# A Miniaturized Dual-Band Short-Ended ZOR Antenna with Backed Ground Plane for Improved Bandwidth and Radiation Efficiency

Authors: Rajkishor Kumar; Avinash Chandra; Sreenath Reddy Thummaluru; Mohammad Monirujjaman Khan; Raghvendra Kumar Chaudhary
Journal: International Journal of Antennas and Propagation
 (2023)

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## Abstract

This paper presents a miniaturized planar dual-band short-ended metamaterial antenna with the backed ground plane to improve antenna bandwidths and radiation characteristics. The proposed dual-band metamaterial (MTM) antenna has been made up of the composite right- or left-handed transmission line (CRLH-TL) concept. Here, the backed ground plane has been employed to generate an extra coupling capacitance (CC), which shifts the ZOR frequency in the lower band while also improving ZOR matching and increasing the impedance bandwidth of the higher-order mode. In this proposed MTM antenna, interdigital capacitance (IDC) has been used in place of a simple series gap, which shifts the higher-order impedance bandwidth into a lower frequency band for second-band Wi-MAX applications (3.3–3.7 GHz). The proposed antenna offers a short-ended MTM, and hence the ZOR frequency is controlled by a series of LC lumped parameters. The proposed antenna offers dual-band behavior with measured −10 dB impedance bandwidths of 5.55% and 41.57% at centered frequencies of 2.70 GHz and 4.33 GHz, respectively. The overall electrical size of the designed antenna is 0.225λ0 × 0.144λ0 × 0.0144λ0 at ZOR (f0 = 2.70 GHz), where λ0 is the free space wavelength; therefore, it is applicable for different Wi-MAX application bands (2.5–2.7 GHz/3.3–3.8 GHz). Furthermore, the proposed dual-band MTM antenna provides compactness, low loss, stable gain, and radiation efficiency, and also offers omnidirectional radiation patterns in the E-plane and dipolar type radiation patterns in the H-plane, respectively.


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## Body

## 1. Introduction

In a metamaterial antenna, the zeroth-order resonance is a special property derived by plotting the dispersion characteristics of composite right- or left-handed transmission lines (CRLH-TL). In the zeroth-order resonant (ZOR) mode, the physical size of the antenna is independent of the resonant frequency [1–8]. Since metamaterial (MTM) is an artificial structure, it has some unnatural properties, such as group and phase velocities being in opposite directions and a nonlinear progressive phase [9, 10]. Several works have been performed based on the ZOR property to miniaturize the size of the antenna, despite the fact that it has a narrow bandwidth, negative gain, and poor radiation efficiency [11, 12]. In recent years, numerous methods have been introduced to improve the impedance bandwidth, gain, and radiation efficiency of metamaterial antenna [13]. Short-ended MTM antenna offers designed flexibility for the antenna community to control ZOR frequency by varying series parameters [14]. Due to the recent requirement of different wireless communication bands in one system, dual or multiband antennas are more demanding to fulfill these requirements. The advantages of dual or multiband antennas are that they reduce the number of antennas and space usage and also overcome interference between other frequency bands [15–17]. Many dual-band compact ZOR antennas have been reported in recent years, but these dual-band MTM antennas have suffered from impedance bandwidth peak gain and radiation efficiency [18–22]. So, there are very big challenges for the antenna community to design such a type of dual-band MTM antenna to remove all these drawbacks.In this context, a compact short-ended CPW-fed ZOR antenna for dual-band applications has been presented. The proposed dual-band MTM antenna improved the ZOR and other higher-order mode impedance bandwidths and radiation efficiencies without affecting its compactness. The designed antenna is basically based on short-ended termination; thus, ZOR frequency depends on LC series parameters and can be controlled by varying series elements of the antenna. Here, an interdigital capacitor is provided, shifting the second resonating frequency towards the lower band. In addition, the backed ground plane improves matching, shifting the frequency on the lower side, and creating wideband nature. The advantages of the proposed antenna are its compactness, low loss, CPW feeding, via-less design, easy fabrication process, low cost, realization of series parameters, and good radiation capacity.


