1 Introduction

Recently, the new wireless communication systems, 5G communications, and the Internet of Things (IoT) technology have several demands, such as high data rate, low latency, low cost, small size, and universal connectivity [1,2,3,4]. The Federal Communications Commission (FCC) has approved several bands from 24 to 50 GHz for 5G technology [5]. So, many researchers moved their research to these bands.

Metamaterials have unique features [6]. They have unusual electromagnetic properties, so they attract researchers to use them in many millimeter (mm) wave applications, such as perfect lensing, antennas, SAR reduction, sensing, and electromagnetic absorption [7,8,9,10]. Metamaterials can provide high absorption and small size characteristics, which make them a good candidate for a microwave absorber. MMA is considered one of the most promising fields for researchers in recent years. MMA has attracted more consideration due to its absorbing properties and simple fabrication [11]. To create metamaterials, different structures with highly conductive metals are used [12]. There are many works conducted on MMA for several applications operated at different frequency bands [13,14,15,16,17,18]. However, there is low work operated at 5G mm-wave frequency bands, especially at 28/38 GHz [3, 11, 19,20,21].In [3], an MMA with a meander line shape printed on an 8 mm × 8 mm FR4 substrate with 1.6 mm thickness and a large size to operate at 24/28 GHz 5G applications is studied and investigated. Simulated results of MMA with a four rectangular ribs shape printed on 5 mm × 5 mm FR4 substrate with 1.7 mm thickness and operated at four bands 17.32 / 21.87 / 29.43 GHz / 37.1 GHz is introduced in [11]. Simulated results of MMA with three L-shaped printed on 2.81 mm × 2.81 mm FR4 substrate with 1 mm thickness and operated at four bands 21.8 GHz to 53.2 GHz are investigated in [20]. In [21], an MMA with a fractal shape split rings printed on 12 mm × 6 mm FR4 substrate with 1.6 mm thickness and a large size to operate at four bands between 22 to 36 GHz 5G applications is presented. An MMA with a complex structure printed on 26 mm × 26 mm FR4 substrate with 1.6 mm thickness and operated at wideband from 20 to 30 GHz is introduced in [22].

In this article, a dual-band MMA with a simple configuration and compact size is introduced. The absorber supports the 28 GHz and 38 GHz bands that are suitable for 5G applications. To know about the proposed structure absorption technique, the currents, and electric field distributions are studied. The suggested MMA is fabricated and tested, and its outcomes are compared to the simulations. The CST Microwave Studio is utilized in the simulation, modeling, and optimization of MMA. The novelty of this work can be concluded as follows; first, the MMA has a simple structure with a compact size which is recommended in future 5G systems. Second, the MMA has fabricated a low-cost FR4 substrate. Third, the MMA is operated at dual bands 28/38 GHz with high absorption rates. The construction of the paper is divided into different sections. The overview of the metamaterial definition and applications are investigated in Section 1. Section 2 discussed the unit cell structure design. The fabrication and measurement setup of the suggested MMA and its outcomes as well as the comparison between our work with others are studied in Section 3. Finally, the conclusion is added in Section 4.

2 Unit cell Layout

The suggested dual-band MMA finite sheet geometry and the unit cell layout are illustrated in Fig. 1(a) and (b). The MMA is composed of two metallic rings on the top and a metallic sheet as a ground plane to block the transmission on the back with a thickness and conductivity of 0.035 mm and 5.96 × 107 S/m, respectively. The low-cost 5.8 mm × 5.8 mm FR-4 substrate is used in the design with a dielectric constant, thickness, and loss tangent of 4.3, 1.5 mm, and 0.025, respectively.

Fig. 1
figure 1

a Configuration of the total size of MMA. b Unit cell with optimized dimensions of d1 = 5.46 mm, d2 = 3.94 mm, w1 = 0.12 mm, w2 = 0.44 mm, h = 1.5 mm (c) Equivalent circuit

Full size image

The MMA depends on the resonance phenomenon. When the resonating conditions are achieved, the top layer accepts the incoming wave; this occurs because of the impedance matching between the free space and the MMA impedance. Also, the back layer has prevented the wave from transmission then all the transmission and reflection = 0, which leads the material to absorb the wave. The absorptivity \(\mathrm{A}\left(\upomega \right)\) can be calculated as [22]:

$$A\left(\omega \right)=1-T\left(\omega \right)-R\left(\omega \right)= 1-{\left|{S}_{21}\left(\omega \right)\right|}^{2}-{\left|{S}_{11}\left(\omega \right)\right|}^{2}$$
(1)

