Gain and Directivity enhancement of microstrip patch antenna using metamaterial superstrate containing rectangular split ring resonators

doi:10.21203/rs.3.rs-1421742/v1

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Gain and Directivity enhancement of microstrip patch antenna using metamaterial superstrate containing rectangular split ring resonators

https://doi.org/10.21203/rs.3.rs-1421742/v1

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In this work a compact high gain, high directivity and large bandwidth microstrip patch antenna incorporated with left-handed material is designed, simulated and fabricated in x band for use in space applications. First a rectangular microstrip patch antenna is designed which operates at 9.97 GHz. Microstrip patch antenna has several disadvantages like low gain, low bandwidth and low directivity. A metamaterial inspired superstrate layer above the microstrip patch antenna is inserted to increase these parameters. The metamaterial inspired superstrate layer consists of array (5x5) of two rectangular split ring resonators on the top of that layer. The split rings resonator works a metamaterial which improves the performances of conventional antenna. The superstrate acts as a lens which convergences all the electromagnetic radiation coming from the patch and increases the radiations. The metamaterial properties are verified. This antenna is design in HFSS.

Conventional antenna

left- handed material

gain

split ring resonator

permeability and permittivity

The left-handed material (LHM) or Negative Refractive-Index (NRI) material is a class of artificial material which is utilized along with normal Right-Handed material (RHM) for improving profile and performance of conventional antennas, like size reduction, higher gain, wider bandwidth and higher directivity etc. Victor Veselago in 1968 introduced the concept of left-handed material which was hypothesized to exhibit a Negative Refractive Index, due to simultaneously negative effective magnetic permeability and electric permittivity [1]. According to him, due to the counter progressive direction of phase (actual wave motion) and group velocity (direction of energy propagation) of wave motion through such media, it can support backward wave propagation. As the basic constitutive properties required for such media is simultaneously negative effective permittivity and permeability, which are not observed in nature directly, these were called metamaterial (beyond normal material). John pendry in 1999 developed a structure of arrays of split ring resonators (SRRs) which had the property of effective negative permeability [2]. His structure exhibited the elusive property due to strong magnetic coupling between the ring structures of SRR. In 2000 D.R smith implemented the first artificial structure of left-handed material [3] as a 3-D prism of SRRs arranged in regular fashion. Periodic structure and strong magnetic coupling of SRR blocks coupled with effective negative permittivity due to plasma like conductor density yielded the desired property of negative refractive index in a band of microwave frequencies. In this paper, a 5 × 5 array of 2 rectangular rings is placed on a layer which is above the conventional antenna. This modified antenna increases the gain from 6.12 dB to 7.18 dB and bandwidth from 1 GHz to 1.01 GHz. The rings exhibit the properties of left-handed material. In Table 3., some references have been mentioned on similar work to show comparative analysis with current work. Some of these designs suffer from low gain, large size, lower bandwidth etc.

2.1. Design of the Conventional Patch Antenna

The top plane of the designed conventional antenna in HFSS is depicted in Figure 1. It consists of ground, substrate and patch respectively. The ground is below the substrate and the patch is on the substrate. The inset cuts on the both sides of the microstrip transmission feedline give the impedance matching between antenna and transmission feedline. The materials of patch as well as ground are copper and the substrate is taken as FR4. The permittivity and the thickness are 4.4 and 1.6 mm respectively. The length and width of the microstrip feedline has a length of 6.5 mm and width 1.

The length and width of patch and substrate are calculated using following Eqs. (1)-(6) as described in [4]. The dimensions are shown in Table 1

$\text{W}=\frac{{\upsilon }_{0} }{2{f}_{r} }\sqrt{\frac{2}{{ϵ}_{r}+1}}$ (1)

$L=\frac{{\upsilon }_{0}}{2{f}_{r}\sqrt{{ϵ}_{reff}} }-2\varDelta L$ (2)

$${ϵ}_{reff}=\frac{{ϵ}_{r}+1}{2}+ \frac{{ϵ}_{r}-1}{2}{\left[1+12\frac{h}{w} \right]}^{-\frac{1}{2}}$$

$$\varDelta L=h0.412\frac{\left({ϵ}_{reff} + 0.3\right) \left(\frac{W}{h} + 0.264\right)}{\left({ϵ}_{reff }- 0.258\right) \left(\frac{W}{h} + 0.8\right)}$$

$${W}_{s}=6h+W$$

$${L}_{s}=6h+L$$

$\text{W}$ is the width of the patch

$\varDelta L$ is extension of length of the patch

$L$ is the length of the patch

${f}_{r}$ is the resonant frequency

υ₀ is the speed of light

$h$ is the thickness of dielectric substrate

${ϵ}_{r}$ is the dielectric constant of substrate

${ϵ}_{reff}$ is the effective dielectric constant

Table 1

Dimensions of microstrip patch antenna
Parameters	Dimensions (In mm)
$\text{W}$	9.15
$L$	6.4
${W}_{s}$	18.75
${L}_{s}$	16

