1 Introduction

Millimeter-wave (mm-Wave) and terahertz (THz) spectrums represent the new frontiers of wireless communications technology, expected to facilitate seamless interconnection between ultra-high-speed wired networks and personal wireless devices. This advancement will simplify the utilization of bandwidth-intensive applications, particularly in indoor and local access scenarios [1]. Several different THz antenna arrays have been investigated recently. The THz antenna array reported in [2] combines metasurface technology with artificial magnetic conductor (AMC) technology. The metasurface structure was implemented by constructing a periodic arrangement of dielectric unit-cells on a thin conductive layer on top of a dielectric substrate. The unit-cells were engineered to interact with the impinging electromagnetic waves in a specific way to modify the dispersion characteristics of surface. The AMC was implemented on a dielectric medium of a specific thickness. The 2 × 2 antenna array was constructed on a 35 μm thick silicon wafer. The array has an impedance bandwidth of 4.56% and reflection coefficient of <  − 16 dB over 300–314 GHz. In [3] the radiation element comprising the 2 × 1 Thz antenna array resembles a petal shaped structure. The array is fabricated on a polyimide substrate with a thickness of 10 µm, a relative permittivity of 3.5, and has dimensions of 2.920 × 1.055 mm2. Simulation using CST Studio Suite® was used to verify the array’s performance. The array is shown to have an impedance bandwidth of 0.63% and reflection coefficient of <  − 23 dB at 714 GHz. The THz patch antenna array reported in [4] consists of 11 patch elements that are interconnected to each other and printed on a single-layer high frequency laminate with a power distribution network realized by a hollow waveguide made from a copper block. The simulated results show that the subarray can achieve an impedance bandwidth of 37% for the reflection coefficient of <  − 10 dB over 150–205 GHz.

Metasurfaces, the two-dimensional counterparts of metamaterials, are artificially engineered to manipulate electromagnetic waves, exhibiting a negative refractive index not found in nature [5, 6]. This methodology has been applied to reduce the physical dimensions and enhance the specifications of microwave components such as antennas, filters, couplers, and mixers [7]. Their unique properties have enabled the development of novel applications, concepts, and devices [8, 9]. As a result, metasurfaces are now widely utilized in the design of antenna devices [10, 11]. Furthermore, the exceptional specifications of metasurfaces allow antenna designers to implement new antenna systems that are unattainable using traditional techniques. Research indicates that metasurface-based antennas offer extended operational bandwidth and increased radiated power [12,13,14,15]. The primary advantages that make metasurfaces appealing for antenna design compared to conventional methods include (i) the implementation of more compact antennas, (ii) larger bandwidth, (iii) improved radiation properties, and (iv) the ability to model multiband functionality.

The paper describes an innovative antenna array using metasurface-inspired techniques and defected ground structure (DGS) technology for operation across the mm-wave and THz spectrum. It is demonstrated that these proposed approaches can realize a high-performance antenna in a compact footprint area for various high-frequency wireless communications, including 6G. The array consists of closely spaced conductive radiating slabs that include sub-wavelength slots, exciting resonant modes within the structure, which in turn radiate energy into free space. The ground plane is perforated with rectangular slots to mitigate unwanted coupling between the radiating elements, resulting in improved array performance.

2 Design of the proposed antenna array

The Fig. 1 illustrates the proposed antenna array, which comprises a 2 × 4 matrix of rectangular conductive slabs loaded with sub-wavelength dielectric slots. These conductive radiating boxes draw inspiration from metasurface structures, where the multiple slots on each patch serve as sub-wavelength scatters. This feature distinguishes metasurfaces from traditional frequency-selective surfaces (FSS). A traditional FSS has individual elements (periodicity) which are of the order of the operating wavelength (generally λ/2). The matrix size was selected to achieve directional radiation. The antenna array is excited via a single microstrip line on one side, with the radiating elements interconnected by microstrip lines. The ground plane features rectangular slots, each positioned directly beneath a radiating element. These slots are designed to mitigate unwanted mutual coupling between the radiating elements, thereby improving the array's overall performance. For fabrication, we used a polyimide substrate with a dielectric constant (\({\varepsilon }_{r}\)) of 3.5 and a thickness of 20 microns.

