In this study, a rectangular microstrip feed antenna on a fire-retarded substrate is presented. The antenna is designed using Computer Simulation Tool (CST) software. Different materials are used for the substrate, and the electromagnetic properties of the proposed structure are analyzed for each material. The EM property variation for each material is comparatively presented. The proposed antenna has CSRR (Complementary Split-Ring Resonator) which is engraved on the radiating element, and the shape of the CSRR is the hexagon. The proposed metamaterial-inspired antenna exhibits multiband operation at 2.1 GHz, 2.6 GHz, 4.6 GHz, 5.5 GHz, 6.1 GHz, and 7.1 GHz. With the help of material analysis, the fire-retarded substrate is chosen for the design, its excellent EM properties, and cheap cost. The photolithography-based printed antenna is validated with the help of S11, VSWR, gain, directivity, surface current distribution, and radiation pattern. Simple, compact structure, good gain, stable radiation pattern, and excellent EM properties of the fire-retarded substrates make it suitable for the GHz application.

1. Introduction

The property of the materials used in the construction of any microwave device will directly affect the performance of the microwave device, the prerequisite for any microwave device design is the material selection knowledge. There are plenty of microwave materials available to manufacture microwaved devices, such as CNT, magnetic and ferrite materials, flexible materials, and metamaterials [1]. The selection of materials based on the range of operating frequency is another major requirement. There are a variety of microwave devices [24] such as antennas [5], filters, couplers, isolators, and mixers. Out of which, the antenna plays a vital role in all wireless applications. The antenna is a device made up of conductors that can convert the vibrating electrons into the EM signal during transmission and vice versa during the reception [6, 7]. Two decades before the antenna is of larger size; there are plenty of antennae-like dipoles, reflectors, slots, horns, etc. During the past two decades, there has been an enormous development in antenna design due to the development of wireless communication [8, 9]. The primary requirement of an antenna is compact size and multifunctionality. Both these essential requirements will depend on the antenna substrate. The amount of space available for the antenna in the present wireless devices is minimal, and they are utilized for multiple applications simultaneously. The size of the antenna will depend on the dielectric constant of the substrate material used.

Due to the compact size, ease of MIC (Microwave Integrated Circuits), integration, and efficiency at the GHz frequency of operation, the printed antenna [1012] replaces the conventional antenna. The antenna has a conductive radiating element and ground. The substrate material is sandwiched between the radiating element and the ground. The radiating element, otherwise called the patch, is of any shape such as rectangular, square, circular, and triangular. The substrate materials that are widely used are Arlon, Roger, FR4, composite materials, magnetic materials, and other semiconductor materials [13, 14]. Another primary requirement for the antenna is the multiband functionality achieved with various techniques such as slotting, defected ground structures, integrating passive elements, and coupling patches. The major drawback of this type of technique is that it increases the design complexity [15] and affects the radiation pattern of the structures.

To overcome this drawback, researchers use metamaterials [16, 17] in the antenna structure. The materials with unnatural electromagnetic properties are considered metamaterials. They have a negative refractive index, permittivity, and permeability. These properties are due to their periodic structure that satisfies the homogeneity property [1823]. Various structures such as Split-Ring Resonator (SRR), Complementary SRR, Electric Inductive Capacitive (ELC), Complementary ELC, and many other structures exhibit the unnatural property [24, 25]. These materials are incorporated in the antenna to improve the antenna parameters [2629] to achieve the wide bandwidth, improve impedance bandwidth, use as a superstrate to improve gain, and use the substrate to tune the radiation. However, the insufficient number of works available concerning the multiband operation uses the complementary split ring resonator.

In this study, a rectangular printed antenna inspired with hexagonal metamaterial is proposed for wireless application. The substrate for the design is FR4. The substrate is chosen based on the analysis of various substrates, and their results are presented. The critical parameters of the structure are finalized with the help of parametric analysis. In Section 2, the materials and methods used for the design are presented, followed by the result discussion in Section 3. In Section 4, the findings are concluded.