## 2. Antenna Design Techniques

### 2.1. Composite Left- and Right-Handed Transmission Line (CRLH-TL) Theory

Figure1 shows an equivalent circuit diagram of the proposed dual-band MTM antenna. It consists of left-handed shunt inductance LL, right-handed series inductance LR, left-handed series capacitance CL, and right-handed shunt capacitance CR. As per the CRLH-TL theorem, the resonance frequencies have been found using these four CRLH-TL parameters (LL, LR, CL, and CR), and they are independent of the size of the antenna.Figure 1
Equivalent circuit diagram (a) without backed ground plane, and (b) with backed ground plane of the proposed dual-band MTM antenna.
(a)(b)From the Bloch and Floquet theory, the dispersion relation is determined for the unit cells of periodic structures, and it is reported in [9, 10].(1)βCRLHTLω=1pcos−11+ZCRHLω×YCRLHω2,where(2)ZCRLHω=jωLR−1ωCL,YCRLHω=jωCR−1ωLLWithoutbackedgroundplane,YCRLHω=jωCR+ωCc−1ωLLWithbackedgroundplane.Here, the phase constant (β) is related to the Bloch wave, and p shows the unit cell length of the proposed dual-band MTM antenna.The resonance frequency of the designed CRLH-TL based on a dual-band MTM has been found using the given equation(3)βnp=nπpl=nπNn=0,1,2,3,4,…………,N−1,where l (=Np), n, and N are the length of the resonator, the resonance modes, and the unit cell number.The designed dual-band MTM antenna is based on short-ended boundary conditions; therefore, the input impedance (Zin) is found from one side of the resonator towards the other side, whereas the load impedance is zero (ZL = 0). So, the input impedance (Zin) is obtained by [9](4)Zinshort=−jZ0×tanβl∼jZ0×βl=jZCRLH′/YCRLH′ZCRLH′YCRLH′jl=ZCRLH′×l=Z×Np,where Z0 is the characteristic of CRLH-TL, ZCRLH′ and YCRLH′ are the series impedance and shunt admittance per unit periodic length.Whenn = 0 (from (3)), the propagation constant β = 0 shows the ZOR mode, and the resonance frequency is obtained by(5)ωse=ωZOR=1LRCL.From (5), the ZOR frequency has been contained by variation of series capacitance (CL) and inductance (LR) of the proposed dual-band MTM.


### 2.2. Antenna Configuration

The configuration of the proposed dual-band MTM antenna is shown in Figure2. The proposed MTM antenna has been fabricated on an FR4 glass-epoxy substrate (εr = 4.4, tanδ = 0.02) with a compactness of 1.6 mm.Figure 2
Geometry of proposed dual-band metamaterial antenna (L = 25, W = 16, PG = 10, WG = 7.1, WF = 1, GF = 0.4, LF = 13.5, R = 4.5, GR = 0.7, GD = 0.6, WL = 2, D1 = 0.8, D2 = 0.5, LD = 2.2, WB = 11, h = 1.6: all dimensions are in mm).
(a)(b)It consists of a circular patch having an interdigital capacitor, a CPW ground plane, and a partially backed ground plane. The designed MTM antenna is fed by a 50 Ω microstrip line. The interdigital capacitance forms a series capacitor (CL), while the small stripline acts as a shunt inductor (LL), which verifies the composite right- or left-handed transmission line-based MTM. CC is the coupling capacitance between the CPW feed and the partially backed ground plane, whereas CR is the shunt capacitance obtained between the upper ground plane and the CPW feed. The series inductance (LR) is realized by a signal patch. The overall electrical size of a designed dual-band MTM antenna is 0.225λ0 × 0.144λ0 × 0.0144λ0 at ZOR (f0 = 2.70 GHz), where λ0 is the free space wavelength.


### 2.3. Validation of ZOR Mode

To verify the short-ended condition of ZOR frequency, parametric analysis has been done based on the series element, i.e.,LR. Figure 3 shows the input reflection coefficient with different values of patch width called series inductor (LR) with respect to frequency. It is found that ZOR frequency decreases towards the lower frequency band when patch width (WF) increases, which confirms the behavior of ZOR mode. There is another way to verify the ZOR conditions of the proposed antenna by plotting the electric field at ZOR frequency. It is clearly observed from Figure 4 that the distributions of the electric field are showing in phase at 2.72 GHz. Therefore, it verified the ZOR condition occurred at a 2.72 GHz resonance frequency.Figure 3
Simulated reflection coefficient of the proposed dual-band MTM antenna by varyingWF3.Figure 4
Electric field distribution to confirm the ZOR mode at 2.72 GHz.


