A Modified Circular Disc Monopole Ultra Wide Band Antenna Soubhi Abou Chahine, Maria Addam, Hadi Abdel Rahim, Areej Itani, Hiba Jomaa

Electrical Engineering Department, Beirut Arab University

P.O. Box: 11 50 20, Beirut, Lebanon

Tel: 961 1 300 110, Fax: 961 1 818 402, achahine@bau.edu.lb Abstract: In this paper we present a Modified Circular Disk Monopole Ultra Wide Band (UWB) antenna. The antenna covers a very large band from 1.5 to more than 15 GHz. The solution of this antenna consists in varying the radius of the patch and the ground dimensions. The proposed antenna designs and performances are analyzed using Ansoft High Frequency Structure Simulator (HFSS) . The radiation characteristics are as expected for monopole antennas.

I. INTRODUCTION Recently, due to the miniaturization of the personal communication devices, the need of compact antennas has become a necessity. Moreover, the growth of wireless systems operating in multiple independent bands or wide bands leads to the use of multi-functional antennas such as UWB. In general, The FCC in the USA has allocated a frequency band from 3.1 GHz to 10.6 GHz for UWB transmission [1]. Due to their capabilities for high data rate information transmission, Ultra-Wideband communication systems are highly promising. In addition, these systems have low power consumption and low interference and an immunity to multipath fading [2].

Conventional UWB antennas having spiral or log periodic geometries cause distortion to the radiated signal, since they radiate each frequency component form different part of the antenna; these antennas are said to be dispersive[3]. Lately, UWB antennas having adequate radiation characteristics, simple structure and fabrication have been proposed [4, 5]. Such antennas are broadband dipoles with circular, square, elliptical, pentagonal and hexagonal configurations [6]. In this paper, a microstrip circular patch antenna, based on previous studies [7], is proposed and investigated. The geometric antenna dimensions are controlled such that an ultra wide band response is obtained with a bandwidth of about 10 GHz and acceptable radiation pattern properties. Furthermore, the antenna can cover more applications such that (GSM1900, DCS, PCS1900, WCDMA/UMTS (3G), 2.45/5.2/5.8-GHz-ISM, UWB (3.1 - 10.6 GHz) etc. The paper is organized as follows: Section II describes the antenna geometry and its frequency response. Section III analyzes the characteristics of the antenna taking into account the effect of each parameter. In Section IV, some practical results are presented. Finally a conclusion summarizes the main characteristics of the proposed antenna.

II. ANTENNA GEOMETRY

The geometry of the proposed disc dipole antenna is illustrated in Figure-1.

Figure.1. Geometry of the proposed circular disc monopole antenna

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This antenna has a circular patch fed by a microstrip line which is tapered at the end to enhance the bandwidth. The circular disk with a radius r and the 50Ω microstrip feed line are printed on the same side of the FR4 substrate of thickness th=1.6 mm. L and W are the length and the width of the substrate respectively, the microstrip line has a width Wf and length Lf which is tapered for a distance S with an angle α. On the other side of the substrate a ground plane of length Lg is printed to cover the section of the microstrip feed line.

III. ANTENNA PARAMETER EFFECTS III-1 Return loss The initial design [7] with r = 10mm, W=42mm th=1.5mm and no tapering angle, had frequency range from 2.69 to 10 GHz as shown in figure-2.

Figure 2: Return Loss for Circular Disk Monopole (initial

design) The most design parameters affecting the performance are the tapering angle, the radius of the patch and the ground plane width. So these parameters are going to be investigated to reach the optimum design. Before reaching the optimum design, we passed through many steps of simulation. We started with the initial design dimensions without tapering in the feed line. But the thickness of the substrate (th) is 1.6 mm instead of 1.5 mm.

A. Effect of the Patch Radius: The first step of optimization was to increase the bandwidth by the variation of the patch radius. Figure-3 shows the return losses simulated at

different values of the radii (8, 10, 12, 12.5, 13 and 14mm)

Figure 3: Return Losses due to the Radius (r) Variation

As seen from the previous figure, r=12.5mm has the wider bandwidth but there is a small stop band, to get rid of it we will change other deign parameters such as the ground plane width W.

B. Effect of Ground Plane Width: In this step we varied the width of the ground plane to enhance the bandwidth. The different values of the ground width are: 30, 40, 42 and 50mm.

Figure 4: Return Loss due to Ground Width (W) variation

From figure-4 we can conclude that the optimum ground width (W) is 50mm which is equivalent to double the diameter of the circular patch.

