A Reconfigurable PIFA Using a Switchable PIN-Diode and a Fine-Tuning Varactor for USPCS WCDMA m-WiMA
A Recon?gurable PIFA Using a Switchable PIN-Diode and a Fine-Tuning Varactor for
USPCS/WCDMA/m-WiMAX/WLAN
Jong-Hyuk Lim,Gyu-Tae Back,Young-Il Ko,Chang-Wook Song,and Tae-Yeoul Yun,Member,IEEE
Abstract—A recon?gurable planar inverted-F antenna using a switchable PIN-diode and a?ne-tuning varactor is presented for mobile communication applications.Selection of operating modes is achieved by switching the PIN-diode between radiators and tuning the varactor on an antenna’s shorting line.Mode I,with the PIN-diode off and tuning the varactor,operates for USPCS(1.85–1.99GHz),WCDMA(1.92–2.18GHz),and WLAN (5.15–5.825GHz).As a result,the varactor used to achieve fre-quency?ne-tuning does not need a DC bias circuit and can expand the bandwidth without increasing the physical size.Mode II,with the PIN diode on and the?xed0-V varactor,operates for USPCS and m-WiMAX(3.4–3.6GHz).To optimize the antenna structure, a parametric analysis is performed by sweeping the length and width of the radiators.Furthermore,equivalent models of a PIN diode and a varactor are presented for accurate prediction of antenna performances which are also analyzed by varying diode parameters.All simulated results are con?rmed with measured data.The peak gains show2.84,2.81,1.25,and1.49dBi at USPCS, WCDMA,m-WiMAX,and WLAN,respectively.
Index Terms—Planar inverted-F antenna(PIFA),PIN-diode,re-con?gurable,switching,tuning,varactor.
I.I NTRODUCTION
D U
E to rapid growth in mobile handset markets and cus-
tomers’needs,it is necessary to merge diverse wireless communication systems such as United States personal commu-nications services(USPCS),wideband code division multiple access(WCDMA),mobile worldwide interoperability for mi-crowave access(m-WiMAX),and wireless local area network (WLAN).In order to merge these service bands,the design of an antenna is needed and must be either of the multiband or fre-quency recon?gurable type.The frequency recon?gurable an-tenna offers many advantages such as compact size,similar ra-diation pattern,and proper gain for all desired frequency-bands, compared to the multiband antenna.For this reason,many an-tenna types have recently been developed in recon?gurable sys-tems[1]–[5].
Design of internal antennas is very important for the miniatur-ization and aesthetical appearance of the mobile handset.Cur-rently,the planar inverted-F antenna(PIFA)is generally used for internal antennas because of its easy fabrication,low pro?le,low
Manuscript received March16,2009;revised December14,2009;accepted January31,2010.Date of publication April22,2010;date of current version July08,2010.This work was supported by the Technology Innovation Program (or Industrial Strategic technology development program,00007812)funded by the Ministry of Knowledge Economy(MKE,Korea).
The authors are with the Department of Electrical and Computer Engineering, Hanyang University,Seoul133-791,Korea(e-mail:taeyeoul@hanyang.ac.kr). Digital Object Identi?er10.1109/TAP.2010.2048849cost,and reduction of special absorption rate(SAR)[6].In ad-dition,the PIFA offers small size and easy multiband operations by inserting the slot and slit on the radiator[6],[7].However, the PIFA has a narrow bandwidth.To overcome this drawback,a recon?gurable PIFA was presented in[8]using a PIN-diode and discrete passive components.However,this antenna resulted in a size increase due to an added radiating element with a tuning circuit.
In this paper,we propose a recon?gurable PIFA using a var-actor and a PIN-diode.By varying capacitance of the varactor on an impedance matching short-line,frequency?ne-tuning is easily achieved without increase of the antenna size or the addition of a bias circuit.Also,based on the PIN-diode on and off status between radiating elements,the antenna is able to select a very separate frequency band.The proposed antenna covers four service bands:USPCS(1.85–1.99GHz),WCDMA (1.92–2.18GHz),m-WiMAX(3.4–3.6GHz),and WLAN (5.15–5.825GHz).
