Abstract
A novel metamaterial-inspired patch antenna is proposed, wherein a 2-segment SRR Labyrinth metamaterial is embedded inside the antenna substrate. It is observed that upon incorporation, the bandwidth widens to around 600% and VSWR improves by approx. 1.5% and the antenna is miniaturized by 400%. The Nicolson-Ross-Weir (NRW) method has been used to retrieve the material parameters from transmission and reflection coefficients.
1. Introduction
Nowadays, the trend is developing devices for wireless communication systems which have inherent high bandwidth, gain, and multiresonance. Rectangular microstrip patch antenna, i.e., RMPA, is not a good candidate as it has a narrow bandwidth, in sufficient gain and directive properties. Thus, various optimization techniques have been introduced to make patch antenna a successful candidate in wireless communication applications.
With the development of metamaterials came the concept of attaining altered behaviour of electromagnetic radiations, i.e., , , and , thus providing a better solution for attaining enhanced antenna’s performance. “Metamaterials” (MTMs) are engineered to modify the bulk permeability and/or permittivity of the medium [1]. It is realized by placing, periodically, structures that alter the material parameters, with elements of size less than the wavelength of the incoming electromagnetic wave. It results in “meta,” i.e., “altered,” behaviour or behaviour unattainable by natural materials. In this paper, a novel antenna structure is proposed with enhanced performance parameters and miniaturization using metamaterial.
2. Metamaterial Design
The study of microstrip patch antennas has made progress in recent years. Compared with conventional antennas, microstrip patch antennas have more advantages and better prospects. They are lighter in weight and have low volume, low cost, low profile, small dimension, ease of fabrication, and conformity. Moreover, the microstrip patch antennas can provide dual and circular polarizations, dual-frequency operation, frequency agility, broad bandwidth, feed line flexibility, and beam scanning omnidirectional patterning. In literature, the following structures have been discussed to improve the bandwidth of the patch antenna: (1)Defected ground structure(2)Electromagnetic band-gap structures(3)Metamaterials
The basic element of DGS is a resonant gap or slot in the ground metal. It helps in the reduction of surface waves, but the disadvantage of using DGS is reduction in antenna efficiency due to back lobe formation. EBGs are a periodic arrangement of dielectric or metallic elements in one-, two-, or three-dimensional manner. By introducing the structure, the surface waves are reduced as the ground/patch is loaded with inductance and capacitance, but the disadvantage of using EBG is that the area of the antenna is increased. By using metamaterials, the antenna’s bandwidth is enhanced and also multiresonance may be observed by studying its response to the incident EM wave. The behaviour of the metamaterial unit cell can be studied in respect of polarization when the incident electromagnetic wave is perpendicular to it, which is depicted by a resonating curve between its permittivity and permeability with respect to frequency. In negative index materials, phase and energy velocities follow the left hand rule. In order to observe the negative group and phase velocities, a positive index material slab is embedded with negative index material in the middle as shown in Figure 1.
The group velocity follows the right hand rule and propagates towards the right irrespective of the medium as shown in Figure 1. When and both are negative, phase velocity follows the left hand rule but the direction of propagation is reversed. This leads to the formation of “backward wave” in materials with negative refraction. However, if either or is negative, then evanescent waves occur. Many structures have been analyzed in literature [3–10] to establish this theory of left-handedness and composite material concept. This analysis of various 2D and 3D structures is based on several factors like spacing, size, arrangement, shapes, compositions, density, or materials used. In Figure 2, a generic view of metamaterials made with periodic inclusions resulting in an effective bulk permittivity and permeability is shown.
3. Evolution of the Shape of the Proposed Metamaterial
In literature [11, 12], the geometry of the SRR consists of two square shape rings with their slit displaced by 180° or 2π as shown in Figure 3.
(a)
(b)
The split or gap () of SR is 0.8 mm wide, the ring width () is 35 microns, and substrate thickness () is 1.6 mm. The spacing () between the and th turn is 0.4 mm. The SRR is printed on a FR4 substrate having relative permittivity, , as 4.4.
A unit cell is designed with the above dimensions, and the simulation result is shown in Figure 4.
By introducing one split each in respective arms, capacitances in gaps are introduced. As per the equivalent circuit, the more the gaps, the more will be the Cg introduced in parallel, which means reduced effective capacitance. Thus, the resonant frequency increases. Therefore, resonant frequency is calculated to be 2.08 GHz.
If the square ring is single, i.e., only the outer ring is considered with a single split, then the resonant frequency increases. This is because the capacitance reduces as there is no coupling between the rings. The resonant frequency is calculated to be 2.83 GHz.
Therefore, the physical explanation is in coherence with the theoretical explanation of the change in resonant frequency of different configurations of square SRR. The above unit cells are designed, and the simulated result is shown in Figure 5.
Thus, an analysis can be made on the two different shapes of the SRR, i.e., circular and square, with single split and double split each keeping the outer perimeter same. A curve in Figure 6 summarizes the inferences.