2. Materials and Methods

2.1. Design of the Multiband Metamaterial GHz Antenna

The proposed HCSRR rectangular printed antenna has a simple rectangular radiating structure. The entire structure is fed with a 50-ohm microstrip feed line. The proposed structure has three stages of evolution. The first stage is a simple microstrip patch antenna A; then, a two-ring hexagonal slot is introduced in the radiating element, which is termed antenna B, and finally, the slit in the hexagonal slot is introduced to design the HCSRR rectangular printed antenna. The proposed structure is fabricated on the FR4 substrate with a dielectric constant of 4.4. The substrate has the size of L x W x h mm3. The radiating element and the feed line are printed on one side, and on the other side, the ground is printed. The feed, radiating element, and ground are made up of copper. In Figure 1, the evolution of the proposed metamaterial-inspired multiband antenna for GHz application [27, 28] is presented. The parameter values are presented in Table 1, and the final proposed structure along with its parameters is presented in Figure 2.

The antenna A is a simple rectangular microstrip patch antenna designed to operate in a 5.2 GHz band. The designed structure operates in the frequency range of 5.00 GHz to 5.68 GHz resonating at 5.3 GHz. The return loss and the bandwidth are equal to 0.68 GHz and -24.18 dB. The antenna B is designed by introducing a dual-ring hexagonal slot in the radiating element precisely at the center concerning the substrate. With the introduction of the hexagonal slot, the current path direction is shifted, and as a result, with good impedance matching, the 5.3 GHz bands are shifted downward. The design antenna B is also a single band antenna that is operating at 4.6 GHz. The proposed structure's operating band is 4.52 GHz to 5.18 GHz with 0.66 GHz as bandwidth and -34.98 dB as return loss.

Then, the hexagonal complementary split-ring resonator is introduced in the radiating element, which enables the multiband characteristics of the proposed structure. The Antenna C resonates at 2.1 GHz, 2.6 GHz, 4.6 GHz, 5.5 GHz, 6.1 GHz, and 7.1 GHz. The proposed structure's operating band ranged from 2.08 GHz to 2.38 GHz, 2.58 GHz to 2.63 GHz, 4.52 GHz to 5.12 GHz, 5.32 GHz to 6.24 GHz, and 6.92 GHz to 7.46 GHz. In Figure 3, the comparison of various evolution antennas is presented. From this, we can observe that the metamaterial is the reason for multiband characteristics. The proposed structure operates five different resonating bands.

The antenna is designed using the following equation:where c is speed of light, is resonant frequency, h is substrate height, and is effective dielectric constant, respectively.

2.2. Substrate Material Analysis

The materials used as the substrate for the printed antenna design decide the proposed antenna's electromagnetic properties. Here, materials such as Arlon AD 250 C, Arlon AD 255C, Arlon AD 320 A, Roger 3010, Roger RT5880, Roger RT6002, and FR4 are analyzed as substrates. The simulated results are presented in Figure 4. From Table 2, we observe that the FR4 substrate can achieve a higher number of operating bands with decent gain and directivity. Therefore, the proposed antenna with FR4 material as the substrate can be used for wireless devices that can be utilized for multiple applications simultaneously.

In Figure 5 (a), comparison of the number of operating bands is presented. In Figure 6, the gain and directivity of the proposed antenna structure when utilizing various types of substrate materials are presented. From Figure 6, we observe that the antenna has FR4 material as the substrate has good reasonable gain and directivity compared to the other materials used.

2.3. Parametric Study

The antenna's critical parameters are analyzed with the help of the parametric analysis of the CST software. The analysis helps identify the optimum dimension of the proposed structure; the critical parameters are the ground length L, feed width wf, and HCSRR split width s and are the critical parameters chosen. First, the ground length L is chosen; the ground length is varied in terms of full ground, half ground, and quarter ground. Out of the above combinations, the full ground can support the multiband characteristics in all operating frequencies with better matching of impedance. Therefore, for the final fabrication of the proposed structure, the complete ground is chosen. The response of s11 concerning various ground sizes is depicted in Figure 7. The split width of the hexagonal CSRR is increased from 0.3 mm to 0.7 mm in steps of 0.2 mm. The split width s = 0.5 mm has better matching, and it is the final value for the split width. The effect of the split width on the return loss is depicted in Figure 8, it is pragmatic that the higher resonant frequency is more affected, which evidences that the introduction of hexagonal CSRR is responsible for the higher-order resonant frequency. The feed width wf is augmented in stages of 1 mm from 2 to 4 mm. Feed width of 3 mm is capable of having excellent impedance around the operating bands as presented in Figure 9.

