Research Article Frequency-Adjustable Small Zeroth-Order …downloads.hindawi.com/journals/ijap/2013/923259.pdf · Research Article Frequency-Adjustable Small Zeroth-Order Resonant

Hindawi Publishing CorporationInternational Journal of Antennas and PropagationVolume 2013, Article ID 923259, 5 pageshttp://dx.doi.org/10.1155/2013/923259

Research ArticleFrequency-Adjustable Small Zeroth-Order Resonant Antennawith Capacitor-Loaded Rectangular Slot on Ground Plane

Youngsoo Jang and Sungjoon Lim

School of Electrical and Electronics Engineering, Chung-Ang University, 221 Heukseok-dong Dongjak-gu,Bobst Hall no. 534, Seoul 156-756, Republic of Korea

Correspondence should be addressed to Sungjoon Lim; [email protected]

Received 17 April 2013; Revised 1 August 2013; Accepted 2 August 2013

Academic Editor: Francisco Falcone Lanas

Copyright © 2013 Y. Jang and S. Lim. This is an open access article distributed under the Creative Commons Attribution License,which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

A capacitor-loaded electrical small zeroth-order resonant (ZOR) antenna is proposed. The proposed antenna is designed on thebasis of a mushroom structure for zeroth-order resonance. To obtain a compact size, the proposed antenna has a rectangular sloton the ground plane, and the chip capacitor is mounted on the slot. The resonant frequency is easily controlled from 2.82GHz to2.29GHz by changing the capacitance from 1 pF to 7 pF, respectively.Therefore, the proposed antenna has the advantages of a smallantenna size as well as easy frequency adjustment.

1. Introduction

Since portable wireless devices such as laptops, mobilephones, personal digital assistants (PDAs), and global posi-tioning systems (GPSs) require increasing functions, thespace to integrate the necessary components has becomesmaller. Antenna size is especially critical because it is depen-dent on the operating frequency. A number of techniqueshave been proposed to reduce the antenna size, among whichthe most straightforward approach is to use a substratewith a high dielectric constant. However, this leads to poorefficiency and narrow bandwidth.Much effort has been givento further reducing the antenna size, such as creating ameandering edge of the patch [1] or using stacked patches[2]. A defected ground structure antenna in one study wasable to achieve miniaturization by about 68% compared toa conventional antenna [3]. In another study, fractal-shapeddefects were presented to reduce the antenna size [4].

It has also been presented that additional loads can beused tominiaturize an antenna. Shorting posts have also beenused in different arrangements to reduce the overall size of apatch antenna [5]. Capacitive loads are an alternative way toreduce antenna size. It has been reported that a monopole orplanar inverted F antenna (PIFA) with a capacitive load canprovide both much lower profile and miniaturization [6, 7].

Recently, the metamaterial concept was applied to severalresonant antennas.The zeroth-order resonant (ZOR) antennahas been presented to reduce antenna size [8]. Its zeropropagation constant allows the antenna size to becomeindependent of the resonant frequency because of the infinitewavelength. It has been reported that the frequency can befurther decreased by distributing inductive and capacitiveloads [9].

In this study, we propose a simple technique to furtherreduce antenna size by introducing capacitor-loaded slots onthe ground and controlling the resonant frequency by thewayof the capacitances of lumped elements.The S-parameter andradiation patterns will be shown at different capacitances.Thezeroth-order resonance phenomenon will be demonstratedwith electric-field distributions along the aperture from full-wave simulation.

2. Antenna Design

ZOR can be achieved by the way of a composite right- /left-handed transmission line (CRLH TL). A mushroom-likestructure is well known to realize ZOR [8]. The unit cell of ageneral CRLHconsists of a series inductance and capacitance,as well as a shunt inductance and capacitance, as shown inFigure 1(a). The conventional CRLH TL consists of a series

2 International Journal of Antennas and Propagation

Shuntparameters

LR CL

LL CR

(a)

LR CL

LL CR CVShunt

parameters

(b)

Figure 1: Equivalent circuit model. (a) General CRLH TL unit cell. (b) CRLH TL unit cell with slot and additional capacitor.

Substrate

Patch

Slot

Via

Chipcapacitor

Microstripfeed line GND

y

x xz

y

z

0.2

0.2

19.8

19.8

9.8

Figure 2: Illustration of the proposed antenna geometry with dimensions (units: mm).

inductance (𝐿𝑅)/capacitance (𝐶

𝐿) and shunt capacitance

(𝐶𝑅)/inductance (𝐿

𝐿).When the terminals of the antenna are

open, the ZOR frequency is given by

𝑓ZOR =1

2𝜋√𝐿𝐿𝐶𝑅

. (1)

Therefore, the frequency is determined by the shuntparameter (CR and LL) [8]. In order to design a small-size antenna, the CR should be increased, but this is dif-ficult. In order to do so, larger patch size, lower substratethickness, or higher permittivity is necessary. These causethe antenna size to increase the patch size, while degradingthe antenna performance to increase the permittivity. Withdecreased substrate thickness, the bandwidth of the antennais decreased. Also, it is difficult to realize a higher shuntinductance (LL) without changing the dimensions. In orderto overcome the limitations associated with decreasing theresonant frequency, we propose a slot and additional capac-itor on the ground plane. A more compact and frequency-adjustable ZOR antenna is proposed by loading slots and chipcapacitors on the ground plane of themushroom structure, asshown in Figure 2. Its equivalent circuit model is modified asin Figure 1(b).

