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220 PIERS Proceedings, Moscow, Russia, August 1923, 2012

A Reduced-size Antipodal Vivaldi Antenna with a Reconfigurable

Band Notch

L. Safatly, M. Al-Husseini, A. El-Hajj, and K. Y. KabalanECE Department, American University of Beirut, Beirut 1107 2020, Lebanon

Abstract The paper presents the design and the implementation of a reduced-size antipodalVivaldi antenna with a reconfigurable band notch. The antenna could be mainly used for UWBapplications since it covers a very wide frequency band. It is also suitable for Cognitive Radio(CR) systems because it is capable to sense the spectrum to determine the bands used by primaryusers, and to communicate pulses that ensure interference avoidance to primary users. For that,a reconfigurable band stop filter, based on three nested complementary split-ring resonators(CSRRs), is integrated on the ground plane. The band notch occurs at 3.5GHz, 5.2GHz or7.3 GHz, and is controlled using electronic switches.

1. INTRODUCTION

The emerging broadband wireless communications and the huge number of medical and military

applications have put increasing demand on a technology suitable for high data rates in smalldistances. For that, the Ultra-Wide Band (UWB) [1] protocol has been gaining a lot of atten-tion. According to FCC regulations, UWB spectrum is limited between from 3.1 to 10.6 GHz. Inthe UWB environment, the transmitter and receiver antennas must be compact and lightweightand characterized by a gain stability, low distortion and low delays. Vivaldi antennas representan adequate candidate to these types of communications [2, 3] since they are listed as frequencyindependent antennas. In [4], the antipodal Vivaldi antenna is introduced and its broadband char-acteristic is highlighted. Although antipodal Vivaldi antennas satisfy UWB requirements, theirdimensions are normally greater than 10 cm. Therefore, size reduction of those UWB antennasis very challenging and was investigated by several researchers to include further enhancement onthe size and on the bandwidth. In [5], a compact directive antipodal Vivaldi antenna is designedwith dimensions of 52 52mm2. Also, a miniaturized 32 35mm2 antipodal Vivaldi antenna isintroduced in [6].

On the other hand, the current crowding of unlicensed spectra necessitates urgent solutions.Cognitive Radio (CR) [7] is a promising technology to solve the shortage problem and exploit theexisting spectrum in a revolutionized way. In a CR environment, unlicensed users are allowed toshare the frequency resources with the licensed or primary users. For that, CR implementationspave the way to new challenges in reconfigurable antenna design and RF front ends in general.The essential requirement in reconfigurability is to maintain a constant gain at different resonantfrequencies. This could be achieved by integrating a reconfigurable filter within the antenna struc-ture [8]. The antenna surface current distribution will not be affected and hence the radiationpattern will be preserved. The filter will modify adequately the bandwidth of the antenna byadding frequency nulls at desired frequencies.

In this paper, the design and the implementation of a reduced-size antipodal Vivaldi antenna ispresented. The antenna is equipped by a reconfigurable filter engraved on its feed line. The overallfilter-antenna system provides a UWB response with a reconfigurable frequency band notch.

2. ANTENNA CONFIGURATION

As depicted in Figure 1, the designed Vivaldi antipodal antenna is printed on a 60 35 1.6 mm3

FR4-epoxy substrate with a relative permittivity r = 4.4. The dual-sided structure of the antennais designed from the intersection of the quarter of two ellipses. The major and the secondary radiiof these ellipses are calculated according to [5]. The dimensions of the different parts are optimizedfor an impedance bandwidth covering frequencies starting from 1.5 GHz as shown in Figure 2.

To induce the band notch and achieve reconfigurability, a reconfigurable band stop filter, basedon three nested complementary split-ring resonators (CSRRs) is integrated on the ground plane.CSRRs are used in the literature to design reconfigurable and tunable bandstop and bandpassfilters, which in turn will be embedded in antennas used for cognitive radio applications [9]. In [10],a bandstop filter implemented in the patch of a UWB antenna and based on nested CSRRs ispresented. Compared to the work in [10], this proposed design has the advantage of implementing

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Progress In Electromagnetics Research Symposium Proceedings, Moscow, Russia, August 1923, 2012 221

(a) (b)

Figure 1: Antenna configuration. (a) Patch. (b) Ground plane.

Figure 2: Reflection coefficient of the antenna without the filter.

the notching mechanism as a filter, in the ground plane, which makes connecting the biasing linesa simple task and limit their degrading effect on the antenna performance. The filter is equippedwith three electronic switches S1, S2 and S3 having a size of 1 0.25mm2. Depending on the state

of a switch, the corresponding split-ring slot does or does not induce a band notch. When a switchis activated, the functioning of a larger CSRR is launched, and thus a notch at a lower frequencyoccurs. The CSRR sizes, depicted in Figure 3, are optimized so that the notch occurs at 3.5 GHz,5.2 GHz or 7.3 GHz. The following switching cases are considered: Case 1 when all three switchesare ON, Case 2 when only switch S3 is deactivated, Case 3 when S2 and S3 are deactivated, andCase 4 when all switches are OFF.

3. RESULTS AND DISCUSSION

The proposed antenna is designed and simulated using Ansoft HFSS [11]. A prototype is fabri-cated and shown in Figure 3, and the reflection coefficient is measured for the possible operationscenarios. As illustrated in Figures 4 and 5, adequate analogy is shown between the simulatedand measured results. For Case 1, in which all switches are ON, a UWB notch-free response isobtained. The results for Cases 2, 3 and 4, where the switches are sequentially activated, reveal

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222 PIERS Proceedings, Moscow, Russia, August 1923, 2012

(a) (b)

Figure 3: (a) Filter configuration. (b) Fabricated antenna photo.

Figure 4: Computed reflection coefficient for the different switching cases of the antenna.

Figure 5: Measured reflection coefficient for the different switching cases of the antenna.

a single notch in the 3.5 GHz, 5.2 GHz or 7.3 GHz band, respectively. A notch in a certain bandhelps to prevent interference to a licensed user or the service operated in that band. The antennahas an omnidirectional pattern and good gain values in its operation band. However, the patterns

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Figure 6: Radiation patterns at 3.5 GHz for Case 2 in the H-plane (blue) and E-plane (red).

are subject to slight degradation at high frequencies. The normalized computed radiation patternsfor Case 2, at 3.5 GHz, are shown in Figure 6. A clear omnidirectional pattern is revealed.

4. CONCLUSION

In this work, an antipodal Vivaldi antenna was proposed. It is compact-size and can operate overa very wide bandwidth, starting from 1.5 GHz. Due to the integrated filter on its feed line, theantenna has a reconfigurable band notch induced by CSRRs and controlled by electronic switches.Such design can be a possible candidate to be utilized in CR systems or in UWB applications.

ACKNOWLEDGMENT

This work was supported by the University Research Board (URB) of the American University ofBeirut (AUB).

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