Keywords

1 Introduction

The growing demand for wireless communications in recent years requires antennas which can provide large impedance bandwidths, high data rate, and higher gain. Ultra-wideband (UWB) is one such technology that provides very large bandwidth and higher data rates [1]. But the main disadvantage of using UWB technology is electromagnetic interference (EMI). To eliminate the interference problems, UWB antennas are designed to achieve band notch characteristics [2,3,4,5,6]. Designing an antenna for wireless applications requires compactness and cost-effectiveness. Using the microstrip antennas for these applications is the right choice because of the numerous advantages provided by microstrip antennas [7,8,9,10]. Several designs have been proposed to achieve UWB response using microstrip antennas [11,12,13,14,15]. Single-, double-, and triple-band rejections are achieved by rearranging the radiating element or by cutting slots on patch, ground plane [16] or by introducing additional stubs on any of the radiating patch or ground plane [17]. Achieving wide bandwidth offers a low gain in the resonant band. The compromise between bandwidth and gain should be made before designing the antenna. Several techniques have been proposed to improve the gain while maintaining the required bandwidth [18].

The antenna miniaturization is getting much attention in the recent years since the fractal geometries are introduced into antenna design [19]. Fractal structures are used widely to reduce the size of the antenna and to achieve multi-band resonant behavior. Several fractal designs have been proposed to get multiple band resonances and at the same time more compact and high-impedance bandwidth structures [20,21,22,23]. The idea of fractals can also be used to achieve UWB range. By using the different fractal geometries, UWB with band rejection can also be designed [24].

In this paper, a rectangular printed patch antenna is designed to achieve the ultra-wideband behavior. By introducing additional stub on ground plane, band notch characteristics are achieved. Frequency band from 5 to 6 GHz is rejected from the ultra-wideband to eliminate the interference with WLAN. Implementing Koch fractal in antenna design provided the multiple band resonances. Different Koch fractal geometries were designed and compared.

2 Design Methodology

The designed antenna was fabricated using a low-cost FR4 substrate with 0.8 mm thickness and 4.4 relative permittivity. The dimensions of patch, ground plane, and slots were given in Table 1. Making use of defected ground structures and slots suppresses the surface waves, which in turn improves the bandwidth significantly.

Table 1 Dimensions of UWB antenna
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Figure 1a shows the slotted ultra-wideband patch antenna. Frequency notch band characteristics are obtained with the addition of radiating stub as shown in Fig. 1b.

Fig. 1
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a UWB antenna without notch element. b UWB antenna with notch element

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2.1 Design of Fractal Antenna

By using the fractal geometries in antenna design, multi-band response can be achieved. To achieve multi-band characteristics, this design Koch fractal geometry as shown in Fig. 2a–c is considered, and the results were compared for different fractal designs. The dimensions of slots were presented in Table 2.

Fig. 2
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a Koch fractal antenna with upper and bottom slots. b Koch fractal antenna with right and left slots. c Koch fractal antenna with slots in all sides

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Table 2 Dimensions of fractal antenna
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3 Results and Discussions

3.1 Parametric Study

Ultra-wideband response for the microstrip antenna is obtained by introducing bevel slots on both radiating patch and ground as shown in Fig. 1a. The return loss plot for design 1 (UWB antenna without notch element) is given in Fig. 3a.

Fig. 3
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a Return loss of UWB antenna without stub. b Effect of stub length on return loss

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Notch band characteristics for the rectangular microstrip printed patch antenna are obtained by optimizing the width and length of the stub placed on ground plane. The parametric analysis results for UWB antenna with additional stub in ground plane were presented in Fig. 3b, c. The optimized values for stub length and width are given in Table 1. For stub length and width A and B, designed antenna operates in UWB range, rejecting the band from 5 to 5.9 GHz. The optimized return loss plot was given in Fig. 3d.

Fig. 3
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c Effect of stub width on return loss. d Optimized return loss plot with single notch

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To achieve multi-band resonance, a Koch fractal is designed by two iterations. The return loss characteristics were given in Fig. 3e.

Fig. 3
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e Koch fractal antenna return loss for designs 1, 2, and 3, respectively

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Fractal geometries act as combination of inductors and capacitors. By properly designing the fractal structure, required multi-band resonance can be obtained. By using Koch fractal with all side slots, dual-band resonance at frequencies 3.3 and 5.5 GHz is obtained. The fabricated antenna and return loss plot for Koch fractal antenna are shown in Fig. 3f, g. Table 3 gives the comparison between three Koch fractal designs and notch band antenna design.

Fig. 3
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f Fabricated and simulated antenna. g Return loss plot for Koch fractal antenna with all side slots (red curve-simulated, blue curve-measured) and gain versus frequency plot

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Table 3 Comparison of resonant frequencies
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3.2 Return Loss

Return loss plots for simulated and fabricated antenna are shown in Fig. 3g. Results were same except for few deviations. From Fig. 3g, it should be noted that designed antenna works for both WiMAX (3.3 GHz) and WLAN (5.5 GHz) bands.

3.3 Gain Versus Frequency

As shown in Fig. 3h, gain values obtained at frequencies 3.3 GHz and 5.5 GHz are 3 dB and 2 dB, respectively.

Fig. 3
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h Electric field distributions at frequencies 3.3 and 5.5 GHz

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3.4 Electric Field Distribution

Field distribution at two resonant frequencies is shown in Fig. 3h. Maximum current is distributed at the edges of the patch which is responsible for maximum radiation at the edges.

3.5 Radiation Pattern

The two-dimensional far-field pattern for UWB antenna without and with stub is presented in Fig. 3i, j. Figure 3k shows the radiation patterns for resonant frequencies and lower and higher cutoff frequencies. It shows that in both cases, the antenna radiates in nearly omni-directional manner.

Fig. 3
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i 2-D radiation pattern for UWB antenna. a Without stub and b with stub

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Fig. 3
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j Radiation patterns in both E-plane and H-plane for frequencies 3.14, 3.3, 3.4, 5.16, 5.5, and 5.76 GHz

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The radiation plots for Koch fractal antennas were given in Fig. 3a–c.

Fig. 3
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k 2-D radiation pattern for Koch fractal antenna. a Upper and bottom slots, b right and left slots, and c all side slots

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4 Conclusion

The performance of the proposed antenna is evaluated for different fractal configurations. The antenna is simple and compact. The results show that the antenna can be used for multiple bands. As a future work, printed fractal antenna can be further extended to achieve the ultra-wideband characteristics.