In Section II,a new recon?gurable PIFA design is presented with a parametric analysis and surface current distributions. Section III shows equivalent circuit models for a PIN-diode and a varactor.Section IV describes simulated and measured impedance characteristics,gains,ef?ciencies,and radiation patterns.Finally,a conclusion is given in Section V.
II.D ESIGN OF R ECONFIGURABLE PIFA
A.Antenna Design
As shown in Fig.1,the proposed recon?gurable PIFA con-sists of main and additional radiating elements on an FR4sub-strate with a relative
permittivity of4.4,a feeding con-ductor,a folded part,a short line,a PIN-diode[9],and a varactor [10].The size of the ground plane has the dimension of
30 70mm for a typical mobile handset.A top view of the pro-posed antenna is shown in Fig.1(b).The switching PIN-diode is located between the main and additional radiators as a con-ducting bridge.Fig.1(c)shows the right side view including the folded
part of the main radiator for the antenna size de-crease and the short line for the impedance matching.The tun-able varactor is placed on the short line.Fig.1(d)depicts the front view of the proposed antenna.
When the PIN-diode is off(0V),the antenna operates at the USPCS and WLAN bands.On the contrary,when the PIN-diode is on(1V),the antenna operates in the USPCS and m-WiMAX bands because the surface current path of the antenna is lengthened into the additional radiating element
0018-926X/$26.00?2010IEEE
Fig.1.Geometry of a proposed PIFA of unit in mm:(a)3-dimensional view,(b)top view,(c)side view,and (d)front view.
through the PIN-diode.The varactor,which operates between approximately 5pF at 4V and 52pF at 0V ,is used for ?ne frequency tuning between USPCS and WCDMA because the inductance of the shorting line is reduced by the capacitance of the varactor.Thus,frequency tuning can be obtained without change of antenna structure.
To design and optimize the proposed antenna,many param-eters were considered.First,it is important to determine the overall size of the antenna at a resonant frequency,which is dependent on the width and length of the radiator.As a re-sult,quarter-wavelengths of the proposed PIFA are chosen as approximately 39,21.4,and 13.6mm at USPCS (1.92GHz),m-WiMAX (3.5GHz),and WLAN (5.5GHz),respectively.In such a quarter-wavelength structure,
50impedance matching of the antenna can easily be obtained by a proper choice of the
feed
spacing
and location of the short line.The antenna was simulated with Microwave Studio of the CST [11].B.Parametric Analysis of Radiating Elements
In order to optimize the physical parameters of the antenna,the parametric analysis was performed,as shown in Fig.2.First,at the lowest frequency band (USPCS),
parameters ,
and are analyzed.According to an increase
in ,the resonant frequency shifts downward without change of the highest band (WLAN),as shown in Fig.2(a).It can be seen that the optimized
value of
is 12.5mm in the USPCS band.By increasing not
only
but
also ,the resonant frequency moves downward,as shown in Fig.2(b).The optimized values
of
and are 5and 5.5mm,respectively.Thus,the overall size can be reduced by the folded radiating
element,.
Next,at the highest frequency band (WLAN),parameters
of
and are analyzed.By
increasing of the radiating el-ement,the resonant frequency shifts downward,as shown in
Fig.2(c).It can be seen that the optimized value
of
is 2mm for WLAN.By
increasing ,the resonant frequency shifts downward for the lower frequency,as shown Fig.2(d).It is
shown that the optimized value
of
is 4.5mm.Other parameters were similarly optimized.As a result,the optimized values for the PIFA design are described in Table I.C.Surface Current Distributions
To explain operations of the recon?gurable PIFA,the excited surface current distributions on the radiating elements were studied.Fig.3shows CST simulation results of the PIFA surface current distributions at 1.9/2.0,3.5,and 5.5GHz,respectively.As shown in Fig.3(a),the surface current ?ows from the feed conductor to the end of the folded patch,in which the path
length
is close to the
quarter-wavelength of 1.92and 2.0GHz for the USPCS and WCDMA bands.The path length,in this paper,was chosen as the inside surface current path.Fig.3(b)shows that the surface current distributions for a PIN-diode on-status ?ow from the feed
Fig.2.Parametric analysis of radiating elements varying:(a)W ,(b)L and L ,(c)W ,and (d)L .
