Abstract

1  Introduction

Metamaterial-inspired antennas have concentrated on the special effect of electromagnetic wave property. Split Ring Resonator (SRR), Complementary Split Ring Resonator (CSRR) and Electric-LC (ELC) are the basic unit elements of metamaterial for enhancing the antenna performances. Planar monopole antenna loaded with SRR structure can be used to achieve bandwidth improvement [1] and miniaturization [2]. CSRR-loaded substrate has spurred for good impedance matching [3], multiband [4], and gain improvement [5]. ELC-based metamaterial offers important applications in phase shifters [6], wideband antenna design [7], and circular polarization [8]. The structural orientation of the ELC metamaterial element introduces magnetic resonance for improving antenna performance [9]. ELC structure

*\ R. Samson Daniel

\samson.rapheal@gmail.com

\R. Pandeeswari

\rpands@nitt.edu

\S. Raghavan

\raghvan@nitt.edu

1\ Department of ECE, K. Ramakrishnan College

of Engineering, Samayapuram, Tiruchirappalli, India

2\ Department of Electronics and Communication Engineering,

National Institute of Technology, Tiruchirappalli 620015,

India

exhibits a quasi-static resonant frequency for achieving miniaturization and multiband antenna design [10]. An array of ELC resonators loaded with varactor diode is used to design broadband tunability and resonance broadening due to harmonic free and phase independent response [11]. Different ELC structures are evolved to improve the simplicity of antenna design [12]. ELC eliminates the cross polarization due to effective medium regime [13] for obtaining the desired power level.

In this paper, a ring monopole antenna coupled with ELC metamaterial element is presented. Dual band with better return loss characteristics are achieved by the use of ELC resonator. The antenna is proposed to operate at 3.45 and 5 GHz for WiMAX and WLAN applications.

2  Proposed antenna design and simulated results

The proposed antenna is expanded from a ring monopole antenna (A) as shown in Fig. 1. To achieve WiMAX and WLAN resonance frequencies from this ring monopole antenna, the ELC metamaterial element is introduced as shown in the proposed antenna (B) of Fig. 1. ELC metamaterial element alters the resonance characteristics of the ring monopole antenna and creates dual-band characteristics for achieving required frequency bands. ELC resonator and its equivalent circuit model [14] is shown

Fig. 2  ELC resonator and its equivalent circuit model

in Fig. 2. It has loop inductance and capacitive elements. The capacitive element is governed by a dielectric gap, and the inductance (L) is governed by conducting loops. For the SRR-based ELC metamaterial unit cell, the inductive element (the loop) is coupled to the magnetic field, and the capacitive element (the dielectric gap) is coupled to the electric field. This modifies the current direction and provides dual-band resonance frequency.

The ring monopole antenna is used to create 3 GHz resonance frequency and ELC metamaterial offers 5 GHz resonance frequency. This resonance frequencies are calculated by [15],

For ring monopole antenna,

(2) Here, S is the side length of the hexagonal ring monopole antenna, S1 is the side length of the ELC metamaterial element and R is the dielectric constant of the FR-4

substrate.

Fig. 3  Detailed layout. a Proposed antenna, b side view

The proposed antenna geometry is shown in Fig. 3, and its parameters are depicted in Table 1. The fabricated structure of the proposed antenna is shown in Fig. 4.

The simulations are examined by Ansoft High-Fre- quency Structure Simulator (HFSS) V.15.0 electromagnetic software. The simulated S11(dB) characteristics of configuration A and B are shown in Fig. 5. It is exposed that the ring monopole antenna offers dual-band characteristics of 3 and 5.33 GHz. ELC resonator shifts the resonance frequencies from 3 to 3.45 GHz and 5.33 to 5 GHz with better return loss characteristics for attaining required frequency bands (WiMAX and WLAN). The proposed antenna covers a bandwidth of 1350 MHz (2.77–4.12 GHz) and 970 MHz (4.45–5.42 GHz) centred

Fig. 5  Simulated return loss characteristics of the conventional antenna and proposed antenna

at 3.45 and 5 GHz, respectively. This dual-band resonance is attributed to ELC-based metamaterial element.

The comparison between previously reported ELC metamaterial antennas with a proposed antenna is described in Table 2. It is understood that, the prototype antenna

covers the least possible area to attain dual-band antenna for WiMAX and WLAN applications.

The simulated current distribution of the proposed antenna with and without ELC metamaterial element is shown in Fig. 6. It shows that at the lower resonance frequency of 3.45 GHz, the current is concentrated around the ring monopole and at the upper resonance frequency of 5 GHz the current is concentrated around the ring monopole, ELC metamaterial element and the feed line of the monopole.

3  Analysis of ELC resonator

The classical waveguide theory is used to explore the S-parameters (S11 and S21) for studying, stop band and pass band characteristics of metamaterial element [3, 4]. The transmission coefficient S21 creates stop band characteristics due to band stop filter and the reflection coefficientS11 performs pass band characteristics due to band pass filter for generating a new resonance frequency. The transmission coefficient S21 of the ELC metamaterial element is shown in Fig. 7. It is observed that antenna without ELC element has no stop band, but antenna with ELC element the stop band is inferred at 4.3 GHz. Figure 8 depicts the effective permeability ( ) characteristics of the

Table 2  Comparisons of the existing antennas with the proposed antenna

ELC metamaterial, where the negative permeability ( ) is exposed at 4.3 GHz due to stop band behaviour of the ELC element.

Similarly, the reflection coefficient (S11) of the ELC is depicted in Fig. 9. It shows that antenna with ELC element generates two pass bands at 3.45 and 5 GHz. This two pass band creates a new resonance frequency in the return loss characteristics of the proposed antenna.

4  Experimental results and discussion

The simulated and measured return loss characteristics are compared and shown in Fig. 10. Numerical results are exhibited in Table 3. Measured result coincides with the upper frequency band and slight difference at the lower frequency band. This variation caused by fabrication error and soldering effect. The experimental return loss covers

Fig. 7  Simulated transmission coefficient S21 of the antenna without ELC element and antenna with ELC element

Fig. 10  Comparison of simulated and measured return loss characteristics

Table 3  Evaluation of simulated and measured values of the proposed antenna

Fig. 8  Effective permeability (µ) of ELC metamaterial at 4.3 GHz

Fig. 9  Simulated reflection coefficient S11 of the antenna without ELC element and antenna with ELC element

the bandwidth of 500 MHz (3.55–4.05 GHz) and 860 MHz (4.73–5.59 GHz) with a resonance frequency of 3.74 and 5.1 GHz, respectively, which is suitable for WiMAX and WLAN applications.

The E-plane and H-plane radiation patterns of measured resonance frequencies at 3.74 and 5.1 GHz are shown in Fig. 11a, b, respectively. Measured radiation patterns are almost matched with simulated radiation patterns. It depicts the dipole pattern at E-plane and omnidirectional pattern at H-plane, which covers required directions for WiMAX and WLAN frequency bands.

The gain of the proposed antenna is measured by gain transfer method. The measured gain is compared with simulated gain, which is shown in Fig. 12. The measured peak gains 1.23 and 1.57 dBi are observed at 3.74 and 5.1 GHz, respectively. The simulated efficiency of the proposed antenna is shown in Fig. 13. The average radiation efficiency of 82% has been inferred around the resonance frequencies of the proposed antenna.