Metamaterial Inspired Microstrip Antenna Investigations Using Metascreens

A dual layer periodically patterned metamaterial inspired antenna on a low cost FR4 substrate is designed, simulated, fabricated, and tested. The eigenmode dispersion simulations are performed indicating the left handed metamaterial characteristics and are tunable with substrate permittivity.The same metamaterial unit cell structure is utilized to fabricate a metascreen.This metascreen is applied below the proposed metamaterial antenna and next used as superstrate above a simple patch to study the effects on impedance bandwidth, gain, and radiation patterns. The experimental results of these antennas are very good and closely match with the simulations. More importantly, the resonance for the proposed metamaterial antenna with metascreen occurs at the left handed (LH) eigenfrequency of the metamaterial unit cell structure. The measured −10 dB bandwidths are 14.56% and 22.86% for the metamaterial antenna with single and double metascreens, respectively. The metascreens over the simple patch show adjacent dual band response. The first and second bands have measured −10 dB bandwidths of 9.6% and 16.66%. The simulated peak gain and radiation efficiency are 1.83 dBi and 74%, respectively. The radiation patterns are also very good and could be useful in the UWB wireless applications.


Introduction
Patch antennas have been the most demanded candidates for research activity due to their multiple advantages like low cost, low profile, being easy to fabricate, compact size, and conformability with the monolithic microwave integrated circuits (MMIC) [1,2].It is the most widely used component in the wireless industry.
Metamaterials are artificially engineered homogeneous media with controllable electric and magnetic responses to give unusual and useful phenomena [3].These materials are characterized by either dispersion relations or by constitutive electromagnetic parameters.The first formal theoretical studies on media with simultaneous double negative parameters were done by Veselago who predicted some novel phenomena inferred from these metamaterials in his famous paper [4].After long time, Pendry el al. in 1990s demonstrated electric plasma (negative permittivity) by wire structures [5] and then magnetic plasma (negative permeability) by ring structures [6].The first experimental demonstration for double negative structures was performed by Smith et al. [7].Since the practical advent of metamaterials, these have been keenly studied as a potential artificial material for a number of applications in the microwave and optical region [8][9][10][11].One such application is the substrate material for microstrip patch antennas.Researchers have investigated extensively to improve its features like return loss, VSWR, bandwidth, gain, and directivity [12][13][14][15][16][17].
Previously Li et al. [31] demonstrated a patch antenna based on the modified form of the planar metamaterial structure proposed by Matsunaga et al. [43].These structures were proposed to avoid the use of via hole in the unit cell for the ease of fabrication purposes.These structures were designed using left handed (LH) dispersion engineering approach.Recently Xiong et al. [44] also demonstrated a patch antenna based on similar structure with modified ground pattern for enhanced performance.
The aim of our work is to design, simulate, fabricate, and investigate a novel metamaterial antenna realized by the metamaterial unit cell structure proposed by Matsunaga et al. [43].The same metamaterial structure is further utilized to fabricate a metascreen.The size of the metascreen is kept equal to the size of antenna to miniaturize the overall design.The proposed metascreen is used in the near field of the proposed metamaterial antenna and also over a simple patch antenna to investigate its effect on impedance bandwidth, gain, and radiation patterns.

Geometrical Model
The metamaterial unit cell is a dual layer planar copper structure on FR4 substrate for which the dimensions for the top and bottom layers are shown in Figure 1.The unit cell top layer consists of a small square copper patch of (5 × 5) mm with disconnected triangular shaped slit gaps on the edges.The bottom layer consists of similar copper patch with cross slits, having gap width of 0.64 mm.This unit cell is repeated twice in the  plane to generate the pattern of the patch antenna.The dimensions of the bottom and top antenna layers are shown in Figures 2(a) and 2(b), respectively.The substrate thickness is 1.51 mm.
The proposed metamaterial (MTM) antenna is parametrically analyzed in commercial FEM based simulation software HFSS to get the optimum values for different antenna dimensions.The design gives FeedLoc = 0.5 mm, FeedL = 4.2 mm, FeedW = 3 mm,  = 5 mm,  = 10 mm, SubL = 17.2 mm, SubW = 26 mm, SubV = 8 mm, and GndGap = 0.5 mm.Further we designed, simulated, fabricated, and investigated the MTM antenna and the simple patch antenna with metascreens made of the same metamaterial unit cell structure.Figure 3 shows the perspective views for the MTM antenna, MTM antenna with single and double metascreens.The gap between the antenna and the metascreen is also parametrically varied to investigate the effect of SubGap.

