2014 IEEE Students’ Conference on Electrical, Electronics and Computer Science

Performance Enhancement of Coaxial FeedMicrostrip Patch Antenna UsingLeft-Handed Metamaterial Cover

Pradeep Paswan, Vivekanand Mishra, P. N. Patel, Surabhi DwivediElectronics Engineering Department, S. V. National Institute of Technology, Surat-395007, Gujarat, India

paswan_pradeep@yahoo.co.in, vive@eced.svnit.ac.in, pnp@eced.svnit.ac.in, sur_2310@yahoo.co.in

Abstract—The conventional microstrip patch an-tenna (MSA) have low gain and low directivity. This pa-per presents coaxial feed rectangular microstrip patchantenna along with left-handed metamaterial (LHM)cover. The Proposed metamaterial cover increases thegain and directivity of the antenna in comparison toconventional microstrip patch antenna alone. The an-tenna has been designed for 8-10 GHz, hence it canbe used for X-band application. S-Parameters (S11 andS22) are used for verifying the double-negative proper-ties of the proposed metamaterial cover. The proposedantenna is simulated by using Ansoft HFSS.

Keywords—Microstrip patch antenna, Coaxial feed-ing, metamaterial, LHM cover, Directivity.

I. INTRODUCTIONThe microstrip patch antenna is widely used in wireless

communication equipment because of its numerous advan-tages such as low profile, low weight and low cost etc.,but it suffer from low gain, low directivity and narrowbandwidth [1]. To overcome this problem, in this paperwe have presented the metamaterial cover over patch atheight 10.1 mm which improves the gain and directivity ofantenna [2]. Metamaterials are artificial materials, whichhas simultaneously negative permittivity (ε) and perme-ability (µ) over some frequency range [3]. The conceptof metamaterial were proposed theoretically by Veselago(1968) [3] but it was realized experimentally by D. R.Smith et al. in 2000 [4]. Metamaterial has unusual electro-magnetic properties, which is usually not present in nature[5]. Because of some interesting properties of metamaterial,it has several names like left-handed metamaterial (LHM),double negative materials (DNG) and negative index ma-terials (NIM) [6]. The refractive index by using Maxwellequation is given by n=±√εµ. As metamaterials has bothε and µ negative, n takes negative root hence refractiveindex of LHM is negative over some frequency range. Thisnegative refractive index properties of metamaterial is usedto realize a focusing flat lens [7].The left-handed metamaterial as cover over patch antennaact as lens when it is illuminated by the electromagnetic(EM) fields radiated from the patch antenna, which issimilar to the focusing of convex lens on the propagationof light waves [8][9]. Many papers have been publishedbased on focusing effect of left-handed metamaterial coverto improve gain and directivity of antenna [10][11].

II. ANTENNA DESIGN

Fig. 1 represents conventional coaxial feed microstrippatch antenna. The basic microstrip patch antenna consistof three layers. The dielectric substrate is placed betweena ground plate (lower layer) and radiating metallic patch(top layer). The proposed microstrip patch antenna isrealized on FR4 substrate εr=4.4 and thickness (t) ofsubstrate is 1.56 mm and ground plate and radiating patchis made of copper. The resonant frequency of proposedantenna is 9 GHz. The dimension of radiating patch iscalculated by equation (1), (2), (3), (4) and (5) whereL, W are length and width of radiating patch [12]. Thedimensions of ground plate is calculated by equation (6)and (7) where Lg,Wg are length and width of ground plate[13].

εeff = εr + 12 + εr−1

2 [1 + 12 hw

]−1/2 (1)

W = c

2fo

√(εr+1)

2

(2)

Leff = c

2f0√εeff

(3)

∆L= 0.412h (εeff + 0.3)(w/h+ 0.264)(εeff −0.258)(w/h+ 0.8) (4)

L= Leff −2∆L (5)

Lg = 6h+L (6)

Wg = 6h+W (7)

The patch dimension (W × L) have been calculated as(10.14 mm × 7.27 mm) and for ground plate dimension(Wg×Lg) have been calculated as (19.5 mm × 16.63 mm).The proposed microstrip patch antenna is excited by coax-ial feed. Coaxial feed can be placed at any desired locationinside patch for perfect matching with input impedance.To find the antenna feed position the antenna inputimpedance is matched to 50 Ω by formula cos2(πy0/L).Antenna feed location (y0) is obtained 1.47 mm.

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Fig. 1: Geometry of coaxial feed microstrip patch antenna(a) top view (b) side view

The Left-handed metamaterial cover consist of periodicarrays of metallic split ring resonators (SRR) and thinmetallic wires [14]. Split ring resonator (SRR) and wiretogether form a material with both negative permittivity(ε) and permeability (µ) over some frequencies [15]. Fig.2 represents LHM cover consist of 3X3 unit cell (SRR +wire) of metamaterial. The length and width of LHM coveris same as ground plate and lattice distance x and y is 5mm. If we increasing the number of periodic structure,the bandwidth is decreases. To get the better antennacharacteristics the optimum dimension of LHM cover ischosen. The LHM cover is placed on patch antenna atheight (hm) as shown in fig. 3. hm can be varied to obtainthe better antenna characteristics, so the best height afteroptimization was 10.1 mm.

