Gain Enhancement of Microstrip Patch Antenna Using Fractal Metamaterial Unit Cell
Parviz Hajizadeh
Electrical and Electronic Eng. Department Shahed University
Tehran, Iran [email protected]
H. R. Hassani
Electrical and Electronic Eng. Department Shahed University
Tehran, Iran [email protected] Abstract— A single-layer printed fractal metamaterial
(MTM) unit cell for gain enhancement of microstrip patch antennas is proposed. The proposed MTM unit cell behaves as a Zero-Refractive Index Metamaterial ZIM over the frequency band of 13.4-14.3 GHz. For considering effect of the proposed unit cell on gain of microstrip patch antennas, a rectangular patch antenna is designed at 13.5 GHz. The proposed MTM unit cell is placed over the rectangular patch antenna with an air spacer. To have the highest gain and return loss of the patch antenna the thickness of the air spacer is set at 10.5 mm. The microstrip rectangular patch antenna with the proposed unit cell is simulated by commercially available CST software. The proposed single layer MTM unit cell can improve the gain of microstrip patch antenna from 7.67 dB to 9.1 dB.
Index Terms— Metamaterial; Zero-Refractive Index Metamaterial ZIM; microstrip patch antenna; gain enhancement
I. INTRODUCTION
Microstrip patch antennas are one of the most attractive antennas in many communication systems due to their light weight, low cost and ease of fabrication. One of the most important limitations of such antennas is their low gain. An ordinary microstrip patch antenna has a gain of order 6 dB.
As the miniaturization process of electronic and electrical components continuous, the need for compacting antennas particularly microstrip antennas are felt. However, compacting of antennas is at the expense of reduced gain by a few dBs.
Several different methods for gain enhancement of microstrip patch antennas have been proposed in the literature.
Covering the microstrip patch antennas with a dielectric superstrate [1], using electromagnetic band gap structures within dielectric substrate and putting a dielectric parabola structure as a lens in front of the surface of the substrate [2]
and arraying of such antennas are the most usual methods that can improve the gain of patch antennas. Such techniques are bulky structures and are difficult to fabricate.
Recently, metamaterials are suggested for gain enhancement of the antennas. For gain enhancement of microstrip patch antennas using metamaterials, a two layer wire grid of rectangular loops is proposed in [3] as a metamaterial lens. In reference [4], a horn antenna filled with a metamaterial structure that is composed of three layers of metal grid acting as a lens on the inner aperture of the horn is
presented. Split ring resonators (SRR) are the other type of metamaterials that are used as lens [5]. Metamaterials are also used as a substrate. In reference [6], a dipole antenna is embedded in metamaterial substrate that is composed of a periodic collection of rods, or of both rods and rings. A dipole antenna is placed within a grid structure in [7]. A two layer printed metamaterial cover is presented for gain enhancement of microstrip patch antenna in [8]. A patch antenna is loaded with a split H-shaped structure as metamaterial lens in [9].
Almost, all of these previously published papers for gain enhancement of antennas are mostly bulky and multi-layer structures.
In this paper, a new simple single-layer printed fractal metamaterial unit cell for gain enhancement of microstrip patch antenna is proposed. The proposed MTM unit cell behaves as a Zero-Refractive Index Metamaterial ZIM over the frequency band of 13.4GHz to 14.3 GHz. The original idea is inspired from metal grids with a square lattice reported in [10]. Then, a probe fed rectangular patch antenna designed and simulated by commercially available MWS CST software.
The rectangular patch antenna resonates at 13.5 GHz and has a gain of 7.67 dB. In the next stage, we placed the proposed fractal metamaterial unit cell above designed patch antenna with an air spacer. The thickness of air spacer is optimized at 10.5 mm with respect to highest gain and good impedance matching. The simulation of patch antenna with the proposed MTM unit cell demonstrates a gain improvement up to 9.1 dB.
II. ZERO-REFRACTIVE INDEX METAMATERIAL UNIT CELL DESIGN
The electromagnetic waves from a microstrip patch antenna are radiated obliquely from the surface of the patch. For gain enhancement of such antennas towards the broadside direction, a method has to be implemented that can direct the radiated electromagnetic waves in the broadside direction.
Zero-refractive index metamaterials are artificial structures that can realize this fact. These metamaterial structures with respect to Snell’s law can improve the gain of patch antennas.
If two medias lie besides each other as shown in Fig. 1, the Snell’s low is defined as:
Figure 1. Propagation of an electromagnetic wave in sin
sin
Figure 2. The geometry of proposed fractal metam Now if n1 becomes zero, θ2 will b consequently the emerging wave vector in m along the normal to the substrate plane.
One of the popular ways for realization index metamaterial is use of multilayer met been reported in the literature. At the first s of ZIM unit cell, two square thin metal rin both sides of a dielectric substrate. The effe each square ring is chosen λg (guided wav dielectric substrate). Then for more compa area, the space filling Hilbert fractal is used.