The above curve has been proved using physical analysis. Also, the reason for the lower resonant frequency in square shape SRR than in circular SRR can be justified from the current distribution as shown in Figure 7.
The square SRR has greater current density than the circular one. It means more inductance, hence the lower resonant frequency. Thus, different shapes can be derived from these basic two shapes, i.e., square SRR and circular SRR. The advantages of the changing shape of the split-ring resonator from primitive circular shape to square are: (1)Better degree of control: a circular SRR has a radius and width of the ring as the parameters that can be varied to control the constitutive material parameters of the medium. However, using square shape SRR, an extra variable parameter is available. Thus, a better degree of control is achievable using length, breadth, and width of the ring in square SRR.(2)Better candidate as metamaterial: using a square shape SRR, the resonant frequency of the unit cell is reduced. Thus, (), which is the ratio of the unit cell dimension to the wavelength of operation, increases. This makes the metamaterial a better candidate over circular SRR.
The epsilon-negative group (i.e., negative permittivity) of metamaterials, shown in Figure 2(a), provided an appreciable increase in bandwidth, which are analogous to a medium consisting of thin wires arranged in a periodic array [12]. It is also observed that the permittivity varies with frequency of operation of the unit cell as presented in Equation (1): where is the plasma frequency and is a damping term.
In Equation (1), if the plasma frequency is less than the operating frequency, then the permittivity becomes negative.
In this paper, the mu-negative group (i.e., negative permeability) metamaterial shown in Figure 2(b) is explored as a 2-segment labyrinth metamaterial. If the rod and ring materials are combined, a left-handed material [13] is formed having simultaneously negative and . The reason is that the wire strips affect the and the split-ring resonators (SRRs) alter the of the medium. Thus, a frequency-dependent negative material with both the parameters being negative is obtained.
4. 2-Segment SRR Metamaterial
The evolution of square SRR into double-split SRR has been explained in the previous section. The 2-segment labyrinth metamaterial design is as shown in Figure 8. The values of the design parameters are conductivity of 0.034 × 10−3 ῼm, side length of the external ring of 10.2 mm, width of the strips of 1.8 mm, separation between two adjacent strips of 0.4 mm, gap width of 0.8 mm, and thickness of the conducting metallic inclusions of 0.16 μm.
This metamaterial is embedded in the middle of the antenna’s substrate such that the center of patch coincides with the center of the metamaterial. The structure is then simulated using HFSS software. The top view of the 2-segment SRR metamaterial RMPA is shown in Figure 9.
4.1. Mathematical Analysis of the 2-Segment SRR Metamaterial
A 2-segment labyrinth metamaterial discussed in the previous section in Figure 8 is analyzed theoretically, and the results are compared with mathematical analysis using an equivalent circuit in this section. The Nicholson-Ross-Weir (NRW) method [14, 15] is used to retrieve or extract the parameters to observe the permeability region of SRR. The metamaterial in the dielectric substrate is bounded by a box on either side as shown in Figure 10 and has air and radiation boundary on the top and bottom.
Using the NRW equation ((2)), the resonance in permeability is obtained as shown in Figure 6. The variables of Equation (2) are defined, and values of and are derived through the HFSS software. where is a wave number, is the thickness of the substrate, and and are the composite terms to represent the addition and subtraction of scattering parameters and are given by Equation (3):
Using Equations (2) and (3), the resonance in permeability is obtained in Figure 11.
It is observed in Figure 11 that the magnetic resonance is obtained at 4.2 GHz. The resonance in permeability is because a 2-segment SRR MTM has magnetic properties due to internal inductances and capacitances. Thus, the 2-segment labyrinth metamaterial behaves as a mu-negative group (MNG) having negative permeability over some frequency region [2].
The equivalent circuit of the above unit cell can be drawn as shown in Figure 12. It can be made as a combination of parallel RC and series of RL components using transmission line theory. The inductance is taken for unit length of the loop, and capacitance can be taken for the gap in the loop. The series resistance describes the losses in the conductor and the shunt resistance for the losses in the dielectric.
The inductance per unit length of the loop () is given by Equation (4) [16]: where is the vacuum permeability and is the average strip length and is calculated over all the rings using Equation 5: where is the filling ratio of the SRR between the cross-links.
Using Equation (5), is calculated as 36 mm. Therefore, substituting it in Equation (4), inductance is obtained as 3 × 10−6 H.
Similarly, the capacitance of the gap () is given by Equation (6): where is the per-unit-length capacitance between two parallel strips and is given by Equation (7):
is the effective relative permittivity related to the dielectric filling of the substrate and is given by Equation (8): where
is the width of the segment of MTM
is the separation between the 2 segments of MTM
is height of substrate of MTM
is the vacuum permittivity
is the complete elliptic integral of the first kind and is given by Equation (9):
Using the above equations, is 14.15 × 10−7 F.
The resultant impedance of the circuit in Figure 12 is given by Equation (10): where and where is the resonant frequency.