Fig. 2: Left-Hand Metamaterial (LHM) cover

Fig. 3: Microstrip patch antenna with LHM cover

Fig. 4 represents single SRR and wire structure ofmetamaterial cover. The dimensions of this model are thesame as described in [16]. A 0.25 mm thick substratewith ε= 4.4, loss tangent of 0.02 is assumed. The copperthickness is 0.017 mm. The width of the wire structure is0.14 mm and its length is 2.5 mm. The outer ring lengthof the SRR is 2.2 mm and line-width of both rings is 0.2mm. The gap in each ring is 0.3 mm and the gap betweenthe inner and outer rings is 0.15 mm [16].

Fig. 4: Single SRR and Wire structure (unit cell of LHMcover )

III. RESULTS AND DISCUSSIONAnsoft High frequency structure simulator (HFSS)

software is used to model and simulate the microstrippatch antenna. HFSS has been widely used in design ofpatch antenanas, microwave filters and other microwavecomponents. It can be used to calculate and plot S-Parameters (S11 and S22), bandwidth, gain and directivity.The effective medium theory is used to extract the ε and µfrom the reflection (S11) and transmission (S21) coefficientparameters (S-parameters) using smith approach [16]. Thefollowing equations are used to determine the effectiveparameters where where n is the refractive index, z is theimpedance and d is the height of slab as described in [16].

n= 1kdcos−1[ 1

2S21(1−S2

11 +S221)] (8)

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z =

√(1 +s11)2 +s2

21(1−s11)2−s2

21(9)

The permittivity (ε) and permeability (µ) are then calcu-lated from µ= nz and ε= n/z [16]. By using S-parametersof unit cell of metamaterial cover and MATLAB code, themetamaterial characteristics have been verified. The valueof (ε) and (µ) is simulatensouly negative for short rangeof frequnecy as shown in fig. 5 , it confirms left-handednature of metamterial.

Fig. 5: The retrieved permittivity (ε) and permeability (µ)of unit cell

The retrieved refractive index in fig. 6 confirms thenegative index band that lies roughly between 8 and 11GHz. Hence, the proposed metamaterial cover can be usedas lens for the microstrip patch antenna, which operatesin X- Band (8-12 GHz).

Fig. 6: The retrieved refractive index (n) of unit cell

Fig. 7 shows the return loss (S11) characteristics ofcoaxial feed microstrip patch antenna without LHM cover.The result shows that antenna has about -30.00 dB returnloss at resonant frequency of 9 GHz.

Fig. 7: Simulated return loss of MSA without LHM cover

Fig. 8 shows the radiation pattern of microstrip patchantenna without LHM cover. The result shows that an-tenna gain and directivity at resonant frequency of 9 GHzis 6.80 dB and 7.80 dB respectively. The half-power (-3dB) beamwidth (HPBW) is 80 and main lobe directionis 0.

Fig. 8: Radiation pattern of MSA without LHM cover

Fig. 9 shows the return loss (S11) characteristics ofcoaxial feed microstrip patch antenna with LHM cover.From the plot it can be observed that there is shift of 0.16GHz in resonant frequency towards the left. The resultshows that antenna has about -24.35 dB return loss atresonant frequency of 8.84 GHz.

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Fig. 9: Simulated return loss of MSA with LHM cover

Fig. 10 shows the radiation pattern of microstrip patchantenna with LHM cover. The result shows that antennagain and directivity at resonant frequency is 7.32 dB and8.18 dB respectively. The HPBW is 76 and main lobedirection is 0.

Fig. 10: Radiation pattern of MSA with LHM cover

To compare the performance of the microstrip patchantenna with and without LHM, their measurement resultare tabulated in table I .

TABLE I: Comparison of simulated results

Parameters MSA without LHM MSA with LHM

Return Loss (dB) -30 -23

Gain (dB) 6.80 7.32

Directivity (dB) 7.80 8.18

HPBW 80 76

From the table I, it can be seen that the gain anddirectivity of antenna with LHM cover is increasing andalso the HPBW is narrowing from 80 to 76, this impliesthat the LHM cover has a focusing effect on EM radiation.

IV. ConclusionIn this paper, a coaxial feed antenna for X-band with

LHM cover above the patch is presented to improve thegain and directivity of antenna. The simulated results formicrostrip patch antenna with LHM cover at resonantfrequency (9 GHz) show the improvement in gain and di-rectivity. The increment in gain is 7.6% and for directivityit is 4.5%. Based on a study of radiation pattern of theantenna, it is concluded that LHM cover acts as lens forEM radiation. It is obvious that multiple layers of LHMcover can be used to further enhance the performance ofmicrostrip patch antenna.

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