A Compact Dual Band Meta-material Antenna for ... - IEEE Xplore

2012 Loughborough Antennas & Propagation Conference

12-13 November 2012, Loughborough, UK

A Compact Dual Band Meta-material Antenna for Wireless Applications Mahmoud A. Abdalla*

Zhirun Hu

Electronic Engineering Department MTC College University Cairo, Egypt [email protected]

Microwave and Communication Group Electrical and Electronic School, Manchester University Manchester, UK [email protected]

Abstract—This paper presents a compact dual band metamaterial antenna for WiMAX applications. The reported antenna was based on using a metamaterial modified left handed transmission line unit cell. The designed antenna was introduced to operate with the WiMAX lower 2.4 GHz band and upper 5.8 GHz band. The designed antenna has the advantages of its compact size (only 2 X 3) cm2. Compared to the conventional patch length, the designed patch radiator is only 25% at 2.4 GHz and 60% 5.8 GHz. The antenna designed was discussed theoretically and its performance was verified using the electromagnetic full wave simulations. The results confirm that the antenna can introduce the dual bands operation with return loss better than 15 dB at each operating band. Moreover, the radiation pattern is almost omnidirectional at the two bands. Keywords-Metamaterial, multi-band antenna, dual band, left handed, WiMAX.

I.

INTRODUCTION

Electromagnetic metamaterials were first investigated by the Russian physicist Victor Veslago [1]. Recently, there is a great interest from electromagnetic waves community in investigating metamaterial structures. Specifically, a great interest has been paid on studying the characteristics of using these artificially constructed metamaterials in possible RF/microwave circuit applications. The studies show that these materials have unique electromagnetic properties at microwave frequency bands which are not found using conventional materials. Examples of metamaterials include artificial dielectrics [2] and magnetic materials [3] which can exhibit positive or negative permittivity or permeability properties, respectively, and Left handed metamaterials (LHMs) which can exhibit simultaneously negative permittivity and permeability properties [4]. Planar left handed (LH) transmission media have been realized using different techniques. They can be based on loading a hosting transmission line (TL) with split ring resonator (SRR)/wire-array [5], complementary SRR (CSRR)/capacitive gap pairs [6] or series capacitors and shunt inductors [7]-[9]. The latter is a non resonant approach and has advantage of compactness, its broad bandwidth, in addition to its suitable with planar microwave circuit applications.

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A practical left handed transmission line has been realized in either microstrip or coplanar waveguide (CPW) configuration [10]-[14]. In general, left handed CPW structures have many attractive features such as their easy and inexpensive fabrication procedures in addition to their simple series and shunt connections. Also, active elements have a planar structure nature, and hence they can be connected easily to coplanar waveguide configurations. Accordingly, many different guided and radiated microwave components such as filters, power divider, couplers, and antennas have been proposed as applications of left handed transmission lines [15], [16]. The major aspects of these components are its size miniaturization and new performance properties. In Wireless applications, low profile, compact planar antennas are preferred. Moreover, they should be characterized by simple design, and omni-directional radiation pattern. The conventional microstrip patch antenna length is approximately λ/2 which is large specially at lower frequencies. Therefore several miniaturization trails have been used to fulfill the above wireless antenna requirements. From the different types of planar antennas, the planar monopole antenna is attractive for WLAN antenna design [17][19]. This can be claimed due to it can provide either broadband or multi-band operation [20]. In order to achieve multi-band antenna operation, the basic technique is to use strips of different lengths to achieve resonance at different frequencies. This mainly requires large antenna size. In this paper we introduce a compact dual band metamaterial antenna for WiMAX lower and upper bands applications, (2.4 GHz band, and 5.8 GHz band). The designed antenna has the compactness advantage. Its overall size is only (only 2 X 3) cm2. Moreover, its radiator patch length is only 25% at 2.4 GHz and 60% 5.8 GHz. In other words, the antenna length has reduced by more than 40% and 75% compared to conventional antenna. The theoretical explanation of the designed antenna is introduced. The performance of the proposed antenna is evaluated using the electromagnetic full wave simulations. The commercial software HFSS was employed. All radiated antenna parameters were extracted.

