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A kind of novel multi-frequency monopole antenna with sector-nested fractal is proposed and designed, which is nested with a series of similar circular sector elements. By means of the trapeziform ground plane with the tapered CPW (coplanar waveguide) feeder in the middle, the antenna’s radiation performance is greatly improved. The antennas can synchronously operate in three frequencies, covering the working frequency bands of WLAN/WiMAX, 2.44 GHz/3.5 GHz/5.2 GHz - 5.8 GHz. The pattern and impedance measurements of antenna show a good performance over the WLAN/WiMAX band; it possesses a near omni-directional characteristic and good radiation efficiency. Moreover, the antenna is miniature and its design idea can be easily applied into other types of nested structure, the features of which make the proposed antenna have a promising application in other fields.

Development of high data rates wireless communication system and the increase of the communication frequency bands put forward the demand for multi-band antennas. In a fixed space, there usually exist many different communication systems, such as wireless local area network system, mobile communication system, etc. These wireless systems need the antennas which could work on different frequencies and modes. For example the working frequencies for the wireless local area network (WLAN) are 2.4 GHz and 5.2 GHz, and those for the GSM wireless mobile terminal are 900 MHz and 1800 MHz. So we really need an antenna, it can meet the requirements of different applications for the band and also can guarantee the gain of antenna. Specifically, an antenna can work on different frequency bands, and the corresponding radiation characteristics within each frequency band can be guaranteed.

There are many methods proposed to realize the miniaturization characteristic of the antennas, including resonant units [

Fractals have self-similar shapes and can be subdivided in parts such that each part is a reduced size copy of the whole. The self-similarity of fractals is the cause of multi-band property and their complicated shapes contribute to the design of antennas with smaller size. Fractals have convoluted and jagged shapes, and these discontinuities will increase bandwidth and weaken the effective radiation of antennas. The space-filling property of fractal makes curves which have long electrical length be fitted into a compact physical volume. The typical fractal antennas in recent years include Sierpinski fractal antenna [

In this paper, a novel sector-nested fractal antenna is proposed, which is a single-plane printed monopole antenna fed by the trapeziform ground plane with CPW. The antenna can synchronously operate in three frequencies, covering the working frequency bands of WLAN/WiMAX (2.44 GHz/3.5 GHz/5.2 GHz - 5.8 GHz). The paper is organized as follows. Section 2 describes the proposed design method and short formulation. Section 3 presents results of simulations using Ansoft HFSS. Section 4 concludes the ﬁndings of the paper.

In recent several years, fractal has caused a fast-growing interest of scientists. These structures (systems) description cannot be presented using traditional derivative equations of integer order. More exactly these processes and objects are quantitatively described by integration-differentiation operators of fractional order

The fractal and processes into them can be described by operators with real degree [

where

termined by the following integral relationship

Therefore integration-differentiation nature of fractional operator of Riemann-Liouville

The sector-nested fractal antenna is evolved from multiple-ring fractal antenna [

The novel sector-nested fractal antenna is shown in

According to previous research, fractal fan-nested antenna with the fan unit in different sizes will have an effect on the reflection loss of multiband, and the mainly factors that influence the pass-band center frequency and bandwidth are the height of the nested fan and vertex angles. Below we will change the height and angle of the sector, and analyze how these two variables affect the performance of the antenna.

Set the fan-nested fractal parameters as follows:_{11} and bandwidth in 1 mm steps are shown in

The experiential formula of deciding antenna size can be established by simulation, which indicates the relation between the outer fan side-length

In formula (3), the speed of light

Keep the outer fan height and other variables constant and respectively change the second and the third height of the nested fan, the results of which are shown in

With fan-shaped nested structure, the distance between each unit is reduced, which will lead to a coupling increase. Under the interaction of different nested loop and especially for the high frequency pass-band, mainly influenced by the internal sector in gand affected by inner and outer ring simultaneously, the relationship between the center frequency for the high frequency pass-band and the ring size is complex, while the adjustment for the low frequency pass-band is relatively easy.

