Isolation Enhancement in Quad Port MIMO Antenna Using PBG Substrate in THz Region

Abstract

In this paper, the design of a 4 element MIMO antenna system for Terahertz (THz) communication applications is reported. The proposed antenna system is engineered with a focus on enhancing isolation between closely spaced antenna elements. Our approach leverages a photonic bandgap (PBG) substrate to suppress mutual coupling and improve isolation performance. 2x2 MIMO with patch dimension of 510×570×40 µm³, is designed to operate in the frequency of 2.4 THz. The return loss, isolation, radiation pattern, Envelope Correlation Coefficient (ECC) and Total Active Reflection Coefficient (TARC) are further analyzed by incorporating PBG. The proposed antenna produces the return loss as -32.97 dB and a gain of 3.69 dB. Moreover, the mutual coupling is less than -32.54 dB. The proposed antenna system offers promising prospects for advancing THz communication networks, enabling high-speed data transmission and reliable connectivity in future wireless communication systems.

Index Terms
MIMO antenna; THz communication; Photonic Bandgap Structures; Isolation.

I. INTRODUCTION

The Terahertz (THz) band, positioned between the microwave and infrared regions of the electromagnetic (EM) spectrum, spans frequencies from 0.1 to 10 THz [¹, ²]. This frequency range is utilized in imaging, networking, the medical field, short-range wireless systems, space science, and remote sensing [³]. A key advantage of THz waves is their non-ionizing and biologically safe nature, making them safer than X-rays [⁴]. THz communication systems promise to revolutionize wireless technologies by supporting ultra-high data rates and wide bandwidth capabilities. Microstrip patch antennas are commonly preferred for THz applications due to their compact and low-profile structure. However, at THz frequencies, their performance is limited by surface wave propagation, which leads to reduced gain and narrow bandwidth [⁵-⁷]. THz antennas are central to future technologies that demand high data throughput, such as artificial intelligence (AI), machine learning (ML), virtual and augmented reality (VR/AR), the Internet of Things (IoT), and applications in satellite, medical, and defense domains [⁸-¹⁰]. Despite their promise, THz systems face challenges such as high atmospheric path loss. While thicker dielectric substrates and larger ground planes can improve performance, they also exacerbate surface wave issues [¹¹]. To overcome these limitations, techniques such as increasing substrate thickness [¹²], employing multilayer substrates [¹³], and optimizing dielectric materials [¹⁴] have been explored to improve bandwidth and radiation performance.

As the demand for high-speed data transmission increases, there is a pressing need for compact, high-performance Multiple-Input Multiple-Output (MIMO) antennas operating in the THz band. MIMO technology enables multiple data streams to be transmitted and received over the same channel, enhancing spectral efficiency and communication reliability. A significant challenge in THz MIMO design is maintaining high isolation between closely spaced elements. Mutual coupling can degrade system performance by introducing interference and reducing channel capacity. Traditional isolation methods-such as increasing inter-element spacing or using decoupling structures-are often impractical in the THz domain due to size constraints and fabrication complexity.

Various techniques have been proposed to mitigate mutual coupling in MIMO antennas, including defected ground structures (DGS), open-loop resonators such as split rings (SR) and complementary split rings (CSR), and metamaterial absorbers [¹⁵], as well as the incorporation of parasitic and decoupling elements [¹⁶-¹⁸]. However, these methods are primarily suited for MIMO antennas with inter-element spacing of approximately λ/2. An emerging approach is the use of Photonic Bandgap (PBG) structures, which have shown promise in enhancing antenna performance [¹]. PBG substrates are particularly effective at suppressing surface waves [¹⁹, ²⁰], guiding EM energy along the substrate [²¹, ²²], and redirecting surface currents to substrate edges [²³]. This suppression of substrate modes helps improve isolation between antenna elements. Moreover, PBG structures offer control over radiation patterns and EM field distribution, enabling precise antenna optimization. At microwave frequencies, they are used to reduce side lobes [²⁴] and to achieve band switching capabilities [²⁵].

PBG structures-also known as photonic crystals-exhibit a forbidden band in which electromagnetic waves cannot propagate. They can be realized in one-dimensional (1D), two-dimensional (2D), or three-dimensional (3D) periodic forms [²⁶]. Conventional metallic patches suffer from conductor losses, prompting the use of CMOS-compatible antennas with low-loss substrates like SiO₂ [¹, ²⁷], which offer high-speed and low-noise communication performance [²⁸, ²⁹]. Graphene-based antennas are also increasingly preferred at THz frequencies for their superior EM properties compared to traditional metallic counterparts [³⁰, ³¹]. Several studies have demonstrated the benefits of integrating PBG structures into THz antenna designs. For instance, a square ring patch antenna loaded with PBG structures was shown to achieve improved directivity [³²]. A hexagonal MIMO antenna using star-shaped fractal slots on a SiO₂ substrate was presented in [¹]. In [³³], a patch antenna was designed on a polyimide substrate, and the authors analyzed performance variations using square, circular, and triangular PBG lattices. They found that circular PBG structures yielded the best results in terms of gain (>9.4 dB), directivity (>10 dB), and impedance bandwidth (>29 GHz). Other studies have explored trapezoidal microstrip patch antennas with PBG substrates for THz applications, demonstrating improvements in return loss, gain, and bandwidth [³⁴]. A polyimide-based MIMO antenna using PBG for the 0.640-0.664 THz range was presented in [³⁵]. PBG structures were also used in curvature patch antennas to enhance gain and efficiency up to 86.96% [³⁶]. Compact antennas for 5G bands (3.1-3.5 GHz and 24-27 GHz) with PBG and PIN diode-based switching were proposed in [²⁵]. A graphene-based PBG patch antenna achieved 7.28 dB directivity at 1.1 THz using a high-permittivity silicon substrate in [³⁷]. In [³⁸], a THz antenna demonstrated high impedance bandwidth (797 GHz), return loss (-50.98 dB), VSWR (1.022), and gain (5.40 dB) at 2.96 THz. An elliptical microstrip patch antenna for THz applications using a polyimide substrate was reported in [³⁹].