2012 Loughborough Antennas & Propagation Conference II.


THORY

As explained above, the realization of LHM based on the TL approach, in effect, comprises both LH contribution due to the loading element effect and RH contribution due to the parasitic series inductance and shunt capacitance effect of the hosting TL elements. This means that the explanation of TL approach LHM as an ideal LH TL is not accurate; however, it can be accepted for short sections of hosting TL. According to this contribution a practical TL approach LHM is referred as a composite right / left handed (CRLH) TL [15]. In other words, a practical LH TL is a combination of both left handedness due to the effect of the loading elements and right handedness due to parasitic elements of the hosting TL medium, hence composite right / left handed TL. The equivalent circuit approach of a lossless CRLH unit cell can be introduced as shown in Fig. 1. In the equivalent circuit model, the series inductor and the shunt capacitor are corresponding to the RH parameters of the hosting TL while the series capacitor and shunt inductor are corresponding to the loading LH elements. At lower frequency, the RH elements, CR and LR, will be open circuited and short circuited respectively. Thus, the CRLH unit cell equivalent circuit reduces to that of LH equivalent circuit and hence it acts as a nearly ideal LH TL. Similarly, at higher frequency, the CRLH unit cell equivalent circuit reduces to that of RH equivalent circuit and hence it acts as an ideal RH TL. The basic concept of transmission line resonating antenna is based on achieving resonance at frequencies which fulfill the full wave conditions as β l = n π,

(1)

where l is the total physical length of the resonating antenna , n is an integer specifies the resonating order and β is the guided propagation constants within the antenna substrate. In conventional microstrip patch antenna, the constant integer n is always positive. In conventional transmission line, the propagation constant is directly proportional to frequency; changes linearly with frequency. Thus, the operating condition for resonant antenna can be satisfied only at the harmonic frequencies of the antenna.

Figure 2. The equivalent block diagram of metamaterial left handed

antenna

On the other hand, the propagation constant is varying with the frequency in non linear way. Therefore, it can be implemented to introduce arbitrary operating multiband which is not essentially to be multiple of the fundamental frequency harmonic. Also, by cascading metamaterial transmission line with conventional transmission line, the propagation constant can be arbitrary positive or negative or even zero. This can be illustrated in Fig.2. Hence, the oscillation radiation condition can be modified as

β l = β l d l + β r d r = n π,

where βl and βr are the propagation constants of LH and RH sections, respectively, whereas dl and dr represent the length of these sections, respectively. In this case, the integer can be expressed as n = 0,±1, ± 2,±3, ............ ± ( N − 1)

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(3)

Basically, such metamaterial antenna can demonstrate shunt or series resonance [21]. The shunt resonance mode is formed by the shunt branch elements LL and CR. The series resonance mode is due the series resonance branch formed by LR and CL. In case of designing the antenna based on the resonance of the series branch, shown in Fig. 1 can be given as f 0 = f ser =

III.

Figure 1. The equivalent circuit model of CRLH transmission line.

(2)

1 2 π C L LR

(4)

DESIGN PROCEDURES

The proposed antenna was designed in CPW configuration over a duroid substrate with εr=2.2, and 1.59 mm thickness. The antenna was by loading a hosting CPW transmission line by only one metamaterial unit cell formed by using a series interdigital capacitor and a patristic strip inductor. In this section we introduce the initial design which fulfills the design goal. Next, we proceed to introduce the final proposed antenna whose size was reduced by half compared to initial design.



Figure 3. The layout of the intial design dual band antenna (b) The layout of the final design dual band antenna.

