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A μ negative metamaterial using spiral resonator (SR) with an electromagnetically coupled (EMC) feeding system is proposed as a novel antenna structure. The proposed antenna is designed and fabricated on a FR4 dielectric substrate with a thickness of 1.6 mm and relative permittivity of 4.0 to achieve its radiation characteristic. The antenna is operated at frequency 2.4 GHz. To improve the antenna gain, a matching circuit is inserted into the feed line. The µ negative metamaterial is achieved by using a spiral resonator with spiral numbers N = 3, 5, 7, and 10. It is found that the negative imaginary part tends to shift leftward as the value of N increases. The simulation result of the proposed antenna structure with spiral number N = 3, strip width w = 3.1 mm, and gap width s = 0.5 mm provides the best performance with S11 = -15 dB, VSWR < 2 bandwidth of 30 MHz and gain of –0.5 dB at frequency of 2.43 GHz. The proposed antenna with matching circuit provides the antenna gain of 2.21 dB, which is better than that without the matching circuit. The dimensions of the proposed antenna are reduced by 53% compared with those of the conventional patch. Both the simulation and measurement results of the radiation characteristics of the proposed antenna show good agreement.

Recently, antennas used in many modern wireless communication systems tend to have the characteristics of compact dimensions and light weight. A variety of antenna miniaturization approaches have been developed to improve the radiation characteristics. One technique to reduce the antenna dimension is the use of metamaterials. A metamaterial is a material that has negative properties of permittivity and permeability (Double Negative or DNG) of the dielectric [

The MNG metamaterial can be composed of two shapes: a split ring resonator (SRR) and spiral resonator (SR). Bilotti et al. [_{0}/30 - l_{0}/40 and l_{0}/65 - l_{0}/250, respectively. Huang et al. [_{0}/50 and used the structure as an embedded spiral microstrip implantable antenna. Similarly, Soontornpipit et al. [_{0}/25 - l_{0}/50. However, the metamaterial properties with µ negative were not discussed in [

Moreover, Kevin fabricated a magnetic material by using a spiral structure as a substrate for applying small patch antenna; this approach can provide an adjustable reduction factor [

Therefore, this paper discusses the radiation characteristic of SR structure of m negative metamaterial as a novel antenna. The discussion begins with a parametric study of the SR structure dimensions to determine the spiral turn number N, strip width w, gap width s of the single patch of the SR structure. The selected dimension of the planar antenna that provides the highest gain will be implemented for developing the proposed antenna. Furthermore, the MNG properties of the SR is obtained through the formulations in [

The background, purpose, and research methodology are described in section one. The design of the spiral resonator on metamaterial MNG and matching ciricuit to obtain a better characteristics of the radiation pattern are discussed in section two. The simulation and measurement result of the SR structure are shown in section three. Furthermore, the discussion and conclusion are presented in section four and five, respectively.

The SR structure as a single patch of the planar-antenna with the EMC feeding system is shown in

A key attribute of the EMC feeding system is capacitive in nature of its coupling mechanism. This is in contrast

to the direct contact methods, which are predominantly inductive. The difference in coupling significantly affects the obtainable impedance bandwidth due to the inductive coupling of the edge and probe-fed geometries limits the thickness of the material useable [

The SR structure dimension is defined by variables of the spiral turn number of N, the width of the strips of w, and the separation between two adjacent turns of s. The proposed antenna is assumed to have the constraint of the side length of the external turn of l which is constant. So that, the w and s values are adjusted to the N variations of 3, 5, 7, and 10.

The MNG metamaterial properties of the SR structure as a planar antenna for the dimensional variations can be approximately obtained by equivalent circuit models of the spiral resonator [_{SR}), capacitance (C_{SR}), losses resistance in the metallic conductor (

where:

K(・) = The complete elliptic integral of the first kind.

_{0} is the per-unit-length capacitance between two parallel strips having width w and separation s.

The obtained parameters of L_{SR}, C_{SR},

This impedance is used to calculate the magnetic polarizability of the individual magnetic inclusion α_{mm} defined as [

Next, the effective permeability of the SR structure is obtained with the variation of the spiral turn number N, the strip width w, and the gap width s based on a first-order approximation of the permeability function, which is derived using the Clausius-Mosotti [

The SR structure functions as an antenna, therefore it must be connected to the feeding system. To enhance antenna gain and bandwidth, a matching circuit is inserted in the feed line, as discussed in [

The SR structure is simulated with the dimensional variation of spiral turn number N, strip width w, and gap width s. By keeping l constant, then w and s values are adjusted to the N variations of 3, 5, 7, and 10.

The MNG metamaterial properties of the SR structure with the N, w, and s variation, which will be characterized as the planar-antenna, is obtained by using Equation (1) to Equation (9), as shown in

_{eff}) of the proposed antenna has the negative real part of at least 0.1 GHz, maintaining relatively similar value for the N variations of 3, 5, 7, and 10, and the negative imaginary part tends to shift leftward with the increasing value of N. This case shows that the negative value of m_{eff} can be achieved at a lower frequency for larger values of N.

