Circularly polarized Ultra-wideband MIMO/Diversity antenna with triple band-rejection characteristic

DOI: https://doi.org/10.21203/rs.3.rs-1562248/v1

Abstract

This paper presents a Circularly polarized (CP) Ultra-Wideband (UWB) multiple-input-multiple-output (MIMO) antenna with three rejected bands. To produce triple band-notched characteristics, two compact Electromagnetic bandgap cells are placed near the feedline of the planar antenna. A broad Axial ratio bandwidth (ARBW) is obtained by adding a slot and a stub expanded from the ground plane. The ability to produce wide ARBW is an additional feature of the suggested structure. This MIMO antenna has a -10dB bandwidth of 3.1–10.6 GHz and axial ratio bandwidth of 3-10.4 GHz with three-rejected bands at 3.5, 5.5 and 8.2 GHz. Moreover, the suggested structure shows good diversity characteristics properties like an envelope correlation coefficient (ECC) > 0.03, total active reflection coefficient (TARC) > 10 dB, and diversity gain (DG) of approximately 9.99. The considered configuration is fabricated using an FR4 dielectric substrate with a total size of 42.7×55×1.6 mm3.

1. Introduction

Modern wireless communication systems are always pushing the capability of transferring high data rates to improve user experiences (i.e. videos, gaming, etc). One way to improve data rate transfer is to use Ultra-Wideband (UWB) technology that requires antennas covering 3.1 to 10.6 GHz that provides various advantages such as low profile, wide spectrum, and low spectral density. One of the major challenges in such a technology is the polarization mismatch loss among the transmitting and the receiving antennas [1]. A circularly polarized (CP) antenna eliminates the requirement to align the placement of the receiving and transmitting antennas thus helps in reducing the polarization mismatch issue. Faraday rotation effect and the multipath interference can also be minimized using a CP antenna. CP waves can be produced by engendering dual orthogonally electric fields having the same amplitude with a 90˚ phase difference [2]. This CP wave can be created by using dividers, modified antenna structures coupler [3] in addition to creating current rotations on the surface of patch antennas. Some narrowband wireless communication system standards partially overlap with the UWB frequency range, such as, WiMAX that lies between 3.3–3.6 GHz, WLAN that lies between 5–6 GHz and part of X band that lies between 7.9–8.4 GHz [4] will cause an interference problem with UWB systems. Consequently, it is essential to propose UWB antennas with band notch features to suppress the undesired interferences. In literature, various CP antennas were designed [5–8], without band-rejection features. To generate notched bands, the traditional approach etches slots in the antenna structure, feedline, or ground plane to suppress the signals within specific bands. In reference [6] band rejection features were employed in a linearly polarized UWB antenna.

In the multipath environment, the major concern for the UWB system is signal fading. This can be resolved by utilizing MIMO technology that gives enhanced channel capacity. Port and field isolation between elements needs to be reduced between UWB MIMO antennas. Port isolation can be improved by Defected Ground Surfaces [12] [13] such as dumb-bell [11] [14] and T- shaped [6] among others. One more method to realize improved mutual coupling is to prolong the ground plane for example with a Y-shaped stub [15]. Authors have also used neutralization lines [15] and decoupling strips [16] to reduce the isolation among the axial ratio bandwidth of the proposed structure in [9] is from 3.2 to 8.2 GHz with dual-band notch from 4.8 to 5.4 GHz and 5.6 to 6.2 GHz. Dual band notches are obtained by using two slots in the radiating patch. Other approaches based on planar resonators like a ring-shaped resonator [10], and meander line resonator [11] were also used to achieve band-notched characteristics. The methods for obtaining band rejection rely on antenna geometry. Thus, these methods are not applicable to all existing UWB antennas. Moreover, the radiation properties get deformed by etching slots in the patches. Consequently, to isolate the wireless system from unwanted interferences, the use of Electromagnetic Band Gap [8] [13] (EBG) has been proposed. An EBG structure is a structure that assist/prevent the transmission of waves in a specific frequency range. Owing to the space constraint in practical applications, a compact EBG structure is required to be incorporated on the UWB antenna to achieve band notch characteristics. The antenna elements. Numerous researchers have designed dual and triple rejected bands features with linearly polarized UWB MIMO antenna [11–13]. Subsequently understanding the differences and limitations of various methods, it is realized that attaining a band notch UWB MIMO antenna with small dimensions remains a challenge.

In this work, we introduce a CP UWB MIMO antenna with three rejected bands. To realize the three band-notches a Two Via Compact Electromagnetic Band Gap (TVC-EBG) structure is employed near the feed line of the UWB structure. A broad ARBW is obtained by inserting a slot under the feed line and expanding a stub from the ground surface. Tested results show that the constructed antenna is covers 3–11 GHz of impedance bandwidth whereas the 3-dB ARBW covers 3-10.4 GHz. The three rejected bands are 3.3–3.6 GHz, 5–6 GHz and 7.9–8.4 GHz. In the meantime, the presented structure has the smallest size, modest design, CP based, and broadest ARBW when compared to previously UWB MIMO antennas.

