An Ultra-wideband Printed Monopole Antenna with a Fractal. Based Reduced Ground Plane. Jawad K. Ali, Ali J. Salim, Ali I. Hammoodi, and Hussam Alsaedi.
Progress In Electromagnetics Research Symposium Proceedings, Moscow, Russia, August 19–23, 2012 613
An Ultra-wideband Printed Monopole Antenna with a Fractal Based Reduced Ground Plane Jawad K. Ali, Ali J. Salim, Ali I. Hammoodi, and Hussam Alsaedi Microwave Research Group, Department of Electrical Engineering University of Technology, Baghdad, Iraq
Abstract— Recently, the ultra-wideband (UWB) systems have attracted much attention because of its advantages including high speed data, small size, low cost, and low complexity. Consequently, the UWB antenna has received an increased attention due to its impedance bandwidth, simple structure and omni-directional radiation pattern. In this paper, the effects of the ground plane of a printed monopole UWB antenna, fed with a 50 Ω microstrip line, have been investigated. A Koch fractal based ground plane structure has been proposed as a means to enhance the UWB antenna performance. Different ground plane structures and feeding methods have been applied to a notched band monopole antenna structure that is a nearly square with embedded E-shaped slot. The proposed antenna has been supposed to be etched using a substrate with relative permittivity of 4.6 and thickness of 1.6 mm. Modeling and performance evaluation of the presented antenna designs have been carried out using a method of moments based EM simulator, IE3D. Simulation results have shown that the antenna with Koch based ground plane and asymmetrical feed offers larger fractional bandwidth of about 124%. By this increment in the antenna bandwidth, it is expected that by suitable dimension scaling of the enhanced bandwidth UWB antenna, many communication services below 3.1 GHz could be integrated with the UWB systems. 1. INTRODUCTION
Ultra-wideband (UWB) communication system is attracting more and more attention because of its advantages such as low power consumption, high data rate transmissions as in the multimedia communications, robustness against jamming, high degree of reliability etc. [1]. Consequently, an increased interest has been reported to the UWB antenna design. For portable devices, an additional challenge is encountered; the antenna has to be miniaturized. The printed UWB antenna has been found to be a good option because it can be easily embedded into wireless devices or integrated with other RF circuits [2]. In 2002, the Federal Communication Commission (FCC) officially released the regulations for UWB technology with allocated spectrum from 3.1 to 10.6 GHz for unlicensed UWB indoor medical, measurement and communication applications [3]. Since then, intensive research work has been devoted to the UWB antenna design. Regarding the purpose of reducing the potential interference between the UWB system and others operating at 5/6 GHz, the antennas reported in the literature can be classified into three categories. The first one includes antenna that are not characterized with a band notch in their return loss, or VSWR, responses [2–5]. In this context, microstrip fed printed monopole antennas having radiators with E-shape [2], swan-like shape patch with reduced ground plane [3], circular shape monopole with trapezoid shape ground [4], and octagon shape [5], are presented for UWB applications. In the other hand, the CPW feed line has been also used for UWB antennas with various possible slotted patch structures [6, 7]. The UWB antennas of the second category are characterized with a single 5/6 GHz band notch in their return loss responses [8–12]. Again, almost similar techniques have been adopted to achieve the UWB impedance bandwidth. Slotted elements of various shapes have been added to create the required notch in the antenna response. In addition, the use of the electromagnetic-bandgap (EBG) structure is proven to be effective create the required band notched response [13]. Elliptical monopoles with CPW feeds were fabricated on liquid crystal polymer (LCP) with reconfigurable 5/6 GHz band-notch characteristics has been presented in [14]. In the third category, antennas are characterized with two band notches in their return loss, or VSWR, responses [15–20]. In this paper, the effects of the ground plane of a printed monopole UWB antenna have been investigated. In an attempt to enhance the UWB antenna performance a new fractal based ground plane structure has been proposed. Many ground plane structures and feeding methods have been applied to a notched band monopole antenna structure that is a nearly square with E-shaped slot embedded in it. It is expected that by suitable dimension scaling of the enhanced bandwidth UWB antenna, many communication services below 3.1 GHz could be integrated with the UWB systems, as recently reported in [21–23].