### 2.4. Effect of Backed Ground Plane

In this subsection, the effect of with and without backed ground plane on the input reflection coefficient has been demonstrated. It is observed from Figure5 that, due to the backed ground plane, one more coupling capacitance (CC) is introduced to shift the ZOR frequency from 3.3 to 2.72 GHz and also improve the higher-order impedance bandwidth of the designed antenna. This coupling capacitance helps to miniaturize the size of the antenna as well as the backed ground plane coupled with the radiator to improve the matching and enhanced impedance bandwidth.Figure 5
Input reflection coefficient of proposed dual-band MTM with and without backed ground plane.


### 2.5. Effect of Series Gap and Interdigital Capacitance

Here, the effect of the series gap and interdigital capacitance on the reflection coefficient has been studied, as shown in Figure6. The spiral-shaped is nothing but interdigital capacitance, which provides more capacitive values compared with the inductive values in order to get good performance at the lower band of the proposed antenna structure. It is clearly observed from Figure 6 that the interdigital capacitance provides a better response compared to series gap capacitance and shifts the impedance bandwidth frequency to the lower side.Figure 6
Input reflection coefficient of proposed dual-band MTM with and without IDC.


## 2.1. Composite Left- and Right-Handed Transmission Line (CRLH-TL) Theory

Figure1 shows an equivalent circuit diagram of the proposed dual-band MTM antenna. It consists of left-handed shunt inductance LL, right-handed series inductance LR, left-handed series capacitance CL, and right-handed shunt capacitance CR. As per the CRLH-TL theorem, the resonance frequencies have been found using these four CRLH-TL parameters (LL, LR, CL, and CR), and they are independent of the size of the antenna.Figure 1
Equivalent circuit diagram (a) without backed ground plane, and (b) with backed ground plane of the proposed dual-band MTM antenna.
(a)(b)From the Bloch and Floquet theory, the dispersion relation is determined for the unit cells of periodic structures, and it is reported in [9, 10].(1)βCRLHTLω=1pcos−11+ZCRHLω×YCRLHω2,where(2)ZCRLHω=jωLR−1ωCL,YCRLHω=jωCR−1ωLLWithoutbackedgroundplane,YCRLHω=jωCR+ωCc−1ωLLWithbackedgroundplane.Here, the phase constant (β) is related to the Bloch wave, and p shows the unit cell length of the proposed dual-band MTM antenna.The resonance frequency of the designed CRLH-TL based on a dual-band MTM has been found using the given equation(3)βnp=nπpl=nπNn=0,1,2,3,4,…………,N−1,where l (=Np), n, and N are the length of the resonator, the resonance modes, and the unit cell number.The designed dual-band MTM antenna is based on short-ended boundary conditions; therefore, the input impedance (Zin) is found from one side of the resonator towards the other side, whereas the load impedance is zero (ZL = 0). So, the input impedance (Zin) is obtained by [9](4)Zinshort=−jZ0×tanβl∼jZ0×βl=jZCRLH′/YCRLH′ZCRLH′YCRLH′jl=ZCRLH′×l=Z×Np,where Z0 is the characteristic of CRLH-TL, ZCRLH′ and YCRLH′ are the series impedance and shunt admittance per unit periodic length.Whenn = 0 (from (3)), the propagation constant β = 0 shows the ZOR mode, and the resonance frequency is obtained by(5)ωse=ωZOR=1LRCL.From (5), the ZOR frequency has been contained by variation of series capacitance (CL) and inductance (LR) of the proposed dual-band MTM.


