276 IEEE ANTENNAS AND WIRELESS PROPAGATION LETTERS, VOL. 9, 2010

Band-Notched Modified Circular Ring MonopoleAntenna for Ultrawideband Applications

Saswati Ghosh, Member, IEEE

Abstract—This letter proposes a planar modified circular ringantenna for ultrawideband (UWB) applications with band notchperformance. By suitably adjusting the antenna parameters,a return loss of 10 dB is achieved over the desired frequencyrange (3.1–10.6 GHz) except the notch frequency band. Theband-notched characteristic is achieved by introducing a tuningstub inside the ring monopole. The results for return loss andradiation pattern are simulated using electromagnetic simulatorWIPL-D. The time domain behavior of the antenna in terms of thefidelity factor is evaluated using Computer Simulation Software(CST) transient solver. A prototype antenna is fabricated, andthe simulated result for return loss is compared to the measuredresult.

Index Terms—Band notch antenna, modified ring monopole an-tenna, ultrawideband (UWB) antenna.

I. INTRODUCTION

T HE increased growth of ultrawideband (UWB) wire-less technology has motivated researchers to design

UWB antennas [1]–[5]. The planar monopole antennas—e.g.,circular, elliptical, annular, rectangular, square, hexagonal,pentagonal, etc.—are used to cover the entire UWB band-width (3.1–10.6 GHz) defined by the FCC [1]–[4]. Besides theUWB properties, another important requirement of the antennadesign is the band rejection characteristics, which aer usedto reduce the interference between the UWB transmitters ondifferent communication systems with overlapping frequencyband (e.g., Wireless LAN). Different novel antennas withband notch characteristics are designed and studied by otherresearchers [6]–[13]. The band rejection characteristics of theUWB antennas are achieved using 3D structures, stub, or twofolded-strip lines to the UWB antennas. The band-notchedmonopole antennas vertically kept on the ground plane are ofhigh demand due to their simple structure, ease of fabrication,and the desired UWB characteristics [8]–[11]. The UWB per-formance of an antenna in frequency domain is characterizedby the impedance matching characteristics, stable radiationpattern, and high efficiency over bandwidth. However, for the

Manuscript received January 16, 2010; revised February 24, 2010; acceptedMarch 09, 2010. Date of publication March 22, 2010; date of current versionApril 19, 2010. This work was supported in part by the Department of Scienceand Technology, New Delhi, India.

The author was with the Kalpana Chawla Space Technology Cell, IndianInstitute of Technology, Kharagpur 721302, India. She is now with the De-partment of Electronics and Communication Engineering, Durgapur Instituteof Advanced Technology and Management, Rajbandh, Durgapur 713212, India(e-mail: saswatikgp@gmail.com).

Color versions of one or more of the figures in this letter are available onlineat http://ieeexplore.ieee.org.

Digital Object Identifier 10.1109/LAWP.2010.2046391

Fig. 1. Geometry of the proposed antenna.

desired UWB performance, the time domain characterizationof the antenna is also required. The UWB antenna with notchfrequency may distort the time domain behavior of the originalantenna. The UWB performance of the transmit or receiveantenna in the time domain is characterized by the fidelity,which measures distortion between the time derivative of thevoltage applied to the antenna terminals to the transmittedwaveform or, equivalently, between the transient field incidentupon the antenna to the received voltage [14], [15].

In this letter, a simple, easy-to-fabricate, high-performancemodified circular ring monopole antenna placed vertically ona circular ground plane with UWB operating bandwidth and aband-notched characteristic is proposed. The band-notched fea-ture is achieved and easily controlled by introducing a tuningstub inside the ring monopole. For the simulation of the returnloss, current distribution, and radiation pattern data, the electro-magnetic software WIPL-D Pro ver. 5.1 is used [16]. The Com-puter Simulation Software (CST) is used for the simulation oftransmit and receive signal in time domain, which is used tocalculate fidelity of the antenna [17]. The prototype antenna isfabricated, and the result for the return loss of the prototype an-tenna is compared to the simulation data.