The literature review shows that while PBG substrates have been widely used to improve bandwidth, gain, efficiency, and surface wave suppression, there has been limited work focused specifically on isolation enhancement in THz MIMO antennas. This paper proposes a novel design of a 4-element MIMO antenna for THz applications that incorporates a PBG substrate to significantly improve isolation, particularly for compact arrays with inter-element spacing less than λ/4. Through detailed simulations and analysis, we demonstrate the effectiveness of PBG structures in minimizing mutual coupling and enhancing overall antenna performance. Additionally, we investigate the influence of PBG geometry and slot configurations on return loss, bandwidth, and radiation characteristics. Our proposed solution is intended to support the development of compact, high-performance MIMO systems for next-generation THz wireless networks.

The paper is organized into five sections, with the current section discussing the requirements of THz antenna and various antennas reported in the literature. Section II provides the method of designing conventional patch, and Section III covers design of 2x2 MIMO antennas with PBG. Parameters of the proposed antenna with and without PBG are discussed in Section IV, and conclusions are drawn in Section V.

II. DESIGN OF THZ ANTENNA

The rectangle and circle are commonly used shapes because of less complicated analysis and ease in fabrication. For the resonance frequency 2.4 THz, the width (W), length (L) and other dimensions of the rectangular patch antenna are calculated using the following equations (1-6) as defined in [⁴⁰].

The wavelength of the antenna can be calculated as,

(1) $$\lambda =\frac{c}{f_r}$$

where, c is the speed of the light, f_r is the resonant frequency. Using this value the width of the patch (W) antenna can be calculated as,

(2) $$W=\frac{c}{2f_r}\sqrt{\frac{2}{1+{\epsilon }_r}}$$

where, ε_r is relative permittivity (dielectric constant). From this the effective permittivity ɛ_eff can be calculated as

(3) $${\epsilon }_{eff}=\frac{{\epsilon }_r+1}{2}+\frac{{\epsilon }_r-1}{2}{\left(1+12\frac{h}{W}\right)}^{-\frac{1}{2}}$$

where, h is the thickness of the substrate. Further the length of the patch (L) can be found as,

(4) $$L=\frac{C}{2f_r\sqrt{{\epsilon }_{eff}}}-0.824\frac{\left(\frac{W}{h}+0.264\right)\left({\epsilon }_{eff}+0.3\right)}{\left(\frac{W}{h}+0.8\right)\left({\epsilon }_{eff}-0.258\right)}h$$

Finally, the width (Ws) and length (Ls) of the substrate is defined as

(5) $$W_s=W+\left(6*h\right)$$

(6) $$L_s=L+\left(6*h\right)$$

A simple patch antenna as shown in Fig. 1, is designed on the dielectric substrate which has dielectric constant 3.5, thickness 40 μm, loss tangent = 0.0027 [⁴⁰-⁴²]. The size of the substrate is 252 μm × 283 μm. Type 1 patch has length and width of 12 μm and 43 μm, respectively as shown in Fig. 1(a). Inset feed with 50 ohm matching impedance. Length and width of the feed line is chosen as quarter guided wavelength and half of the feed length, respectively. Dimension of the proposed antenna is listed in Table I. As shown in Fig. 1(b), in Type 2 design two slots are inserted with the radius of 1.5 μm. Each slot is separated by 16 μm distance. As shown in Fig. 1(c), in Type 3, another one slot with the thickness of 0.4 μm is inserted at the center of the patch. Functional parameters of the each type of proposed antenna are listed in Table II.