TABLE I
O PTIMIZED P ARAMETER V ALUES FOR THE PIFA D
ESIGN
conductor to the end of the additional radiator,in which the path
length is close to the quarter-wavelength of 3.5GHz for the m-WiMAX band.Finally,Fig.3(c)shows the surface current distributions for a PIN-diode off-status.Then the antenna operates at the highest frequency band around 5.5GHz for the WLAN band.It can be observed that the surface current
path
in this case is close to the quarter-wavelength at 5.5GHz.
III.E QUIV ALENT C IRCUIT M ODELING OF D IODES
In order to accurately predict the recon?gurability of the pro-posed antenna,it is essential to extract the diode’s characteris-tics.Therefore,we performed an equivalent-circuit modeling
of
Fig.3.Simulated PIFA surface current distributions at (a)1.9GHz (USPCS)and 2.0GHz (WCDMA)(b)3.5GHz (m-WiMAX),and (c)5.5GHz
(WLAN).
Fig.4.Equivalent circuit for a PIN-diode.
the diodes based on the through-delay-line de-embedding to ob-tain accurate measurement data.As shown in Fig.4,we adopted a simpli?ed RLC equivalent circuit for a PIN-diode without the
Fig.5.Simulated and measured S-parameters for the PIN-diode at:(a)0V (off),(b)1V
(on).
Fig.6.Equivalent circuit for a varactor.
surface mounting effect because the CST simulation tool can’t apply a complex RLC model.It consists of a series parasitic in-
ductance
and an intrinsic
capacitance in parallel with an intrinsic
resistance
.Parameter values of the equivalent-cir-cuit model are calculated by Agilent Advanced Design System (ADS).As a result,when the PIN-diode is off (0V),the values
of
,
and are 3
k ,0.45nH,and 0.08pF,respectively.On the contrary,when the PIN-diode is on (1V),the values
of
,
and are
3.5,0.45nH,respectively.
Fig.5shows simulated and measured S-parameters for a micro-semi MPP4203PIN-diode [9]from 1to 6GHz.When the PIN-diode is off (0V),it has an isolation of 11.32dB at 5.5GHz due to the small total capacitance (0.08pF).On the contrary,when the PIN-diode is on (1V),it has an insertion loss of 0.66dB at 3.5GHz due to the small resistance
(3.5).The insertion loss caused by intrinsic resistance diminished an antenna gain.Fig.6shows an adopted equivalent circuit for the varactor,which omits a surface mounting effect for the same reason as in
the PIN-diode case.It consists of a parasitic inductance
,a variable resistance
,and a variable capacitance in
series.Fig.7.Simulated and measured S-parameters for the varactor biased at:(a)0
V ,(b)2V ,and (c)4V.
TABLE II
P ARAMETER V ALUES OF E QUIV ALENT C IRCUITS FOR D
IODES
Fig.7validates the equivalent-circuit model for an In?neon BBY59varactor [10]with measured data from 1to 6GHz.
Fig.8.Con?guration of
measurement.
Fig.9.Simulated and measured S for the proposed PIFA when:(a)PIN is off,(b)PIN is on,and (c)PIN is off and a varactor with 0and 4
V.
Fig.10.Simulated gain and ef?ciency variations depending on the PIN-diode capacitance for the mode I;(a)gain,(b)ef?ciency.
When the varactor reverse
voltage is 0V ,the maximum
capacitance is 52.15pF.On the contrary,when the var-actor is biased at 2and 4V ,the capacitances are 14.96and 5.37pF,respectively.These capacitances will compensate the inductance of the short line and change an operating frequency.