## 2.2. Antenna Configuration

The configuration of the proposed dual-band MTM antenna is shown in Figure2. The proposed MTM antenna has been fabricated on an FR4 glass-epoxy substrate (εr = 4.4, tanδ = 0.02) with a compactness of 1.6 mm.Figure 2
Geometry of proposed dual-band metamaterial antenna (L = 25, W = 16, PG = 10, WG = 7.1, WF = 1, GF = 0.4, LF = 13.5, R = 4.5, GR = 0.7, GD = 0.6, WL = 2, D1 = 0.8, D2 = 0.5, LD = 2.2, WB = 11, h = 1.6: all dimensions are in mm).
(a)(b)It consists of a circular patch having an interdigital capacitor, a CPW ground plane, and a partially backed ground plane. The designed MTM antenna is fed by a 50 Ω microstrip line. The interdigital capacitance forms a series capacitor (CL), while the small stripline acts as a shunt inductor (LL), which verifies the composite right- or left-handed transmission line-based MTM. CC is the coupling capacitance between the CPW feed and the partially backed ground plane, whereas CR is the shunt capacitance obtained between the upper ground plane and the CPW feed. The series inductance (LR) is realized by a signal patch. The overall electrical size of a designed dual-band MTM antenna is 0.225λ0 × 0.144λ0 × 0.0144λ0 at ZOR (f0 = 2.70 GHz), where λ0 is the free space wavelength.


## 2.3. Validation of ZOR Mode

To verify the short-ended condition of ZOR frequency, parametric analysis has been done based on the series element, i.e.,LR. Figure 3 shows the input reflection coefficient with different values of patch width called series inductor (LR) with respect to frequency. It is found that ZOR frequency decreases towards the lower frequency band when patch width (WF) increases, which confirms the behavior of ZOR mode. There is another way to verify the ZOR conditions of the proposed antenna by plotting the electric field at ZOR frequency. It is clearly observed from Figure 4 that the distributions of the electric field are showing in phase at 2.72 GHz. Therefore, it verified the ZOR condition occurred at a 2.72 GHz resonance frequency.Figure 3
Simulated reflection coefficient of the proposed dual-band MTM antenna by varyingWF3.Figure 4
Electric field distribution to confirm the ZOR mode at 2.72 GHz.


## 2.4. Effect of Backed Ground Plane

In this subsection, the effect of with and without backed ground plane on the input reflection coefficient has been demonstrated. It is observed from Figure5 that, due to the backed ground plane, one more coupling capacitance (CC) is introduced to shift the ZOR frequency from 3.3 to 2.72 GHz and also improve the higher-order impedance bandwidth of the designed antenna. This coupling capacitance helps to miniaturize the size of the antenna as well as the backed ground plane coupled with the radiator to improve the matching and enhanced impedance bandwidth.Figure 5
Input reflection coefficient of proposed dual-band MTM with and without backed ground plane.


## 2.5. Effect of Series Gap and Interdigital Capacitance

Here, the effect of the series gap and interdigital capacitance on the reflection coefficient has been studied, as shown in Figure6. The spiral-shaped is nothing but interdigital capacitance, which provides more capacitive values compared with the inductive values in order to get good performance at the lower band of the proposed antenna structure. It is clearly observed from Figure 6 that the interdigital capacitance provides a better response compared to series gap capacitance and shifts the impedance bandwidth frequency to the lower side.Figure 6
Input reflection coefficient of proposed dual-band MTM with and without IDC.