TABLE I
DIMENSION OF PROPOSED ANTENNA (ALL DIMENSIONS IN µM)

Parameters	Dimensions (µm)
Size of substrate (Ls×Ws×hs)	252 ×283×40
Size of patch (Lp×Wp)	12×43
Length of the feed (Lf)	12
Width of the feed (Wf)	6
Length of the inset feed (Lf1)	9
Width of the inset feed (Wf1)	0.5
Radius of slot 1 (R1)	1.5
Slot gap 1 (G1)	16
Slot gap 2 (G2)	9
Radius of slot 2 (R2)	0.4

TABLE II
FUNCTIONAL PARAMETERS COMPARISON OF PROPOSED TYPE 1, TYPE 2 AND TYPE 3 ANTENNA

Parameter	Type 1	Type 2	Type 3
Resonant Frequency (THz)	2.4	2.36	2.4
Return Loss (dB)	- 23.54	-18.87	-28.35
VSWR	1.15	1.23	1.19
Gain (dB)	2.31	2.61	2.84
Bandwidth (GHz)	55	55.7	60.1
Bandwidth %	2.29	2.50	2.31

Fig. 1
Front view of (a) Type 1 and (b) Type 2 and (c) Type 3 antenna

Variation in return loss of each type of proposed antenna is shown in Fig. 2. Type 3 antenna produces the return loss as -28.35 dB. As the slots included in the patch, the effective electrical length of the antenna increases. This is because the slots cause the current path to take a longer route which effectively increasing the total distance of the current travels, hence resonance frequency is shifted to lower frequency, i.e., Type 2 antenna resonats at 2.36 THz and Type 3 antenna resonates at 2.4 THz. However, in the case of the Type 3 antenna, although more slots were introduced compared to Type 2, the resonant frequency appears to revert to 2.4 THz-similar to that of the un-slotted Type 1 design. Because, generally the number of slots impacts the resonance, design-specific factors such as slot symmetry, placement relative to feed point, and interaction between slots can cause deviations from this trend as seen in the Type 3 antenna. Further, gain of the Type 3 antenna is increased by 9% compared to Type 1 antenna.

Fig. 2
Return loss of comparison of each type of proposed antenna

A. Design of MIMO Antenna

MIMO antenna is designed and evaluated for 2 elements and 4 elements. The front and back view of the proposed 2 element MIMO antenna is shown in Fig. 3(a) and 3(b). The radiating elements are orthogonally arranged in the 510 μm×570 μm sized substrate. For better isolation each radiating element is separated by the distance of 62.5 µm which is half of the operating wavelength [³⁵]. Partial ground below the patch in the dimension of 255 μm×285 μm is used.

Fig. 3
(a) Front and (b) Back view of 2 element MIMO antenna

Fig. 4 shows the front and back view of 2 element and 4 element MIMO antenna. Similar to 2 element antenna, the 4 elements are arranged orthogonally and spaced by 62.5 µm in both horizontal and vertical direction as shown in Fig. 4(a). Two partial grounds in the dimensions of 510 μm×285 μm as shown in Fig. 4(d) is used.

Fig. 4
(a) Front and (b) Back view of 4 element MIMO antenna

B. Equivalent Circuit of Proposed THz MIMO antenna

The equivalent circuit of a simple microstrip patch antenna can be modeled as a parallel RLC circuit representing its fundamental resonant behavior, as shown in Fig. 5(a). In this model, R denotes the radiation resistance accounting for the power radiated into free space, L represents the inductance due to the current path on the patch, and C is the capacitance that arises from the gap between the patch and the ground plane.

Fig. 5
Equivelent circuit of (a) simple patch and (b) 4 element MIMO antenna

When slots are introduced into the patch, they alter the current distribution by lengthening the current path, which effectively increases the inductive and capacitive reactance. This is modeled by adding additional series inductance and capacitance components in the equivalent circuit, modifying the antenna's impedance and bandwidth. For MIMO configurations such as the 2×2 arrangement, mutual coupling occurs between adjacent and diagonal elements due to electromagnetic interaction, which is represented in the equivalent circuit as mutual RLC elements between individual patch elements, as shown in Fig. 5(b).The resistance (R), inductance (L) and capacitance (C) can be calculated using the following formula [³⁹] which incorporates the impedance characteristics of the circuit.

(7) $$S_{11=\frac{1}{1+\left(\frac{2}{Z_0}\right)+\left(R+j\omega L+\frac{1}{j\omega C}\right)}}$$

This expression emphasizes how the combined resistance, inductance, and capacitance contribute to impedance mismatch and, consequently, reflection characteristics. The presence of ω (angular frequency) highlights the resonant nature of the system: as frequency varies, the net impedance changes, influencing S11.

III. DESIGN OF MIMO ANTENNA WITH PBG STRUCTURES

In compact MIMO antenna systems, reducing the spacing between radiating elements to less than λ/2 is often necessary to meet small footprint requirements, especially in THz applications. This has been demonstrated in prior works where closely packed arrays, when properly decoupled, have enabled higher-order MIMO configurations without sacrificing performance [¹⁵-¹⁸]. However, the interference between adjacent antennas increases when the elements are closely placed. To reduce the interference, PBG structures are introduced. This PBG structures that can control the propagation of electromagnetic waves in a similar way to how a semiconductor controls the flow of electrons. It creates band gaps, where certain frequency of signal cannot propagate. Here two different structures namely Uniform as shown in Fig. 6(a) and non-uniform PBG structures as shown in Fig. 6(b) are designed. Uniform PBG structures are periodic structures where the periodicity and the properties of the materials are consistent throughout the structure. Non-uniform PBG structures, also known as graded or aperiodic PBG structures, involve varying the periodicity or the material properties in a controlled manner. Equally spaced cylindrical holes in the height of 40 μm are drilled in square lattice, hence square lattice exhibits significant directional dependence compared to triangular. The distance between the centers of adjacent holes (x), is a fundamental parameter in defining the PBG structure. The radius (r) is typically expressed as a fraction of the lattice constant (r/x). The choice of this ratio significantly affects the band gap properties. The lattice constant is fixed as 7 μm and 10 μm for uniform and non-uniform PBG structures respectively and the band structure for each radius is analyzed to get optimal result.