When the
varactor
changes from 0to 4V ,it has an insertion loss of 0.18dB around 2GHz.As shown in Figs.5and 7,the simulated and measured results for both diodes agree well.Finally,the parameter values of the equivalent circuits for the PIN-diode and the varactor are listed in Table II.The simulated and measured results for the proposed antenna using PIN-and varactor-diodes will be presented in the following sections.IV .A NTENNA S IMULATION AND M EASUREMENT
Fig.8shows the DC bias setup of the proposed antenna with PIN-and varactor-diodes.The antenna is fed through the bias tee which supplies the RF signal and the DC bias for the varactor.For DC biasing the PIN-diode without leakage of RF signal to the DC bias line,the RF choke inductor (12nH),which has a self-resonant frequency around 3.5GHz and a peak impedance at that frequency,was attached on the additional
radiator.The forward bias
voltage
of the PIN-diode for frequency switching was controlled by the DC bias between 0and 1V.The PIN-diode maximally consumes a forward bias current of 16mA at 1V.The varactor for the frequency ?ne-tuning was controlled by the DC bias between 0and 4V.
Fig.9shows the simulated and measured
of the pro-posed PIFA for the different band operation depending on the
Fig.11.Simulated gain and ef?ciency variations depending on the varactor capacitance for the mode I;(a)gain,and (b)ef?ciency.
DC biases of the PIN-diode and varactor.Both the simulated and measured data satisfy the return loss of more than 6dB for all bands.As shown in Fig.9(a),when the PIN-diode is off,the antenna operates at the USPCS and WLAN.On the con-trary,when the PIN-diode is on,the antenna operation shifts from WLAN to m-WiMAX without a change of the lower fre-quency band of the USPCS,as shown in Fig.9(b).The simu-lated and
measured with varying varactor bias conditions are shown in Fig.9(c).When the varactor is biased at 4V ,the lower band just moves toward WCDMA slightly.As a result,the proposed PIFA can cover four bands:USPCS (1.85–1.99GHz),WCDMA (1.92–2.18GHz),m-WiMAX (3.4–3.6GHz),and WLAN (5.15–5.35GHz and 5.725–5.825GHz).The sim-ulation and measurement results for input impedance matching showed good agreement.
Fig.10shows simulated antenna gains and ef?ciencies by varying the PIN-diode capacitance when the antenna operates in mode I (PIN-diode off).When the capacitance parameter of the PIN-diode increases from 0.08to 0.3pF,the peak frequencies of the gain and ef?ciency decrease from 5.5to 4.5GHz.For the lower bands (USPCS and WCDMA)near 2GHz,we had similar results.This means that the off-state capacitance affects antenna performances very much and should be small.
In addition,when the varactor capacitance decreases,con-trolled by the bias from 0to 4V for ?ne tuning of the
operating
Fig.12.Simulated gain and ef?ciency variations depending on the PIN-diode intrinsic resistance for the mode II;(a)gain,(b)
ef?ciency.
Fig.13.Simulated and measured radiation ef?ciencies for each band.
band,the peak frequencies of the gain and ef?ciency are shifted from USPCS to WCDMA band,as shown in Fig.11(a)and (b).On the contrary,when the antenna operates in mode II (PIN-diode on),the antenna gain and ef?ciency are also simulated with variations of the PIN-diode’s resistance in Fig.12.The antenna gains and ef?ciencies decreases when the resistance of the PIN-diode increases from 0(ideal)to
3.5.It should be noted that variations of the parasitic inductance of both diodes didn’t affect antenna performances.
Fig.14.Simulated(dotted line)and measured(solid line)co-polarization radiation patterns in the xz-,xy-,and yz-planes,respectively at:(a)1.95GHz(USPCS, WCDMA),(b)3.5GHz(m-WiMAX),and(c)5.5GHz(WLAN).
Fig.13shows simulated and measured radiation ef?ciencies for the mode I and II.When the PIN-diode is off,the radia-tion ef?ciencies have near90%.On the other hand,when the PIN-diode is on,the radiation ef?ciency abruptly decreases to 63%for the m-WiMAX(3.35–3.69GHz)band due to the resis-tance effect of the PIN-diode but remains93%for the USPCS (1.85–1.99GHz)band due to no radiation through the Pin-diode for this band.