The frequency domain equivalent of is obtained by substituting in place of in Equation (10) and is given by Equation (11):
At resonance, the imaginary part of is zero. So, Equation (11) becomes:
Neglecting , the value of is given by Equation (13):
Substituting the values of and in Equation (13), the resonant frequency is given as 4.08 GHz. Thus, around 1% error is seen between the simulation and the theoretical study of 2-segment SRR.
5. Antenna Design
A rectangular microstrip patch antenna is designed for 4.12 GHz as per the methodology shown in Figure 13 with design parameters as shown in Figure 14; the dielectric constant of the substrate is 4.2, FR4, and the thickness of the substrate is 3.2 mm.
The rectangular patch is etched on one side of the substrate, and the other side of the substrate acts as the ground plane. This whole antenna is excited by a quarter wave-transformed matched microstrip feed as shown in Figure 15.
The patch is simulated using Ansoft HFSS software, and the simulation results of the different performance parameters of antenna are shown in Figure 16.
(a)
(b)
(c)
The above simulation results are tabulated in Table 1.
For this antenna, the return loss is −13.5 with VSWR of 1.49. This shows that the impedance matching of the antenna is within the limits. The bandwidth of the antenna is 0.4 GHz. To improve the performance parameters of the above antenna, the structure of this conventional rectangular patch is modified by embedding a 2-segment labyrinth metamaterial in the middle of the substrate. This metamaterial is embedded in the middle of the antenna’s substrate such that the center of patch coincides with the center of the metamaterial. The structure is then simulated using HFSS software. The top view of the 2-segment SRR metamaterial RMPA is shown in Figure 17.
The study of performance parameters is shown in the simulation of this designed 2-segment SRR metamaterial RMPA shown in Figure 18.
(a)
(b)
(c)
The performance parameters of the MTM RMPA are shown in Table 2.
By comparing Tables 1 and 2, it can be observed that by embedding a 2-segment labyrinth metamaterial in the middle of the substrate of the patch antenna, the bandwidth increases by 6 times, in the GHz range. This lowers the effective permittivity of the antenna’s substrate, which leads to a spreading of the electric field. This in turn leads to broadening of the bandwidth. Improvement in VSWR by 1.5% indicates better impedance matching of the antenna when it is loaded with the metamaterial than conventional patch antenna. However, the trade-off is reduction in gain by 1.5%. This is because the product of gain and bandwidth is unity. So, in this structure, gain decreases due to an increase in bandwidth. It can be seen that the miniaturization of nearly 1/4th of the conventional patch is obtained as secondary resonance is observed at 3.19 GHz.
The proposed antenna operates at the C-band, which is used in satellite applications and is more compact and broadband than the actual antenna that would be designed at C-band frequencies.
6. Experimental Validation of Simulated Results
This rectangular microstrip patch antenna has been fabricated on FR4 substrate. The fabricated antenna is shown in Figure 19.
(a)
(b)
Using the Keysight Fieldfox RF Network Analyzer as shown in Figure 20, the simulation and fabrication results are compared in Figure 21.
Using AMITECH, the gain measurement set-up is as shown in Figure 22.
Using the gain measurement set-up, a radiation pattern is obtained as in Figure 23.
The patch antenna is fabricated by embedding a 2-segment SRR metamaterial in the middle of the substrate as shown in Figure 24.
Using the Keysight Fieldfox RF Network Analyzer as shown in Figure 20, the simulation and fabrication results are compared in Figure 25.
Thus, “fabricated and simulated antennas” help in achieving miniaturization and bandwidth enhancement which is in coherence with the proposed antenna configurations with nearly 5% error. It is normal to have some differences between simulated and measured results because of the following reasons: (a)Fabrication tolerances (e.g., SMA soldering) and measurement error/limitations(b)In simulation, when the substrate is selected, it shows a perfect material; however, variations in thickness & dielectric constant exist in commercially available material (especially the latter which has more influence).(c)In actual measurement, i.e., if measurement is done inside a room/lab, there may be a lot of metallic objects which will highly affect results due to limited surrounding reflection phenomena. The influence could be on a worst side or best side; i.e., depending upon reflected signal, it can be either additive or subtractive.
In literature [17–20], using 2-segment SRRs, the bandwidth of the patch antenna is reduced, but 24% miniaturization is obtained. However, using 2-segment SRR in the middle of the antenna’s substrate, an increase in the bandwidth by 600% and miniaturization of nearly 1/4th, i.e., 25% of the conventional patch, is obtained as secondary resonance is observed at 3.19 GHz. Thus, the proposed configuration is better than the previous configurations in literature. The findings can be tabulated as shown in Table 3.
7. Conclusion
In this paper, a proposed novel microstrip antenna attains miniaturization by nearly 400% and maximization in bandwidth of nearly 600% with metamaterial embedded in the middle of the substrate of the patch antenna. This is a result of negative permeability of the metamaterial which reduces the effective medium below the patch.
Data Availability
The data used to support the findings of this study are available from the corresponding author upon request.
Conflicts of Interest
The authors declare that they have no conflicts of interest.