch, respectively, l

_{1}and l

_{2}represent the length of the ground plane and radiating patch respectively and g represents the distance between the ground and the radiation patch. All parameters are in the units of mm. For the considered antenna, l

_{1}= 19 mm, l

_{2}= 13.7 mm, g = 1.56 mm, A

_{1}= 42.2 mm

^{2}and A

_{2}= 224.06 mm

^{2}. The calculated and simulated fr have been obtained as 4 and 3.7 GHz, respectively. It can be seen that the estimated value is close to the simulated value.

_{3}, at the top of the patch and the stub connected to the feeder line can both improve the antenna matching to achieve the ultrawideband operation. In particular, the stub that plays an important role in the impedance matching. In addition, the small rectangular notches on the left and right-hand sides of the semicircular protrusion are denoted as W

_{2}and L

_{2}and they help with the impedance matching.

## 3. Influence of Special Structure

#### 3.1. Semi Surround Ground Structure

_{2}. The effects of different values of H

_{2}are studied and the results are shown in Figure 4.

_{2}, the first resonant frequency point shifts from 3.2 to 3.8 GHz. This is because when H

_{2}is increased, the gap between the radiating patch and the surrounding ground plane is reduced, that is to say, g in Equation (5) is decreased, resulting in the shift of the first resonant frequency point. In addition, it can be observed that when H

_{2}is shorter, a narrower bandwidth is achieved since the match is lost around 5 GHz and 8–9 GHz. When H

_{2}is longer, the cutoff frequency is about 3.2 GHz, and the matching is lost around 15 GHz, which fails to meet the requirements. Therefore, by adjusting H

_{2}to a suitable value, the matching can be achieved in the range of 3–20 GHz.

#### 3.2. Length of Radiating Patch

_{11}for various patch lengths.

_{1}in Equation (5) will increase, which will cause the first resonant frequency shift to the lower frequency, meanwhile, the cut-off frequency of low frequency will also shift to the lower frequency. When Lp is small, the cut-off frequency is approximately 3.2 GHz, which fails to satisfy the requirements of UWB. In addition, with the increase of Lp, the antenna matching in the 4–6 and 7–10 GHz bands becomes worse. Selecting appropriate Lp can make the antenna achieve better impedance matching in the UWB operating frequency range.

#### 3.3. A Stub Connected to Feedline

_{11}with and without stub, where it can be observed that a sufficiently wide matching bandwidth has been achieved up to 17 GHz. Moreover, this can be extended by adding the matching stub that has created another resonance point around 15 GHz, which effectively extends the impedance matching to cover the whole frequency range.

## 4. Results and Discussions

#### 4.1. S-Parameters

_{11}of the proposed UWB-MIMO antenna system are not totally identical with the simulated results at high frequencies. This could be attributed to fabrication and experimental tolerances with respect to the antenna printing, welding as well as testing conditions.

#### 4.2. Surface Current Distribution

#### 4.3. Radiation Patterns and Gain

#### 4.4. MIMO Performance

_{eij}is calculated when N = 4 throughout the whole bandwidth. Diversity gain (DG) can also be used to express the correlation of antennas, which can be calculated by Equation (7) [21]. The smaller the ECC value, the larger the diversity gain of the antenna.

_{mux}) and written by Equation (8) [22]:

_{mux}is higher than −3 dB throughout the whole frequency range. In addition, the peak gain is more than 1.3 dBi across the entire bandwidth. Therefore, both parameters satisfy the requirements of MIMO wireless communication systems.

#### 4.5. Performance Comparison

## 5. Conclusions

^{3}. The antenna achieves a considerably wide impedance bandwidth from 3–20 GHz. Easy fabrication decoupling structure is utilized to design the proposed antenna. The isolation between various elements is less than −17 dB. The incorporation of a matching stub is placed on the feeder of the antenna to improve the impedance matching in the high frequency band. All the simulated and measured results demonstrate that the proposed antenna offers important characteristics such as ultra-wide bandwidth, low mutual coupling, stable gain and radiation patterns. In addition, low ECC demonstrates the potential of the proposed antenna with presented diversity characteristics. A comparison of the proposed MIMO antenna with the other reported antenna structures has been presented to highlight the novelty and significance of our proposed work. Therefore, the CPW Fed Compact UWB 4-Element MIMO Antenna can be considered as a promising candidate for UWB applications.

## Author Contributions

## Funding

## Institutional Review Board Statement

## Informed Consent Statement

## Data Availability Statement

## Acknowledgments

## Conflicts of Interest

## References

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**Figure 7.**Fabrication photograph of proposed coplanar waveguide (CPW) fed compact ultrawideband (UWB)-MIMO antenna.

**Figure 9.**Surface current distributions of the MIMO antenna at: (

**a**) 3.5 GHz, (

**b**) 9 GHz, (

**c**) 14 GHz, (

**d**) 19 GHz.

**Figure 10.**Radiation pattern for the proposed MIMO antenna at: (

**a**) 3.5 GHz, (

**b**) 6.5 GHz, (

**c**) 14 GHz.

**Table 1.**Antenna dimensions shown in Figure 1 (unit: mm).

W | Wf | W_{1} | W_{2} | W_{3} | W_{4} | H | H_{1} | H_{2} | H_{3} | g |
---|---|---|---|---|---|---|---|---|---|---|

38 | 1.5 | 0.5 | 1 | 0.1 | 0.4 | 1.6 | 8.6 | 11.2 | 5 | 1.56 |

L | Lp | L_{1} | L_{2} | L_{3} | L_{4} | R_{1} | R_{2} | R_{3} | l_{1} | l_{2} |

38 | 4 | 2 | 0.7 | 1.6 | 5.8 | 3.2 | 7.2 | 1.1 | 19 | 13.7 |

Reference | Antenna Size (mm ^{2}) | Bandwidth (GHz) | Gain (dBi) | Isolation (dB) | ECC | Ports |
---|---|---|---|---|---|---|

[6] | 35 × 35 | 3.0–12 | NA | >20 | <0.3 | 2 |

[9] | 30 × 40 | 3.1–10.6 | NA | >15 | <0.15 | 2 |

[11] | 34 × 18 | 2.93–20 | 0–7 | >22 | <0.01 | 2 |

[12] | 50 × 30 | 2.5–14.5 | 0.1–4 | >20 | <0.04 | 2 |

[15] | 36 × 18 | 3.2–12 | 0–7 | >22 | <0.01 | 2 |

[17] | 58 × 58 | 2.9–40 | 4.3–13.5 | >17 | <0.01 | 4 |

[18] | 80 × 80 | 2.1–20 | 5.8 average | >25 | <0.02 | 4 |

This work | 38 × 38 | 3.0–20 | 1.3–6.2 | >17 | <0.08 | 4 |

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**MDPI and ACS Style**

Yin, W.; Chen, S.; Chang, J.; Li, C.; Khamas, S.K.
CPW Fed Compact UWB 4-Element MIMO Antenna with High Isolation. *Sensors* **2021**, *21*, 2688.
https://doi.org/10.3390/s21082688

**AMA Style**

Yin W, Chen S, Chang J, Li C, Khamas SK.
CPW Fed Compact UWB 4-Element MIMO Antenna with High Isolation. *Sensors*. 2021; 21(8):2688.
https://doi.org/10.3390/s21082688

**Chicago/Turabian Style**

Yin, Wenfei, Shaoxiang Chen, Junjie Chang, Chunhua Li, and Salam K. Khamas.
2021. "CPW Fed Compact UWB 4-Element MIMO Antenna with High Isolation" *Sensors* 21, no. 8: 2688.
https://doi.org/10.3390/s21082688