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2. THE ANTENNA DESIGN
The proposed UWB antenna is a printed monopole with a nearly square shaped radiator. An E-shaped slot has been cut in the radiator to produce the notched band. On the other side of the substrate, a reduced ground plane has been printed. The geometry of the proposed UWB printed monopole antenna is shown in Figure 1. The antenna is to be modeled using an FR4 substrate with thickness of 1.6 mm and relative permittivity of 4.6. For design convenience, the proposed antenna is fed by a 50 Ohm microstrip line printed on the radiator side of the substrate. The feed line width is of about 3 mm, and is symmetrically located with respect to both the radiating element and the ground plane. On the front surface of the substrate, a nearly square radiating patch with initial dimensions of 13.45 × 14.55 mm2 , has been etched; while on the other side of the substrate, a conducting ground plane of 19.90 × 9.20 mm2 is placed. An E-slot is etched on the rectangular radiating element with slot width, WM S = 6.25 mm, slot length, LM S = 9.05 mm, and slot trace width, LT S = 1.5 mm. The slot is symmetrically cut in the X-axis, while it is away from the upper edge of the radiating element by a distance, WT = 1.70 mm. A printed monopole antenna with E-slot has been designed to resonate with the lower frequency is located at 3.1 GHz, as a starting step. After suitable dimension scaling, the resulting antenna radiating element length, LE , has to be determined. Observing the influence of the various parameters on the antenna performance, it has been found that the dominant factor in the antenna is the monopole element perimeter, 2(LE + WE ), in terms of the guided wavelength λg . λ0 λg = √ εeff
(1)
where εeff is the effective dielectric constant. Then the lower resonant frequency, fL , relative to the radiating element length is formulated by fL ≈
Co √ 2(LE + WE ) εeff
(2)
where Co is the speed of light in free space. In this paper, the effects of the ground plane of the printed monopole UWB antenna have to be investigated. The previously designed antenna has been considered as reference for the sake of comparison with other antennas with different feeding and ground planes. This antenna will be referred to as Ref. Ant. In an attempt to enhance impedance bandwidth, for S11 ≤ −10 dB, of this antenna, a modified reduced ground plane is proposed. The proposed ground plane has been modified by making its top edge in the form of the 2nd Koch fractal geometry. Three other antennas have been presented and referred to as Ant. I, Ant. II, and Ant. III, as shown in Figure 2, where the proposed Koch fractal based ground plane structure has been depicted. The top edge of the reduced ground plane has been modified to be in the form o the 2nd iteration Koch fractal geometry for Ant. II and Ant. III.
Figure 1: The geometry of the modeled reference antenna.
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Figure 2: The four modeled UWB antennas with different feed positions and ground planes as depicted in Table 1. Table 1: Summary of the modeled antennas and the corresponding ground planes, feeding method and the realized fractional bandwidths. Antenna Type Ref. Ant Ant. I Ant. II Ant. III
Ground Plane Reduced GP Reduced GP Koch Based Reduced Koch Based Reduced
Feed Method Symmetrical Asymmetrical Symmetrical Asymmetrical
Resulting Bandwidth % 105 113 124 118
Figure 3: Return loss responses of the four UWB antennas with the different ground plane structures.
3. PERFORMANCE ASSESMENT AND SIMULATION RESULTS
The four UWB printed monopole antennas, depicted in Figure 2, have been modeled and their performances have been evaluated using the commercially available software IE3D, from Zeland Software Inc. [24]. These antennas have the same parameters as previously stated; the only change is the ground planes and the feeding methods. Table 1 summarizes the related differences among these antennas. Figure 3 presents the simulated return loss responses of the four antennas. It is implied that at low frequencies, below 6.0 GHz, the four antennas perform equally, since the lowest resonant frequency, according to (2), is primarily determined by the monopole radiating element parameters. Furthermore, the position of the notched band, from 4.41 to 5.98 GHz, has not changed, because it is attributed to E-slot parameters as prescribed. The effects of the ground planes of the modeled antennas started beyond this frequency. In this context, the four antennas offer different fractional bandwidths, as it is clearly shown in Figure 3. The realized fractional bandwidths corresponding to each of these antennas are summarized in Table 1. It is clear that antennas with the modified ground planes, Ant. II and Ant. III, possess the largest fractional bandwidths. However, Ant. II, with Koch based ground plane and symmetrical feed, offers larger fractional bandwidth of about 124%. By this increment in the antenna bandwidth,
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Figure 4: Simulated current distributions on the surface of the Ant. II at different frequencies.
it is expected that by suitable dimension scaling of the enhanced bandwidth UWB antenna, many communication services below 3.1 GHz could be integrated with the UWB systems. 4. CONCLUSIONS
The use of a new fractal based modified ground plane of the printed UWB antenna has been investigated in this paper as a means to enhance its fractional bandwidth. The 2nd iteration Koch fractal geometry has been applied to shape the ground plane of a printed UWB monopole antenna with symmetrical and asymmetrical feeding positions. It is found that at low frequencies, below 6.0 GHz, the modeled antenna perform equally independent on the type of the ground plane and the feeding method. The use of the Koch fractal based ground plane has proved it impact in the resulting fractional bandwidth. Simulation results show that the resulting bandwidth extends to 13.29 GHz, beyond the extent required by the FCC standards. By this increment in the antenna bandwidth, it is expected that by suitable dimension scaling of the enhanced bandwidth UWB antenna, many communication services below 3.1 GHz could be integrated with the UWB systems. Additional work could be carried out to explore the use of other fractal geometries on the UWB antenna performance. REFERENCES
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