Furthermore, using Bilotti’s work [

provides the S_{11} and the realized gain are shown in

The radiation pattern of the proposed antenna is also characterized by the linear array approach, as shown in [

Parameter | Frequency Range of 2.4 - 2.5 GHz | S_{11} (dB) | Gain (dB) | |
---|---|---|---|---|

N = 3 | w = 3.1 s = 0.5 | 2.43 | −15 | −0.5 |

w = 2.5 s = 1.5 | 2.47 | −9 | −0.6 | |

w = 2.5 s = 1 | out of range | not observed | not observed | |

w = 2 s = 2 | out of range | not observed | not observed | |

w = 2 s = 2.5 | out of range | not observed | not observed | |

w = 1.5 s = 2.5 | out of range | not observed | not observed |

Moreover, the realized gain of the proposed antenna with the variation of N, w, and s in the frequency range of 2.0 GHz - 2.5 GHz is shown in

_{11} = −15 dB, and VSWR = 2 bandwidth is 30 MHz. The obtained antenna gain of −0.5 dB is still low and must be improved. To increase the antenna gain, the matching circuit is inserted into the feed line, as shown in _{11} and gain of the proposed antenna with the matching circuit that is inserted in the feed line are shown in

_{11} of −25 dB and a VSWR = 2 bandwidth of 40 MHz, whereas the proposed antenna without the matching circuit has a S_{11} of −15 dB and a bandwidth of 30 MHz.

The simulation result of the proposed antenna compared to the antenna with a conventional rectangular patch is shown in

Parameter | Proposed Antenna | Antenna with Conventional Patch |
---|---|---|

Dimensions of the Ground Plane (mm) | 30 × 30 | 50 × 50 |

Dimensions of the Patch (mm) | 22.6 × 22.6 | 36 × 30 |

Frequency (GHz) | 2.45 | 2.45 |

S_{11} (dB) | −25 | −23 |

Bandwidth (MHz) | 40 | 69 |

Gain (dB) | 2.2 | 3.7 |

Dimensional Reduction of the Patch | 53% |

The simulations are validated by fabrication and characterization of the proposed antenna. The proposed antenna with a matching circuit in the EMC feeding is fabricated on FR4 substrate with thickness of 1.6 mm and relative dielectric permittivity of 4, as shown in

The radiation characteristics of the proposed antenna that measured are S_{11}, radiation pattern and bandwidth. The S_{11} comparison between the measurement and simulation results is shown in

_{11} has good agreement with the simulation results. For convenience, the comparison between the measurement and simulation results of the proposed antenna performance, the S_{11}, bandwidth, and gain are tabulated in

The radiation pattern comparison between the measurement and simulation results of the proposed antenna is shown in

Parameters | Simulation | Measurement |
---|---|---|

S_{11} (dB) | −25 | −19 |

Bandwidth (MHz) at S_{11} of −10 dB | 40 | 49 |

Realized Gain (dB) | 2.21 | 2.0 |

The main issue in the design of the microstrip antenna is the problem of the surface wave that is generated; such a surface wave significantly disrupts the antenna performance by causing an efficiency reduction and a degradation of the radiation pattern. Various methods have been used by researchers to eliminate the surface wave; one of the methods being applied is the use of a material that has a negative permittivity (ENG) or negative permeability (MNG), commonly called a single negative metamaterial. This material does not radiate the waves, as described by Kim [

In his research, Billoti [

For many parameters of an antenna, such as the dimensions, gain, and bandwidth, compromise among these three parameters is found. In our case, the optimum value of the proposed antenna parameters is achieved for the antenna gain at 2.2 dB with a patch dimension reduction factor of up to 53%.

Having aforementioned discussions, the SR has a unique structure that can function as radiator and significantly reduce the dimension of the SR structure. Therefore, the SR structure has good prospects for development of the planar antenna applications. For future work, the SR structure will be expanded as an antennas array to obtain better characteristic of the radiation pattern.

A novel antenna using an SR structure having MNG metamaterial properties was presented. The effective permeability value (m_{eff}) of the proposed antenna has the negative real part at least 0.1 GHz being relatively similar for spiral numbers of N = 3, 5, 7, and 10, and the negative imaginary part tends to shift leftward with increasing values of N. The proposed antenna showed size reduction compared with the conventional rectangular patch. Furthermore the simulation and measurement results show good agreement. Therefore, the proposed SR structure can be used as an effective antenna.

MochamadYunus,Fitri YuliZulkifli,Eko TjiptoRahardjo, (2016) Radiation Characteristics of a Novel µ Negative Metamaterial Spiral Resonator Antenna at the 2.4 GHz. Open Journal of Antennas and Propagation,04,1-11. doi: 10.4236/ojapr.2016.41001