IEEE ANTENNAS AND WIRELESS PROPAGATION LETTERS, VOL. 11, 2012 411

Very Compact Printed Triple Band-Notched UWBAntenna With Quarter-Wavelength Slots

Dang Trang Nguyen, Dong Hyun Lee, and Hyun Chang Park

Abstract—A very compact coplanar waveguide (CPW)-fedultrawideband (UWB) printed monopole antenna (PMA) withtriple band-notched characteristics is presented. The antennauses three open-ended quarter-wavelength slots to create tripleband-notched characteristics in 3.3–3.7 GHz for WiMAX,5.15–5.825 GHz for WLAN, and 7.25–7.75 GHz for downlinkof X-band satellite communication systems, respectively. Theopen-ended quarter-wavelength slot is analyzed in detail. Surfacecurrent distributions are used to show the effect of these slots.The antenna shows broad bandwidth and good omnidirectionalradiation patterns in the passband, with a very compact size of19 24 mm .

Index Terms—Monopole antenna, notched band, planar,quarter-wavelength slot, ultrawideband (UWB).

I. INTRODUCTION

I N THE last decade, the ultrawideband (UWB) technologyhas become one of the most promising technologies for

increasing data rate in wireless communication. The UWBantenna, which is an essential part of the UWB system, hasdrawn heavy attention from researchers. A UWB antennadesign requires broad bandwidth of 3.1–10.6 GHz [1] andgood omnidirectional radiation patterns. Among the proposedUWB antennas, the printed monopole antenna (PMA) is verypromising due to its remarkably small size, simple fabrication,and easy integration with compact RF front ends. In addition,the coplanar waveguide (CPW) feed lines have some attractiveadvantages compared to microstrip feed lines such as lowerdispersion at high frequencies, unipolar configuration, easyintegration with active devices, and minimal dependence onsubstrate thickness [2].For the integration of the UWB technology in the handheld

terminals that are becoming smaller and thinner each day,the design of a very compact UWB antenna covering thewhole operating frequency band is one of the most essentialrequirements. However, reducing the size of a PMA usuallybrings about reduced operating bandwidth. Moreover, there aresome narrow bands for other communication systems existingin the allocated wide bandwidth of the UWB system, suchas 3.3–3.7 GHz for WiMAX, 5.15–5.825 GHz for WLAN,and 7.25–7.75 GHz for downlink of X-band satellite com-munication systems. These bands may cause electromagnetic

Manuscript received November 10, 2011; revised December 25, 2011 andFebruary 20, 2012; accepted March 25, 2012. Date of publication April 03,2012; date of current version April 20, 2012. This work was supported by theBasic Science Research Program through the National Research Foundation ofKorea (NRF) funded by the Ministry of Education, Science and Technology(20110026869).The authors are with the Division of Electronics and Electrical Engineering,

Dongguk University, Seoul 100-715, Korea (e-mail: hcpark@dongguk.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.2012.2192900

interference with the UWB system. Therefore, it is necessaryto design UWB antennas with band-notched characteristicsthat can minimize the potential electromagnetic interferencewith these existing systems. According to these assessments, avery compact CPW-fed UWB PMA with triple band-notchedcharacteristics at the aforementioned bands and good radiationperformance is highly desired. In this letter, we propose anddemonstrate such an antenna.Various UWB PMAs with band-notched characteristics have

been recently presented. These antennas were designed withone notched band [3]–[8], two notched bands [9]–[13], orthree notched bands [14]–[16] at the aforementioned frequencybands. In order to obtain notched bands, slots of various shapeswere embedded in the patch or in the ground plane. In most ofthe cases, the band-notched function was achieved when thelength of the slot was about a half of the guided wavelengthcalculated at the intended notch frequency. The operationalprinciple of these half-wavelength slots was presented in [11].On the other hand, we have demonstrated the possibility ofusing an open-ended slot with the length of about a quarter ofthe guided wavelength to create a notched band at a relativelylow frequency of 3.5 GHz [16], and have anticipated a possiblereduction in size of the triple band-notched antenna with the useof quarter-wavelength slots instead of half-wavelength slots.In this letter, we propose and demonstrate a very compact

CPW-fed UWB PMA with triple notched bands at as high as7.5 GHz using only quarter-wavelength slots. Both the radiatingpatch and the ground plane are beveled to cover the entire UWBband from 3.1 to 10.6 GHz with VSWR less than 2. The antennameasures only 19 24 mm .