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C. Tapering the Feed Line: So far the result is good but tapering the feed line by α=10˚ caused the return loss at high frequencies to drop. Finally the optimum obtained dimensions are summarized in table-1 Dimensions (mm) Dimensions (mm)

L 50 r 12.5 W 50 S 3 Lf 17.3 α 16˚ Wf 2.6 th 1.6 Lg 20

Table 1: Dimensions of Circular Disk MonopoleAntenna of

optimum design

These simulations were performed using the Ansoft High Frequency Structure Simulation (HFSS), [8]. Figure-5 shows the simulated return

loss curve with the optimal design, i.e. The simulated -10 dB return loss bandwidth is from 1.5 to more than 15 GHz, which conforms to the UWB standards. Moreover a considerable increase in bandwidth is obtained relative to the initial design.

Figure 5: Return Loss with the optimum design

III-2 Radiation patterns and Gain The simulated radiation pattern at 2.4, 3, 6.5 and 10.6 GHz are plotted in figure-6 to figure-9.

Figure 6: Gain at 2.4 GHz Figure 7: Gain at 3 GHz

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Figure 8: Gain at 6.5 GHz Figure 9: Gain at 10.6 GHz

It is noticed that, the simulated H-plane patterns are omni-directional at lower frequencies (2.4 and 3 GHz) and is near omni-directional at higher frequencies (6.5 and 10.6 GHz) as shown in the figures. The simulated E-plane patterns show that as the frequency is increased, slightnotches start to form and pattern shows more directionality around 105 o and 160o.

Table-2 illustrates the simulated gain of the proposed antenna with different frequencies. Frequency

(GHz) 3.1 6 9 10.6 13 15

Peak Gain

(dB) 0.86 3.67 4.99 5.38 3.38 5.53

Table 2: Simulated Gain versus Frequency

IV. MEASURED RESULTS The fabricated antenna is shown in figure-10. The antenna was fabricated from a double layer PCB. The radiating patch and the microstrip line where etched on one side and the ground plane was etched on the other side.

Figure10: Fabricated antenna To measure the return losses, a scalar network analyzer, a hybrid coupler , and a sweep generator (F > 20 GHz) are used.

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The simulated and measured return losses are plotted as shown in figure-11.

Figure 11: Measured and Simulated Return Loss for the

Circular Disk Monopole This chart shows a satisfactory agreement between both results regarding the RL (less than -10 dB). To determine the maximum gain of the Circular Disk Monopole antenna, two identical prototypes are fabricated and placed opposite to each other with a separating distance d. This distance is calculated according to the far field condition:

λ

22Dr = , where D is the largest dimension of

the antenna aperture. For two identical antennas, operating at the same center frequency, the gain is defined as:

λπr

dBmtro WWG 421 log10)( +−=

In this equation, Go is the antenna gain, Wt is the transmitted power in dBm, Wr is the received power in dBm, r is the distance between the two antennas, and λo is the wavelength. The equipment used to measure Wt and Wr

comprises a signal generator and a power meter. The measured gain at 2.4 GHz frequency was 0.25 dB, which is near the theoretical value.

V- CONCLUSION In this paper, a Circular Disc Monopole was proposed having a bandwidth from 1.5 GHz to more than 15 GHz. The designed antenna has simple configurations and is easy to fabricate. It has been shown that the performance of this antenna in terms of its frequency domain characteristics is mostly dependent on the patch radius, ground plane width and tapering the feed line. It is demonstrated by simulation that the proposed antennas can yield an ultra wide bandwidth, and that the radiation patterns are nearly omni-directional over the entire 10 dB return loss bandwidth. The Circular Disk Monopole antenna can be used for many applications including 3G, Wi-Fi, WiMAX, as well as UWB applications.

REFERENCES [1] FCC Report and Order for part 15 Acceptance of Ultra Wideband (UWB) Systems from 3.1 -10.6 GHz, Washington, DC, 2002. [2] N. P. Agrawall, G. Kumar, and K. P. Ray, “Wide-band planar monopole antennas”, IEEE Tran Atennas Propag., vol. 46, no. 46, no. 2, Feb. 1998. [3] H. G. Schantz, “Ultra wideband technology gains a boost from new antennas”, Antenna Syst. Technol., vol. 4, no. 1, Jan./Feb.2001. [4] J.Liang, C,Chiau, X.Chen, and C.G. Parini, “Analysis and design of UWB disc monopole antennas”, in The IEE seminar on Ultra Wideband Communications Technologies an System Design, 2004 [5] M. J. Ammann and Z. N. Chen, “Wideband monopole antennas for multi-band wireless systems” IEEE Antennas Propag. Mag., vol. 45, no. 2, Apr. 2003. [6] Z. N. Chen and M. Y. W. Chia, “Broadband planar antennas; design and applications”, John Wiley & Sons, England, 2006. [7] J. Liang, C Chiau, X. Chen and C.G. Parini, “Printed circular disc monopole antenna for ultra wideband applications", IEE Electronic Letters, vol. 40, no. 20, September 30th, 2004, pp.1246-1248. [8] Ansoft High Frequency Structure Simulation (HFSS), Ver. 9.2 Ansoft Corporation.

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