Planar Circular Disc Monopole Antennas Using Compact ImpedanceMatching Networks for Ultra-Wideband (UWB) Applications

Mohamed Nabil Srifi 1, Symon K. Podilchak 2, Mohamed Essaaidi 1, Yahia M.M. Antar 2

1Electronics & Microwaves Group, Faculty of Science,Abdelmalek Essaadi University, Tetuan, 93000, Morocco

srifimn@gmail.com, essaaidi@ieee.org

2Royal Military College of CanadaKingston, Ontario, Canada

antar-y@rmc.ca

Abstract— Two planar circular disc antennas that utilize com-pact impedance matching networks for current and future ultra-wideband (UWB) applications are investigated. The proposeddesigns have a physically small size (30 mm x 35 mm) andsimple geometry, and offer a relatively large bandwidth (BW).Input energy is launched through a 50-Ω feedline (attached toa K-Connector) and broadband antenna matching is improvedby introducing transitions between the feedline and the printedcircular discs. Specifically, two designs are investigated usingsingle- and duel-microstrip line transitions. By using this antennamatching technique, respective BWs (|S11|<-10 dB) of 3.18 to11.74 GHz and 3.47 to 31.94 GHz are obtained. Results arecompared to an analogous UWB monopole with no matchingnetwork transitions. Measured and simulated radiation patternsare presented along with reflection loss values up to 40 GHz.

Index Terms— Planar Circular Disc Monopole Antennas,Ultra-wideband (UWB) Antennas, Impedance Bandwidth (BW).

I. INTRODUCTION

Recently, there has been a considerable interest in the designof ultra-wideband (UWB) antennas for radar, imaging andcommunication systems. One of the strongest contenders interms of impedance bandwidth (BW) and radiation efficienciesare the circular and elliptical disc monopoles [1]-[4]. Thesedesigns can be made printed, allowing for simple fabricationand integration with associated electronics and UWB systems.

To improve the antenna matching over broad BWs varioustechniques have been proposed including feedgap optimization[5], bevels [6], ground plane slits and shaping [7], [8], multiplefeeding configurations and orientations [9], [10], and variationsin monopole shape [4]. In this work we propose a newmatching technique for printed UWB monopole antennas. Byintroducing microstrip transitions between the 50-Ω antennafeedline and the printed circular discs, reflection loss valuesbelow 10 dB can be realized from 3.47 to 31.94 GHz.

II. ANTENNA CONFIGURATIONS AND EXTENDEDBANDWIDTHS USING IMPEDANCE MATCHING

The proposed planar monopole antennas are shown in Figs.1 and 2. Circular discs with a radius of 7.5 mm and 50-Ωfeedlines (1.8 mm x 8 mm) were printed on a 30 mm x 35mm dielectric slab (εr = 3.38, h = 0.83 mm). Partial groundplanes (30 mm x 15.6 mm) were placed on the underside of the

Fig. 1. Investigated circular disc UWB antenna impedance matchedusing a 50-Ω feedline and a single-microstrip line transition.

Fig. 2. Modified circular disc UWB antenna using a duel-microstriptransistion and a 50-Ω feedline for increased impedance matching.

UWB antennas (Fig. 3) and the designs were attached to K-Connectors. Both designs were fabricated and measured, andsimulations were completed using a numerical solver (HFSS).

Reflection loss and beam pattern measurements of the twoUWB antennas were conducted in an anechoic chamber. Re-sults are compared to simulated values as well as an analogousmonopole with no matching transitions and only a 50-Ω feed-line (Figs. 4-10). A good agreement in terms of the antennabeam patterns and the operating BW, |S11|≤-10 dB, wereobserved. Deviations may be attributed to substrate variations,and fabrication tolerances, feed connector misalignment, andcopper cladding thickness variations due to microfabrication.

978-1-4244-2802-1/09/$25.00 ©2009 IEEE 782

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Fig. 3. Layout configuration of the proposed UWB antennas definedin the x−y planes. (a): Impedance matched using a single-microstripline. (b): Impedance matched using a duel-microstrip line.

A. Impedance Matching Using a Single-Microstrip Line

As shown in Fig. 1 and illustrated in Fig. 3 (a), a single-microstrip transition (1.4 mm x 8 mm) was introduced betweenthe feedline and the planar circular disc. Measured [Simulated]reflections loss values are below 10 dB from 3.18 - 11.74 [3.21- 10.48] GHz offering a BW of 8.56 [7.27] GHz as shown inFig. 5. This design offers minor performance improvements incomparison to a monopole with no matching transitions (Fig.4); for instance, simulated values of |S11| are below -10 dBfrom 3.34 - 10.32 GHz offering a BW of 6.93 GHz.