The simulated and measured co-polarization radiation pat-terns of the proposed PIFA are plotted at1.95,3.5,and5.5GHz according to the cutting plane in Fig.14.The radiation patterns of the proposed PIFA were measured in an anechoic shielded chamber.As shown in Fig.14(a),the radiation pattern in the xy-plane for the lower frequency band showed dipole-like characteristics and the radiation pattern in the yz-plane has an omni-directional characteristic because the surface current excited in the x-direction is dominant,as shown in Fig.3(a). As shown in Fig.14(b)and(c),however,the radiation patterns in the xy-plane for higher frequencies have nulls in several directions since the antenna has a dominant operating current to the y-direction and undesired weak surface currents to other directions,as shown in Fig.3(b)and(c).The measured radi-ation antenna peak gains are2.84and2.81dBi at the USPCS and WCDMA bands,respectively.In the diode-on case for the m-WiMAX,the peak gain is1.49dBi.On the contrary, when the diode is off for the WLAN,the peak gain is1.25 dBi.The antenna gains for the m-WiMAX and WLAN bands are lower than the lowest bands because the weak surface currents of other directions contribute to additional radiation in cross-polarized directions.All the calculated and measured gain radiation patterns showed good agreement.Finally,the measured results are summarized in Table III.
V.C ONCLUSION
The recon?gurable PIFA design with a PIN-diode and a tun-able varactor covering the USPCS,WCDMA,m-WiMAX,and WLAN bands has been demonstrated with analysis and mea-surement.This paper also presented equivalent-circuit models of the PIN-diode and varactor.The proposed antenna was able to select a very separate frequency band and also achieved?ne-fre-quency tuning without increasing the physical size.Overall,the
TABLE III
P ERFORMANCE S UMMARY OF THE P ROPOSED R ECONFIGURABLE
PIFA
simulated and measured results showed good agreement.There-fore,the proposed recon?gurable PIFA can be useful for an up-coming generation of mobile systems.
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Jong-Hyuk Lim received the B.S.degree from Hongik University,Korea,in 2004and the M.S.degree from Hanyang University,Seoul,Korea,in 2006,where he is currently working toward the Ph.D.degree.
His research interests include antenna design,mi-crowave circuit design,and wireless communication
systems.
Gyu-Tae Back received the B.Sc.degree in elec-trical engineering from Gyeongju University,Korea,in February 2004.He is currently working toward the M.S.degree at University of Hanyang,Seoul,Korea.
His research interests include recon?gurable/multi input multi output (MIMO)antennas and the design of RF integrated circuits
(RFICs).
Young-Il Ko received the B.Sc.degree in communi-cation engineering from Daejin University,Korea,in 2008.He is currently working toward M.S.degree at the University of Hanyang,Seoul,Korea.
His current research interests include microstrip and recon?gurable antenna
design.
Chang-Wook Song received the B.Sc.degree in information and communication engineering from Dongeui University,Busan,Korea,in February 2007.He is currently working toward the M.S.degree at the University of Hanyang,Seoul,Korea.His research interests are recon?gurable/multi input multi output (MIMO)antennas and the design of RF integrated circuits
(RFICs).
Tae-Yeoul Yun received the B.S.E.E.degree from the Kyungpook National University,Korea,in 1987,the M.S.E.E.degree from KAIST,Korea,in 1989,and the Ph.D.degree from Texas A&M University,Col-lege Station,in May 2001.
From 1989to 1996,he worked for an optical telecommunication system group,ETRI,Korea,where he developed 2.5-Gb/s and 10-Gb/s systems.From 2001to 2003,he was an MMIC Designer at Triquint Semiconductor,Dallas,TX.Since March 2003,he has been a Professor at Hanyang University,
Korea.He has published more than 100technical papers.His research interests are RFICs,MMICs,antennas,and wireless communication systems.