The slot is placed around the via which is connected tothe rectangular slot in series. The chip capacitor on the slot

x

y

z

Bottom viewTop view

x

y

z

Figure 3: Picture of the fabricated antenna prototype: top andbottom views.

increases the capacitance from the original slot. When thechip capacitor is loaded on the rectangular slot on the groundplane, the shunt parameter is changed.The ZOR frequency ofthe modified structure becomes

𝑓ZOR =1

2𝜋√𝐿𝐿(𝐶𝑅+ 𝐶V)

. (2)

International Journal of Antennas and Propagation 3

Frequency (GHz)

Simulation

1.6 32.82.62.42.221.8

Retu

rn lo

ss (d

B)

1pF3pF

0

10

20

5pF7pF

(a)

Measurement

Retu

rn lo

ss (d

B)

5pF7pF

Frequency (GHz)1.6 32.82.62.42.221.8

1pF3pF

0

10

20

30

(b)

Figure 4: Results of S parameters at different capacitance values. (a) Simulated return losses and (b) measured return losses.

Therefore, when the capacitance of the chip capacitors (𝐶V) ischanged, the ZOR frequency can be easily adjusted.

3. Fabrication and Measurement

The proposed antenna is fabricated on Rogers RT/Duroid5880 substrate with a dielectric material constant of 2.2 anda thickness of 1.6mm, as shown in Figure 3. The copper-covered planes have a thickness of 35 𝜇m.The shorting is con-ducted via silver, which is a material with good conductivity.The conductive epoxy is used tomount the chip capacitors onthe slot.

In Figure 4, the measured return losses with differentcapacitances (1, 3, 4, and 7 pF) are compared with the sim-ulated results. For instance, when 1 pF of the capacitor isused, the experimental return loss is 25.02 dB at 2.82GHz.The return loss at 7 pF capacitance is 18.46 dB at 2.29GHz.Thus, the resonant frequency can be tuned from 2.82GHz to2.29GHz by varying the capacitance from 1 pF to 7 pF, respec-tively. The relationship between the resonant frequency andthe loaded capacitance is plotted in Figure 5. Since the pro-posed antenna is designed by using twounit cells of theCRLHTL, it is observed that the additional −1st order resonanceoccurred at the lower frequencies [10]. In this work, onlyzeroth-order resonant modes are used for antenna applica-tions because the negative resonance shows very low antennagain and efficiency. For EM simulation, the parasitic resis-tance and inductance are extracted by a through-reflection-load (TRL) calibration and taken into account.Thus, the sim-ulation and measurement results show excellent agreement.

The radiation patterns of the proposed antenna with 1,3, 4, and 7 pF chip capacitors are measured in the anechoicchamber. The simulated and measured patterns on the XZ

Capacitance (pF)

SimulationMeasurement

Freq

uenc

y (G

Hz)

1 3 5 7

3

2.8

2.6

2.4

2.2

2

Figure 5: Relationship between the resonant frequency and theloaded capacitance.

plane are compared in Figure 6. As expected from a typicalZOR antenna, monopole patterns are observed.

The measured peak gain of the proposed antenna is−0.1 dBi with 1 pF capacitance, which decreases with a highercapacitance. The measured efficiency is 78% when the capac-itance is 1 pF, and it is decreased to 75% with 7 pF.

In order to demonstrate the zeroth-order resonancephenomenon of the proposed antenna, the vectors of theelectric field are plotted in Figure 7. It is observed that thephases of the electric field are all in the same direction in eachcell because of a zero phase constant and infinite wavelength.Figure 7 shows the snapshot of the E-field when the phaseis varied. The phase of the E-field is constant along theantenna aperture. Therefore, it is successfully demonstrated

4 International Journal of Antennas and Propagation

1pF3pF

5pF7pF

90

0

315

270

225

180

135

−5

−5

−15

−15

45

(a)

1pF3pF

5pF7pF

90

0

315

270

225

180

135

−5

−5

−15

−15

45

(b)

Figure 6: Radiation patterns on the XZ-plane: (a) simulated radiation patterns and (b) measured radiation patterns with differentcapacitances.

(a) (b)

(c) (d)

Figure 7: Electric field distributions with zero propagation constant at different phases: (a) 80∘, (b) 110∘, (c) 140∘, and (d) 170∘.

that the proposed antenna operates in the zeroth-orderresonant mode.