B. Increased Matching Using a Duel-Microstrip Line

To obtain an extended BW, the antenna was modified byintroducing an additional transition between the original 50-Ωfeedline and the planar circular disc as shown in Fig. 3 (b).Dimensions of the duel-microstrip transitions are 1.4 mm x 5mm and 1 mm x 3 mm, respectively. Measured [Simulated]reflections loss values are below 10 dB from 3.47 - 31.94 [3.46- 28.61] GHz as shown in Fig. 6 suggesting that indeed theoperating BW (|S11|≤-10 dB) can be extended.

C. Current Distribution & Radiation Characteristics

For the investigated monopoles numerous factors contributeto the broadband 50−Ω impedance match, including groundplane configuration, feedline orientation and transitions, anddisc dimensions. Consequently, by proper configuration ofthese parameters, good antenna matching can be achieved[1]-[4]. Essentially multiple resonances can occur on theplanar monopole configurations [3], and by the addition ofthe aforementioned microstrip transitions, input energy maybe efficiently radiated by the antennas at these frequencies.

Simulated current distributions are shown in Fig. 9 forthe antenna design using the duel-microstrip transition forfrequencies that exhibit reflection loss values below −20 dBand thus suggest good impedance matching. On the bottom ofthe antenna current is mainly distributed on the outer edge ofthe ground plane (near the disc), while on top of the antenna,current is mainly concentrated along the edge of the disc [3]near the ground plane. Multiple maxima can be observed.

Simulated BW of

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Fig. 4. Reflection loss values of the planar monopole using only a50-Ω feedline (no impedance matching). A simulated BW of 6.93GHz can be observed.

SimulationMeasurement

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SimulationMeasurement

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Fig. 6. Reflection loss values of the UWB antenna using theduel-microstrip feed configuration for increased impedance matching.

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Fig. 7. Measured beam patterns of the planar monopole antenna(using the duel-microstrip transition) in dBi at 3.5 and 30.0 GHz.

D. Measured and Simulated Antenna Beam Patterns

Gain pattern measurements were conducted for the twodesigns and results are shown in Figs. 7, 8 and 10. The E-plane beam patterns (x-y plane) illustrate traditional monopolecharacteristics at lower frequencies (f ≤ 8.6 GHz), while withand an increase in frequency, multiple notches are formed.High cross-polarizations levels are observed at 30 GHz in thex-y plane (Fig. 7) suggesting an upper frequency limit forthe proposed planar monopole antenna design using the duel-microstrip transition.

III. CONCLUSION

Two compact circular disc UWB antennas are presented.Measurements illustrate that the operating frequency of themonopole can be increased beyond 30 GHz (3.47 - 31.94GHz, BW of 28.47 GHz) by introducing two microstriptransitions between the 50-Ω feedline and the circular disk.The proposed antennas are planar, easy to fabricate and thusmay be attractive for current and future UWB systems.

REFERENCES

[1] H. Schartz, The Art and Science of Ultrawideband Antennas. House,Inc., 2005.

[2] B. Allen, M. Dohler, E. Okon, W. Malik, A. Brown, and D. Edwards,Ultra Wideband Antennas and Propagation for Communications, Radarand Imaging. John Wiley and Sons Inc. New Jersey: Wiley, 2007.

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Fig. 8. Measured and simulated co-polarization radiation beampatterns of the UWB antenna (using the single-microstrip transition)at 3.5, 5.2, 8.6, 11.4 and 13.8 GHz (values shown in dBi).

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Fig. 9. Simulated current distributions on the ground plane (bottom), the feedline, and the planar monopole disk (top) at 5.2, 9.4, 17.7, 23.6and 32.0 GHz for the duel-microstrip feedline configuration. Simulated frequencies are shown for when |S11| ≤ −20 dB (from Fig. 6).

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Fig. 10. Co-polarization beam patterns of the UWB antenna in dBi using the duel-microstrip transition at 16.1, 22.2, 25.2 and 27.0 GHz.

[4] A.M. Abbosh and M.E. Bialkowski, “Design of Ultrawideband PlanarMonopole Antennas of Circular and Elliptical Shape,” IEEE Trans. onAnt. and Prop., vol. 56, no. 1, pp. 17–23, Jan. 2008.

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[6] M.J. Ammann, “Control of the Impedance Bandwidth of Wideband PlanarMonopole Antennas Using a Beveling Technique,” Micro. Opt. Tech. Lett.,vol. 30, no. 4, pp. 229-232, Jul. 2001.

[7] C. Zhang and A.E. Fathy, “Development of an Ultra-Wideband Ellip-tical Disc Planar Monopole Antenna with Improved OmnidirectionalPerformance using a Modified Ground,” IEEE Int. Anten. Propag. Symp.,

Alburqueque, NM, 1689-1692, 2006.[8] X. L. Bao and M. J. Ammann, “Investigation On UWB Printed Monopole

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[10] M.J. Ammann and Z.N. Chen, “An Asymmetrical Feed Arrangement forImproved Impedance Bandwidth of Planar Monopole Antennas,” Micro.Opt. Tech. Lett., vol. 40, no. 2, pp. 156-158, Dec. 2003.

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