286 IEEE ANTENNAS AND WIRELESS PROPAGATION LETTERS, VOL. 5, 2006

Planar Monopole Antenna With Attached SleevesV. Zachou, C. G. Christodoulou, M. T. Chryssomallis, D. Anagnostou, and S. Barbin

Abstract—The analysis of a new printed antenna is presentedand discussed. This antenna consists of a printed monopole, withone or two sleeves on each side, fed by a coplanar waveguide (CPW)line. Switches are used to control the length of the monopole andthe sleeves and to tune the resonant frequencies of the antenna. Inthe case of the double-sleeved antenna, the switch is used to connector disconnect a second sleeve in the cactus antenna. Measurementresults show that the cactus antenna maintains the dipole-like ra-diation patterns for all the different resonant frequencies.

Index Terms—Coplanar waveguide (CPW), monopole antenna,reconfigurability, sleeves.

I. INTRODUCTION

MONOPOLE antennas have found widespread appli-cations, especially in wireless communications. The

increasing use of mobile communication systems has stimulatedthe interest in the dual-frequency designs [1]–[3]. A planarmonopole has been considered as an attractive printed antennafor wireless communications, because of its simple structureand omnidirectional radiation pattern. The coplanar waveguideapproach is proposed as a feeding transmission line medium forthese antennas. It is well known that coplanar waveguide (CPW)feed lines are uniplanar structures, which are compatible withmonolithic microwave integrated circuits (MMICs), have lessradiation losses and less dispersion than the microstrip line andcan be easily integrated with shunt and series lumped passive el-ements without any need of via holes as in microstrip technologycircuits. Many investigators have studied extensively differentkinds of CPW-fed printed antennas [4]–[7].

Reconfigurability in an antenna system is a desired charac-teristic that has been the focus of several research groups inrecent years. In this work a modified monopole with addedsleeves to obtain more versatility and frequency range is pre-sented. To achieve this goal, switches are introduced in orderto control both, the length of the monopole and of the sleeves.In the double-sleeved antenna (two sleeves on each side of themonopole) the switches are being used in order to keep thesecond sleeve connected or disconnected to the monopole.

II. ANTENNA DESIGN

A. One Sleeve on Each Side of the Monopole

Fig. 1 depicts the proposed layout of the antenna with onesleeve on each side of the monopole. The antenna is printed

Manuscript received March 28, 2006; revised April 21, 2006.V. Zachou, C. G. Christodoulou, and S. Barbin are with the Electrical and

Computer Engineering Department, University of New Mexico, Albuquerque,NM 87131 USA (e-mail: christos@ece.unm.edu).

M. T. Chryssomallis is with the Electrical and Computer Engineering Depart-ment, Democritus University of Thrace, Xanthi, Greece.

D. Anagnostou is with the Georgia Institute of Technology, Atlanta, GA30332 USA.

Digital Object Identifier 10.1109/LAWP.2006.876970

Fig. 1. Single-sleeved antenna configuration (all dimensions in mm).

on a Rogers RO 3203 substrate with a thickness of 1.524 mmand relative permittivity . It is fed by a 50- CPWtransmission line. A 12-mm-long sleeve is attached to each sideof the monopole. Switches are used to connect two additionalpatches of area 1 mm 1 mm to each sleeve. An extra patch oflength 12 mm is also connected to the monopole via a switch.In this way, frequency agility is added to the antenna, since theobtained resonant frequencies will vary depending on the statesof the monopole and the sleeve switches. The switches are mod-eled by tiny copper strips of area mm , so for theOFF switch state, we simply remove the copper strip leaving agap of the same area . These copper strips canbe replaced by radio frequency microelectromechanical systems(RF MEMS) or p–i–n diodes switches. The analysis is carriedout using the method of moment simulator IE3D 9.0.

Figs. 2 and 3 give the simulation result of the return loss (in dB) versus the frequency (GHz) for the different sleeve switchstates when the monopole switch is ON and OFF, respectively.When the monopole switch is ON the first resonant frequencyoccurs at 1.85 GHz, whereas when it is OFF, the resonance oc-curs at 2.4 GHz. These frequencies are chosen due to the widerange of applications in wireless communications and WirelessLocal Area Networks. By changing the state of the two sleeveswitches we can now change the second resonant frequency,which is equal to GHz when both switches are ON,

GHz when only one switch is ON, and GHzwhen none of the switches are activated (Figs. 1 and 2).