Center Freq (GHz) | S_{11} (dB) | Center Freq (GHz) | S_{11} (dB) | Center Freq (GHz) | S_{11} (dB) | ||

22 | 2.61 | −23.4 | ―― | ―― | 5.41 | −17.3 | |

21 | 2.74 | −39.0 | 3.92 | −16.8 | 5.44 | −17.9 | |

20 | 2.83 | −41.3 | 3.58 | −21.3 | 5.49 | −18.7 | |

18 | 2.528 | −21.8 | 3.63 | −23.8 | 4.86 | −14.8 | |

17 | 2.578 | −22.3 | 3.38 | −24.2 | 5.12 | −16.2 | |

16 | 2.586 | −22.6 | 3.41 | −25.1 | 5.23 | −17.6 | |

14 | 2.48 | −20.6 | 3.42 | −27.3 | 5.01 | −13.2 | |

13 | 2.473 | −18.8 | 3.40 | −33.1 | 4.98 | −14.3 | |

12 | 2.46 | −18.6 | 3.43 | −32.8 | 4.96 | −14.9 |

Keep the nested sector remain the same, that is, to meet the conditions:

As _{11}) have non-monotone variation, _{11}. Apex angle size will affect the ultra-wideband antenna characteristics, and in order to achieve multiple frequency characteristics, the internal fan-shaped apex angle should not be too small, and outer fan apex angle should not be too big. Thus in this paper we choose

Through the above analysis, we can draw the following conclusions: the fan nested fractal antenna has the characteristic of multiple frequency, and the height and apex angle of the nested fan will influence the frequency, size of the pass-band, and the depth of resonance of the multi-frequency pass-band. According to the characteristics of the fan nested fractal antenna and the researches of the effects of different parameters on the band, we can design a multi-band antenna with three bands (2.48 GHz, 3.5 GHz and 5.58 GHz) by adjusting values of the parameters. As the fan apex angle has a small contribution on frequency of the antenna, the antenna design is

accomplished mainly by adjusting the fan height. Because the adjustment for the low frequency pass-band is relatively easier than that for the high frequency, so we should first determine the lowest center frequency as 2.48 GHz, and then set the second and the third band center frequency (3.5 GHz, 5.58 GHz) by adjusting the nested triangular height. If the first center frequency is 2.48 GHz, we can determine the outer triangle height according to formula (3). The experiential formula of antenna size can be established from simulation, which indicates the relation between fan side-length and the wavelength of corresponding resonant frequency, as is shown in the following:

According to the above discussion and in order to realize the multi-bands, the structural parameters of this antenna can be set as:

In

Antenna | |||
---|---|---|---|

Center Value of | Range of | Largest Deviation of | |

Proposed | 0.117 | 0.112 - 0.122 | 4.3 |

Diamond Monopole [ | 0.350 | 0.234 - 0.465 | 33.0 |

Antenna [ | 0.375 | 0.369 - 0.381 | 10.6 |

Antenna [ | 0.263 | 0.242 - 0.283 | 7.8 |

frequency. The center value of

A novel sector-nested fractal antenna has been introduced. By combining two techniques: the sector-nested fractal structure and the trapeziform ground plane, the monopole multiband antenna is designed to cover three bands at 2.44 GHz/3.5 GHz/5.2 GHz - 5.8 GHz, which are required by WLAN/WiMAX systems. In addition, the antenna has simple structure, thin profile, low cost and significant gain. Therefore it can be applied for the electronic protection systems, etc., and will be an attractive candidate for various WLAN/WiMAX applications.

This paper is too short to provide sufficient evidence about general properties of fractal antennas and to infer much about their relationship with the antenna fractal dimension and its performance indices. Nevertheless, our results indicate that a good analysis framework has been established for further study of miniature multi-band antennas for practical applications. For future work, we intend to investigate the reduction in size without diminishing the performance of antennas, and to seek its application in wireless systems.

This work was supported by the Natural Science Foundation of Guangdong Province, China (Grant No. 2010B050900016), the fundamental research funds for the Central Universities (Grant No. 21613323) and the Funds for International Cooperation and Exchange of the National Natural Science Foundation of China (Grant No. 21614605).