IEEE ANTENNAS AND WIRELESS PROPAGATION LETTERS 959

Ultrawideband Antenna With Triple Band-NotchedCharacteristics Using Closed-Loop Ring Resonators

Mohammad Almalkawi, Student Member, IEEE, and Vijay Devabhaktuni, Senior Member, IEEE

AbstractThe design of a simple multilayered triple band-notched ultrawideband (UWB) antenna is presented in thisletter. The proposed antenna is compact in size, suitable forultrawideband applications, and exhibits triple narrow frequencyband notches to suppress the interference of the nearby wirelesscommunication systems within a UWB frequency range. Thenarrow band notches are realized by adding closed-loop ringresonators designed to cover the 3.33.7-GHz, 5.155.35-GHz,and 5.7255.825-GHz bands. The center frequencies of theband notches can be adjusted by varying the ring resonatorsmean diameters. The designed antenna has a compact volumeof 33 30 1.524 mm . The antenna is fabricated and testedproviding broadband impedance matching, appropriate gain, andstable radiation pattern.

Index TermsClosed-loop ring resonators, multilayered, tripleband-notched, ultrawideband (UWB) antenna.

I. INTRODUCTION

R ECENTLY, there has been increasing demand in de-signing ultrawideband (UWB) systems, and more partic-ularly so after the release of the frequency band 3.110.6 GHzby the Federal Communications Commission (FCC) [1]. Themain objective of UWB is the handling of high data ratesin the presence of wireless communication standards, whichcause electromagnetic (EM) interference with the UWB sys-tems, such as the Worldwide Interoperability for MicrowaveAccess (WiMAX) system operating at 3.33.7 GHz andwireless local area network (WLAN) system operating at5.155.35 and 5.7255.825 GHz. Since antennas are essentialcomponents for a host system application, it is desirable todesign UWB antennas that comprise narrow band notcheswithin the UWB frequency range. The early works on fre-quency band-rejected UWB antennas were realized by utilizingsmall strip bars [2][4], an open-loop resonator [5], U-shapedslots [6][8], an -shaped slot [9], a half-mode substrate inte-grated waveguide cavity [10], and a pentagonal radiating patchwith two bent slots [11]. In [2][9], however, the elementswere developed on the same layer within the antenna radiatoror on the back side of the same layer for generating single-

Manuscript received August 26, 2011; accepted September 01, 2011. Date ofpublication September 12, 2011; date of current version October 03, 2011. Thiswork was supported in part by the EECS Department and the College of Engi-neering of the University of Toledo, Toledo, OH, under graduate assistantshipsand a startup grant, respectively.The authors are with the Electrical Engineering and Computer Science De-

partment, University of Toledo, Toledo, OH 43606 USA (e-mail: Mohammad.Almalkawi@utoledo.edu).Color versions of one or more of the figures in this letter are available online

at http://ieeexplore.ieee.org.Digital Object Identifier 10.1109/LAWP.2011.2167649

Fig. 1. Structure of the proposed triple band-notched UWB antenna in threelayers configuration. (a) 2-D double-sided view of each layer. (b) Top view.(c) Side view.

and/or dual-frequency band-notched antennas. Therefore, dueto the space limitation, it is difficult to generate multiple bandnotches. On the other hand, in [10] and [11], the designs havecomplicated structures leading to increased fabrication costs,antenna size, and difficulty in the integration with microwaveintegrated circuits.The objective of this letter is to present a simple and com-

pact realization with stable radiation performance of a tripleband-notched planar antenna suitable for UWB applications. Itwill be shown that the proposed antenna in Fig. 1 possessesthe desirable feature of compactness while achieving an accept-able impedance bandwidth performance. Closed-loop ring res-onators have been utilized because of their narrow bandwidth,compact size, and low radiation loss [12] essential to ensuringrelatively omnidirectional far-field radiation pattern (which is

1536-1225/$26.00 2011 IEEE

960 IEEE ANTENNAS AND WIRELESS PROPAGATION LETTERS

originally due to the actual rectangular slot patch monopoleantenna).The organization of this letter is as follows. Section II de-

scribes the configuration of the proposed antenna. Section IIIpresents measured and simulated results, while conclusions aregiven in Section IV.

II. ANTENNA CONFIGURATIONThe monopole UWB antenna illustrated in Fig. 1, top side of

layer 3, is realized using a rectangular slot patch with a 50- mi-crostrip feed line. Triple-frequency band notches are achievedby vertically aligning three ring resonators (with their centersaligned along an axis) on multilayered planes. Here, each ringis responsible for creating a frequency band notch.The antenna is printed on three layers of Rogers substrate

(RO4003) with a dielectric constant of 3.55 and a loss tangentof 0.0027. The thicknesses of the layers have been arbitrarily se-lected, and each layer has a thickness of 0.508 mm as illustratedin Fig. 1(c). The antenna is connected to a 50- SMA connectorbetween the feed line on the top side of layer 3 and the groundplane on the bottom side of layer 1.Full-wave EM simulations are performed using ANSYS-

HFSS [13], leading to the following optimal dimensions for theproposed antenna: mm, mm, mm,

mm, mm, mm, mm,mm, mm, mm, mm,mm, mm, mm, mm,mm, mm.