T(ω) = 0 because of the background plane

$$A\left(\omega \right)=1-R\left(\omega \right)$$
(2)
$$A\left(\omega \right)=1-{\left|{S}_{11}\left(\omega \right)\right|}^{2}$$
(3)

At the resonance conditions, the MMA impedance can be calculated as [23]

$$z=\frac{1+{S}_{11}}{1-{S}_{11}}$$
(4)

The equivalent circuit of the suggested MMA to know the concept of the operation of the design is illustrated in Fig. 1(c) [24, 25]. As Shown in Fig. 1(c), it is noticed that each ring can be modeled as parallel LC which is responsible for distinct resonance frequency. The resistance values are responsible for the losses and their values can be tuned to control the level of absorption. There is a coupling between two rings which can be modeled by coupling capacitance Cm. The values of the lumped components are L1 = 0.1nH, L2 = 0.0045nH and C1 = 0.088pF, C2 = 0.28pF, Cm = 0.054 pF. The simulation outcomes of the dual-band MMA extracted from EM and circuit simulators are shown in Fig. 2. It is seen that the reflection coefficient S11 and absorption present two resonant absorption peaks at 28 GHz and 38 GHz in the upper band of the 5G spectrum with 3-dB bandwidth and absorptivity of 1.5 GHz, 1.6 GHz, 98.4% and 98%, respectively with a resonable matching between the two outcomes.

Fig. 2
figure 2

The simulation outcomes of the reflection and the absorption dual-band absorber

Full size image

To understand the behavior of the MMA, the development of the MMA is illustrated in Fig. 3. Every structure has a unique absorption peak, so by using parametric optimization, the desired band can be achieved. The structure (A) with the outer ring is utilized as the initial design of the MMA, and the responsivity curve is illustrated in Fig. 4. It is observed that it exhibits an absorption peak at 28.3 GHz with a peak absorption of 99.5% (the blue dashed line). The structure (B) with the inner ring is used, and the responsivity curve is illustrated in Fig. 4. It is observed that it exhibits an absorption peak at 35.2 GHz with an absorptivity of 86.3%. (the red dotted line). Finally, by combining the two rings as the suggested structure, two resonant absorption peaks at 28 GHz and 38 GHz, with the absorption peak of 98.4% and 98%, is achieved, and a middle absorption peak appears at 34 GHz with 60% is observed as shown in Fig. 4. This frequency shift is achieved due to the coupling effect between the two rings.

Fig. 3
figure 3

The development of the suggested absorber

Full size image
Fig. 4
figure 4

The simulated outcomes of the absorptivity for different structure

Full size image

To study the characteristics of the suggested absorber, complex permittivity (ε), permeability (µ), and input impedance are extracted and introduced, as illustrated in Fig. 5. Several methods can be used to extract these parameters. However, for simplicity, Nicolson-Ross-Weir (NRW) is utilized [26]. It is seen that the real part of the relative permittivity and relative permeability have negative values within the operating frequency bands. As well the normalized impedance is illustrated in Fig. 5(c). As noticed in Fig. 5(c), the normalized input impedance is close to 1 at 28/38 GHz, and the imaginary part is close to 0 at the two frequency bands, which confirms the matching conditions to the free space, which leads to maximum absorption.

Fig. 5
figure 5

The unit cell MMA outcomes (a) Relative permittivity (b) Relative permeability (c) The Normalized input impedance

Full size image

3 Results and Discussion

To know the best performance of the MMA at the desired frequency bands, parametric investigations are utilized. The FR4 substrate thickness is considered the main parameter which affects MMA performance. The effect of the substrate thickness on the MMA while keeping the other parameters constant is illustrated in Fig. 6. It is observed that when the h = 1.2 mm, there is one resonant absorption peak at 28.6 GHz with an absorptivity of 97%. When the h = 1.5 mm, two resonant absorption peaks at 28 GHz and 38 GHz with absorptivity of 98.4% and 98% are achieved. When the h = 1.8 mm, two resonant absorption peaks at 27.23 GHz and 31.55 GHz with absorptivity of 90.8% and 78% are achieved. Finally, When the h = 2 mm, two resonant absorption peaks at 26.5 GHz and 30.25 GHz with absorptivity of 87.6% and 97% are achieved. So, the substrate with h = 1.5 mm is chosen to achieve the desired frequency bands.

