A Compact Highly Isolated Four-Element Antenna System for Ultra-Wideband Applications

Abstract

A small, orthogonally polarized, ultra-wideband (UWB), four-port multiple-input multiple-output (MIMO) printed antenna is presented in this study. The envisioned antenna is built up of four microstrip fractal-based circular patch elements, each of which is fed by a microstrip line with a 50-ohm impedance. The use of a defective ground plane allows for the ultra-wideband frequency response to be obtained. In order to achieve maximal isolation, the amount of surface current that flow between the antenna’s four components is limited by arranging radiating elements orthogonally. The four-port MIMO system is printed on a FR4 substrate with a loss tangent of 0.02 and an overall dimension of 20 × 30 × 1.6 mm³. A port-to-port isolation of less than 25 dB was achieved as a consequence of this orthogonal orientation of antenna elements, and the impedance bandwidth is achieved up to 158% (3.1–12 GHz). The suggested ultra-wideband multiple-input multiple-output (UWB-MIMO) antenna achieved a maximum gain of 8 dBi over the operational frequency range (3.1–12 GHz); the findings that were measured and those that were simulated accord with one another rather well. The findings also give an overall strong diversity performance, with the ECC < 0.25, DG > 9.9, and CCL < 0.2 values all being within acceptable ranges.

This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

1. Introduction

These days, there is a great deal of demand for needs and requirements that include flow of data without interference. A radio link’s capacity may be increased using the multiple-input multiple-output (MIMO) technology by utilising several transmit and receive antennas to accomplish multipath propagation [1]. The deployment of multiple-input multiple-output (MIMO) systems has revolutionized wireless communication networks by significantly enhancing data throughput and spectral efficiency [2]. By exploiting spatial diversity and multiplexing gains, MIMO technology enables simultaneous transmission of multiple data streams over the same frequency band, thereby increasing the overall system capacity. In practical implementations, the effectiveness of MIMO systems heavily relies on the spatial separation of antennas and the mitigation of interference effects. To achieve optimal performance, antennas need to be strategically positioned to maximize diversity and minimize correlation between antenna elements [3, 4]. This spatial arrangement is crucial for combating multipath fading and improving signal quality, especially in environments with severe channel conditions. Ultra-wideband (UWB) technology complements the capabilities of MIMO systems by offering enhanced data rates, low power consumption, and robust communication links. Operating across a wide range of frequencies, UWB enables high-speed data transmission while adhering to stringent regulatory constraints set by organizations like the Federal Communication Commission (FCC) [5]. The adoption of UWB further extends the applicability of MIMO systems in diverse wireless environments, ranging from personal area networks to industrial IoT deployments. However, the integration of MIMO and UWB technologies introduces challenges associated with antenna design and mutual coupling effects. In compact MIMO antenna configurations, the proximity of antenna elements can lead to mutual coupling, where electromagnetic fields from one antenna influence the performance of neighbouring antennas. Mutual coupling not only degrades the isolation between antenna elements but also affects the radiation pattern and impedance matching of the overall antenna array [6].

Addressing mutual coupling issues is essential for ensuring the robustness and reliability of MIMO-UWB systems, especially in scenarios where signal integrity is paramount. Techniques such as antenna decoupling, isolation enhancement, and adaptive array processing play a pivotal role in mitigating the adverse effects of mutual coupling and optimizing system performance. As we transition into the era of 5G connectivity, the demand for high-speed data transmission, ultra-low latency, and seamless connectivity continues to escalate. MIMO-UWB systems offer a compelling solution to meet these evolving demands by leveraging advanced signal processing algorithms, beamforming techniques, and interference mitigation strategies [7]. This is because radiations from free space, surface currents, and surface waves can all contribute to the formation of mutual coupling. It is possible for the total active reflection coefficient of a MIMO antenna to be altered by a variety of signal processing methods. These techniques include those that reduce carrier frequency offset (CFO), channel estimate, and angle of arrival estimation. When mutual coupling is present, it is possible for large out-of-band (OOB) emissions from power amplifiers (PAs) to interfere with communication on neighbouring channels. In the realm of digital communications, the mutual coupling has been reduced due to improvements made to MIMO precoding and decoding techniques [8]. Before adaptive algorithms can calculate the weight vector, the received voltages must first be adjusted to eliminate any mutual coupling that may have occurred. During postprocessing, one may improve the signal-to-interference noise ratio (SINR) by reducing the amount of relative interference or noise. In order to maintain the SINR, it is not essential to adjust the electromagnetic interactions. Using the approach described above, it is possible to achieve some degree of success in enhancing the electromagnetic suppression in the digital realm. When viewed from the point of view of the antenna, a technology that is efficient in decoupling is more successful at mitigating the consequences of mutual coupling. As a direct consequence of this, approaches for decoupling based on antennas need to be developed [9].