4. Conclusion

A frequency-adjustable ZOR antenna has been presented byintegrating an additional rectangular slot and capacitors. The

ZOR characteristic is used to effectively reduce the antennasize. The rectangular slot on the ground plane is introducedin order to further reduce the antenna size. In addition, thechip capacitor is used to easily adjust the resonant frequency.The addition capacitance from the chip capacitor resultsin further size reduction. From simulation and measure-ment results, the proposed ZOR antenna’s frequency can be

International Journal of Antennas and Propagation 5

adjusted from 2.82 to 2.29GHz by changing the capacitancefrom 1 pF to 7 pF. 79.9% size reduction with a 7 pF capacitoris achieved compared with a conventional half-wavelengthpatch antenna at 2.29GHz. Therefore, the possibility of afrequency tuning capability and miniaturization is success-fully demonstrated in this work. The chip capacitors of theproposed antenna are potentially replaced by varactor diodes.

Acknowledgment

This research was supported by the Basic Science ResearchProgram through theNational Research Foundation of Korea(NRF) funded by the Ministry of Education, Science andTechnology (2011-0022562).

References

[1] A. A. Heidari, M. Heyrani, andM. Nakhkash, “A dual-band cir-cularly polarized stub loaded microstrip patch antenna for GPSapplications,” Progress in Electromagnetics Research, vol. 92, pp.195–208, 2009.

[2] J.-F. Li, B.-H. Sun, H.-J. Zhou, and Q.-Z. Liu, “Miniaturizedcircularly-polarized antenna using tapered meander-line struc-ture,” Progress in Electromagnetics Research, vol. 78, pp. 321–328,2008.

[3] A. Kordzadeh and F.H.Kashani, “Anew reduced sizemicrostrippatch antenna with fractial shaped defects,” Progress in Electro-magnetics Research B, vol. 11, pp. 29–37, 2009.

[4] J. X. Liu, W. Y. Yin, and S. L. He, “A new defected ground struc-ture and its application for miniaturized switchable antenna,”Progress in Electromagnetics Research, vol. 107, pp. 115–128, 2010.

[5] M.-C. Huynh and W. Stutzman, “Ground plane effects on pla-nar inverted-F antenna (PIFA) performance,” IEE Proceedings:Microwaves, Antennas and Propagation, vol. 150, no. 4, pp. 209–213, 2003.

[6] C. R. Rowell and R. D. Murch, “A capacitively loaded PIFA forcompact mobile telephone handsets,” IEEE Transactions onAntennas and Propagation, vol. 45, no. 5, pp. 837–842, 1997.

[7] J. Oh and K. Sarabandi, “Low profile, miniaturized, inductivelycoupled capacitively loaded monopole antenna,” IEEE Transac-tions on Antennas and Propagation, vol. 60, no. 3, pp. 1206–1213,2012.

[8] A. Lai, K. M. K. H. Leong, and T. Itoh, “Infinite wavelengthresonant antennas with monopolar radiation pattern based onperiodic structures,” IEEE Transactions on Antennas and Prop-agation, vol. 55, no. 3, pp. 868–876, 2007.

[9] S. Baek and S. Lim, “Miniaturised zeroth-order antenna onspiral slotted ground plane,” Electronics Letters, vol. 45, no. 20,pp. 1012–1014, 2009.

[10] C.-J. Lee, K. M. K. H. Leong, and T. Itoh, “Composite right/left-handed transmission line based compact resonant antennas forRF module integration,” IEEE Transactions on Antennas andPropagation, vol. 54, no. 8, pp. 2283–2291, 2006.

International Journal of

AerospaceEngineeringHindawi Publishing Corporationhttp://www.hindawi.com Volume 2014

RoboticsJournal of

Hindawi Publishing Corporationhttp://www.hindawi.com Volume 2014


Active and Passive Electronic Components

Control Scienceand Engineering

Journal of



RotatingMachinery


Hindawi Publishing Corporation http://www.hindawi.com

Journal ofEngineeringVolume 2014

Submit your manuscripts athttp://www.hindawi.com

VLSI Design



Shock and Vibration


Civil EngineeringAdvances in

Acoustics and VibrationAdvances in



Electrical and Computer Engineering

Journal of

Advances inOptoElectronics

Hindawi Publishing Corporation http://www.hindawi.com

Volume 2014

The Scientific World JournalHindawi Publishing Corporation http://www.hindawi.com Volume 2014

SensorsJournal of


Modelling & Simulation in EngineeringHindawi Publishing Corporation http://www.hindawi.com Volume 2014


Chemical EngineeringInternational Journal of Antennas and

Propagation




Navigation and Observation



DistributedSensor Networks


Documents

Research Article Frequency-Adjustable Small Zeroth-Order …downloads.hindawi.com/journals/ijap/2013/923259.pdf · Research Article Frequency-Adjustable Small Zeroth-Order Resonant