2. Antenna Configuration And Design

The presented antenna is designed and fabricated using a commercial FR4 substrate with a permittivity and height, loss tangent of 4.4, and 1.6 mm and 0.02, respectively. The modeling and simulation of the antenna were conducted using HFSS (v13).

A. Proposed TVC-EBG Structure

EBG structures act as a band stop filter in a particular frequency range. Thus, EBG structures behave as high impedance surfaces at the resonant frequency. The TVC-EBG structure is made up of the copper patch surface, dielectric, ground surface, and via. The design formulations are given below in (1) and (2) as highlighted in [8], [17–20]

 \(\begin{gathered} L={\mu _0}h \hfill \\ C=\frac{{w{\varepsilon _0}({\varepsilon _r}+1)}}{\pi }{\operatorname{Cos} ^{ - 1}}\left\{ {\frac{{2w+g}}{g}} \right\} \hfill \\ \end{gathered}\) 

 \({f_c}=\frac{1}{{2\pi \sqrt {LC} }}\) 

Where, ɛ0, p, g, ɛr, and µ0 are the permittivity in free space, the width of the patch, relative permeability, and absolute permittivity respectively. To achieve compactness in the EBG unit cell, the equivalent values of the capacitor (C) and the inductor (L) are increased. To increase the value of L and C, a slot and via are inserted on the EBG unit structure. Figure 1(a) shows the proposed EBG unit cell. 

Parameter

Ls

Ws

WF
Table 1

Parameters of the suggested antenna

Units(mm)

42.72

28

1.8

Parameter

W1

L1

Lf

Units(mm)

5.5

11

9

Parameter

d1

d2

d3

Units(mm)

1.7

2.7

6

Parameter

R

R1

R2

Units(mm)

6

10

2.5

Parameter

d5

d4

d3

Units(mm)

3

1

6
  

Ref.

IB (GHz)

ARBW

(GHz)

No. of element

Band notches

(GHz)

No. of band notches

Size

(mm2)

Isolation

(dB)
Table 2

Comparison with previous works

Proposed Antenna

3–11

3-10.4

2

3.5,5.5&8.2

3

42.72×55

-15

[9]

3.2–9.6

3.2–8.8

1

5.2and 5.8

2

65×65

NA

[27]

5.10–5.85

5.10–5.85

2

NA

NA

56×32

-20

[28]

5.772–5.864

5.49–6.024

2

NA

NA

97× 26.72

-33

[29]

5.8

5.6-6

2

NA

NA

110×58

NA

[30]

5.71–8.2

7.72–8.04

2

NA

NA

80 × 80

-15

[31]

4.56–8.5

4.75–8.45

1

NA

NA

40 × 40

NA

[32]

5.02–10.84

5.07–9.22

1

NA

NA

40 × 40

NA

[33]

2.08–3.75

2.28–3.76

1

NA

NA

46.6 × 70

NA

[34]

1.84–2.43

1.89–2.43

1

NA

NA

50 × 50

NA


$${f_{c1}}=\frac{1}{{2\pi \sqrt {{L_1}{C_1}} }}$$ ,
4


$${f_{c2}}=\frac{1}{{2\pi \sqrt {{L_2}{C_{2eq}}} }}$$ ,
5


$${f_{c3}}=\frac{1}{{2\pi \sqrt {{L_3}{C_{3eq}}} }}$$ , 6

B. Triple band rejected CP UWB Antenna

The final geometry of the CP UWB antenna with EBG unit cell is shown in Fig. 2. The design comprises an improved ring- shaped CP UWB antenna fed by a Microstrip feed line. To achieve the three rejected bands, the TVC-EBG cell is positioned close to the feedline of the UWB antenna. To attain wider ARBW, a slot and stub are inserted in the ground surface. The design parameter details of the CP UWB antenna with triple-band rejection are given in Table I. The step-wise evolution of the resonating antenna is shown in Fig. 3. Firstly, a modified ring monopole with a ground plane (Antenna-A) is simulated, as illustrated in Fig. 3(a). For antenna A, the axial ratio and VSWR range from 3–11 GHz with and ARBW between 4–5 GHz. In Fig. 3(b) the TVC-EBG structure is positioned near the feed line (Antenna-B) to achieve three notch bands. Figure 3(c) shows that by inserting stub and slots in the ground surface (Antenna-C) we can widen the ARBW.

C. Triple band rejected Circularly polarized UWB MIMO Antenna

Because of space restrictions in wireless communication devices and terminals, the antenna size must be as compact as possible [21]. A compact two port MIMO antenna is tough to design because of the port and field isolation issues among its radiating elements. The symmetrical arrangement of the suggested structure is introduced and Table 1 presents the specification of the structure. The geometry of the MIMO antenna is presented in Fig. 6. The complete size of the presented antenna is 42.7×55×1.6 mm3.

