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Proc. of Int. Conf. on Control, Communication and Power Engineering 2010

CPW-Fed UWB Slot Antenna with Reconfigurable Rejection Bands J. William and R. Nakkeeran Pondicherry Engineering College, Department of ECE, Pondicherry, India Email: wilsus@rediffmail.com, rnakeeran@pec.edu GHz. In this paper a new method is proposed to obtain the reconfiguration in the frequency notch bands for WiMax, WLAN 802.11a and HYPERLAN/2 systems by making ‘on’ and ‘off’ the diodes placed over the ground slot. This intern changes the tuning length of the slot, which is responsible for the desired frequency notch band. The simulation software used for this analysis is IE3D [16].

Abstract— A coplanar waveguide (CPW) fed Ultra Wideband (UWB) slot antenna with reconfigurable rejection bands is presented in this paper. The antenna has a rectangular slot with triangular structure that acts as a tuning stub with CPW feed. The notch band is achieved by inserting a rectangular slot in the ground plane with a length of λ/2. The rejection of the bands 3.1 GHz – 3.9 GHz, 4 GHz–5.3 GHz and 4.1 GHz–5.9 GHz are achieved by switching the diodes placed over the ground slot. The characteristics of the designed structure are investigated by using MoM based solver, IE3D. Detailed simulation results of the proposed antenna are presented in this paper. The simple configuration and low profile of the proposed antenna leads to easy fabrication that may be built in any wireless UWB device applications where reconfigurable rejection bands are required. Index Terms— coplanar waveguide, reconfiguration, slot antenna, UWB

notch

band,

I. INTRODUCTION The release of UWB by FCC in the year 2002, it gains a lot attention by the researchers due to its inherent properties of low power consumption, high data rate and simple configuration [1]. Designing an antenna to operate in the UWB band is quiet challenging one because it has to satisfy the requirements such as ultra wide impedance bandwidth, omni directional radiation pattern, constant gain, high radiation efficiency, constant group delay, low profile easy manufacturing etc [2]. Interestingly the planar slot antennas with CPW fed posses the above said features [3]. One of the major problems associated with UWB system is the interference from the other communication systems such as WiMax (3.3 – 3.7 GHz), IEEE 802.11a (5.15 - 5.35 GHz and 5.725 – 5.825 GHz) and HYPERLAN/2 (5.15 - 5.35 GHz and 5.47 – 5.825 GHz). To mitigate this effect many antenna designs have been proposed in the literature [4-8]. The rejection of interference is done either by inserting half wavelength slits, stripes in the tuning stub [9], or inserting stub in the aperture connected to the ground planes [10], or inserting square ring resonator in the tuning stub [11], or inserting ‘L’ branches in the ground plane [12], or with complementary split ring resonator [13], or inserting strip in the slot [14]. All the above methods are used for rejecting a fixed band of frequencies. To effectively utilize the UWB spectrum and to improve the performance of the UWB system, it is desirable to design the UWB antenna with reconfigurable notch band. It will help to minimize the interference between the systems. In [15] RF MEMS switches are used for the reconfigurabilty of rejection band between the frequencies 5 GHz to 6

Figure 1. Geometry of the proposed reconfigurable slot antenna.

II. ANTENNA GEOMETRY The structure of the antenna is shown in Fig 1. The antenna consists of rectangular slot with width ‘W1’ and length ‘L1’. The tuning stub comprises a triangular patch with height ‘H’. The distance between the tuning stub and feed line is ‘d’, ‘W’ and ‘L’ are the overall width and length of the antenna respectively. In this study, dielectric substrate FR4 with thickness of 1.6 mm with relative permittivity of 4.4 is chosen. The CPW feed is designed for 50 Ω characteristic impedance with fixed 2.4 mm feed line width and 0.5 mm ground gap. The effective length of the rectangular slot introduced in the ground plane is approximately 0.5 λeff at the desired notch center frequency. The effective wavelength of the slot is,

, ε

ε +1

≈ r

(1) 2 ε n eff where ‘fn’ is the centre frequency of the notch band. The placement of the diodes is desired by the effective wavelength for the different notch frequencies. eff

eff

Proc. of Int. Conf. on Control, Communication and Power Engineering 2010

III. SIMULATED RESULTS AND ANALYSIS

C. Gain The computed gain of the proposed UWB antenna for different tuning lengths of the slot in the ground plane and without slot is compared in Fig. 4. It is observed that there is a gain variation at the notched frequencies.