II. ANTENNA DESIGN

Fig. 1 shows the geometry of the proposed UWB ringmonopole antenna with band notch characteristics. The outerradius of the ring is R, and the inner radius is r. The circularring monopole is mounted on a circular ground plane of ra-dius mm with a feed gap of 2 mm. The band notchis achieved by the introduction of a rectangular tuning stubof length L and width W. The length of the tuning stub istaken approximately as quarter-wavelength at the desired notch

1536-1225/$26.00 © 2010 IEEE

GHOSH: BAND-NOTCHED MODIFIED CIRCULAR RING MONOPOLE ANTENNA FOR UWB APPLICATIONS 277

Fig. 2. Return loss versus frequency plot for various inner radius of the circularring monopole (without notch) with fixed outer radius � � �� mm.

Fig. 3. Return loss versus frequency plot for various stub length inside the cir-cular ring monopole (with notch) with � � ��mm, � � �mm, � � ��� mm.

frequency. At the resonant frequency of the stub, the surfacecurrent distribution of the ring monopole will be disturbed, thuscausing a mismatch of the antenna’s input impedance. All theseparameters are optimized to achieve the best performance.

III. SIMULATION RESULTS

The result for the return loss over the UWB frequency rangeis simulated using electromagnetic simulator WIPL-D [16]. Forthe simulation, the antenna is considered to be excited by adelta-gap source of 1 V. The antenna is placed on finite groundplane of radius of 75 mm. Fig. 2 shows the plot of return lossversus frequency for a ring monopole with different inner radius(without notch). Next, keeping the inner radius mm, thestub is introduced to achieve the band notch characteristics. Theplot of return loss versus frequency for various stub length isshown in Fig. 3. The surface current distributions on the modi-fied ring monopole (with stub) at the notch frequency and alsoat another frequency in the UWB are presented in Figs. 4 and5. The radiation characteristics of the proposed band-notchedUWB antenna are also studied at different frequencies and areshown in Fig. 6.

For the time domain simulation, CST Microwave Studiois used [17]. The two identical antennas are considered to beplaced side by side at 40-cm distances, i.e., in the far-fieldregion over the operating bandwidth (Fig. 7). The transmitantenna is excited by using a waveguide port. The input signal

Fig. 4. Surface current distribution for notch frequency 4.75 GHz of the mod-ified circular ring monopole with � � �� mm, � � � mm, � � ��� mm, � � mm.

Fig. 5. Surface current distribution for frequency 8 GHz of the modified cir-cular ring monopole with stub with � � �� mm, � � � mm, � � ��� mm, � � mm.

Fig. 6. Radiation pattern in the vertical plane at different frequencies of themodified circular ring monopole with stub with � � �� mm, � � � mm,� � ��� mm, � � mm. The angle is measured from the vertical z-axis.

of the excited waveguide port is normalized to 1 sqrt (W) peakpower, and the waveguide port realizes an input power of 1 Wover its entire port face. The receive antenna is connected with

278 IEEE ANTENNAS AND WIRELESS PROPAGATION LETTERS, VOL. 9, 2010

Fig. 7. Simulation of trans–receive antenna system using CST.

Fig. 8. Plot of normalized excitation waveform and the received voltage wave-form at the receiving antenna (without the stub) placed at a distance of 40 cmfrom the transmit antenna (without the stub).

Fig. 9. Plot of normalized excitation waveform and the received voltage wave-form at the receiving antenna (with the stub) placed at a distance of 40 cm fromthe transmit antenna (with the stub).

a lumped element of 50 impedance. The normalized time do-main waveform of the excitation voltage applied to the transmitantenna and the load voltage at the receive antenna without andwith the stub are shown in Figs. 8 and 9, respectively.

The time domain characteristic of the antenna is measured interms of the fidelity of the antenna. The fidelity is defined by thefollowing equation [15]:

(1)

According to (1), the fidelity corresponds to the maximum mag-nitude of the cross correlation between the normalized observedresponse and the ideal response . Here, is calcu-lated as the normalized time derivative of the source waveform

, and is calculated using the normalized versions of thesimulated waveforms plotted in Figs. 8 and 9. This calculation

Fig. 10. Prototype band-notched modified circular ring monopole antenna.