II. ANTENNA DESIGN AND ANALYSIS

Through simulationswith the software Ansoft HFSS, the finaloptimized design of the proposed antenna with a very com-pact size of 19 24 mm was obtained as shown in Fig. 1.This antenna was printed on the FR4 substrate with thicknessof 1.2 mm, relative dielectric constant of 4.4, and loss tan-gent of 0.02. The antenna consists of a 50- CPW feed lineand a planar radiating patch with three slots , , and . Thesingle-layered CPW-fed structures and the small total size of theantenna make it easy to integrate with RF front ends. Bevelededges of the radiating patch and the ground plane result in asmooth transition from one resonant mode to another, ensuringgood impedance match over a broad frequency range [9]. De-tails of the design procedure are as follows.

A. Basic Antenna Design

In this section, the basic antenna (without any of the slots ,, and ) covering the full UWB band is first described. The

effects of the geometric parameters of the radiating patch andthe ground plane are discussed.

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412 IEEE ANTENNAS AND WIRELESS PROPAGATION LETTERS, VOL. 11, 2012

Fig. 1. Geometry of the proposed antenna (units in millimeters) with (a) topview, (b) side view, and (c) fabricated antenna.

Fig. 2. Simulated VSWR of the basic antenna (without the slots , , and) with mm and different values of .

Fig. 3. Simulated VSWR of the basic antenna (without the slots , , and) with and different values of .

Fig. 2 shows the effects of varying the bevel angle of theradiating patch on the simulated VSWR with mm. Itcan be seen that the bandwidth for VSWR less than 2 increasesgreatly as increases from 30 to 40 . However, as in-creases to 50 , the impedance of the radiating patch and theinput impedance become mismatched at the middle frequenciesof the UWB band. Therefore, we decided on of 40 as theoptimum, resulting in the bandwidth from 3.4 to 10.7 GHz.The ground plane also affects the characteristics of the an-

tenna. Fig. 3 shows the simulated VSWR as varies from 0 to5.5 mm with . It can be seen that the bandwidth of theantenna increases as increases from 0 to 4.5 mm, as the loweredge of the bandwidth decreases from 3.4 to 3 GHz. Whenincreases to 5.5 mm, the antenna performance becomes worseat the middle frequencies of the UWB band without further in-crease in the bandwidth. Therefore, we decided on mmas the optimum with the bandwidth from 3 to 11 GHz, coveringthe entire UWB band.This effect can be further investigated in terms of leaky-wave

interaction between the ground plane and the feed line and theradiating patch. The leaky wave was presented through the

Fig. 4. Leakage current distribution in the substrate of the basic antenna withand (a) mm and (b) mm.

leakage current distribution as shown in Fig. 4. It can be seenthat the leakage current distribution from the ground planeis weaker in the edge regions (areas inside the black circlesin Fig. 4) when the ground plane is beveled. As a result, theoperational characteristics of the antenna become better asshown in Fig. 3.