In this design, the first resonant frequency is controlled bythe length of the monopole, while the second one is determinedby the length of the sleeves and their order of activation. Thisis validated by the amplitude of the surface currents. Fig. 4 de-picts a closer look on the surface currents around the area ofthe monopole and the sleeve switches in the first two resonantfrequencies. We have taken as an example the case where onlyone of the sleeve switches is ON. As we can see, around the area

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ZACHOU et al.: PLANAR MONOPOLE ANTENNA WITH ATTACHED SLEEVES 287

Fig. 2. Simulated results of S versus frequency for different sleeve switchstates when monopole switch ON.

Fig. 3. Simulated results of S versus frequency for different sleeve switchstates when monopole switch OFF.

Fig. 4. a) Surface currents at f = 1:85 GHz and b) surface currents at f =

3:2 GHz when the monopole switch is ON and one sleeve switch is ON.

of the monopole switch, the current has greater amplitude inthe first frequency, GHz [see Fig. 4(a)], than in thesecond frequency, GHz [see Fig. 4(b)]. The oppositeoccurs in the area around the sleeve switches. It can also be seenthat in both cases there is no current passing in the upper mostpatch of the sleeves, since the switch is OFF, whereas in the lowerpatch the current passes as shown in Fig. 4.

B. Double-Sleeved Antenna

In this section, we propose a monopole antenna with twosleeves on each side. Fig. 5 shows the configuration of the pro-posed double-sleeved antenna. The length of the monopole iskept equal to 37 mm and two sleeves of equal lengths of 12 mmare added to each side. The same CPW line feeds the antenna.A switch is added to control the activation of the second (upper)sleeve.

The simulation results of the return loss versus the fre-quency for the double-sleeved cactus antenna are presented inFig. 6. From this figure we can see that depending on the state of

Fig. 5. Double-sleeved antenna configuration with the switch in the ON state.

Fig. 6. Simulated results of S versus frequency for the two states of theswitch for the double-sleeved antenna.

Fig. 7. Fabricated double-sleeved antenna with the switch OFF.

the switches we can have different resonant frequencies. Whenthe switch is in the ON state (the second sleeve is also connectedto the monopole) we have three resonant frequencies, one at2.4 GHz, a second one at 2.92 GHz and a third one at 3.35 GHz.When the second sleeve is not connected to the monopole (theswitch is in the OFF state), then we have two resonant frequen-cies, one at 2.65 GHz and one at 3.17 GHz. From these obser-vations we can see that the existence of a second sleeve doesnot only add a third resonant frequency, but also shifts the othertwo resonant frequencies that exist when the second sleeve isnot connected to the monopole (switch state OFF).

III. MEASUREMENT RESULTS

The antennas shown in Figs. 1 and 5 were fabricated andmeasured. Fig. 7 depicts the fabricated double-sleeved antennaprinted on RO 3203 1/2 oz. copper laminate with 1.524 mm(0.060 ) thickness dielectric substrate with relative permittivity3.02. Figs. 8 and 9 show the measured return loss versus thefrequency when the monopole switch is ON and OFF and for thethree different states of the sleeve switches for the single-sleevedantenna. The measured and the simulated results are shown inTables I and II.

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288 IEEE ANTENNAS AND WIRELESS PROPAGATION LETTERS, VOL. 5, 2006

Fig. 8. Measured return loss versus frequency when monopole switch is ON.

Fig. 9. Measured return loss versus frequency when monopole switch is OFF.

TABLE IMEASURED AND SIMULATED RESULTS WHEN MONOPOLE SWITCH ON

TABLE IIMEASURED AND SIMULATED RESULTS WHEN MONOPOLE SWITCH OFF

Fig. 10. Measured return loss vs. frequency for the double-sleeved antenna.

Fig. 10 shows the measured return loss versus the frequencyfor the double-sleeved antenna. The simulated and measurementresults are concerted in Table III.

The radiation patterns of the fabricated antennas were mea-sured for all the different switch states of the proposed models.

TABLE IIIMEASURED AND SIMULATED RESULTS FOR DOUBLE-SLEEVED ANTENNA

Fig. 11. Simulated and measured E- and H-plane when monopole switch ON

and both sleeve switches ON.

All the radiation patterns have a good agreement with the sim-ulated results. The patterns keep the “dipole-like” shape at alldifferent resonant frequencies. In Fig. 11 we give the radiationpatterns of one out of the six different switch states for the cactusantenna with one sleeve (when the monopole switch is ON andboth sleeve switches are ON). For the rest of the switch states weget similar radiation patterns. Finally, Fig. 12 depicts the sim-ulated and measured radiation patterns for the double-sleevedantenna when the switch is ON.

