ACCEPTED VERSION

1

Reactively Steered Ring Antenna Array
for Automotive Application
Shinya Sugiura, Member, IEEE and Hideo Iizuka, Member, IEEE

Abstract—A novel type of smart antenna, called a reactively
steered ring antenna array (RSRAA), is proposed. The antennafeatures a conformal configuration with three overlapping onewavelength ring elements. Directivity can be controlled by changing the values of variable reactance circuits that are embeddedin the antenna. The antenna can be mounted on the windshieldof an automobile without interfering with the view of the driver.
An equivalent model of six dipole elements is derived from theoriginal antenna configuration to enable simple calculation of
directivity when a set of reactance values is given. The validityof the proposed equivalent model is confirmed by comparing
simulation results calculated by the equivalent model and by
the method of moment (MoM). Another simulation shows thatthe conventional reactance domain multiple signal classification(MUSIC) algorithm can be applied to the proposed antenna,
while keeping the estimated direction-of-arrival (DOA) number
equal to the number of reactance circuits. In addition, thecharacteristics of an RSRAA , which is optimized at 2.4GHz
and is mounted on an automobile, are measured in an anechoic
chamber, and directivity control is found to be implemented
effectively.
Index Terms—Active antennas, antenna arrays, direction ofarrival estimation, road vehicles, varactors

I. I NTRODUCTIONR

ECENTLY, the demand for safety applications capableof drastically reducing the number of traffic accidents
has increased. Radar systems and/or imaging sensor systems
have been developed for practical use. These systems arehighly effective for preventing possible collisions with forward
obstacles that are within the line-of-sight (LOS) of the incident
vehicle. However, the systems cannot work in the case of
accidents that occur outside of the LOS, which accounts fora large percentage of traffic accidents in Japan. To solve thisproblem, safety applications using wireless communication,such as inter-vehicle communication (IVC), are presently
being studied extensively [1]-[3]. As IVC is categorized asa high-mobile ad hoc network, space division multiple access(SDMA) by smart antennas can be applied for the purposeof increasing system capacity and gain [4]. Dedicated shortrange communications (DSRC) have already been used incommercial applications in addition to safety applications.
Smart antennas can also be used for sharing system resources
between DSRC and IVC [5].
c 2007 IEEE. Personal use of this material is permitted.
Copyright ⃝However, permission to use this material for any other purposes must be
obtained from the IEEE by sending a request to pubs-permissions@ieee.org
The authors are with Toyota Central Research & Development
Laboratories.,
Incorporated.,
Aichi
480-1192,Japan(e-mail:sugiura@mosk.tytlabs.co.jp; hiizuka@mosk.tytlabs.co.jp).
DOI: 10.1109/TAP.2007.900251

Many types of smart antennas have been designed. Digital beamforming (DBF) antennas are typical examples ofhigh performance antennas, but these antennas have major
disadvantages with respect to cost and size. This is a resultof the requirement for DBF antennas to have multiple antenna elements and the same number of RF chains, including
low-noise amplifiers (LNAs), down converters (DCs), andanalog-to-digital converters (ADCs). In essence, vehicle safety
applications are applied in all types of vehicles, i.e., notonly expensive vehicles but also popular types of vehicles.
Therefore, smart antennas must be low-cost. To realize asimpler configuration of smart antennas, various types ofanalog beamforming (ABF) antennas have been developed.
Typical examples include reactively steered antennas, whichhave only one active element with an RF chain and multiple
parasitic elements terminated by variable reactance circuits[6]-[11]. By changing the values of these reactance circuits,
directivity can be controlled. Here, the key point is that theantenna actively utilizes sufficient mutual couplings among theactive element and the parasitic elements.
From a practical viewpoint with respect to antenna installation on automobiles, on-glass antennas are generally preferredbecause they do not degrade the appearance of the vehicle.
In addition, considering the fact that approximately twenty
antennas are installed inside the vehicle, on-glass antennas aredesirable in order to save installation space. However, there are
difficulties in realizing a conformal type ABF antenna that canbe installed on the windshield. This is mainly because previous
research has primarily focused on three-dimensional ABF
antennas [6], [8], [9]. With regard to the microstrip type ABF
antenna [7], it is difficult to design an on-glass antenna becauseof the ground plate. It is possible to achieve a conformal
configuration by reducing the number of parasitic elements
to two in previous work [6], [8], [9], but this has the effect ofdegrading beamforming performance. In addition, this leads
to lower performance with respect to other capabilities, suchas direction-of-arrival (DOA) estimation using the reactancedomain multiple signal classification (MUSIC) algorithm [11].
The maximum estimated number of DOAs is given by the
number of reactance circuits that are sufficiently coupled witha feed point. Therefore, the estimated number of DOAs also
decreases with the decrease in the number of effective parasitic
elements in the previous study.
The present paper proposes a novel type of ABF antenna,
called a reactively steered ring antenna array (RSRAA). The
concept of the RSRAA was originally presented in [12].
Further numerical and experimental studies are next described.
Note that the operating frequency is chosen to be 2.4 GHz

ACCEPTED VERSION

2

throughout the present paper. The RSRAA is characterized
by a conformal configuration, which maintains the numberof reactance circuits that are strongly coupled with a feed
point as five. The configuration and concept of the RSRAA
are described in Section II. Here, an equivalent model is alsopresented for the purpose of simple calculation of incident
directivity when a set of reactance values is given. The
performance of the RSRAA is described in Section III. Resultsof parameter studies and examples of beam patterns are obtained by Monte Carlo (MC) simulation. Section IV describes
experimental results of a prototype antenna, including patternsof the antenna mounted on a windshield. A brief commenton the antenna module configuration is then presented froma practical viewpoint. An additional discussion regarding theantenna configuration is also presented. Finally, this paper isconcluded in Section V.