B. Slot Analysis

The configuration of the slots is shown in Fig. 1(a). In thedesign, we used guided wavelength and

, where is the free-space wavelength [17]. Threeslots with the same line width were cut in the radiating patch,each with an open gap at the edge of the patch. The length ofslot mm, is about a quarter of the guided wave-length calculated at the center frequency of the WiMAX band,3.5 GHz. The length of slot mm, is about a quarterof the guided wavelength calculated at the center frequency ofthe WLAN band, 5.5 GHz, while that of slot mm,is about a quarter of the guided wavelength calculated at thecenter frequency of the downlink of X-band satellite communi-cation systems, 7.5 GHz. Due to the compact size of the radi-ating patch, we used an inverted L-shaped slot instead of astraight slot to ensure the quarter-wavelength characteristics.In our previous antenna [16], the performance of the open-

ended quarter-wavelength slot in getting the notched band be-came worse at high frequencies as the amplitude of at thenotched band—in other words, the level of band rejection—de-creased. Therefore, we had to use half-wavelength slots to ob-tain good band-notched functions at high frequencies, and theantenna could not be made smaller. To minimize the antennasize, use of quarter-wavelength slots only is much preferred.In order to provide a general guidance for the design of the

UWB antenna with notched bands created by quarter-wave-length slots, we carried out an analysis of the UWB antennashown in Fig. 1(a), but with slot only, to start with. The lengthof the slot was fixed at 8.2 mm. By analyzing the effect of thewidth, the position, and the angle of the slot in the radiatingpatch, we aimed to find the optimized configuration of the slotto get a good level of band rejection even at high frequencies.Fig. 5 shows the influence of the slot width on the simulatedVSWR of the antenna. It can be observed that both the ampli-tude and the bandwidth of the notched band for VSWR greaterthan 2 increase as increases from 0.2 to 0.5 mm. Whenincreases from 0.5 to 1.1 mm, however, the characteristics ofthe notched band show minimal changes. Therefore, the slotwidth was fixed at 0.5 mm. Fig. 6 shows the influence of thedistance on the simulated VSWR of the antenna. It can be

NGUYEN et al.: VERY COMPACT PRINTED TRIPLE BAND-NOTCHED UWB ANTENNA WITH QUARTER-WAVELENGTH SLOTS 413

Fig. 5. Influence of the width on VSWR of the antenna.

Fig. 6. Influence of the distance on VSWR of the antenna.

Fig. 7. Influence of the angle on VSWR of the antenna.

seen that both the amplitude and the bandwidth of the notchedband increase as increases from 2.5 to 6.5 mm. Fig. 7 showsthe influence of the angle on the simulated VSWR of the an-tenna with mm. Obviously, both the amplitude and thebandwidth of the notched band increase when increases from70 to 130 . Through Figs. 6 and 7, it can be observed that boththe amplitude and the bandwidth of the notched band increasewhen the slot gets closer to the antenna feed line (see the insetsof Figs. 6 and 7). In short, by reasonably choosing the positionand the angle of an open-ended quarter-wavelength slot, we canobtain a good notched band even at high frequencies.The proposed antenna with three notched bands was designed

based on the above conclusion. Additional points considered inthe design include the following. First, the angle was fixed at90 for all slots because multiple slots were employed. There-fore, only the positions of the slots were used to optimize theband-notched characteristics. Second, although both the ampli-tude and the bandwidth of the notched band increase when theslot gets closer to the antenna feed line, an unnecessarily widenotched band is not desired as this will reduce the useful band-width of the UWB band. Third, effects of possible interactionsbetween the slots were carefully observed. For example, Fig. 8shows the optimization of slot by changing while thepositions of slots and are fixed at mm and

mm, respectively. It can be seen that both the am-plitude and the bandwidth of the third notched band increaseas increases from 3.3 to 4.9 mm, but those of the secondnotched band decrease at the same time. Therefore, we decided

Fig. 8. Influence of the position of slot on VSWR of the antenna.