IV. CONCLUSION

A new printed cactus-antenna with one and two sleeves oneach side of the monopole was demonstrated. Switches wereintroduced in order to obtain frequency agility. Changing thestates of the switches results in different resonant frequen-cies, since the resonant frequencies depend on the size of themonopole and the sleeves (in the case of one sleeve in eachside of the monopole) and the shape of the antenna (in the case

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ZACHOU et al.: PLANAR MONOPOLE ANTENNA WITH ATTACHED SLEEVES 289

Fig. 12. Simulated and measured radiation patterns in E- and H-plane for thedouble-sleeved antenna when switch ON.

of two sleeves in each side of the monopole). In the case ofthe double-sleeved antenna with the two configurations of theswitch, we were able to cover the whole band from 2.4 GHzto 3.35 GHz. The radiation patterns of the proposed antennasmaintained the “dipole-like” shape for all the switch states. Theagreement between the simulation and the measurement resultswas good.

Even though the reconfigurability as a proof of concept wasthe main goal of this research, commercial use of this appli-cation requires the integration of MEMS switches on the samesubstrate with the antenna. Preliminary studies have been made,with excellent results [8]. Series cantilever MEMS switches can

be used in order to reconfigure the shape/size (and thus the oper-ating frequencies) of this antenna. The switches are connectedin series to the monopole’s sleeves, thus adjust each sleeve’slength. The biasing is achieved using dc pads connected to theswitches with high-resistive lines. The fabrication of MEMSswitches on thin and flexible materials such as RO3203 or liquidcrystal polymers (LCP) has also been achieved and measuredwith excellent results [9].

Since the frequencies of operation are relatively low, p–i–ndiodes can also be used. In this case, the p–i–n diodes need tobe biased using lines (at the frequency of operation of theparticular monopole/sleeve), ending in radial stubs. Both p–i–ndiodes and switches have been used in the past and demonstratedexcellent performance, and they are ideal to achieve the desiredreconfigurability.

REFERENCES

[1] D. Liu, “A dual-band antenna for cellular applications,” in Proc. IEEEAntennas and Propagat. Soc. Int. Symp., vol. 2, Jun. 21–26, 1998, pp.786–789.

[2] R. Schlub, D. V. Thiel, J. W. Lu, and S. G. O’Keefe, “Dual-band six-ele-ment switched parasitic array for smart antenna cellular communicationssystems,” Electron. Lett., vol. 36, pp. 1342–1343, 2000.

[3] M. Ali, M. Okoniewski, M. A. Stuchly, and S. S. Stuckly, “Dual-fre-quency strip-sleeve monopole for laptop computers,” in Proc. IEEE An-tennas and Propagat. Soc. Int. Symp., vol. 47, Feb. 1999, pp. 317–323.

[4] S.-M. Deng, M.-D. Wu, and P. Hsu, “Analysis of coplanar waveguide-fedmicrostrip antennas,” IEEE Trans. Antennas and Propagation, vol. 43,pp. 734–737, Jul. 1995.

[5] R. Q. Lee and R. N. Simons, “Coplanar-waveguide aperture-coupledmicrostrip patch antenna,” IEEE Microwave Guided Wave Lett., vol. 2,pp. 138–139, Apr. 1992.

[6] S.-M. Deng, M.-D. Wu, and P. Hsu, “Impedance characteristics of mi-crostrip antennas excited by coplanar waveguides with inductive or ca-pacitive coupling slots,” IEEE Microwave Guided Wave Lett., vol. 5, pp.391–393, Nov. 1995.

[7] J.-M. Laheurte, L. P. B. Katehi, and G. M. Rebeiz, “CPW-fed slotantennas on multilayer dielectric substrates,” IEEE Trans. AntennasPropag., vol. 44, pp. 1102–1111, Aug. 1996.

[8] D. E. Anagnostou, G. Zheng, M. T. Chryssomallis, J. C. Lyke, G. E.Ponchak, J. Papapolymerou, and C. G. Christodoulou, “Design, fabri-cation, and measurements of an RF-MEMS-based self-similar reconfig-urable antenna,” IEEE Trans. Antennas Propag., vol. 54, pp. 422–432,Feb. 2006.

[9] I. Kim, N. Kingsley, M. Morton, R. Bairavasubramanian, J. Papapoly-merou, M. M. Tentzeris, and J.-G. Yook, “Fractal-shaped microstrip cou-pled-line bandpass filters for suppression of second harmonic,” IEEETrans. Microw. Theory Techn., vol. 54, pp. 2943–2948, Sep. 2005.

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