There are a variety of approaches that may be used to achieve decoupling. Examples of such decoupling methods include things like deflected ground structures [10], decoupling networks [11], electromagnetic band gap structures (EBG) [12], neutralising lines [13], parasitic or slot components [14], and complementary split-ring resonator (CSRR) [15]. There is a description of a UWB-MIMO antenna with twin ports in reference [16]. This antenna has wideband features, including a frequency range of 3.0–11.0 GHz as well as four rejection bands. According to the findings presented in reference [17], slots with inverted L and U forms are able to give quadruple notch characteristics at frequencies of 3.3 GHz, 4.03 GHz, and 5.4 GHz. In addition to this, the antenna has an isolation of more than 25 decibels. In [18], a flexible CPW-fed MIMO antenna is shown; in order to achieve strong polarization diversity, a large impedance width of 125.83%, a maximum gain of 6.8 dBi, and an efficiency of 89%, four identical elements are stacked symmetrically in a sequential rotating fashion. This allows the antenna to fulfil its goals. The authors of [19] were able to reduce the size of a UWB-MIMO antenna by combining the radiation from a rhombic slot with that of four microstrip feeders. In order to achieve the notched properties, C-slots, L-slots, and an H-shaped EBG structure must first be opened. The antenna has a strong performance in terms of variety, with an isolation level of 40 dB, a peak gain of 5.5 dBi, and a radiation efficiency range of 75%–90%. In a letter that was cited in [20], it was proven that a wideband impedance match and dual-band notch capability may be achieved by using a quasi-U-shaped patch on the upper plane and a stepped slot in the ground plane. The use of defective ground structure for an elliptical planar antenna has been explored in [21], where the DGS is used to realise a UWB frequency response. The purpose of this study was to investigate the use of DGS. In the research described in [22], two symmetrical half-slot components with CPW-fed structures were used, together with a Y-shaped slot that was split at the bottom centre of the common ground. Both the current flow at low UWB frequency and the mutual coupling between the MIMO components are significantly reduced due to the slots’ efficient blocking of both. There is a paper in reference [23] that describes a unique four-element MIMO antenna that may be used for UWB applications; the antenna is CPW-fed and is made up of four orthogonal sections. An impedance width ranging from 2.1 to 20 GHz may be accomplished by using notch frequencies in the range of 3.3 GHz–4.1 GHz and 8.2 GHz–8.6 GHz with the antenna that has been presented. When notches are reached, it is possible to filter out interference from WiMAX and radar applications. In [24], the neutralisation line approach is discussed in relation to its use in the construction of MIMO antennas for UWB communications. The bandwidths that were measured were 95.22% (3.51–9.51 GHz) and 96.47% (3.52–10.0 GHz), and the isolation was improved to 23 dB. The MIMO antenna offers a peak gain of 2.91 dBi and has superior diversity metrics than the other antenna types. In addition, the design of the antenna that is discussed in [25] is for an extremely compact 2-element planar antenna. In order to get the UWB response, a rectangular stepped slot and a z-shaped slot are etched into the ground plane. The described construction has a geometrical dimension of 23 mm by 26 mm by 0.8 mm³, and the antenna has a peak gain of 5 dBi with an efficiency of 80%.

In this work, we have proposed a fractal-based compact four-element antenna for UWB applications operating from 3 to 12 GHz with high isolation and gain. By placing antenna elements orthogonally, isolation up to 30 dB is achieved which results in lower values of ECC below 0.25. The antenna also achieves maximum gain up to 8 dB which is highly significant for UWB applications. Moreover, MIMO parameters diversity gain (DG) and channel capacity loss (CCL) have also been computed to verify presented MIMO antenna’s diverse performance over the operating spectrum. The results show that the antenna proposed in this research could be a viable candidate for UWB applications.

The significance of the proposed work could be stated as follows:

(1) The presented antenna occupies only a minimum area of around 20 × 30 mm² which is relatively smaller in size compared to the other works in the literature.

(2) With such low-profile configuration, the antenna achieves ultra-wideband operation spanning between 3 and 12 GHz with 10 dB impedance bandwidth.