Fig. 6
Layout of (a) uniform PBG and (b) non-uniform PBG

Unit cell of uniform PBG structure has cylindrical rods with radius of 1.5 μm (r) arranged with a gap of 7 μm (x). In non-uniform PBG structure, the radius of cylindrical rods in each column is varied as 4 μm, 2 μm, 3.5 μm, 2.5 μm and 1 μm with a gap of 10 μm (x). The outer cylindrical rods are wider and inner rods are narrower. Because wider outer rods create stronger reflection and confinement of electromagnetic waves at the edges of the structure. This helps in achieving a more defined photonic band gap by preventing leakage of waves from the edges and narrower rods in the center, can introduce localized defect modes. Further, the closely spaced (λ/4) 2 elements and 4 elements are investigated with uniform PBG structures as shown in Fig. 7(a) and 7(b). Non-uniform PBG structures based 2 elements and 4 elements are designed as shown in Fig. 8(a) and 8(b) respectively.

Fig. 7
Front view of (a) 2 element and (b) 4 element MIMO antenna with uniform PBG

Fig. 8
Front view of (a) 2 element and (b) 4 element MIMO antenna with non-uniform PBG

IV. RESULTS AND DISCUSSION

Radiation parameters of the proposed 2 element and 4 element MIMO antennas are investigated over the frequency of 1.0 to 3.0 THz. Return loss and isolation of 2 MIMO antenna without PBG structures are shown in Fig. 9(a). It seems that 2 element MIMO has reflection coefficient (S11/S22) of -27.72 dB at the resonant frequency 2.36 GHz and the coupling coefficient (S12/S21) of -20.56 dB. Since the radiators are closed placed, it has poor isolation. Fig. 9(b) shows the S parameters of 4 element antenna. The diagonal elements are having better isolation (S13/S31, S24/S42) compared to adjacent elements.

Fig. 9
Return loss and isolation of (a) 2 element MIMO antenna and (b) 4 element MIMO antenna

S parameters of the 2 element and 4 element MIMO antenna integrated with uniform PBG structures are shown in Fig. 10(a) and Fig.10(b) respectively. Since the surface waves are radiated towards the edges of the substrate the isolation is improved as <-40 dB and <-59 dB for adjacent elements and diagonal elements, respectively. Similarly the performance of the proposed MIMO antenna loaded with non-uniform PBG structures is analysed and shown in Fig. 11(a) and Fig. 11(b). Compared to isolation produced by antenna without PBG the isolation produced by MIMO antenna loaded with non-uniform PBG is improved from -20.56 dB to -36 dB as shown in Fig. 11.

Fig. 10
Return loss and isolation of uniform PBG with (a) 2 element MIMO antenna and (b) 4 element MIMO antenna

Fig. 11
Return loss and isolation of (a) 2 element MIMO antenna with non-uniform PBG and (b) 4 element MIMO antenna with non-uniform PBG

Fig. 12
2 element MIMO antenna Radiation pattern in (a) E plane, (b) H plane and 4 element MIMO antenna Radiation pattern in (c) E plane, (d) H plane

Fig. 13
E plane Radiation pattern of MIMO antenna with uniform PBG (a) 2 element, (b) 4 element and H plane Radiation pattern of MIMO antenna with uniform PBG (c) 2 element, (d) 4 element

Fig. 14
E plane Radiation pattern of MIMO antenna with non-uniform PBG (a) 2 element, (b) 4 element and H plane Radiation pattern of MIMO antenna with non-uniform PBG (c) 2 element, (d) 4 element

Comparison among radiation parameters of proposed antenna with and without PBG structures are listed in Table III. Without PBG, the resonant frequency is stable at 2.35 THz for the 2 element array and 2.36 THz for the 4-element array. With uniform PBG, the resonant frequency slightly increases to 2.42 THz for both arrays, whereas non-uniform PBG causes a significant increase, especially for the 4-element array, reaching 2.56 THz. In terms of return loss, the non-PBG arrays show good performance at -27.72 dB (2-element) and -30.32 dB(4 element). Uniform PBG improves the 4 element array’s return loss to -32.97 dB. Non-uniform PBG improves the 2 element array’s return loss to -29.22 dB. Isolation seems moderate values without PBG at -20.56 dB for 2 element and -22.35 dB for 4 element. With uniform PBG, isolation significantly improves to -36.72 dB for 2 element and -32.54 dB for 4 element. Non-uniform PBG provides the highest isolation, with 30.23 dB for the 2 element array and -39.45 dB for the 4-element array. Gain without PBG is baseline at 2.88 dB (2 element) and 2.99 dB (4 element).