Fig. 6
figure 6

The effect of the substrate thickness on the MMA

Full size image

Also, the simulated outcomes of the absorptivity for different polarization angles (Ø) under normal incidence to check the polarization insensitivity of the suggested MMA are shown in Fig. 7. It is seen that the suggested MMA has the same absorptivity outcomes as illustrated in Fig. 7.

Fig. 7
figure 7

The simulated outcomes of the absorptivity for different polarization angles (Ø) under normal incidence

Full size image

Further, the simulated outcomes of the absorptivity for different incidence angles (ϴ) are shown in Fig. 8. The incidence angles are changed from 0° to 45° with a 15° step size angle. It is noticed that, at 28 GHz, the MMA has an absorption of more than 90% up to 15° of incidence angles, but the absorptivity decreases with absorption ranging from 60 to 80%, and the frequency is shifted to 30 GHz for ϴ = 30° and 45°. Also, for the 38 GHz, the MMA behaves with good absorption of more than 90% up to ϴ = 30o, but the absorptivity decreases around 60%, and the frequency is shifted to 37 GHz for ϴ = 45o

Fig. 8
figure 8

The simulated outcomes of the absorptivity for different incidence angles (ϴ)

Full size image

The simulated outcomes of the electric field and current distributions at the two frequency bands are illustrated in Fig. 9. It is clear that the outer circle has the highest concentration for both electric field and current at 28 GHz, which confirms it's responsible for absorption at 28 GHz. The inner circle is responsible for the second resonance at 38 GHz. It also noted that the current distribution in Fig. 9(a) confirms that the absorption variation of the incident angle is independent. The current distribution is symmetrical on the absorber surface [27].

Fig. 9
figure 9

The simulated outcomes of the field distribution at the two frequency bands (a) Current (b) Electric

Full size image

An array of 10 × 10 of the MMA is fabricated to validate the simulation outcomes. To reduce the difference between the simulated and tested outcomes, the total size of the fabricated prototype should be increased, which increases the overall size of the system. So, trade-off solutions should be utilized. The MMA fabricated prototype is manufactured by utilizing a UV exposure system over a 1.5 mm FR-4 substrate. The suggested MMA has an overall size of 60 mm × 60 mm × 1.5 mm, as illustrated in Fig. 10(b). The MMA is tested inside the anechoic chamber, which contains two standard gain horn antennas A-INFOMW LB-28–10-C-KF (26.5 – 40 GHz), which is connected to a vector network analyzer R&S ZVA 67 (VNA) and a sample holder as illustrated in Fig. 10(a). First, the VNA is calibrated then the horn antennas are added away from the fabricated prototype to ensure the far field distance. To measure the reflection coefficient, a copper sheet as a reference sheet is added to the sample holder. Then the MMA prototype is placed in the same place instead of the copper sheet to measure its reflection coefficient. The difference between the reflected power from the MMA and the copper sheet is considered the actual reflection from the MMA (normalization).

