Small Printed Ultra Wideband Antenna with Coupled Slot - IEEE Xplore

2009 Loughborough Antennas & Propagation Conference

16-17 November 2009, Loughborough, UK

Small Printed Ultra Wideband Antenna with Coupled Slot Yusnita Rahayu #1, Tharek A. Rahman 2, Razali Ngah 2, Peter S. Hall 3 #1

Faculty of Mechanical Engineering, Universiti Malaysia Pahang, Kuantan, Malaysia 2 Wireless Communication Centre (WCC), Faculty of Electrical Engineering Universiti Teknologi Malaysia, Johor Bahru, Malaysia 3 Department of Electronic, Electrical and Computer Engineering University of Birmingham Edgbaston Birmingham, B15 2TT United Kingdom 1

[email protected]

Abstract— This paper presents a small printed ultra wideband (UWB) antenna with a coupled slot. This proposed antenna uses coupled L and U slots to broaden the bandwidth. A truncated ground plane is designed to control the antenna’s impedance matching. Both slots were designed by investigating the behaviour of the current distribution of the patch antenna. It is shown that by proper design of the slot width and length of those coupled slots, without degrading their performance, a broad bandwidth can be obtained. The antenna’s dimension is 30 mm x 30 mm and fed by microstrip line. Both simulated and measured results have shown a good return loss with respect to -10 dB and nearly omni-directional pattern over 2.5 GHz to 10.1 GHz. Thus, this proposed antenna is as potential candidate in UWB application.

I. INTRODUCTION In recent years, UWB antennas have experienced many significant developments and have been getting more and more popular due to its simple structure, low cost, low power consumption, wide bandwidth and high data transmission rate. UWB antennas have to be able to transmit pulses as accurately and efficiently as possible. The spectrum allocated certainly requires transmitters and receivers with wideband antennas. The main challenge in UWB antenna design is achieving the extremely wide impedance bandwidth while still maintaining high radiation efficiency. By definition, an UWB antenna must be operable over the entire 3.1 GHz to 10.6 GHz frequency range [1]. Therefore, the UWB antenna must achieve almost a decade of impedance bandwidth, spanning 7.5 GHz. Today the state of the art of UWB antenna focuses in the microstrip, slot and planar monopole antennas with different matching techniques to improve the bandwidth ratio without loss of its radiation pattern properties [2]. Many research papers on monopole UWB antennas have been reported in many literatures [3-6]. In this paper, a small printed UWB antenna is presented. The proposed antenna consists of a coupled slot, L and U, with truncated ground plane. The slot’s shape is designed by studying the current distribution which will give input impedance improvement. The truncated ground plane is used to control the antenna’s impedance matching. Both simulated and measured results have shown a good return loss over the

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entire bandwidth, the effect of each slot to the current distribution will be discussed as well. II. ANTENNA DESIGN Fig. 1 shows the proposed antenna structure printed on the FR4 substrate of r = 4.6. The pentagonal antenna is vertically installed above a ground plane (lgrd) of 11 mm. The optimum feed gap (h) to the ground plane is found to be 1.5 mm. The dimension of substrate is chosen to be 30 x 30 mm2 (Wsub x Lsub) in this study. The antenna has a pentagonal shape with a width (w) of 15 mm and a length (l) of 12 mm. This shape is variation of the rectangular shape with bevel techniques. The slot size of the proposed antenna is listed in Table 1. The slot width is 0.5 mm in order to improve the bandwidth above 10 GHz.

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(a)

(b) Fig. 1 (a) Geometry and coordinate system of the UWB coupled slot antenna, (b) Photograph of the coupled slot antenna



TABLE I DIMENSION OF L AND U SLOT Description

Slot length

L and U slots Symbol Is1 Is2 Is3 Is4

Size [mm] 6 9 3 6.5

Fig. 2 and Fig. 3 show the effect of a coupled slot on the current distribution of the antenna at 3 GHz and 6 GHz, respectively. Individual L and U slot current distribution is depicted as well. It is shown from the figures; the vertical current is concentrated near to the slots edges for both frequencies. It is also noted for both L and U slots, the current distribution is less on the area between the slots. While for the L slot only, the current intensity decreases on the area below the L slot. However, the U slot has less current on the area opposite to the U slot. It has been investigated that increasing the frequency reduced the current on the area between both L and U slots, below L slot and opposite the U slot. This is due to the current tending to distribute along the slot which correspondence to the resonance frequency.