Fig. 1
figure 1

Geometry and layout of the proposed 2 × 4 antenna array, (a) top view, (b) side view, and (c) back view

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The antenna array’s structural dimensions that are given in Table 1 were chosen for the array to operate across the 80 to 200 GHz range. CST Studio Suite®, which is a high-performance 3D electromagnetic tool, was used to analyze and determine the antenna’s optimum dimensions. CST Studio Suite® is a powerful industry standard tool that provides accurate and detailed analysis in a virtual environment, reducing the need for costly and time-consuming physical prototyping. Essentially slots are implemented on each of the conductive patches. The patch itself acts as a resonant structure. When properly designed, the dimensions of the patch and the dielectric substrate it's placed on determine its resonant frequency. This resonance causes a strong localized electric field at the patch edges and, therefore, the edges become the primary regions for radiation. Slots, or openings, that are introduced into the patch create breaks or discontinuities in the conducting surface of the patch. When electromagnetic waves from the feedline or incident from free space interact with the slots, they couple energy into the microstrip structure. This interaction induces current flow on the conductive patch. The presence of the slots changes the current distribution on the patch compared to a solid patch. This non-uniform current distribution causes radiation from the antenna. Moreover, the electric field inside the slots interacts with the dielectric material, causing polarization of the dielectric. Depending on the frequency and dimensions of the slots, this polarization can lead to the generation of secondary currents within the slots. These slot currents act as sources of radiation, and they emit electromagnetic waves into free space.

Table 1 Antenna structural dimensions
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CST Studio Suite® was used to analyze the performance of the antenna array. This simulation tool is well-calibrated and validated against known measurement benchmarks, which is crucial in establishing the reliability of the simulated results presented here. The reflection coefficient of the proposed 2 × 4 antenna array is shown in Fig. 2. By defecting the ground plane, the impedance matching (for |S11|< − 10 dB) is improved and this is over a wider bandwidth. In the design of antenna arrays, as is the case here where the radiating elements are very closely spaced, mutual coupling between adjacent radiators can severely limit the array’s bandwidth. This is because the mutual coupling causes energy from one antenna to interfere with the operation of neighboring antennas. By defecting the ground plane, the electromagnetic environment is altered resulting in reduction in the mutual coupling between the elements. As the results in Fig. 2 show the consequence of this is improved bandwidth performance of the array. The array operates over a very wide frequency range from 80 GHz to above 200 GHz for |S11|< − 10 dB, which corresponds to a fractional bandwidth of 85.71%.

Fig. 2
figure 2

Reflection coefficient of the proposed 2 × 4 array using metasurface-inspired techniques and DGS technology

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The realized radiation gain and efficiency of the array are presented in Fig. 3. The gain exceeds 4 dBi, with efficiency consistently above 55% in the frequency range from 80 to 200 GHz. At its peak, the radiation gain reaches 7.5 dBi, while the efficiency reaches 75% at 91 GHz. Beyond 91 GHz, both the gain and efficiency decline. This is because the array was optimized at 91 GHz. At higher frequencies, this spacing between the radiating elements is longer be optimal, leading to reduced antenna array performance. A table summarizing the gain and efficiency at various frequencies within the 8 GHz to 200 GHz range is provided in Table 2.

Fig. 3
figure 3

The gain and efficiency of the proposed antenna array, (a) realized gain and (b) radiation efficiency

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Table 2 Radiation properties
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The 2D radiation patterns of the proposed 2 × 4 antenna array is shown in Fig. 4 at various spot frequencies between 8 and 200 GHz. The antenna radiates energy essentially directionally, and the cross-polarization gain is more than 10 dB below the co-polarization over the 3 dB beamwidth. The good cross-polarization discrimination demonstrates that the array should reject signals with polarizations different from the desired polarization. This rejection of cross-polarized signals should reduce interference. The radiation patterns show relatively low sidelobes.

Fig. 4
figure 4

Radiation patterns of the proposed 2 × 4 array slotted ground antenna at various spot frequencies, (a) 80 GHz, (b) 100 GHz (c) 120 GHz, (d) 150 GHz, (e) 180 GHz, and (f) 200 GHz

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3 State-of-the-art comparison

The proposed antenna array’s characterizing parameters are compared with the state-of-the-art in Table 3. It is evident from the table that compared to the cited references the proposed antenna array design, which is metasurface-inspired and uses DGS technology, exhibits a much wider impedance bandwidth from 80 to 200 GHz and much higher average radiation efficiency of ~ 65%. A wider bandwidth is advantageous, especially for mm-Wave and THz applications, where a broad operating frequency range is often desirable. These properties make this antenna suitable for existing and future mm-Wave and THz wireless systems.

Table 3 Specification comparison of the proposed antenna with state-of-the-art
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4 Conclusion

A proof-of-concept is demonstrated for an innovative 2 × 4 antenna array designed for applications in the millimeter-wave and THz spectrum. The antenna array design draws inspiration from metasurface techniques and incorporates defected ground structure principles to enhance its radiation performance. The array consists of radiating elements comprising conductive slabs loaded with sub-wavelength slots. It is shown that the slots in the defected ground plane effectively mitigate unwanted mutual coupling between closely spaced radiating elements, thereby enhancing the impedance bandwidth of the array. The proposed antenna exhibits directional radiation between 80 and 200 GHz, with an average gain of approximately 5.5 dBi and a radiation efficiency of around 65%. The proposed antenna has dimensions of 20 × 10 × 0.6 mm3.