(3) The designed antenna is implemented as a four-element MIMO antenna system which provides high isolation better than 25 dB without seeking additional decoupling mechanism.

(4) Most importantly, the antenna attains maximum gain up to 8 dB which is highly sufficient for UWB applications.

(5) Furthermore, the antenna exhibits considerable MIMO features which include ECC, DG, and CCL over the operating spectrum.

1.1. Single Antenna Design and Analysis

The proposed UWB antenna is illustrated along with its schematic representation as shown in Figures 1(a) and 1(b), respectively. The radiating element of size L × W is printed on front side of 1.6 mm thick (H) FR4 substrate. The radiating element consists of two main components, namely, modified circular patch and feeding strip, as shown in Figure 1(a). To obtain the wide bandwidth, 15 equal steps of size L4 have been etched from the corners of the circular radiating element and connected with a 50-Ω feeding strip of size L1 × W1. On the one hand, defected ground has been printed on the backside of the substrate with the dimension of L1 × W. In order to achieve better impedance matching as well as wider bandwidth, portions of width W3 have been etched away from top of the ground plane. Furthermore, to improve the resonance over the desired band of spectrum, a slot of L3 × W2 has been removed from the centre of the ground plane as shown in Figure 1(b). The detailed dimension of the single antenna element is listed in Table 1.

[figure(s) omitted; refer to PDF]

Table 1

Various dimensions of the proposed antenna.

Parameters	Values (mm)
L	20
W	30
L1	11
L2	8.0275
L3	2.475
W1	2.5
W2	3
W3	8.4
H	1.6

The reflection coefficient of the proposed UWB antenna is illustrated in Figure 2. It is clearly shown that the proposed antenna is capable of covering UWB spectrum ranging from 3 to 10 GHz over the 10 dB impedance scale.

[figure(s) omitted; refer to PDF]

1.2. Design Process

To provide a better understanding of the proposed design, evolution stages have been studied in this section. The design stages have been categorized as Antenna 1, Antenna 2, Antenna 3, and Proposed Antenna as shown in Figure 3. Antenna 1 consists of full ground plane with a full circle along with feeding strip. Then, Antenna 2 is obtained after reducing the size of ground plane. Ground plane of the Antenna 2 is etched at top left and right edges and named as Antenna 3. The proposed antenna is derived from Antenna 3 by incorporating 15 equal steps over the edges of the circular radiator. The corresponding reflection coefficients are illustrated in Figure 4. Antenna 1 is only able to provide dual and narrow band resonance at 8.5 and 10.25 GHz, respectively. The reduction of ground plane size influences resonance to be created at three spectrums that include 4 GHz, 7.8 GHz, and 10.5 GHz. It proves that defected ground could cause spectrum of operation with reasonable impedance matching. Even better impedance matching with wider spectrum of operation is obtained after making changes along top edges of the ground plane (Antenna 3). The UWB performance is achieved by incorporating fractal-based structure (equal steps) over the circular radiating element as shown in Figure 4.

[figure(s) omitted; refer to PDF]

1.3. Parametric Study

The parametric analysis has been provided in this section to understand the design concepts of the proposed UWB antenna. However, the proposed antenna does have many parameters; only three have been taken for analysis including length of the feed (L1), length of the ground plane (L2), and length of square slot in the ground plane (L3). As shown in Figure 5(a), the parameter L1 has been altered from 10 mm to 13 mm with the equal step size of 1 mm. It is noticed that L1 has influenced wideband resonance while it is progressively increased. The proposed antenna is capable of exhibiting UWB resonance only at 11 mm with significant reflection coefficient below 10 dB. Altering the stub length (L1) results in variations in the frequency of operation. Similarly, the effect of changing ground plane length (L2) is also studied and plotted in Figure 5(b). Increasing length (L2) affects impedance matching of the antenna throughout the bandwidth. On the other hand, variation in slot length (L3) in the ground plane is studied as shown in Figure 5(c). Ground slot length (L3) does not affect the wideband resonance but influences the frequency shift as well as impedance matching of the designed antenna.