## 3. Simulated and Measured Results

A prototype antenna has been fabricated to verify the measured and simulated near- and far-field results, as shown in Figure7. The Keysight programmable network analyzer (N5221A) has been used to measure near-field results, i.e., the input reflection coefficient. The simulated dual-band MTM antenna offers −10 dB input impedance bandwidth of 5.14% (2.65–2.79 GHz) and 30.37% (3.63–4.93 GHz) for the first and second bands, respectively, whereas the measured −10 dB input impedance bandwidth (S11 < −10 dB) offers 5.55% (2.63–2.78 GHz) and 41.57% (3.43–5.23 GHz) for the first and second band, respectively, as shown in Figure 8. The simulated and measured resonant frequency of ZOR is found to be 2.72 GHz and 2.70 GHz, respectively, and also higher-order impedance bandwidth increases. This happens due to imperfections in antenna fabrication and tolerance in soldering.Figure 7
Photograph of proposed dual-band MTM antenna: (a) top view, (b) ground plane, and (c) 3D view.
(a)(b)(c)Figure 8
Simulated and measured input reflection coefficients of the proposed dual-band MTM antenna.Figure9 illustrates the normalized radiation patterns of the proposed antenna in the xz- and yz-plane at 2.72 GHz and 4.58 GHz, respectively. It is observed that at ZOR frequency (2.72 GHz) and 4.58 GHz, the proposed antenna shows an omnidirectional radiation pattern in the xz-plane. At frequencies 2.72 GHz and 4.58 GHz, it shows a dipolar type radiation pattern in yz-plane. The difference between copolarization and cross-polarization levels in the xz-plane is −46.53 dB at 2.72 GHz and −40.03 dB at 4.58 GHz.Figure 9
Normalized radiation patterns: (a)xz-plane, 2.72 GHz; (b) yz-plane, 2.72 GHz; (c) xz-plane, 4.58 GHz; (d) yz-plane, 4.58 GHz of proposed dual-band MTM antenna.
(a)(b)(c)(d)The maximum gain of the proposed dual-band MTM antenna is 0.72 dBi for the first band and 2.0 dBi for the second band, respectively, whereas it shows a measured maximum peak gain of 0.59 dBi for the first band and 2.10 dBi for the second band, respectively [23], as shown in Figure 10(a). The designed dual-band MTM antenna shows an average radiation efficiency of 71.52% for the first band and 84.02% for the second band, as shown in Figure 10(b). Table 1 shows the comparison among dual-band MTM antennas with recently published work. It is observed that the advantages of the proposed dual-band MTM antenna are an overall improvement in impedance bandwidth, a high level of miniaturization, and via-less design.Figure 10
Far-field results of proposed dual-band MTM antenna in the broadside direction. (a) Simulated and measured gain; (b) radiation efficiency.
(a)(b)Table 1
Comparison of dual-band MTM antennas with recently published work.
Ref.Antenna sizeResonance frequency (GHz)Via process−10 dB impedance bandwidth (%)Gain (dBi)Radiation efficiency (%)[13]0.27λ0 × 0.23λ02.6/3.6Not needed2.29/28.570.53/1.98NA#/NA#[16]0.33λ0 × 0.098λ02.45/3.37Not needed8.16/23.740.3/1.168/77.3[24]1.05λ0 × 1.05λ06.6/10.9Needed3.03/5.047.29/2.97NA#/NA#[25]1.27λ0 × 1.27λ01.73/5.36Not needed17.34/11.758.23/8.30NA#/NA#[26]0.021λ0 × 0.018λ00.91/2.45Needed17.8/35.8−17.1/−9.81NA#/NA#[27]0.35λ0 × 0.45λ02.58/5.41Not needed20.6/15.73.5/3.53NA#/NA#PW∗0.22λ0 × 0.14λ02.72/4.58Not needed5.50/41.570.72/2.071.52/84.02NA# = not available, PW∗ = proposed work.


## 4. Conclusion

A miniaturized CPW-fed dual-band short-ended ZOR-based antenna with the backed ground plane is investigated. The proposed antenna is short-ended, so the ZOR mode is dependent upon the series LC parameters of the proposed MTM antenna. The designed antenna consists of the backed ground plane to introduce extra capacitance, which is used to shift ZOR frequency towards a lower frequency band and miniaturize the size of the antenna. Here, instead of a series gap, an interdigital capacitance is used in the proposed antenna to lower the higher-order impedance bandwidth. The designed antenna offers dual-band behavior with measured −10 dB impedance bandwidths of 5.55% (centered at 2.70 GHz) and 41.57% (centered at 4.33 GHz) for the first and second bands, respectively. The proposed antenna shows a measured maximum peak gain of 0.59 dBi for the first band and 2.10 dBi for the second band, respectively, whereas it shows an averaged simulated radiation efficiency of 71.52% and 84.02% for the first and second bands, respectively. Furthermore, the proposed MTM offers a high level of compactness and an omnidirectional and dipolar type radiation pattern, which is suitable for Wi-MAX band applications.



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*Source: 2478853-2023-02-09.xml*