TABLE III
FUNCTIONAL PARAMETERS PROPOSED THZ MIMO ANTENNA WITH AND WITHOUT PBG STRUCTURES

Parameter	Without PBG (Spacing λ/2)		Without PBG (Spacing <λ/2)		With uniform PBG		With non-uniform PBG
	2 element	4 element	2 element	4 element	2 element	4 element	2 element	4 element
Resonant Frequency (THz)	2.35	2.36	2.35	2.36	2.42	2.42	2.42	2.56
Return Loss (dB)	-27.72	-30.32	-28.35	-29.85	-21.80	-32.97	-29.22	-24.32
Isolation	-20.56	-22.35	-18.32	-17.42	-36.72	-32.54	-30.23	-39.45
VSWR	1.08	1.22	1.08	1.22	1.23	1.21	1.15	1.2
Gain (dB)	2.88	2.99	2.88	2.99	3.22	3.69	3.03	3.58
Bandwidth (GHz)	61.9	64	61.9	64	55.7	62.4	55.7	70.9

Uniform PBG improves gain to 3.22 dB (2 element) and reaches the highest gain at 3.69 dB (4 element). Non-uniform PBG also shows improved gain at 3.03 dB (2 element) and 3.58 dB (4 element). Bandwidth without PBG is wide at 61.9 GHz (2 element) and 64 GHz (4 element). Uniform PBG slightly decreases bandwidth to 55.7 GHz (2 element) but maintains it at 62.4 GHz (4 element). Non-uniform PBG maintains bandwidth at 55.7 GHz (2 element) and significantly increases it to 70.9 GHz (4 element).

Co-polarisation and cross-polarisation in E plane and H plane of proposed THz antenna at resonant frequency without PBG are given in Fig. 12(a), 12(b), 12(c) and 12(d). with uniform PBG and without uniform PBG are given in Fig. 12, 13 and 14, respectively.

The radiation pattern is consistently omnidirectional at the operating frequency of the proposed MIMO as shown. The consistency of the omnidirectional radiation pattern across the operating frequency range indicates that the MIMO system maintains its performance characteristics regardless of the specific frequency being used. Hence this MIMO system is useful for diverse indoor communication applications. Compared to non-uniform, antenna exhibits better performance due to uniform PBG structures. Since, uniform PBG structures maintain consistent bandgap characteristics across the antenna structure. This consistency ensures better control over the propagation of electromagnetic waves within the antenna's operating frequency range. As a result, unwanted frequencies are efficiently suppressed, leading to improved antenna performance.

The Envelope Correlation Coefficient (ECC) and Total active refection coefficient (TARC) are the important parameters of MIMO antenna. ECC is used to measure the mutual couplinbg of electric field from the simulated port to another port. TARC represents the radiation and frequency response of multiple ports antenna. These values can be calculated from S parameters using the following equations.

(8) $${}_{{\rho }_{ij}=\frac{{\left|S_{ii}X{S}_{ij}+S_{ji}X{S}_{jj}\right|}^2}{\left(1-{\left|S_{ii}\right|}^2-{\left|S_{ji}\right|}^2\right)\left(1-{\left|S_{jj}\right|}^2-{\left|S_{ij}\right|}^2\right)}}$$

(9) $${}_{{\tau }_{ij}=\sqrt{\frac{{\left(S_{ii}+S_{ij}\right)}^2+{\left(S_{ji}+S_{jj}\right)}^2}{2}}}$$

The acceptable value of ECC is <0.5 and TARC is <-10dB. A high ECC value indicates strong correlation between antenna elements, which can lead to reduced diversity gain and potentially degrade system performance, particularly in terms of spatial multiplexing. Similarly, provides insights into how efficiently the antenna elements radiate electromagnetic energy and how well they match the impedance of the transmission line or system to which they are connected. A lower TARC value indicates better radiation efficiency and impedance matching, leading to improved antenna performance and reduced signal reflections. Fig. 15 and 16 show that the ECC and the TARC value at the resonant frequency. It is noticed that ECC and TARC are less than 0.5 and -10dB, respectively. Since, ECC values are below 0.5, the MIMO antenna system can effectively mitigate interference and improve channel capacity, resulting in enhanced system performance and reliability and low TARC reflects minimal signal reflections and ensures maximum power transfer between the antenna and the transmission line or system.

Fig. 15
Envelope correlation coefficient (ECC) of 4 element MIMO antenna with (a) uniform PBG and (b) non-uniform PBG

Fig. 16
Total active reflection coefficient (TARC) of 4 element MIMO antenna with (a) uniform PBG and (b) non-uniform PBG

A comparison of previous work with proposed PBG based THz MIMO antenna is reported in Table IV. The proposed system operates at a significantly higher frequency of 2.4 THz compared to the other references, indicating its suitability for high-frequency communication applications. Additionally, while the return loss of the proposed system (-32.97 dB) falls within the range of the existing references, it maintains a competitive level of signal reflection.