[figure(s) omitted; refer to PDF]

1.4. Surface Currents

The antenna surface current distributions at various frequencies are illustrated in Figures 6(a)–6(d). At 4 GHz, current concentration maximizes along the feeding strip as well as in the corners of circular radiating element as shown in Figure 6. It proves that length of the feeding strip affects performance of the antenna at lower frequencies. Figure 5 also shows current distribution at 6 GHz. It is noticed that peak current concentration exists at bottom of the radiating element and sliding edges of the ground plane which influences impedance matching of the antenna at middle range of the spectrum. At 8 GHz, the current concentrated at the edges of the ground slot, whereas at 10 GHz, peak current is observed along the feeding line along with sliding edges on the ground plane as shown in Figure 6.

[figure(s) omitted; refer to PDF]

1.5. MIMO Antenna Structure

The proposed single antenna element is converted into four-port MIMO antenna system which is printed on 65 mm × 65 mm FR4 substrate in orthogonal fashion. All the antenna elements in the MIMO system have covered UWB spectrum ranging from 3 to 12 GHz over the 10 dB impedance scale. Since all the antenna elements are placed orthogonally, significant isolation up to 20 dB is achieved between any adjacent pair. It results in considerable improvement radiation efficiency and envelope correlation coefficient (ECC). The structure of the four-port MIMO antenna is illustrated in Figure 7(a). The fabricated prototype of the MIMO antenna is depicted in Figure 7(b). The simulated and measured reflection and transmission coefficients of the proposed MIMO antenna system are illustrated in Figures 8(a)–8(d). It is observed that both simulated and measured S-parameters of the antenna synchronized well with each other.

[figure(s) omitted; refer to PDF]

It is mentioned earlier that isolation among antenna elements is better than 25 dB as all elements are placed orthogonally. It is verified with surface current distribution of the proposed antenna excited at various frequency of operation as shown in Figures 9(a)–9(c). It is observed that when Antenna 1 is excited, other elements have not been affected by the current which flows in Antenna 1, thereby improving isolation significantly.

[figure(s) omitted; refer to PDF]

Similarly, measured two-dimensional radiation patterns at $\phi =0^{°}$ and 90° at 4 GHz, 6 GHz, and 8 GHz are illustrated in Figures 10(a)–10(c). At all the frequency ranges, the proposed antenna exhibits unique pattern over $\phi =0^{°}$ and 90° directions with cross-polarization better than 20 dB. The simulated and measured gain of the proposed antenna is illustrated in Figure 11. The proposed MIMO antenna achieves maximum gain up to 8 dBi over the desired operation spectrum which is highly significant for UWB applications. It is noteworthy that any dedicated structure or mechanism has not been included in this work to improve the gain.

[figure(s) omitted; refer to PDF]

1.6. MIMO Performance

In this section, various MIMO features have been discussed to validate the MIMO performance of the proposed antenna. Envelope correlation coefficient is an important parameter which reveals how far antenna elements are displaced, thereby achieving superior performance. Figure 12(a) illustrates the ECC of the proposed antenna which lies in the range between 0.1 and 0.2. The obtained ECCs are lower than the acceptable threshold of 0.5. ECC values of the proposed antenna are calculated using the following equation:$$\begin{array}{}\text{(1)}& \text{ECC}{\rho }_e=\frac{{\iint \stackrel{⟶}{A_1}Ɵ,\phi {\rm X}\stackrel{⟶}{A_2}Ɵ,\phi d\Omega }^2}{{\iint \stackrel{⟶}{A_1}Ɵ,\phi }^{2}d\Omega {\iint \stackrel{⟶}{A_2}Ɵ,\phi }^{2}d\Omega }.\end{array}$$

[figure(s) omitted; refer to PDF]

Also, diversity gain of the proposed MIMO antenna system has been computed using (2) and plotted in Figure 12(b). Generally, both DG and ECCs of the antenna are dependent parameters which represent gain of the antenna when radiated wave propagates in diverse directions. Generally, diversity gain of the ideal MIMO antenna system should be 10 dB over the operating spectrum. For the proposed antenna, it is as closer as 9.8 dB between 3 and 10 GHz.$$\begin{array}{}\text{(2)}& \text{Diversity\hspace{0.17em}Gain}DG=\sqrt{1-{{\rho }_e}^2}.\end{array}$$