To evaluate the performance of the proposed antenna with reconfigurable slot, the optimal parameter values of the antenna suggested in our paper [17] are considered. However to study the impact of the slot, the slot length ‘L2’ and width ‘W2’ are varied by keeping one as constant. In the simulation switching diodes were simulated as a capacitor for the ‘Off’ state and as a resistor in the ‘On’ state. The current distribution, gain and group delay are also studied. The simulated return loss, with different states of the diode are shown in Fig. 2, indicates that the notch frequency and notch bandwidth is varied when the slot length is varied.

Figure 4. Simulated gain for different biasing conditions of diodes.

D. Radiation Pattern The radiation pattern for the E plane and H plane at frequencies 3.4 GHz, 5.2 GHz and 5.6 GHz are simulated for all the three cases of diode states are compared and displayed in Fig.5, which disclose that the directivity gain of the radiation pattern is reduced at the notch frequencies without affecting the shape of the radiation pattern. Figure 2. Simulated response curves for different biasing condition.

A. Effect of Slot on the Ground Plane The length of the ground slot is adjusted by switching the diodes placed appropriately over the slot. The switching diodes ‘D1’ and ‘D2’ are placed over the ground plane slot. Table 1 shows the states of the diode switches and notch frequency with notch band width.

(a) 3.4 GHz

TABLE I. DESCRIPTION OF SWITCHING OF THE DIODES D1

Notch Freq (GHz)

Notch band

Off Off On

Off On Off

3.6 GHz 5.1 GHz 5.6 GHz

3.1-3.9 GHz 4 – 5.3 GHz 4.1-5.9 GHz

(b) 5.2 GHz

B. Current Distribuition The simulated current distribution at different notch frequencies according to the switching condition of the diodes is presented in Fig.3; it is witnessed from the figure the current distribution around the radiation slot is disturbed by the introduction of ground slot, which is responsible for the notch in the frequency band.

E. Group Delay The group delay ‘τ’ of the antenna is calculated from the phase of the computed ‘S21’ by using the following equation and plotted in Fig. 6, dφ τ=(2) df where ‘ φ ’ is phase of S21 in radians /sec and ‘f’ is frequency in GHz. It is noticed that the variation in the group delay for the antenna with and without slot is around 2 ns only.

(a) (b) (c) Figure 3. Current distribution for different switching conditions of diodes. a) ‘D1’, ‘D2’ ‘Off’ b) ‘D1’ ‘On’ & ‘D2’ ‘Off’ c) ‘D1’ ‘Off’ & ‘D2’ ‘On’.