In this design, the fundamental stopband center frequencycorresponding to each ring is approximately given by

(1a)

Therefore

(1b)

where is the mean diameter of the ring, is the guided wave-length, is the speed of light in free space, and is the ef-fective relative dielectric constant. The arrangement of the ringresonators could be either above and/or below the actual an-tenna radiator. Each ring in the structure resonates at differentresonant frequencies by varying the mean diameter. Rings onthe top and bottom side of layer 1 are responsible for generatingthe rejection bands for upper WLAN and WiMAX standards,respectively, while the ring on top of layer 2 is responsible forgenerating the rejection band for the lower WLAN standard.

III. EXPERIMENTAL RESULTS AND DISCUSSIONSIn this section, parametric studies have been carried out using

ANSYS-HFSS for providing a better understanding of the an-tenna operation. Moreover, measured impedance bandwidthand radiation pattern are discussed. For convenience, resultsin Figs. 2 and 3 have been obtained using one ring resonatorlocated on the top side of layer 2, with the other resonatorsremoved.Fig. 2 shows the variation of the VSWR with frequency for

different mean diameters of the ring resonator. Increasing the

Fig. 2. Simulated VSWR versus frequency by varying mean diameter of thering resonator located on the top side of layer 2.

Fig. 3. Simulated VSWR versus frequency by varying the axial position alongthe -direction and adjusting both width andmean diameter of the ring resonatorlocated on the top side of layer 2.

Fig. 4. Measured and simulated VSWR performance of the proposed antenna.

TABLE ICALCULATED AND OPTIMIZED RING RESONATORS PARAMETERSCORRESPONDING TO THE REJECTED CENTER FREQUENCY

AND THE REQUIRED BANDWIDTH

mean diameter of the ring resonator leads to negative shift inthe resonant frequency.

ALMALKAWI AND DEVABHAKTUNI: UWB ANTENNA WITH TRIPLE BAND-NOTCHED CHARACTERISTICS 961

Fig. 5. Measured far-field radiation pattern of the proposed antenna at (a) 4, (b) 7, and (c) 9 GHz.

From Fig. 3, it can be seen that for a given rejected centerfrequency, the bandwidth is a function of the axial position alongthe -axis ( ) as well as the mean diameter ( ) and the ringwidth ( ). By fixing the axial position ( ) and applying theabove analysis, the optimized dimensions for each of the ringresonators ( and ) were found using ANSYS-HFSS aftercalculating the mean diameter using (1b) as listed in Table I.Fig. 4 shows both measured and full-wave EM simu-

lated VSWR characteristics of the proposed antenna. Thefrequency performance demonstrates three rejection bands

with VSWR covering all of WiMAX and lower/upperWLAN applications, while maintaining VSWR out of therejection bands. A very good agreement between measuredand simulated results is observed. Slight discrepancies couldbe attributed to the effects of the SMA connector, which is notconsidered in our simulation.Measurements are carried out for the far-field radiation pat-

tern on the triple band-notched UWB antenna in the custom-built anechoic chamber with a triple-ridged horn antenna as areference at WRTest Labs, Belleville, MI [14]. Fig. 5 depicts

962 IEEE ANTENNAS AND WIRELESS PROPAGATION LETTERS

Fig. 6. Measured and simulated gains of the proposed antenna versus fre-quency.

Fig. 7. Photograph of the fabricated antenna. (a) Top view. (b) Bottom view.

the measured radiation patterns of the proposed antenna along-, -, and -planes at 4, 7, and 9 GHz, respectively. The-plane (i.e., -plane) patterns are almost omnidirectional forthe three frequencies, in a manner similar to the conventionaldipole antenna. On the other hand, the -plane (i.e., -plane)patterns are relatively similar to those of a monopole.Fig. 6 shows themeasured and simulated antenna gains, while

Fig. 7 shows a photograph of the fabricated antenna. The gainwas measured at the broadside direction as shown in Fig. 6,where two identical antennas are separated by a distance of0.98 m. The transmission coefficient was measured after afull two-ports calibration was carried out to a Rhode & SchwarzZVB20 vector network analyzer (VNA) and used to calculatethe antenna gain using the following formula [15]:

(2)

where , are respectively the transmitter and receiver gainand are equal in this case. , are the transmitted and re-ceived powers, and is free-space wavelength in meters.It is observed that the average gain of the proposed antenna is

about 24.5 dB over the entire operating band, exhibiting gen-eral flat gain performance. As well, it is clearly evident that atthe notch bands, the antenna gain drops sharply.

IV. CONCLUSION

A compact and simple multilayered triple band-notchedUWB antenna for WiMAX and lower/upper WLAN appli-cations has been proposed. The overall antenna volume is

33 30 1.524 mm . The antenna radiator design comprisesa rectangular slot patch fed with a 50- microstrip line.Triple band-notched frequencies are realized by adding threeclosed-loop ring resonators sharing the same vertical axial andimplemented on multiple layers. Having the ring resonatorssharing the same vertical axis led to straightforward fabrication.The antenna has been fabricated and measured for the purposeof validating our design. The performance of the proposedantenna exhibits a good impedance matching and radiationperformance.It is therefore possible to conclude that the presented antenna

will prove advantageous inmodernmultilayeredmicrowave cir-cuits such as microwave monolithic integrated circuits (MMIC)or low-temperature co-fired ceramic (LTCC) technologies.

ACKNOWLEDGMENT

The authors would like to express their gratitude toDr. M. Alam, Chair of the EECS Department, Universityof Toledo, Toledo, OH.

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