TABLE IV
COMPARISON OF PREVIOUS WORK WITH PROPOSED PBG BASED THZ MIMO ANTENNA

Ref	Substrate	No. of elements	Resonant Frequency (THz)	Return Loss (dB)	Isolation (dB)	Gain (dB)	Bandwidth (GHz)
[1]	SiO2	2	1.89	-	-25	4.6	1590
[7]	Polymide	4	0.64	-40.3	-18	7.5	363
[40]	Polymide	2	0.51	-41	-54	5.49	400
[41]	Polymide	2	0.472	-20.75	-52	3.9	435
[42]	Polymide	8	0.63	-23.98	-14	8.2	36.2
[43]	Polymide	4	0.445, .540	-32.45, -35.13	-66	7.9, 6.6	22.48, 41.56
[44]	Graphene	4	0.6	-34.85	-24.66	5	550
[45]	Graphene	2	6.51, 7.48, 8.46	-38,23, -48.21, -60.13	-32, -44, -45	13,53	700, 690, 890
Proposed	Polymide	4	2.4	-32.97	-32.54	3.69	62.4

High isolation of -66dB was achieved in [66], when the 4 elements are placed at 0.86 mm, which is higher than the operating wavelength. Further, it is observed that the proposed THz MIMO antenna using Polymide substrate compared to [⁷, ⁴⁰, ⁴¹, ⁴²], better isolation <-32.54 dB achieved even though the elements are closely spaced. However, the bandwidth is greater than antenna designed in [⁴³], the proposed antenna has narrow bandwidth. Furthermore, the proposed system demonstrates a gain of 3.69 dB, ensuring efficient signal amplification and reception.

V. CONCLUSION

In this paper, closely spaced MIMO antenna to operate at THz frequency is designed and analysed. The proposed 2x2 MIMO antenna has the dimension of 510×570×40mm³, to operate in the frequency of 2.4 THz. Through comprehensive simulation studies, the effectiveness of the PBG-based approach in reducing mutual coupling and enhancing isolation between antenna elements is demonstrated. The proposed antenna produces the return loss as -32.97 dB and a gain of 3.69 dB. Moreover, the mutual coupling is less than -32.54 dB. Also, it has ECC and TARC as less than 0.5 and -10dB. Further, the radiation pattern is consistently omnidirectional hence this MIMO system is useful for diverse indoor communication applications.