G

z
x

#3

#2

#4

#1

#5

H

Feed

y

W

: variable reactance circuit(a) Top view

L2

#2

x
z

II. R EACTIVELY S TEERED R ING A NTENNA A RRAY

L1

#4

#3

#5
#1

Feed

y

This section presents the configuration and concept of the
RSRAA.

D: variable reactance circuit(b) Side viewx’

φ’

A. Configurationy’

A configuration of the proposed reactively steered ring
antenna array is shown in Fig. 1(a) and Fig. 1(b), which are top
and side views, respectively. In addition, a mounted RSRAA
on a windshield is shown in Fig. 1(c). Three overlapping ringshaped wires are provided without any mutually nodal points.
The circuit length of each of the ring wires is given by one
wavelength λ. A balanced feed point and five reactance circuits
are embedded in the wires. These elements are positioned
along the same straight line when viewed from the x-axis.
By changing the values of the reactance circuits, directivity
can be controlled in the xy-plane. In practical terms, thereactance circuits are made of chip parts such as varactor
diodes, the values of which are controlled by changing their
input voltages. In this way, the proposed smart antenna realizes
a simple and conformal configuration.

θ’z’(c) Over view of a mounted antenna
Fig. 1. Configuration of proposed antenna. Each figure shows (a) top view,(b) side view, and (c) over view of a mounted antenna on a windshield.

B. Equivalent model
The directivity of previous ABF antennas can be formulated
as a function of values of reactance circuits [9]. The methodof applying the formulation to RSRAA is given here. As
described in [13], a one-wavelength ring antenna is represented
by two in-phase dipole antennas with a spacing of 0.27 λ. Thisis because large current distributions are concentrated on boththe feed point and the opposite point of the feed on the ringelement. Using this relation, RSRAA is assumed to have an
equivalent model as shown in Fig. 2, which is composed of six
dipole elements. Accordingly, the directivity D (ϕ, x) in thexy-plane is represented by a function of values of reactanceT
circuits x = [x1 · · · x5 ] as follows [9]:T

D (ϕ, x) = αw (x) a (ϕ)

(1)

z
x

y

R

Fig. 2. Equivalent model of RSRAA. The model is composed of a dipole
element array.

ACCEPTED VERSION

3TABLE IPARAMETERS OF C ALCULATED M ODEL .
Height H of the antenna
Length D
Length G
Line width L1
Line height L2

39.8 mm
0.125 mm312.5 mm
1.5 mm0.03 mm(= λ/π)(= λ/4)

where α is a constant, and the weight vector w (x) andsteering vector a (ϕ) are given by()−1
w (x)= Y −1 + Xr
(2)
u0
[
{
}
2π
a (ϕ) = exp j r T0 L (ϕ)
λ
}]T
{
2π T
,
· · · exp j r 5 L (ϕ)(3)λ
respectively, where
Xr

=

[
]
diag zs jxT

u0=[1 0 0 0 0 0]

T

(5)

T

(6)

(4)

L (ϕ) = [ cos ϕ sin ϕ 0 ] .

Here, components of the admittance matrix Y are mutual
admittances among the ports (the feed point and the fivereactance circuits). Position vectors ri (i = 0, · · · , 5) represent
the positions of the feed (i = 0) and the reactance circuits(i = 1 · · · 5) in the equivalent model, respectively. In addition,
zs is the characteristic impedance. From the equivalent model,
the antenna is found to be polarized vertically. Note that Y
has to be calculated by full-wave simulations before applying
the formulated directivity (1), as noted in [9]. In general,
reactively steered types of smart antennas actively utilize
mutual couplings among elements [6]. The RSRAA realizes
strong mutual coupling among each port because the ring wires
are overlapping and share magnetic fields through the areas.
It should be noted that these strong mutual couplings of the
RSRAA cannot be obtained by a real dipole array that is
identical to the array of the equivalent model.
III. P ERFORMANCEIn this section, numerical analysis is carried out in orderto clarify the performance of the RSRAA. First, beamforming
abilities were examined by a MC simulation. The conventional
reactance domain MUSIC algorithm [11] was then applied
to the RSRAA for estimating DOAs at a high resolution.
Admittance matrix Y , as well as radiation properties of
the antenna, are calculated in free space using commercial
simulator ”FEKO”, based on the method of moment (MoM).
Note that reactance values are assumed to be excluded in thecase of Y calculation. The basic parameters of the antennaconfiguration are shown in Table I. Unless otherwise noted,
these parameters are used throughout the paper.

