Compact Ultrawide Band Metamaterial-Inspired Split Ring Resonator Structure Loaded Band Notched Antenna

A novel compact size ultrawide band planar antenna with band notched characteristics is present. (e band rejection characteristic is achieved by loading a pair of metamaterial inspired rectangular split ring resonator (SRR) near the feed line and by etching the SRR slots on a radiating patch. (e simulated and measured results reveal that the proposed antenna exhibits the impedance bandwidth over the ultrawide band (UWB) frequency range from 3.1 to 14GHz with the voltage standing wave ratio less than 2 except for band stop bands at 3.29 to 3.7GHz (WiMAX band), 3.7 to 4.10GHz (C-band), 5.1 to 5.9GHz (WLAN band), and 7.06 to 7.76GHz (downlink X-band satellite communication), respectively. (e proposed antenna fabricated on low-cost FR4 substrate has compact size of 24× 20×1.6mm. (e simulation results are compared with measured results and demonstrate good agreement with stable gain over pass bands. (e proposed antenna also exhibits dipole-like radiation pattern in E-plane and omni-directional pattern in H-plane. (ese results led to conclusion that the presented antenna is a suitable candidate for ultrawide band UWB applications with desired band notch characteristics.


Introduction
Since the allocation of the frequency range for unlicensed radio application by FCC (Federal Communications Commission) having the bandwidth of 7.5 GHz (3.1-10.6 GHz), ultrawide band UWB technology has attracted much attention in communication system. is technology is growing rapidly day by day with advancement in the variety of applications such as radio sensing, body area networking, and short-range broadband communication [1]. e patch containing radiator antennas is widely used in order to implement ultrawide band communications which have been analyzed by using various feeding techniques viz. microstrip line feed, coplanar waveguide feed, coaxial feed, etc. [2][3][4][5]. However, it is always a challenge to design ultrawide band antenna along with stable gain, compact size, and filtering characteristics which can eliminate the coexisting narrow band communication such as WiMAX band (3.3-3.7 GHz), C-band (3.7-4.2 GHz), WLAN band , and X-band satellite communication (7.1-7.9 GHz) [6][7][8]. Metamaterials are the artificial engineered magnetic materials which can be used to obtain and improve the band notching characteristics in ultrawide band antennas.
e MTMs are made up of subwavelength structures which help in attaining negative permittivity and permeability in the structures at certain resonant frequencies. Split ring resonators (SRR) and complementary split ring resonators (CSRR) are examples of mostly utilized MTM structures for achieving band notching in ultrawide band antennas [9,10]. ese structures are capable of rejecting the interfering narrow band frequencies without degrading the other performance characteristics antenna. Coupling between the two ring resonator structures having different resonant frequencies is done to achieve new resonant frequency and obtain desired results.
Nowadays, metamaterial-inspired resonator viz. SRR and CSRR [27] structures are commonly used to accomplish the filtration of interfering bands. Split ring resonator (SRR), sometimes also called Negative refractive index material (NRIM), has capability to control and channelize electromagnetic waves [28,29] which was presented in [30]. e SRR structure can also be used as a slot for rejecting bands as carried out in [31,32]. Understand the further concepts on optimization [33][34][35][36]. e complementary split ring resonator (CSRR) which is negative reflection of the split ring resonator (SRR) can be used as periodic structure of metamaterial and has attracted much attention of various researchers to be used as frequency selective structure [37][38][39][40][41][42][43].
In this paper, a novel miniaturized ultrawide band UWB antenna with unwanted band rejection application is proposed. Two circular shape stub and rectangular stub is introduced on the ground plane to achieve the large impedance bandwidth from 3 GHz to 14 GHz. e rejection of worldwide interoperability for microwave access WiMAX band (3.3 to 3.7 GHz) and C-band (3.7 to 4.2 GHz) is acquired by etching the metamaterial-inspired split ring resonator (SRR) slot on the radiator. e wireless local area network WLAN band (5.1 to 5.8 GHz) is achieved by embedding the metamaterial resonator (RSRR) near the microstrip feed line, and the downlink of X-band satellite communication frequency is rejected by employing single CSRR slot on the radiator patch. e performances of designed antenna are studied by using HFSS, and simulated results are compared with the measured results.
e presented paper is arranged in the following manner. Section 2 presents the configuration and design of proposed antenna. Section 3 presents and analyzes the simulated and measured results and further the experimental results of the presented antenna are compared with existing antennas, and Section 4 concludes the paper.