REFERENCES

• ^[1] Vasu Babu K, Gorre Naga Jyothi Sree, Tanvir Islam, Sudipta Das, Mohammed El Ghzaoui, R. Agilesh Saravanan, “Performance Analysis of a Photonic Crystals Embedded Wideband (1.41-3.0 THz) Fractal MIMO Antenna Over SiO2 Substrate for Terahertz Band Applications”, Silicon, vol. 15, pp. 7823-7836, 2023. https://doi.org/10.1007/s12633-023-02622-0
  » https://doi.org/10.1007/s12633-023-02622-0
• ^[2] Das S, Anveshkumar N, Dutta J, Biswas A, “Advances in Terahertz Technology and Its Applications”, Springer, ISBN: 978-981-16-5730-6, 2021, https://doi.org/10. 1007/ 978-981-16-5731-3_1
  » https://doi.org/10. 1007/ 978-981-16-5731-3_1
• ^[3] Subodh Kumar Tripathi, Ajay Kumar, “High Gain Miniaturised Photonic Band Gap Terahertz Antenna For Size-Limited Applications”, Australian Journal of Electrical and Electronics Engineering, vol. 16, no. 2, pp. 74-80, 2019. https://doi.org/10.1080/1448837X.2019.1602944 .
  » https://doi.org/10.1080/1448837X.2019.1602944
• ^[4] Chun-Hsing Li, Te-Yen Chiu, “340-GHz Low-Cost and High-Gain On-Chip Higher Order Mode Dielectric Resonator Antenna for THz Applications”, IEEE Transactions on Terahertz Science and Technology, vol. 7, no. 3, pp. 284 - 294, 2017.
• ^[5] Temmar, M.N.E, Hocini, A., Khedrouche, D., Zamani, M., “Analysis and Design of A Terahertz Microstrip Antenna Based on A Synthesized Photonic Bandgap Substrate using BPSO”, Journal of Computational Electronics, vol. 18, pp. 231-240, 2019.
• ^[6] Singh P. K. Saini J, “Effect of Varying Curvature and Inter Element Spacing On Dielectric Coated Conformal Microstrip Antenna Array”, Progress In Electromagnetics Research M, vol. 58, pp. 11-19, 2017.
• ^[7] Singh P. K., Saini J, “Reconfigurable Microstrip Antennas Conformal To Cylindrical Surface”, Progress In Electromagnetics Research Letters, vol. 72, pp. 119-126, 2018.
• ^[8] Yadav R., A. Parmar, L. Malviya, D, Nitnaware, “Graphene Based Multiband Stack Patch THz Antenna with Proximity Feed”, IEEE conference on Antenna and Propagation, pp. 544-547, 2021. https://doi.org/10.1109/InCAP52216.2021.9726472
  » https://doi.org/10.1109/InCAP52216.2021.9726472
• ^[9] Moghe S., Yadav R., Malviya L, “Sub-Terahertz MIMO Array Antenna for Future Wireless Applications” 11th IEEE International Conference on Communication Systems and Network Technologies,pp. 49-53, 2022. https://doi.org/10.1109/CSNT54456.2022.9787576 Malviya L., J. Malik, R.K. Panigrahi, M.V., Kartikeyan, “Design of A Compact MIMO Antenna with Polarization Diversity Technique For Wireless Communication”, International Conference on Microwave, Optical and Communication Engineeringpp. 21-24, 2015. https://doi.org/10.1109/ICMOCE.2015.7489681
  » https://doi.org/10.1109/CSNT54456.2022.9787576» https://doi.org/10.1109/ICMOCE.2015.7489681
• ^[10] Hocini A, Temmar MN, Khedrouche D, Zamani M, “Novel Approach For The Design And Analysis of A Terahertz Microstrip Patch Antenna Based on Photonic Crystals”, Photonics and Nanostructures - Fundamentals and Applications, vol. 36, pp. 100723, 2019.
• ^[11] Sharma, V.K. Dwivedi, Singh G, “THz Rectangular Patch Microstrip Antenna Design Using Photonic Crystal As Substrate”, Progress in Electromagnetics Research Symposium, pp. 161-165, 2008.
• ^[12] Sharma, V.K. Dwivedi, Singh G, “THz Rectangular Microstrip Patch Antenna on Multilayered Substrate For Advanced Wireless Communication Systems”, Progress in Electromagnetics Research Symposium, pp. 627- 631, 2009.
• ^[13] Rajni Bala, Anupma Marwaha, “Analysis of Graphene Based Triangular Nano Patch Antenna Using Photonic Crystal As Substrate For Wireless Applications”, 2nd International Conference on Recent Advances in Engineering & Computational Sciences, 2015.
• ^[14] Maria Denise Astorino, Fabrizio Frezza, Nicola Tedeschi, “Ultra-thin Narrow-Band, Complementary Narrow-Band and Dual-Band Metamaterial Absorbers for Applications in the THz Regime”, Journal of Applied Physics, vol. 121(063103), pp. 1-11, 2017.
• ^[15] Malviya L., Panigrahi R. K., Kartikeyan M.V, “MIMO Antennas with Diversity and Mutual Coupling Reduction Techniques: A Review”, International Journal of Microwave and Wireless Technologies, vol. 9, no. 8, pp. 1763-1780, 2017.
• ^[16] Rao L., Pant M., Malviya L., Parmar A., Charhate S.V, “5G Beamforming Techniques For The Coverage of Intended Directions In Modern Wireless Communication; In Depth Review”, International Journal of Microwave And Wireless Technologies, vol. 13, pp. 1039-1062, 2021.
• ^[17] Yadav R., Malviya L, “UWB antenna and MIMO Antennas with Bandwidth, Band-Notched and Isolation Properties For High-Speed Data Rate Wireless Communication: A Review Article”, International Journal of RF and Microwave Computer-aided Engineering, vol. 30, pp. 1-25, 2019.
• ^[18] Zaidi, A., Baghdad A., Ballouk A., Badri A, “High Gain Microstrip Patch Antenna, with PBG substrate and PBG cover, for Millimeter Wave Applications”, 4th International Conference on Optimization and Applications, pp. 1-6, 2018.
• ^[19] AbuTarboush, H. F., Al-Raweshidy H. S., Nilavalan R, “Bandwidth Enhancement For Small Patch Antenna Using PBG Structure For Different Wireless Applications”, IEEE International Workshop on Antenna Technology pp. 1-4, 2009.
• ^[20] Lin L.L., Li Z.Y, “Interface States In Photonic Crystal Heterostructures”, Physics Review B, vol. 63, no. 3, pp. 3101-3104, 2001.