The proposed metamaterial unit cell is sho composed of third order Hilbert fractal meta both sides of RO5880TM substrate with hei and εr = 2.2. As a low value of 0.3 mm is ch of the strip, the highest order for the Hilbe can be used would be third.
The final dimension on the substrate of optimization process is 4.93×8.13 mm2, a parameters of the structure are: w1=0.2, w w4=4.93, w5=18, w6=8, l1=0.4, l2=8.13,
two different medias.
(1)
material unit cell.
become zero and media II will direct
of zero-refractive tal grids that have stage of designing ngs are printed on ective perimeter of ve number in the acting of substrate own in Fig. 2. It is al rings printed on ight of 0.787 mm hosen for the width rt fractal ring that the unit cell after and the optimized w2=0.15, w3=0.4, l3=6.5, l4=16.5,
Figure 3. The (a) magnitude and (b) p Transmission coefficient S21of the p
versus fr l5=10.5 (all in mm). The size o less than those reported in the l
The phase and magnitude o and transmission coefficient S2 unit cell are shown in Fig. 3.
As explained in [11] throug can obtain the relevant parame effective permittivity, effectiv impedance and the refractive in shown in Fig. 3.
As one can see from Fig. 4 the proposed metamaterial unit GHz. This frequency is the pl unit cell. Also, the permeabili refractive index is almost zero From Fig. 7 one can also notic takes place over a wide frequen I. CONFIGURATION OF M
WITH METAMAT A probe fed rectangular pat 6.5×8 mm2 simulated by comm
(a)
(b)
phase of Reflection coefficient S11 and proposed fractal metamaterial unit cell frequency.
of the unit cell proposed here is literature.
of the reflection coefficient S11 21 of the proposed metamaterial gh the S11 and S21 results one eters of the unit cell, such as the ve permeability , characteristic
ndex. The calculated results are 4 and Fig. 5, the permittivity of t cell becomes zero around 13.4 asma frequency of this specific ity is zero at 14.3 GHz and the between these two frequencies.
ce that the zero refractive index ncy range, 13.4-14.3GHz.
MICROSTRIP PATCH ANTENNA TERIAL UNIT CELL
tch antenna with dimensions of mercially available CST software
Figure 4. Calculated permittivity of the proposed m
Figure 5. Calculated permeability of the proposed m
Figure 6. Calculated characteristic impedance of the p unit cell.
is found to resonate at 13.5 GHz. The single a gain of 7.67 dBi. The applied dielec Roger5880TM that has a relative permittiv height of 0.787 mm. The proposed sin metamaterial unit cell is then placed over the antenna with an air spacer. After optimizatio of air spacer with respect to high gain and
etamaterial unit cell.
metamaterial unit cell.
proposed metamaterial
patch antenna has ctric substrate is vity of 2.2 and a ngle layer fractal e rectangular patch on of the thickness d good impedance
Figure 7. Calculated permeability o matching, the respective heigh account the S11 parameter is fo
The configuration of the des with the proposed MTM unit c parameter of single patch ant metamaterial unit cell is shown shown in Fig. 10 in both si metamaterial unit cell.
Figure 8. The configuration of rect fractal metama
Figure 9. S11 parameter of rectangu proposed fractal m
of the proposed metamaterial unit cell.
ht for highest gain taking into ound to be 10.5 mm.
signed microstrip patch antenna cell is shown in Fig. 8. The S11 tenna with and without fractal n in Fig. 9. A gain comparison is
ituations with and without the
tangular patch antenna with proposed aterial unit cell.
ular patch antenna with and without the etamaterial unit cell.
Figure 10. simulated gain of microstrip patch antenna proposed MTM unit cell.
Figure 11. simulated E-plane pattern of the microstrip p without the proposed MTM unit ce
Figure 12. simulated E-plane pattern of the microstrip p without the proposed MTM unit ce It is clearly seen that the gain of the s metamaterial unit cell is around 9.1 dBi wh 1.43 dB as compared to that of the sing Placing metamaterial unit cell over the antenna affects both patch’s current and con impedance matching of the antenna.
For clarifying the gain enhancement of antenna using the proposed MTM unit ce patterns are shown in Fig. 11 and Fig. 12.
with and without the
patch antenna with and ell.
patch antenna with and ell.
structure with the hich is higher than gle patch antenna.
microstrip patch nsequently detunes f microstrip patch
ll, the E- and H-
II. CO A compact single layer fract a zero refractive index over th GHz has been proposed. Whe placed above a rectangular pa between, the structure can pro least 1.43 dBi as compared to t
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ONCLUSION
tal metamaterial unit cell having he frequency band of 13.4-14.3 en this single layer unit cell is atch antenna with an air gap in ovide a gain enhancement of at that of a single patch antenna.
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