Configuration and Design of Band Notched
UWB Antenna e schematic of proposed band notched UWB antenna is shown in Figure 1. e prototype is designed and augmented using high-frequency simulation software HFSS 15 and is represented in Figures 1(a) and 1(b). e proposed antenna consists of the rectangular patch with two elliptical stubs at two lower corner of the patch and two circular stubs at the upper edge of the ground plane and one rectangular cut at the centre of the ground plane. e suggested antenna has miniaturized size of 24 mm × 20 mm (L s × W s ) and is mounted on low-cost FR-4 substrate having thickness of 1.6 mm, relative dielectric constant of 4.4, and loss tangent (tanδ) of 0.02. e metamaterial unit cell near the feed line and metamaterial-inspired SRR slot on the top of the radiating patch are incorporated in order to attain band notching characteristics. e detailed dimensions of proposed metamaterial-inspired SRR loaded band stop antenna is represented in Table 1. Table 2 represents the detailed geometry of the metamaterial unit cell. e proposed antenna started with the rectangular radiator of size 10 mm × 12 mm (L p × W p ) which is fed by 50 ohm microstrip feed line of size 10 mm × 2.5 mm (L f × W f ). e ground plane of size 8 mm × 20 mm (L g × W g ) is printed on the back side of the substrate. It can be seen that, by inclusion of the elliptical shaped notches at the lower corner of the radiator patch and circular notches at upper edges of the ground plane, the enhanced impedance bandwidth of the suggested antenna is achieved due to electromagnetic coupling between the rectangular radiator and the partial ground plane. e two band notches at WiMAX (3.29-3.7 GHz) and C-band (3.7-4.1 GHz) are achieved by etching the metamaterial-inspired split ring resonator SRR slot on the top of the radiating patch. e next frequency notch at the WLAN band (5.1-5.9 GHz) is obtained by embedding a pair of rectangular metamaterial-inspired SRR near the feed line of the antenna.
e downlink X-band (7.06-7.76 GHz) is rejected by employing the split ring circular slot at the centre of the radiator. e fabricated prototype is represented in Figure 2. We further used the methods to optimise the technique [44,45]. e length of etched slots should be one half of guided wavelength at desired notched band frequency [46]. e length of each slot can be premeditated using equations (1) and (2) as follows: Figures 3 and 4 illustrate the reflection coefficient and voltage standing wave ratio (VSWR) results for the evolution of the proposed antenna. It can be seen from figures that the proposed antenna exhibits the band notching at WiMAX band, C band, WLAN band, and downlink X-band by employing the metamaterial-inspired SRR structures and slots on the radiating patch. e characterisation of the metamaterial unit cell is represented in Figures 5 and 6. e scattering parameters such as reflection coefficient (S 11 ) and transmission coefficient (S 21 ) of the SRR structure are achieved by exciting the structure with electromagnetic waves. e simulated result shows that the metamaterial unit cell exhibits the stop band characteristics at 5.5 GHz. e impedance (Z) versus frequency graph depicts the stop band characteristics of the unit cell. At band notched frequency, the value of input resistance and input reactance are not around nominal values, i.e., 50 ohms and zero ohm. e negative permittivity and permeability of the metamaterial-inspired SRR unit cell are extracted by calculating the values of refractive index and wave impedance from the S-parameters. Figure 7 represents the extracted negative permittivity and negative permeability of the metamaterial-inspired SRR unit cell.