• ^[21] John D. Joannopoulos, Steven G.Johnson, Joshuoa N. Winn, “Photonic Crystals Molding the Flow of Light”, Princeton University press, 2008.
• ^[22] Gonzalo R, Mart Nez B, Maagt P, Sorolla M, “Improved Patch Antenna Performance by Using Photonic Bandgap Substrate”, Microwave and Optical Technology Letters, vol. 24, no. 4, pp. 213 - 215, 2000.
• ^[23] Watts M., X. Liu, W. J. Padilla, “Metamaterial Electromagnetic Wave Absorbers”, Advanced Optical Materials, vol. 24, no. 98, pp. 1-23, 2012.
• ^[24] Jasmine Saini and Manoj K. Garg, “PBG Structured Compact Antenna with Switching Capability in Lower and Upper Bands of 5G”, Progress In Electromagnetics Research M, vol. 94, pp. 19-29, 2020.
• ^[25] Kushwaha, R.K., Karuppanan, P, “Enhanced Radiation Characteristics of Graphene-Based Patch Antenna Array Employing Photonic Crystals And Dielectric Grating for THz Applications”, Optik, vol. 200(163422), pp. 1-12, 2020.
• ^[26] Chashmi, M.J., Rezaei, P., Kiani, N., “Y-Shaped Graphene-Based Antenna with Switchable Circular Polarization”, Optik, vol. 200(163321), pp. 1-11, 2020.
• ^[27] Peterson C.L, “Nanotechnology from Feynman to the Grand Challenge of Molecular Manufacturing”, IEEE Technology and Society Magazine, vol. 23, no. 4, pp. 9-15, 2004.
• ^[28] Rajni Bala, Anupma Marwaha, “Investigation of graphene based miniaturized terahertz antenna for novel substrate materials”, Engineering Science and Technology an International Journal, vol. 19, pp. 531-537, 2015.
• ^[29] Avez Syed, Mansour H. Almalki, “Graphene-Based Full-Duplex Antenna for Future Generation Communication in THz Frequency Range”, Hindawi Journal of Computer Networks and Communications, vol. 9285354, pp. 1-7, 2023.
• ^[30] Badr, N.S., Moradi, G., “Graphene-Based Microstrip-Fed Hexagonal Shape Dual Band Antenna”, Optik, vol. 202 (163608), pp. 1-12, 2020.
• ^[31] Debin Hou, Jixin Chen, Hong W, “A 270 GHz×9 Multiplier Chain MMIC With On-Chip Dielectric-Resonator Antenna”, IEEE Transactions on Terahertz Science and Technology, vol. 69, no. 9, pp. 3621 - 3632, 2020.
• ^[32] Inzamam Ahmad, Sadiq Ullah, Shakir Ullah, Usman Habib, Sarosh Ahmad, Adnan Ghaffar, Mohammad Alibakhshikenari, Salahuddin Khan, Ernesto Limiti, “Design and Analysis of a Photonic Crystal Based Planar Antenna for THz Applications”, Electronics, vol. 10(1941), pp. 3-12, 2021.
• ^[33] Allen M. J., Tung V. C., Kaner R. B, “Honeycomb Carbon: A Review of Graphene”, Chemical Review, vol. 110, pp. 132-145, 2010.
• ^[34] Ujjwal Tripathi, Deepak Solanki, Priyanshi Malviya, Ajay Parmar, “MIMO antenna design with PBG structure for THz Communication”, 1st International Conference on Microwave, Antenna and Communication, 2023.
• ^[35] Rashmi Pant, Leeladhar Malvia, Nineeta Choudry, “Design and Analysis of Gain Enhancement THz Microstrip Curvature Patch Antenna with Inset Feed”, Mobile Radio Communications and 5G Networks, vol. 140, pp. 707-715, 2020.
• ^[36] Nasrin Shoghi Badr, Gholamreza Moradi, “Graphene-Based Microstrip-Fed Hexagonal Shape Dual Band Antenna”, Optik, vol. 202(163608), pp. 1-13, 2020.
• ^[37] Shamim S. M., Youssef Trabelsi, Nahid Arafn, Anushkannan N. K., Umme Salma Dina, Arafat Hossain MD., Nazrul Islam, “Design and Analysis of Microstrip Patch Antenna With Photonic Band Gap (PBG) Structure for High-speed THz Application”, Optical and Quantum Electronics, vol. 55, no. 618, pp. 1-15, 2023.
• ^[38] Jablan M., H. Buljan, M. Soljaˇci ́c, “Plasmonics in Graphene at Infrared Frequencies”, Physical review B, vol. 80, no. 24, pp. 1-8, 2009.
• ^[39] Balanis, C. A., “Antenna Theory: Analysis and Design”, John Wiley & Sons, 2015.
• ^[40] Babu KV, Das S, Sree GNJ, “Design and Optimization of Micro-Sized Wideband Fractal MIMO Antenna Based on Characteristic Analysis of Graphene For Terahertz Applications”, Journal Optical Quantum Electronics, vol. 54, no. 281, pp. 1-20, 2022.
• ^[41] Vasu Babu K, “A Micro-Scaled Graphene-Based Tree shaped Wideband Printed MIMO Antenna for Terahertz Applications”, Journal of Computational Electronics, vol. 21, no. 1, pp. 289-303, 2022.
• ^[42] Singh M, Singh S, Islam MT, “Highly Efficient Ultra-Wideband MIMO Patch Antenna Array For Short Range THz Applications”, Emerging Trends in Terahertz Engineering and System Technologies Devices, Materials, Imaging, Data Acquisition and Processing, pp. 193-207, 2021. https://doi.org/10.1007/978-981-15-9766-4_10
  » https://doi.org/10.1007/978-981-15-9766-4_10
• ^[43] Youssef Amraoui a, Imane Halkhams, Rachid El Alami, Mohammed Ouazzani Jamil, Hassan Qjidaa, “High isolation integrated four-port MIMO Antenna for terahertz communication”, Results in Engineering, vol.26, pp.1-18, 2025
• ^[44] Narges Kiani, Farzad Tavakkol Hamedani, Pejman Rezaei, “Graphene‑Based Quad‑Port MIMO Reconfgurable Antennas for THz Applications”, Silicon, vol.16, pp.3641-3655, 2024
• ^[45] Ashraful Haque, Kamal H.N, Jamal H.N, KawsarAhmed, Narinderjit S.S.S, Liton C.P, Abeer D.Algarni, Mohammed El, Ahmed A. Abd El-Latif, Abdelhamied, “Multiband THz MIMO antenna with regression machine learning techniques for isolation prediction in IoT applications”, Scientific Reports, vol.15, pp.1-23, 2025

Publication Dates

• Publication in this collection
  03 Nov 2025
• Date of issue
  2025

History

• Received
  05 Jan 2025
• Reviewed
  22 Apr 2025
• Accepted
  14 July 2025