Fig. 10
figure 10

The MMA (a) The measurement setup (b) The fabricated prototype

Full size image

In the TE polarization, the \(\left(\left|{\mathrm{S}}_{21}\right|\right)\) in dB was measured using the VNA when the MMA prototype was rotated from 0° to 30°. Similarly, In TM polarization, the \(\left(\left|{\mathrm{S}}_{21}\right|\right)\) in dB was measured using the VNA when the horn antennas were rotated by 90o, and the MMA prototype was rotated similarly from 0° to 30°. Then the absorptivity can be calculated from (1). While the \(\left(\left|{\mathrm{S}}_{11}\right|\right)\) is the reflected wave by the transmitted horn antenna, which approximately tends to zero While \(\left(\left|{\mathrm{S}}_{21}\right|\right)\) is the received wave by the receiving horn antenna after the wave is reflected from the MMA prototype. The simulated and tested outcomes of the absorptivity for TE polarization are shown in Fig. 11. The outcomes of the normal incident are achieved with ϴ = 0o, as shown in Fig. 11(a). It is seen that two resonant absorption peaks at 28 GHz and 38 GHz with absorptivity of 98.4% and 98% are achieved. As well there is another peak at 34 GHz with an absorptivity of 63.86% is seen as shown in Fig. 11(a). A reasonable matching between the simulated and tested outcomes has been achieved. However, there are some ripples which are due to the small size of the fabricated prototype, the tolerance of fabrication, and the testing setup. When ϴ is increased to 15°, the MMA outcomes are illustrated in Fig. 11(b). Five absorption peaks in the 5G band are introduced with absorptivity of 99.31%, 83.44%, 61.10%, 81.10%, and 83.74% at 28.4 GHz, 30.2 GHz, 33.8 GHz, 37.4 GHz, and 39.2 GHz, respectively. Finally, when ϴ is increased to 30°, the MMA outcomes are illustrated in Fig. 11(c). It is seen that three absorption peaks in the 5G band are presented with absorptivity of 95%, 83.7%, and 99.68% at 29.6 GHz, 33 GHz, and 37.6 GHz, respectively.

Fig. 11
figure 11

The simulated and tested outcomes of the absorptivity for TE polarization at (a) 0° (b) 15° (c) 30o

Full size image

Similarly, the simulated and tested outcomes of the absorptivity for TM polarization are shown in Fig. 12. The outcomes of the normal incident are achieved with ϴ = 0°, as shown in Fig. 12(a). It is seen that two resonant absorption peaks at 28 GHz and 38 GHz with absorptivity of 98.4% and 98% are achieved. As well there is another peak at 34 GHz with an absorptivity of 63.86% is seen as shown in Fig. 12(a). A reasonable matching between the simulated and tested outcomes has been achieved. When ϴ is increased to 15°, the MMA outcomes are illustrated in Fig. 12(b). Three absorption peaks are presented with absorptivity of 96.31%, 67.5%, and 99% at 28 GHz, 34.4 GHz, and 37.2 GHz, respectively. Finally, when ϴ is increased to 30°, the MMA outcomes are illustrated in Fig. 12(c). It is seen that three absorption peaks are introduced with absorptivity of 94%, 66.5%, and 99% at 28.6 GHz, 34.2 GHz, and 37 GHz, respectively. In the end, it is concluded that the absorptivity of the MMA has more peaks of more than 80% when the ϴ is changed from 0 to 30° for both TE and TM polarization with a slight shift in the frequency bands; this is due to the MMA's small size, fabrication tolerance, and testing setup as we mentioned before.

Fig. 12
figure 12

The simulated and tested outcomes of the absorptivity for TM polarization at (a) 0° (b) 15° (c) 30o

Full size image

The suggested 5G MMA, in comparison with other 5G mm wave works, is presented in Table 1. To achieve a fair comparison, all metamaterials have to be produced on the same FR-4 substrate and have to work in the same frequency range from 20 to 40 GHz. It can be concluded that the suggested MMA has a compact size, low cost, simple configuration, and manufacturing compared to others which makes it to be utilized in the 5G mm-wave applications.

Table 1 The suggested 5G MMA in comparison with other 5G works
Full size table

4 Conclusion

The design, characterization, fabrication, and testing of a dual-band MMA have been introduced in this work. The structure consists of two circular resonators added on an FR4 substrate with a compact size of 5.8 mm × 5.8 mm × 1.5 mm to work at 28 /38 GHz which is applicable for 5G applications. The MMA has achieved absorption peak values of 98 and 98.4% at 28/38 GHz, respectively, under normal incidence. Because of the symmetry of the MMA structure and compactness of 0.54 \({\lambda }_{0}\) × 0.54 \({\lambda }_{0}\) with a thickness of 0.14 \({\lambda }_{0}\) at 28 GHz, the design has achieved polarisation insensitivity. To know about the proposed structure absorption technique, the currents, electric field distributions, and normalized impedance are studied. The TE and TM polarizations at different angles have been investigated. The MMA structure has been fabricated and tested to validate the simulated outcomes. A good match between the two outcomes is observed. The suggested work has been compared with others to validate the structure's compactness and simplicity. The suggested absorber can be utilized in 5G communication applications.