D. Generation of circular polarization

The suggested structure is fabricated with polarization diversity rather than working with the same CP mode. This type of structure results in both the RHCP and LHCP modes. To understand the CP of the presented antenna surface currents are used. Figure 7 shows the surface currents distribution at different stages 0˚, 90˚, 180˚, and 270˚. For 0°, the predominant vector is in -Y direction, though for 90° the vector direction is - X direction. The vector direction is inverse to 0° and 90° for 180° and 270°. In the Fig. 7 when observed from + Z direction it is seen that the surface current moves in the left to right direction. The suggested antenna with Port 1 is operating with LHCP. Correspondingly, when Port 2 is working, the current distribution moves in opposite way as the phase moves from 0˚ to 270˚, as specified in Figure. 8. This produces the RHCP in the CP UWB MIMO antenna. Thus, polarization diversity is achieved which will reduce the ECC values automatically

3. Results And Discussion

A. Simulated results of the proposed unit cell.

To verify the simulated curves, a prototype of the presented structure has been fabricated. Curves like VSWR, mutual coupling are achieved from the Vector Network Analyzer (Agilent N5230C) and radiation pattern are tested in anechoic chamber. The fabricated prototype of the CP UWB-MIMO antenna is displayed in Fig. 9. Measured VSWR ranges from 3.1 to 11 GHz excluding three interfering bands as presented in Fig. 10. The ARBW covers 3 to 10.4 GHz eliminating narrow frequencies shown as shown in Fig. 11. Figure 12 shows the variation of mutual coupling with Wg = 1mm, 1.5mm, 2mm, 2.5mm and 3mm. It can be observed from the Fig. 12 that as the value of Wg increase the axial ratio beam width decreases. Likewise in Fig. 13 as the Wg increases the isolation between the antenna elements also increases. Figure 14 shows the Radiation pattern measurement in anechoic chamber. Tested and simulated radiation plots at 5 and 9 GHz are specified in Fig. 15.

Figure 16 shows the isolation amid the two elements that is less than -15dB for entire frequencies. MIMO performance has been calculated by determining the ECC, DG, and TARC as highlighted in [7], [19-20]. ECC in an isotropic environment can be found using [19].

Where,  and are the complex 3D radiated fields from ports 1 and 2, respectively, and * signifies the Hermitian product operator. DG can be evaluated using [23]

$$DG=10\sqrt {1 - EC{C^2}}$$
8

As reflection coefficient is considered for only single element antennas, TARC can needs to be evaluated for multi-port antenna systems as specified in [22],[24–26]

$$TARC=\sqrt {\frac{{{{({S_{11}}+{S_{12}})}^2}+{{({S_{21}}+{S_{22}})}^2}}}{2}}$$
9

Where, S11, S22 S12 and S21 are S parameters of the Scattering matrix. Figure 17 shows acceptable TARC values in the band of operation while Fig. 18 shows that the DG is around 9.98 dB for the whole resonating band with very low ECC values due to the polarization diversity of the proposed antenna system.

Comparative study of different CP UWB MIMO antenna

The relative study of numerous CP UWB MIMO antenna with the suggested one is illustrated in Table II. The dimension, the number of notched bands, ARBW, and − 10dB bandwidth are significant factors deliberated for the comparison. It is seen from the table, that reference [9] attains two bands only, while the suggested antenna provides three-band notches with a simple geometry. The suggested structure has a small size, as imparted to the ref [9], [27–34] with broader ARBW of 3-10.4 GHz.

4. Conclusion

In this work, a CP UWB MIMO system is designed and fabricated. The MIMO antenna comprises two CP antennas opposite to one another to reduce size while having good port isolation and polarization diversity performances. Furthermore, the TVC-EBG structure achieves triple rejected narrow band frequencies. A broad − 10dB BW as well as ARBW from 3 to 11 GHz with three notched bands was achieved. Moreover, the presented structure achieves good MIMO performances with isolation more than 15 dB,

ECC less than 0.03, and TARC is below − 4 dB. Hence, the presented structure can be a possible solution for future UWB-CP MIMO front-ends in wireless terminals and access points.

Declarations

Acknowledgments

Sincere, thanks to Prof. Binod K. Kanaujia for permitting an easy approach to the laboratory facilities.

Funding: This research received no specific grant from any funding agency in the public, commercial, or not-for-profit sectors. 

Conflicts of Interest: All co-authors have seen and agree with the contents of the manuscript and there is no conflict of interest to report. We certify that the submission is original work and is not under review at any other publication. 

Availability of Data and Material: Data sharing is not applicable to this article as no new data were created or analyzed in this study.

Code Availability: Code Availability is not applicable to this article as no new code were created or analyzed in this study.

Author’s Contribution: Dr. Naveen Jaglan finalized the idea of the paper and Dr. Ekta Thakur contributed in paper writing.

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