Dispersion Plots. Different eigenmode dispersion plots
for the same structure on different substrate materials are shown in Figure 4.The left handed material (LHM) characteristic is indicated by the negative slope of these curves.The permittivity values of the substrates Roggers, FR4, and Duroid are 10.2, 4.4, and 2.2, respectively, giving eigenfrequencies approximately around 6 GHz, 9.1 GHz, and 11.5 GHz, respectively.The curves illustrate that the eigenmode response shifts towards the higher frequency side with the decrease of substrate permittivity.So permittivity variation keeps left handed characteristics of the curve intact but only scales the frequency response to the desired frequency band giving an option for substrate based tunability.All the curves give LH characteristics between points  and .The steepest descent makes a valley close to point .These curve variations suggest the nonhomogeneous and anisotropic nature of the unit cell structure.The LH eigenfrequency band is very narrow confirming the band limited and dispersive nature of the metamaterial structures.The effect of substrate thickness is studied and it does not change the left handed characteristics of the curve.Interchanging the top and bottom structure in the unit cell also does not change the characteristics of the eigenresponse.

Simple Patch Antenna.
A simple patch antenna with dimensions for length  = 5 mm and width  = 10 mm is designed, simulated, fabricated, and tested.The purpose is to compare the performance of simple patch antenna with MTM antenna and MTM antenna with metascreens, all having the same dimensions.This antenna gives resonance at 12.5 GHz with off-centered feed location at FeedLoc = 0.5 mm, as shown in Figure 5(a).This is about 3 GHz away from the eigenresponse.Plots for various FeedLoc are shown in Figure 5(a).The measured −10 dB bandwidth is 8.4%.The simulated peak gain and radiation efficiency are 3.9 dBi and 69%, respectively.The fabricated antenna at 13 GHz has a total size of (0.216 0 × 0.433 0 × 0.065 0 ).

MTM Antenna with Single and Double Metascreens.
When we simulated the proposed MTM antenna having the dimensions (5 mm × 10 mm) with FeedLoc at 0.5 mm, it gave a resonance at 8.85 GHz, as shown in Figure 6(a) (solid line).The response is very weak but very close to the eigenresponse at 9.1 GHz.When we made simulation with a metascreen made of the same structure and size below this metamaterial antenna, it also gave resonance very close to the eigenfrequency but stronger than the simple MTM antenna, as shown in Figure 6(a) (dash line).The weak response is due to the lossy nature of these metamaterials and this fact  is already reported in literature [30].Another factor may be the loss tangent of FR4 material which is high at higher frequencies.
We fabricated the MTM antenna and metascreens.Experiments were performed by using single and then double metascreens below the metamaterial antenna.We got the measured resonance at the LH Eigenfrequency of 9.1 GHz as shown and compared in Figure 7 with simulation results.The overall curve shapes are similar with some shift seen in the resonant frequencies.The reason for this frequency shift is due to the difference in boundary conditions [30].The dispersion plots are obtained by using periodic boundary conditions while antennas use limited unit cells with radiation boundary conditions.Another factor is the permittivity variation of commercial FR4 substrates.It decreases with frequency and this causes the resonance to move towards higher side as depicted in Figure 7 (blue cross line).The comparison shows that the measured results for the single (solid line) and double metascreens (solid square line) under the MTM antenna closely match with each other in the frequency range from 8 GHz to 9.5 GHz.However there is another resonance that appeared near 7.6 GHz in the measured results for double metascreen which does not appear in the measured results for the single metascreen.The simulated results for the double metascreen are not shown since they follow the results for single metascreen case.The measured −10 dB bandwidths at 9.1 GHz are 14.56% (simulated 22%) and 22.86% for the MTM antenna with single and double metascreens, respectively.The simulated peak gain and radiation efficiency are 0.7 dBi and 90% for single metascreen case.The fabricated MTM antenna with single metascreen at 9.1 GHz has a total size of (0.152 0 × 0.303 0 × 0.106 0 ).   the eigenfrequency of the MTM unit cell, that is, 9.1 GHz.Nevertheless, these metascreens can be used to broaden the impedance bandwidths around the normal resonances of the simple patch antennas.The simulated peak gain and radiation efficiency are 1.83 dBi and 74%, respectively, at 13.95 GHz.The fabricated antenna with single metascreen at 13 GHz has a total size of (0.216 0 × 0.433 0 × 0.13 0 ).