Fig. 9. Surface current distributions on the radiating patch at (a) a passbandfrequency of 4.5 GHz, (b) the first notched band at 3.5 GHz, (c) the secondnotched band at 5.5 GHz, and (d) the third notched band at 7.5 GHz.

on mm as the optimum position of slot . Theseprocedures had also been used to come up with mmand mm as the optimum positions of slots and ,respectively.Physical effects of the open-ended quarter-wavelength slot

have been explained in [16] using the concept of the effectivelength and transmission-line models. Each slot is modeled asa short-circuit-terminated stub in the transmission-line modelof the antenna. The first notched band, stub1, corresponding toslot , works as a quarter-wavelength transmission line termi-nated in a short circuit. Therefore, it behaves as an open-cir-cuited series stub with infinite input impedance, causing a totalimpedance mismatch between the feed line and the radiatingpatch. At the second and the third notched bands, slots and, respectively play the same role as .In order to observe the effects of slots , , and in getting

the notched bands, the surface current distributions on the radi-ating patch of the proposed antenna at four different frequen-cies are shown in Fig. 9. At a passband frequency of 4.5 GHz(outside the notched bands), the distribution of the surface cur-rent is uniform [Fig. 9(a)]. Meanwhile, in Fig. 9(b)–(d), we cansee stronger current distributions concentrated near the edges ofslots , , and at the center frequency of the first notchedband 3.5 GHz, the second notched band 5.5 GHz, and the thirdnotched band 7.5 GHz, respectively. These clearly show thepositive effects of the slots upon obtaining the band-notchedcharacteristics.

III. RESULT AND DISCUSSION

Fig. 10 shows the simulated and the measured VSWR ofthe proposed antenna. The measurement was performed with

414 IEEE ANTENNAS AND WIRELESS PROPAGATION LETTERS, VOL. 11, 2012

Fig. 10. Simulated and measured VSWR of the antenna with three notchedbands.

Fig. 11. Measured and simulated radiation patterns in the H- and the E-planesat (a) 3.5, (b) 4.5, (c) 5.5, (d) 6.5, (e) 7.5, and (f) 9.5 GHz.

Fig. 12. Calculated (solid line) and measured (square marker) maximum gainof the antenna.

an Agilent 8719A network analyzer. Measured data showgood agreement with the simulation. The antenna with threeopen-ended quarter-wavelength slots successfully exhibits threedesigned notched bands of 3.2–3.7, 4.8–6.3, and 7.0–8.15 GHz,otherwise maintaining broadband performance from 2.45 to10.65 GHz with VSWR less than 2, covering the entire UWBfrequency band. The measured and simulated radiation patternsin the E- ( -) and the H- ( -) planes at 3.5, 4.5, 5.5, 6.5, 7.5,and 9.5 GHz are shown in Fig. 11(a)–(f), respectively. At thepassband frequencies out of the notched bands (4.5, 6.5, and9.5 GHz), the antenna displays good omnidirectional radiationpatterns in the H-plane and dipole-like radiation patterns inthe E-plane as shown in Fig. 11(b), (d), and (f). Meanwhile,at the notched-band frequencies (3.5, 5.5, and 7.5 GHz), theantenna displays distorted and unstable radiation patterns asshown in Fig. 11(a), (c), and (e). The calculated and measuredmaximum gain of the antenna is shown in Fig. 12. Threesharp reductions at the three notched bands clearly confirmthe signal-rejection capability of the proposed antenna. Thecalculated average radiation efficiency using Ansoft HFSS

is about 94.6%. By using only quarter-wavelength slots tocreate band-notched characteristics, this antenna was realizedin a much smaller size compared to the previous antenna withsimilar functionality [16].

IV. CONCLUSION

A very compact CPW-fed UWB printed monopole antennawith triple band-notched characteristics was proposed, fabri-cated, and discussed. The three designed notched bands wererealized by etching three open-ended quarter-wavelength slotsin the radiating patch. The effects of the width, position, andthe angle of the slot in the radiating patch were analyzed to findthe optimized configuration of the slot to get a good level ofband rejection even at high frequencies. Surface current distri-butions were used to show the effect of these slots in getting thenotched bands. The fabricated antenna showed good agreementbetween measured and simulated results with a wide bandwidthfrom 2.45 to 10.65 GHz and three intended notched bands in asmall size.

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