Results and Discussion
In this section, the simulated result of the proposed antenna is discussed and the proposed antenna is optimized by analysing the simulated results of voltage standing wave ratio, gain of antenna, radiation pattern, and radiation efficiency, respectively. e parametric study is carried out for optimization of performances of the proposed band notched antenna. e proposed band notched antenna is analyzed by using high-frequency simulation software (HFSS). During the parametric study, the effect of variation of dimensions of the rectangular split ring resonator (SRR) structure, SRR slot, and circular shaped split ring slot is investigated. Figure 8 illustrates the optimization of gap g 1 and g 2 of the metamaterial-inspired SRR structure on the basis of voltage standing wave ratio (VSWR). e dimension of gap g 1 and g 2 is varied from 1.12 mm to 1.24 mm for optimization of WLAN band rejection frequency. It can be observed that rejected WLAN band (5.1-5.9 GHz) exhibits the peak VSWR value of 6.04 at frequency 5.5 GHz. e increase in the gap value g 1 and g 2 of the metamaterial-inspired SRR structure leads to small decrease in the total inductance and the gap capacitance of the resonator structure, which helps in achieving the desired rejected frequency of the WLAN    band. Figure 8 depicts the gap variation of the split ring resonator metamaterial slot for optimization of WiMAX band and C-band notch characteristics. e value of g 3 is increased from 0.5 mm to 2 mm with step size of 0.5 mm for achieving the WiMAX band and C-band notching. It has been observed from Figure 9 that when the value of gap is taken equal to 0.5 mm, the rejected band exhibits peak VSWR 5.11 at frequency 3.3 GHz. When the gap value is increased in step size of 0.5 mm, rejected band exhibits the peak VSWR 5.08 at 3.5 GHz, 4.94 at 3.5 GHz, and 5.07 at 3.6 GHz. It is noticed that the rejected band does exhibit required band notching frequency when the gap value g 3 is taken as 0.5 mm, 1.5 mm, and 2 mm, and it can be concluded that, at g 3 � 1 mm, WiMAX and C-band rejection is stronger and hence accomplished. Figure 10 describes the VSWR variation of gap g 4 for optimization of downlink X-band satellite communication rejection characteristics. e value of gap g 4 is varied from 2.16 mm to 2.2 mm with the increase in the value having step size of 0.2 mm. It can be seen from Figure 9 that the rejected band exhibits peak VSWR 3.96 when the gap value is taken as g 4 � 2.2 mm and the circular shaped split ring is able to achieve desired X-band notching. e effective inductance of the circular-shaped split ring decreases when its length decreases, i.e., the gap size g 4 increases, which helps in attaining the higher resonance frequency. e simulated radiation patterns are also obtained in E-plane (phi � 0°) and H-plane (phi � 90°) (principal planes) at selected frequencies 3.18 GHz, 4.74 GHz, 6.38 GHz, and 8.8 GHz, and these results are represented in Figures 11-14, respectively. e simulated result shows that the proposed antenna exhibits dipole-like radiation pattern in E-plane and omni-directional radiation pattern in H-plane. e small variation at higher frequency is due to cross polarisation and radiation pattern is distorted, whereas in H-plane the radiation pattern is perfectly omni-directional at all frequencies of pass bands. e gain versus frequency variation for the proposed band notched antenna is illustrated in Figure 15. It can be seen from Figure 15 that there is a sharp decrease in gain at notched frequencies with the negative value of gain −8.45 dB at 3.4 GHz, −13.04 dB at 5.5 GHz, and −3.42 dB at 7.6 GHz along the peak gain value of 3.7 dB with stable gain over the pass band frequencies. Figure 16 shows the radiation efficiency of the suggested antenna which is nearly 85-90% over pass bands and at the notched bands; radiation efficiency is 34%, 22%, and 48% at 3.42 GHz, 5.5 GHz, and 7.58 GHz, respectively.
Furthermore, the comparison of proposed prototype planar band notch antenna with few other existing antennas is done and represented in Table 3. From this Table 3, we can conclude that the proposed UWB band notched antenna loaded with metamaterial-inspired SRR and slots is compact in dimensions and novel and functional for ultrawide band applications with rejected band characteristics at WiMAX, C-band, WLAN band, and downlink X-band satellite communication. e present work also exhibits a good gain of 3.7 dB compared to other existing antennas.

Conclusions
A compact size metamaterial-inspired split ring resonator loaded antenna with band stop characteristics is proposed. e proposed antenna is able to provide a wide bandwidth from 3.1 GHz to 14 GHz with frequency rejection characteristics. By etching split ring resonator slot on top of patch, two band notches, one at WiMAX band and other at C-band, are achieved. e WLAN band stop characteristic is accomplished by entrenching the metamaterial-inspired SRR structure near the feed line of the suggested antenna. e next notching of downlink of X-band satellite communication is achieved by utilizing the split ring circular-shaped slot at the centre of the radiating patch. It is observed from the parametric study that the band notching at different frequencies can be tuned and controlled independently by adjusting the dimensions of SRR structure and slots. e presented antenna is compact and also excellent to suppress the interference of existing bands (WiMAX, C-band, WLAN-band, and X-band satellite communication signal) from the UWB pass band. e proposed antenna exhibits negative gain at notch frequencies with a peak gain of 3.7 dB. e presented work also exhibits the good radiation efficiency of nearly 85-90% over pass bands and 34%, 22%, and 48% at 3.42 GHz, 5.5 GHz, and 7.58 GHz, respectively. e simulated results show good agreement with the measured result. e results of the proposed antenna show that it is a suitable candidate for wireless applications.

Data Availability
e data used to support the findings of the study are available from the corresponding author upon request.