Fig. 11. Plot of return loss versus frequency of band-notched modified circularring monopole antenna.

yields fidelity of approximately 0.99 and 0.97 for the antennawithout and also with the tuning stub, respectively.

IV. MEASURED RESULTS

The prototype antenna is fabricated for experimental verifi-cation (Fig. 10). The design parameters of the prototype an-tenna are as follows: mm; mm; mm;

mm. A circular finite ground plane of radius of 75 mmis used for the measurement. The circular ring with the stub issupported on the ground plane by an SMA connector. The cir-cular ring is soldered with the pin of the connector protrudingthrough the hole made in the ground plane. The length of thepin is 2 mm, which determines the height of the ring above theground plane. For the simulation, this pin is taken as a wire ofdiameter 1 mm and length 2 mm. A comparison between thesimulated (using WIPL-D and CST) and measured return lossusing HP 8757 C network analyzer is presented in Fig. 11.

V. DISCUSSION

The return loss versus frequency plot of the ring monopole(Fig. 2) shows that by adjusting the inner radius of the ring fora fixed outer radius, an impedance bandwidth defined by 10 dBreturn loss is achieved over 3.1–10.6 GHz. Fig. 2 also shows thatthe desired bandwidth is achieved for mm. It is noticedthat a hike in the return loss over a certain frequency band isachieved by including the stub. By tuning the length of the stub,the notch frequency can be adjusted (Fig. 3). At the resonantfrequency of the stub, the surface current distribution of the ringmonopole is disturbed, thus causing a mismatch of the antenna’s

GHOSH: BAND-NOTCHED MODIFIED CIRCULAR RING MONOPOLE ANTENNA FOR UWB APPLICATIONS 279

input impedance (Figs. 4 and 5). The radiation patterns at dif-ferent frequencies within the UWB range (Fig. 6) show goodomnidirectional radiation patterns, which are about the sameas those of the corresponding simple circular-disk monopoleantenna. The time domain characteristics of the antenna arestudied in terms of the fidelity of the antenna, which is achievedfrom the excitation and received voltage waveform using CSTfor antenna structures without and with the stub (Figs. 8 and9. The fidelity values calculated for the displayed waveformsare equal to 0.99 and 0.97 for the antenna structures withoutthe stub and also after inclusion of the tuning stub for a par-ticular distance of 40 cm. The result indicates a good matchingbetween the excitation and received waveform. Also, it is no-ticed that the band notch characteristic is achieved for the mod-ified planar ring monopole antenna without major compromisein the fidelity. It can be observed from Fig. 11 that a good agree-ment between the measurements and simulations is obtained,though a small shift of the notch frequency band is noticed forthe measured data. Though sincere effort was made to minimizethe measurement error, the deviation of the inner and outer radiiin some portions of the fabricated antenna from the actual an-tenna structure causes the difference in the measured and sim-ulated results. For better matching, a refined antenna prototypeis required. However, the measured return loss data shows thesame impedance bandwidth as that of the simulation data.

VI. CONCLUSION

This letter presents the initial investigation on the perfor-mance of a modified planar ring monopole antenna as an UWBband-notched antenna. The simulated results for the return lossshow that the desired bandwidth with desired frequency notchcan be achieved and easily controlled by adjusting stub lengthand the other parameters of the antenna. The omnidirectionalradiation pattern of a conventional monopole antenna is main-tained over the frequency range. The high fidelity value of theantenna proves that the notch structure maintains the desiredtime domain characteristics of the original antenna.

ACKNOWLEDGMENT

The useful comments of the reviewers are gratefully appreci-ated. The author is grateful to Prof. A. Chakrabarty, Departmentof Electronics and Electrical Communication Engineering(E & ECE), Indian Institute of Technology, Kharagpur, India,for his valuable comments on various technical aspects of thiswork. The author would also like to thank the technical staffs

of the Microwave Measurement Laboratory, Department ofE & ECE and Kalpana Chawla Space Technology Cell, IndianInstitute of Technology, Kharagpur, for helping in performingthe experiments.

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