3. Results and Discussion

3.1. Effect of the Metamaterial Structure

HCSRR rectangular printed antenna’s distributions of current at different operating frequencies are presented in Figure 10. It is observed from the figure that the current is evenly distributed over the entire radiating surface. At the frequency of 2.1 and 4.6 GHz, the current is concentrated on the hexagonal CSRR external ring. At 2.6 and 5.6 GHz, the current is concentrated more on the innermost ring. With the introduction of metamaterial structure, the current path is altered and enables the multiband characteristics of the proposed antenna.

3.2. Analysis of FR4 Substrate Printed Antenna

In Figure 11, the 3D radiation pattern, along with the E plane and H plane, is presented. At all frequencies except 6.1 GHz, the E plane radiation has a dipole radiation pattern. The proposed antenna H plane has an omni direction radiation pattern. At 6.1 GHz, the E plane has an end-fire pattern. It is observed from the three-dimensional radiation pattern that the exhaustive radiation is upright to the antenna.

In Figure 12, the HCSRR rectangular printed antenna’s 11 characteristics are depicted, from which we can observe that the HCSRR rectangular printed antenna is operating at different bands. The VSWR of the proposed antenna at the operating frequency is less than 2.

In Figure 13, the directivity of the GHz antenna is presented. The maximum directivity is about 8 dB, and the directivity is above 5.4 dB in the entire operating region. In Table 3, the simulated result of the proposed metamaterial GHz antenna being tabulated.

3.3. Proof for Metamaterial Property

The designed CSRR is placed in a waveguide setup, as shown in Figure 14. An appropriate boundary condition is specified for the extraction of the transmission and reflection coefficient. Through the input port, the EM wave is used to excite the CSRR, and at the output port, the s parameter coefficients are retrieved.

In Figure 15, the extracted S parameter is presented. From the figure, the reflection and transmission coefficient of the proposed antenna are presented. In Figure 16, the proposed antenna permittivity and permeability characteristics are presented. It is observed that the CSRR acts as an inductor; CSRR can have a permittivity value greater than 1 at the bands created by it. In Table 4, the proposed antenna is compared with the antennas in the literature.

4. Conclusion

A rectangular hexagonal CSRR-inspired monopole antenna is presented. The antenna is designed on the substrate called FR4. The choice of the substrate is based on the material analysis and its effect on the electromagnetic properties. Here, materials such as Arlon AD 250 C, Arlon AD 255C, Arlon AD 320 A, Roger 3010, Roger RT5880, Roger RT6002, and FR4 are analyzed as substrates. Material analysis results are presented in Section 2, from which it is found that the proposed structure with FR4 as the substrate material is capable of achieving multiband functionality with reasonable gain and directivity. Metamaterial hexagonal complementary split-ring resonator is the reason for the multiband operation that can be validated with the help of the parametric analysis and surface current distribution. Five different operating frequencies, stable radiation patterns, and compactness make the proposed structure useful for the GHz application, such as material sensing, material characterizing, GHz, microwave, and communication applications.

Data Availability

The data used to support the findings of this study are included within the article. Further data or information can obtained from the corresponding author upon request.

Conflicts of Interest

The authors declare that there are no conflicts of interest regarding the publication of this paper.

Authors’ Contributions

The authors confirm their contribution to the paper as follows: Dr. B. Thiyaneswaran, B. Aruna Devi, and Dhanagopal conceptualized and designed the study;S Palanivelrajan and Suresh Kumar Muthuvel collected data; S Priyadharshini, Samson Alemayehu Mamo, and B. Aruna Devi analyzed and interpreted the results; Dr. B. Thiyaneswaran, B. Aruna Devi, and S Palanivelrajan drafted and prepared the manuscript; Samson Alemayehu Mamo and B. Aruna Devi validated the study. All authors reviewed the results and approved the final version of the manuscript.