Conclusion
In conclusion a dual layer metamaterial inspired microstrip antenna on a low cost FR4 substrate is designed, simulated, fabricated, and tested.The metamaterial structure is dispersion engineered and its eigenfrequency is around 9.1 GHz for the FR4 material.This eigenfrequency can be tuned to higher frequencies by using other substrate materials with lower permittivity values.This structure is utilized to fabricate a metascreen.This metascreen is applied under the proposed MTM antenna and also over a simple patch antenna to study their performance parameters such as impedance bandwidths, gains, and radiation patterns.The measured −10 dB bandwidths at 9.1 GHz are 14.56% (simulated 22%) and 22.86% for the MTM antenna with single and double metascreens, respectively.The simulated peak gain and radiation efficiency are 0.7 dBi and 90% for single metascreen case.The −10 dB bandwidth for the simple patch with inverted metascreen is measured as 9.6% and 16.66% (21% simulations) for dual band operation.The simulated peak gain and radiation efficiency are 1.83 dBi and 74%, respectively.The radiation patterns are also very good and could be useful for the UWB wireless applications.

FrequencyFigure 4 :
Figure 4: (a) Eigenfrequency dispersion plots for the metamaterial unit cell and (b) Brillouin zone showing the phase traversal path for dispersion plots in the -plane.

Figure 5 :
Figure 5: (a) Measured and simulated return loss for simple patch and (b) photograph for the fabricated simple patch (5 mm × 10 mm); FeedLoc = 1 mm.

Figure 6 :
Figure 6: (a) Response for simulated MTM antenna and MTM antenna with single metascreen SubGap = 1.5 mm.(b) Fabricated MTM antenna, metascreen top and bottom layers.(c) Side view showing the adhesive foam tape of 1.5 mm between MTM antenna and metascreen.

Figure 7 :
Figure 7: Return loss for metamaterial antenna with single and double metascreens.

3. 4 .Figure 8 :0Figure 9 :
Figure 8: Perspective views for (a) simple patch antenna, (b) metascreen over simple patch antenna, (c) photograph for the fabricated simple patch antenna with metascreen bottom and top layers, and (d) side view.

3. 5 .
Radiation Patterns.The two-dimensional radiation patterns for the MTM antenna, MTM antenna with single metascreen, and simple patch with inverted metascreen are plotted inFigures 10,11,and 12, respectively.The simple MTM antenna radiates at 8.85 GHz which is mostly along the bore side and back side in the  plane while it is from 0 to −180 degrees in the  plane.The single metascreen under the MTM antenna also radiates along the bore side and back side in the  and  planes.The radiation pattern is also broadband as can be seen from response of two frequencies within the band.The radiation pattern for the metascreen over the simple patch is directional with maximum from −30 to −120 degrees in the  plane and 0 to −60 degrees in the  plane.