The design of the proposed antenna has been initially started by loading a 50 Ω hosting CPW of center line width of 1.5 mm by a metamaterial unit cell. The employed unit cell was only designed as the series branch of left handed configuration formed by series interdigital capacitor and two different strips of different widths as shown in Fig. 3 (a). The interdigital capacitor represent the left handed capacitor whose fingers width and spaces are all equal to 0.25 mm. The parasitic strips have widths of 1.5 mm and 0.5 mm and lengths equal 12 mm and 14 mm, respectively. The reason of changing the strip width is to enable the two resonance frequencies to occur at desired bands. The overall antenna size is 4 X 3 cm2. The simulated reflection coefficient for this antenna is shown in Fig. 4. As shown, the antenna can illustrate better than -10 dB reflection coefficient at the two desired frequencies (2.4 GHz and 5.8 GHz). Such results can be considered as a starting for the second step in the antenna design and miniaturization objective. The final antenna layout is shown in Fig. 3 (b).

Figure 4. The simulated Return loss of the initaial designed dual band metamaterial antenna.

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Figure 5. The simulated Return loss of the final designed dual band metamaterial antenna.

In the final proposed antenna design, we suggested forming the second loaded patristic strip inductance as a T shape. Accordingly, we have succeeded in reducing the antenna size by half to become only 3 X 2 cm2. The length of the T slot is 18 mm and the thin patristic inductance strip length has been reduced to only 5mm. Also, the antenna performance was further optimized and the final simulated antenna reflection coefficient is shown in Fig. 5. As shown, the antenna can demonstrate almost identical and better than -15 dB in the reflection coefficient at the two designed frequencies. The antenna has better than 10 dB bandwidth from 2 -2.8 GHz in the first 2.4 GHz band and 5.74-5.95 GHz which both satisfy the required two standard bandwidth requirements. IV.

ANTENNA RADIATION PROPERTIES

The antenna radiation performance at the two operating band has been checked by investigating the radiation pattern at each band centre frequency. The 3D normalized radiation pattern at 2.4 GHz and 5.8 GHz of the metamaterial dual band antenna is shown in Fig. 6. It is obvious that the dual band antenna has an omni-directional radiation pattern with maximum radiation in Z direction, perpendicular to the antenna. Such radiation pattern is appreciated in the WiMAX applications.

Figure 6. 3D normalized radiation pattern at 2.4 GHz and 5.8 GHz of the metamaterial dual band antenna.


12-13 November 2012, Loughborough, UK [2]

[3]

[4]

[5]

[6]

[7]

[8] Figure 7. Comparison between the co-polarized and cross-polarized gain in H/E planes at 2.4 GHz and 5.8 GHz of the metamaterial dual band antenna.

For investigation of the polarization properties of the antenna, the co-polarized and cross-polarized gains were compared and plotted in Fig. 7 at the two previous frequencies in both H plane (YZ plane, phi=90) and E plane (XZ plane, phi=0). From our observation E plane, co polarized antenna gain component is directional one with maximum along z direction (θ=0). The difference between the co polarized and cross polarized antenna gain components is better than 15 dB, at 5.8 GHz, and it reaches better than 40 dB in the case of operating frequency 2.4 GHz. On the other hand in the H plane, the co-polarized antenna gain is isotropic. Similar results can be observed for the difference between the co-polarized and cross polarized antenna gain components is better than 15 dB. Finally, the antenna radiation efficiency was computed at the two operating frequencies. Excellent agreement was achieved

as 0.99 at the lower band (2.4 GHz) and 0.92 at upper frequency (5.8 GHz).

[9]

[10]

[11]

[12]

[13]

[14]

[15]

V.

CONCLUSION

A Compact dual band metamaterial antenna for lower 2.4 GHz and upper 5.8 GHz WiMAX frequency bands has been presented. The antenna design and performance has been introduced using electromagnetic full wave simulations. The results illustrate that the proposed antenna can demonstrate better than 15 dB return loss at the two operating bands. Also, antenna demonstrate almost typical omni-directional radiation pattern at the two operating frequencies with better than 15 dB difference in co polarized and cross polarized component. Finally, the proposed antenna has been miniaturized up to 75% compared to conventional patch antenna. REFERENCES [1]

V. G. Veselago, "The electrodynamics of substances with simultaneously negative values of ϵ and &mu," Soviet Physics Uspekhi, vol. 10, pp. 509-14, 1968.

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