Channel capacity loss (CCL) is an important metric being measured in terms of bits per seconds per Hz using (3) and (4). It measures loss of data during transmission of electromagnetic waves. For ideal antenna system, CCL should be zero but practically it could be less than 0.4 bits/Sec/Hz. The proposed antenna system exhibits CCL around 0.25 bits/Sec/Hz over the operating spectrum as shown in Figure 12(c) that ensures designed antenna has minimum transmission loss during radiation.$$\begin{array}{}\text{(3)}& \text{Channel\hspace{0.17em}Loss\hspace{0.17em}}\text{CL}={-\log }_2\det {\phi }^{\mathrm{R}},\text{(4)}& {\phi }^R=\begin{array}{ccc}{p}_{\text{ii}}& \cdots & p_{\text{ij}}\\ ⋮& \ddots & ⋮\\ p_{\text{ji}}& \cdots & p_{\text{jj}}\end{array},\end{array}$$where$$\begin{array}{c}\text{(5)}& p_{ii}=1-{S_{ii}}^2+{S_{ij}}^2,p_{ij}=-{S_{ii}}^{\ast }{S}_{ij}+{S_{ji}}^{\ast }{S}_{jj},\hspace{1em}\text{\hspace{0.17em}for\hspace{0.17em}}\mathrm{i},\mathrm{j}=1,2\dots \dots 4.\end{array}$$

Figure 13(a) represents total active reflective coefficient of the proposed antenna. It is obvious that TARC will be measured when phase of all ports will be changed when phase of the port 1 is kept constant. It is observed that frequency response of port 1 maintains same bandwidth when altering phase of all other ports. Similarly, multiplexing efficiency has also been calculated and plotted in Figure 13(b). The proposed antenna exhibits multiplexing efficiency lower than 2.8 dB over the operating bandwidth which is within the acceptable limit of the MIMO antenna system. The overall performance of the designed antenna reflects that it could be a suitable candidate for UWB applications.

[figure(s) omitted; refer to PDF]

State-of-the-art comparison is presented in Table 2.​ It is observed that size of an individual antenna has been comparatively minimized about 20 × 30 mm² with significant gain up to 8 dB. By placing antenna elements orthogonally, the proposed antenna achieves isolation up to 25 dB with significant MIMO parameters including ECC, DG, and CCL.

Table 2

State-of-the-art comparison of the proposed antenna with the literature.

Ref. no	Antenna size (mm2)	Frequency of operation (GHz)	Electrical dimension of the antenna (λ2)	Isolation (dB)	Max. gain (dB)	ECC	DG	CCL
[9]	56 × 68	3.89–17.09	0.726λ × 0.88λ	15	5.87	<0.02	NA	<0.5
[10]	34 × 34	3.3–3.9, 5-6, 7.4–8.5	0.374λ × 0.374λ	15	5.5	<0.05	NA	NA
[14]	80 × 80	3 to 4	0.8λ × 0.8λ	25	5.8	<0.02	9.9	<0.4
[15]	48 × 34	2.1–20	0.336λ × 0.238λ	23	2.91	NA	9.81	<0.29
[17]	74 × 74	3.37–27.71	0.831λ × 0.831λ	34	7.68	<0.039	NA	NA
[21]	36 × 35	2.6–12.2	0.312λ × 0.303λ	17	NA	NA	9.99	NA
[22]	40 × 40	2.6–12.2	0.346λ × 0.346λ	15	3.5	<0.012	9.95	0.22
[23]	40 × 40	1.71–3.94	0.228λ × 0.228λ	25	2.36	<0.4	9.71	0.13
[26]	30 × 30	3.1–12	0.31λ × 0.31λ	17	6.2	<0.001	9.9	0.1
Proposed work	20 × 30	3.1–12	0.206λ × 0.31λ	25	8	<0.2	9.9	0.25

2. Conclusion

This study presents a quad-port antenna system for ultra-wideband applications. Four modified circular radiating components positioned orthogonally across the substrate and each supplied by a 50-microstrip line make up the planned antenna. By including a defective ground structure with two triangular slots at the top side borders of the ground plane and two square slots in the ground plane’s centre, ultra-wideband response is made possible. By restricting the current flow between the antenna components through orthogonal designs, interport isolation up to 25 dB is accomplished. Each antenna element is printed on a conventional FR4 substrate with dimensions of 20 × 30 × 1.6 mm³ and a dielectric constant and tangential loss of 4.6 and 0.02, respectively. The suggested antenna has an impedance bandwidth of 10 dB and covers the frequency range of 3.1 to 12 GHz. The suggested antenna has considerable radiation characteristics and a peak gain of up to 8 dBi for the operating frequency. Both the measured and the simulated outcomes agreed with one another extremely well. The ECC < 0.25, DG > 9.9, and 2 3 CCL < 0.2 are within the allowed bounds of variation, which further suggests a strong overall performance in terms of variety.

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