Proc. of Int. Conf. on Control, Communication and Power Engineering 2010

REFERENCES FCC NEWS(FCC 02-48), Feb. 14,2002. FCC News release. [2] M. Ghavami, L.B. Michael and R. Kohno, Ultra Wideband Signals and Systems in Communication Engineering, New York: John Wiley and Sons, USA, 2004. [3] K.L. Wong, Compact and Broadband Microstrip Antenna, John Wiley and sons. Inc., NY, USA, 2001. [4] I.-J. Yoon, H. Kim et al, “Ultra-wideband tapered slot antenna with band cutoff characteristic,” Electronics Letters, vol. 41, no. 11, pp. 629-630, May 2005. [5] A. M. Abbosh, M. E. Bialkowski et al, “ A Planar UWB antenna With Signal Rejection Capability in the 4–6 GHz Band,” IEEE Microwave and Wireless Comp. Letters, vol. 16, no. 5, pp. 278-280, May 2006. [6] Keyvan Bahadori and Yahya Rahmat-Samii, “Miniaturized Elliptic-Card UWB antenna with WLAN Band Rejection for Wireless Communications,” IEEE Trans. Antennas and Propag., vol.55, no.11, pp.3326-3332,Nov. 2007. [7] Chong-Yu Hong, Ching-Wei Ling et al, “ Design of a Planar Ultra- wideband Antenna With a New BandNotch Structure ,” IEEE Trans.Antennas and Propag. vol.55, no.12, pp.3391-3397, Dec. 2007. [8] Q.-X. Chu and Y.-Y. Yang , “ 3.5 / 5.5 GHz dual band-notch ultra-wide band antenna,” Electronics Letters, vol. 44, no. 3, pp. 172-174, Jan. 2008. [9] Yi-Cheng Lin and Kuan-Jung Hung, “Compact Ultrawideband Rectangular Aperture Antenna and BandNotched Designs,” IEEE Trans. Antennas Propag., vol.54, no.11, pp.3075-3081, Nov.2006. [10] Hyung Kuk Yoon, Yohan Lim et al, “ UWB Wide Slot antenna with Band -notch Function,” Proc. IEEE Antennas and Propag. society Intl Symposium, pp. 30593062, July 2006. [11] Wen-jun Lui, Chong-hu Cheng and Hong-bo Zhu, “Improved Frequency Notched Ultra wideband Slot Antenna Using Square Ring Resonator,” IEEE Trans. Antennas Propag., vol.55, no.9, pp.2445-2450, Sept.2007. [12] Yunlong Cai and Zhenghe Feng , “ A UWB Antenna with novel L branches on ground for Band-Notching Application ,” Proc. Of IEEE Intl. Conference on Microwave and Millimeter wave Tec., vol. 4, pp.16541657, April 2008. [13] The-Nan Chang and Min-Chi Wu , “ Band-Notched Design for UWB Antennas , ” IEEE Antennas Wirel. Propag. Letters, vol.7, pp.636- 639, 2008. [14] C.- Y. Huang, S.- A. Huang et al , “ Band-notched ultra-wideband circular slot antenna with Inverted Cshaped parasitic strip,”Electronics Letters , vol. 44, no.15, pp. 891-892, July 2008. [15] Symeon Nikolaou Nickolas D. Kingsley , George E. Ponchak, John Papapolymerou, and Manos M. Tentzeris, “UWB Elliptical Monopoles with a Reconfigurable Band Notch Using MEMS Switches Actuated without Bias Lines,” IEEE Trans. [16] IE3D 14, Zeland Software, Ins., Fremont, USA. [17] J. William and R. Nakkeeran , “A new compact CPW-fed wideband slot antenna for UWB applications,” Proc. of IEEE First Himalayan Intl. Conference on Internet , Nov. 2009. DOI 10.1109/ AHICI.2009.5340282. [18] Stanislas Licul and William A Davis, “Ultrawideband(UWB) antenna measurements using vector Network analyzer,” IEEE Antennas and Propagation International Symposium, pp. 1319-1322, 2004.

[1]

Figure 6. Group delay response.

IV. TIME DOMAIN ANALYSIS In ultra wideband systems, the information is transmitted using short pulses. To study the temporal behavior of the transmitted pulse, the channel is assumed to be linear time invariant (LTI) system. The transfer function of the entire system is computed using simulated value of ‘S21’ parameter [18]. The received output pulse is obtained by taking the Inverse Fourier Transform (IFT) of the transfer function and spectrum of the test input pulse. While computing ‘S21’, two identical antennas are placed face to face at a far-field distance of the antenna. The cosine modulated Gaussian pulse is considered for this analysis with centre frequency of 6.85 GHz and pulse width of 220 ps. The comparison of input and output responses of the system for the antenna with different notches are shown in Fig.7, which ensures the distortion less pulse transmission and also guarantees that the designed antenna is capable of transmitting and receiving short pulses.

Figure 7. Comparison of input and received pulses in time domain.

CONCLUSIONS This paper describes the simulated analysis of UWB slot antenna with reconfigurable rejection bands. By switching the diodes, the effective length of the ground slot is tuned to the notch center frequency. The simple configuration and low profile of the proposed antenna leads to easy fabrication that may be built for any wireless UWB device applications where reconfigurable rejection bands are required. The rejection of WiMax, IEEE 802.11a and HYPERLAN/2 bands can be achieved using the proposed antenna.