Fig. 3 The simulated current distribution of L and U slotted antenna at 6 GHz

Fig. 2 The simulated current distribution of L and U slotted antenna at 3 GHz

Fig. 4a shows the simulated return loss comparison between antenna with and without L and U slotted on the monopole. The lower resonances for all antenna models are shifted slightly, but they are still around 5 GHz. The individual L slotted antenna has degraded the return loss at upper frequency rather than the individual U slotted antenna. It is clearly shown that L and U coupled slotted antenna has a good return loss performance with respect to -10 dB. To validate the simulation results, an antenna prototype was fabricated and tested. The comparison simulated and measured return loss for proposed a coupled slot UWB antenna is shown in Fig. 4b. The measured return loss is slightly shifted to the simulated one, but they still cover 2.5 GHz to 10.1 GHz as required. From the simulation, the U slot improves the upper dip resonance of 10.3 GHz and the L slot improves the lower dip resonance of 5.3 GHz. The coupling of both slots has shown a very good return loss below -10 dB. The length of L slot is 14.5 mm approximately equal to 0.25 at 5.3 GHz, and the length of U slot is 11.5 mm approximately equal to 0.4 at 10.3 GHz. In this prototype, measurements are done by using a coaxial port which is soldered at the bottom edge of microstrip line.

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Return Loss (dB)

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-20

-30 L slot only U slot only L and U slot without L and U slot

-40

E-plane at 4 GHz

-50 0

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Frequency (GHz)

(a)

Return Loss (dB)

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H-plane at 4GHz -30

Fig. 5 Simulated and measured radiation pattern at 4GHz

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Simulated L and U slotted antenna Measured L and U slotted antenna

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Frequency (GHz)

(b) Fig. 4 (a) Simulated return loss comparison between antenna with and without L and U slotted on patch radiator, (b) comparison measured and simulated return loss of a coupled slot antenna

However, some differences in the simulated and measured results are expected, since in the simulation model a discrete, and not coaxial, port is used. In reality the coaxial cable has a considerable effect, especially the length of its inner conductor, which is connected to the input of the antenna, creating an additional inductance. In addition, since the antenna is fed by a micro-strip line, misalignment can result because etching is required on both sides of the dielectric substrate. The alignment error results degradation to the antenna performance. Fig. 5 and Fig. 6 show the simulated and measured E and H planes for 4 GHz and 5.8 GHz, respectively. The results show that the radiation patterns are changing as the frequency increases. The H planes show a good omni-directional for both frequency ranges. The E-planes are relatively broad and slightly distorted at 5.8 GHz.

E-plane at 5.8 GHz

H-plane at 5.8GHz Fig. 6 Simulated and measured radiation pattern at 5.8GHz

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III. CONCLUSION In this paper, a small printed coupled slot UWB antenna has been presented. This antenna uses two slots, L and U, with a truncated ground plane for bandwidth enhancement. An experimental prototype has been designed, fabricated and tested. The measured return losses cover the UWB bandwidth requirements of 2.5 GHz – 10.1 GHz with respect to -10 dB. The measured radiation patterns of this prototype are also presented at frequencies 4, and 5.8 GHz, respectively. REFERENCES [1] Federal Communications Commission, “Revision of part 15 of the commission's rules regarding ultra-wideband transmission systems,” First Report and Order, ET Docket 98-153, FCC 02-48, April 2002. http://www.fcc.gov. [2] M.A. Peyrot Solis, G.M. Galvan Tejada, and H. Jardon Aguilar, ”State of the art in ultra-wideband antennas,” in Proc. 2nd International Conference on Electrical and Electronics Engineering (ICEEE) and XI Conference on Electrical Engineering (CIE). Mexico city, Mexico, 7th - 9th September 2005. [3] Yusnita Rahayu, Tharek A. Rahman, Razali Ngah, P.S. Hall, “Slotted UWB antenna for bandwidth enhancement,” Loughborough Antennas & Propagation Conference (LAPC), Loughborough University, UK, 17th 18th March 2008. [4] Daniel Valderas, et al, “Design of UWB folded-plate monopole antennas based on TLM,” IEEE Transactions on Antennas and Propagation, Vol. 54, No. 6, pp. 1676-1687, June 2006. [5] A. A. Eldek, “Numerical analysis of a small ultra wideband microstrip-fed tap monopole antenna,” Progress In Electromagnetic Research, PIER 65, pp.59-69, 2006. [6] Yusnita Rahayu, Tharek A. Rahman, Razali Ngah, P.S. Hall,” A small novel ultra wideband antenna with slotted ground plane, ”International Conference on Computer and Communication Engineering (ICCCE), Kuala Lumpur, Malaysia, 13th - 15th May 2008.

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