2.2 Design of The Unit Cell

The unit cell is designed with FR4 substrate with relative permittivity 4.4. The length and width of substrate are 2.5 mm and 2.5 mm respectively. The length outermost square is 2.5 mm and the length of inner most square is 1.5 mm as shown in Fig. 2.2. The line width of each ring is 0.2 mm. The gap between two rings is 1.5 mm. The split in each ring is 0.3 mm and they are opposite to each other. The perfect electric plane and the perfect magnetic plane are assigned to Z and Y axes and the excitation is given to X axis as shown in Fig. 2.1. The dimensions of the unit cell are shown in Fig. 2.2. A wire is also place below the substrate for metamaterial properties extractions.

2.3 Extraction of Metamaterial Properties

Figure 2.3 shows transmission and reflection coefficient of unit cell. The unit cell resonates at 9.4 GHz and the S11 and S12 curves intersect each other at 9.9 GHz. The unit cell has permeability negative properties in the range of 9.8 GHZ to 11.6 GHz as shown in Fig. 2.5. The imaginary value of permittivity of unit cell is negative from 9.5 GHz to 1.5 GHz as show in Fig. 2.6. The effective refractive index is also near to zero as shown in Fig. 2.4. All the parameter mentioned blow are calculated using the following Eqs. (7)-(10) as described in [5].

$${n}_{eff}=-j.ln\left\{\frac{{s}_{21}}{1-{s}_{11}\left(\frac{z-1}{z+1}\right)}\right\}\frac{1}{{k}_{0}d}$$

$${z}_{eff}=\pm \sqrt{\frac{{\left(1+{s}_{11}\right)}^{2}-{{s}^{2}}_{21}}{{\left(1-{s}_{11}\right)}^{2}-{{s}^{2}}_{21}}}$$

$${\mu }_{eff}={n}_{eff}.{z}_{eff}$$

$${ϵ}_{eff}=\frac{{n}_{eff}}{{z}_{eff}}$$

${n}_{eff}$ is the effective refractive index

${k}_{0}$ is the number of wave

$d$ is thickness of unit cell

${z}_{eff}$ is the effective impedance

${\mu }_{eff}$ is the effective permeability

${ϵ}_{eff}$ is the effective permittivity

2.4 Design of Metamaterial Antenna

An additional dielectric layer called superstrate consisting of array of rectangular split ring resonator on the top layer is inserted at a height of 16 mm above the conventional patch antenna. The height is varied and the height of 16 mm gives best results and this height is taken for fabrications of proposed antenna. The material of the superstrate is FR4 epoxy dielectric whose relative permittivity is 4.4 with a thickness measuring 1.6 mm. The width and length of the superstrate are 18.75 mm and 16 mm respectively as shown in Fig. 2.7. An array of (5x5) of 2 rectangular split ring is placed on the superstrate. These rings show the metamaterial properties and as the metamaterial superstrate has negative permeability characteristics, it suppresses the unwanted surface wave and thus results into a significant antenna performance.

The conventional and metamaterial antenna are resonating at 9.97 GHz and 9.86 GHz as depicted in Fig. 3.1 and 3.2. The conventional antenna has reflection coefficient of -24.58 dB and metamaterial antenna has − 26.17 dB. The reflection coefficient in metamaterial antenna is improved as compared to the conventional antenna. Reflection coefficient indicates that there is a good matching between impedance of transmission line and the antenna load. The bandwidth of the conventional antenna is 1 GHz i.e., from 9.5 GHz to 10.5 GHz and for metamaterial antenna it is 1.01 GHz which indicates that there is some improvement in bandwidth. Figure 3.2 and Fig. 3.3 show the E and H field at resonating frequency of proposed antenna i.e., 9.86 GHz. The gain in E and H plane is higher in metamaterial antenna as compared to conventional antenna. The 3 dB beamwidth in E and H plane become narrower. The polar plots of gain for antennas without and with LHM are shown in Figure. 3.4 and Fig. 3.5. The gain without and with LHM antenna are 6.12 dB and 7.18 dB and they are along the + Z-axis. The gain is improved by 1.06 dB. The directivity of the convention antenna and metamaterial antenna are 7.27 dB and 8.36 dB as depicted in Fig. 3.6 and Fig. 3.7. The directivity is higher after adding LHM in the conventional antenna. The proposed antenna is suitable for space application purpose. The VSWR for each antenna is shown in Fig. 3.8 and they are 1.12 for conventional and 1.1 for metamaterial antenna. The VSWR of the proposed antenna is less which indicate there is less standing wave in transmission line and a good matching between the antenna and the transmission line. A comparison table of conventional and metamaterial antenna is shown in Table 3

Table 2

The comparison of the results of conventional and metamaterial.
Parameter	Conventional Antenna	Metamaterial Antenna
Resonating Frequency	9.97GHz	9.86 GHz
Return loss	-24.58 dB	-26.17 dB
Gain	6.12 dB	7.18 dB
VSWR	1.12	1.1
Bandwidth	1 GHz	1.01 GHz
Directivity	7.27 dB	8.36 dB

Table 3

Comparison of proposed work with other works
References	Size(mm³)	Operating Frequency (GHz)	Array	Gain(dB)
6	32.5$\times$16$\times$ 1.6	2.4	3$\times$4	1.2
7	117$\times$127$\times$ 1.6	2.4	8$\times$7	4.22
8	60$\times$50$\times$ 1.6	5.26	7$\times$7	6.04
9	19.5$\times$16.6$\times$ 1.56	7.32	3$\times$3	7.32
10	61.25$\times$61.25$\times 1.6$	2.5	5$\times$5	7.92
11	9$\times$55$\times$ 1.6	5.7	9$\times$3	8.3
Proposed antenna	18.75$\times$16$\times$ 1.6	9.86	5$\times$5	7.18

4.1. Conventional antenna

4.2 Metamaterial antenna

Both conventional and metamaterial antenna were fabricated after all the simulation and the measurement for reflection coefficient was taken from vector network analyzer. The prototype model of conventional antenna and the proposed antenna is depicted in Fig. 4.1 and 4.4. The measured results of reflection coefficient for both conventional and metamaterial antenna are shown in Fig. 4.2 and Fig. 4.5. From Fig. 4.3 we can see that the measured reflection coefficient of conventional antenna has some shifting of resonant frequency to right side i.e., higher frequency as compare to simulated results. These shifting occur because there were some manufacturing defects during fabrication process. The resonating frequency of the proposed antenna is 9.86 GHz. The comparison of measured and simulated results of proposed antenna is shown in Fig. 4.6. The reflection coefficient is -10.5 dB at 9.8 GHz as observed from Fig. 4.5. From these measured results it can be observed the measured reflection coefficient is decreased due to fabrication defects and mismatch between feedline and patch of the antenna. The VSWR is also measured for proposed antenna and it is shown in Fig. 4.7. It is found that VSWR of metamaterial antenna is 1.1.

It is observed from the paper that performance of the proposed antenna is improved in terms of gain, bandwidth and directivity in frequency range of satellite applications. The gain of the proposed antenna becomes 7.18 dB from 6.12 dB and directivity increases from 6.12 dB to 7.18 dB. From this work can be concluded that the gain and directivity of conventional antenna is increased after inserting left-handed material. This proposed antenna can be used for space applications.

Acknowledgement

This work is done under the research project entitled “Applicability of Electromagnetic Metamaterials in Antennas and Microwave Circuits for use in Space Applications” funded by Indian Space Research Organization, Dept. of Space, Govt. of India. The authors sincerely acknowledge the support of Indian Space Research Organization (ISRO) and also the National Institute of Technology, Agartala.

Funding: The research is supported by Indian Space Research Organization.

Conflicts of interest/Competing interests: The authors have no conflicts of interest to declare.

Availability of data and material: Data sharing not applicable to this article as no datasets were generated or analysed during the current study.

Code availability: Code sharing not applicable to this article as no code sets were generated or analysed during the current study.

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You are reading this latest preprint version

Gain and Directivity enhancement of microstrip patch antenna using metamaterial superstrate containing rectangular split ring resonators

Status:

Version 1

Abstract

Figures

1. Introduction

2. Antenna Design

2.1. Design of the Conventional Patch Antenna

2.2 Design of The Unit Cell

2.3 Extraction of Metamaterial Properties

2.4 Design of Metamaterial Antenna

3. Simulated Results

4. Fabrication And Measurement

4.1. Conventional antenna

4.2 Metamaterial antenna

5. Conclusion

Declarations

References

Supplementary Files

Status:

Version 1

References	Size(mm³)	Operating Frequency (GHz)	Array	Gain(dB)
6	32.5\(\times\)16\(\times\) 1.6	2.4	3\(\times\)4	1.2
7	117\(\times\)127\(\times\) 1.6	2.4	8\(\times\)7	4.22
8	60\(\times\)50\(\times\) 1.6	5.26	7\(\times\)7	6.04
9	19.5\(\times\)16.6\(\times\) 1.56	7.32	3\(\times\)3	7.32
10	61.25\(\times\)61.25\(\times 1.6\)	2.5	5\(\times\)5	7.92
11	9\(\times\)55\(\times\) 1.6	5.7	9\(\times\)3	8.3
Proposed antenna	18.75\(\times\)16\(\times\) 1.6	9.86	5\(\times\)5	7.18