]B 6
d[ 5
ina 4
g
de 3
zli
a2
m
ro 1
N
0
-90

-60

-30

0

Angle [deg.]

30

60

90

Fig. 3. Dependence of variable range of reactance values on beamforming
toward each direction in the case of (xm , xM ) = (-50Ω, 0Ω), (-100Ω, 0Ω),and (-150Ω, 0Ω), respectively.

Here, each of the simulated gains Gn (ϕ, x) is normalized in
the horizontal plane, which is given by the following equation:
Gn (ϕ, x) = ∫ π/2

D (ϕ, x)−π/2

D (ϕ′ , x) dϕ′(7)

In order to determine each beam pattern Gn (ϕ, x), 100,000
random sets of x were generated. Here, directivity D (ϕ, x) inequation (1) was chosen as the cost function to be maximized
for each ϕ. The following three cases were selected as thesimulation conditions of the variable range of each reactance value: (xm , xM ) = (−50 Ω, 0 Ω), (−100 Ω, 0 Ω),
and (−150 Ω, 0 Ω), where xm and xM are minimum andmaximum values of the variable range, respectively. First,
it was found that beamforming ability differs depending on
direction. In the case of (−150 Ω, 0 Ω), for example, themaximum normalized gain reaches 5 dB at ϕ =0 deg. On
the other hand, the normalized gains toward the directionsof approximately ϕ =30 deg and -45 deg are lower incomparison with the average. Second, the calculated gain isgreatly influenced by the range of variable reactance values.
As shown in the figure, the case of the narrowest reactance
range, i.e., (xm , xM ) = (−50 Ω, 0 Ω), shows a much lower
performance than the other two cases. The range of thevariable reactance circuits must be higher than a certain value.
Next, the influence of the spacing G between the centersof the closest ring elements on the beamforming ability was
examined. The simulation results are shown in Fig. 4. Here,the average gain in the figure is defined by averaging the calculated normalized gain in (7) for each direction. Parameters
other than G were kept constant, and the range of the reactancevalues was set to (xm , xM ) = (−150 Ω, 0 Ω). The figure
shows that the beamforming ability is highest when G = λ/4.
Thus, G was set to λ/4.
B. Beamforming

A. Parameter Study
The dependence of the antenna gain on the beam direction
was calculated. Fig. 3 shows the gain toward each direction at
every 10 deg from ϕ =-90 deg to ϕ =90 deg with θ = 90 deg.

Fig. 5 shows beam patterns toward ϕ =-90 deg, -30 deg,
0 deg, 30 deg, and 90 deg, each of which is calculated
by both MoM and the equivalent model represented by (1).
In terms of MoM simulations, the values of the reactanceACCEPTED VERSION

4-30 deg-90 deg-20 -15 -10 -5

0 [dB]

0 deg-20 -15 -10 -5

0 [dB]-20 -15 -10 -5

30 deg0 [dB]-20 -15 -10 -5

0 [dB][dB] 0-5 -10 -15 -20

Equivalent model
MoM

Equivalent model
MoM

Equivalent model
MoM

Equivalent model
MoM

Equivalent model
MoM(a) -90 deg(b) -30 deg(c) 0 deg(d) 30 deg(e) 90 deg

90 degFig. 5. Comparison between patterns calculated by equivalent model and by MoM.

TABLE IIIS IMULATION PARAMETERS FOR MUSIC.

4.5

]B
d[
ina
ge 4
ga
re
vA

Number of Waves
DOAs
SNR

3.5
3.5

3.75

4

ٛٛٛٛG

4.25

4.5

Fig. 4. Calculated average gain of each direction for parameter study in thecase of G = λ/3.5, λ/3.75, λ/4, λ/4.25, and λ/4.5.
TABLE III NCIDENT VALUES OF R EACTANCE C IRCUITS U SED IN PATTERNS IMULATION .
Direction-90 deg-30 deg
0 deg
30 deg90 degDirection-90 deg-30 deg
0 deg
30 deg90 degx1-157.3 Ω
36.3 Ω-63.1 Ω
-122.2 Ω-49.0 ΩHPBW
93 deg64 deg45 deg57 deg92 degx2
106.2 Ω
156.0 Ω
-17.5 Ω
56.9 Ω
39.1 Ω

x3-168.4 Ω
34.5 Ω
7.8 Ω
193.0 Ω
-107.8 Ω

x4-153.2 Ω
268.6 Ω
-64.9 Ω
9.7 Ω
-123.2 Ω

x5
162.4 Ω
295.3 Ω
-79.5 Ω
-229.2 Ω
24.7 Ω

circuits are built into the model as lumped-parameter elements.
Each incident x for each beam pattern is obtained by MC
simulation of 100,000 random x in the same way as the former
simulation of the parameter study. Here, (xm , xM ) is setby (−300 Ω, 300 Ω). The reactance values obtained by MC
simulation are listed in Table II. Each half-power beam width(HPBW) is also described in the table. The following two
findings were observed in the simulation results. First, typicalbeam patterns toward the desired directions were formed.
Thus, an effective beam control technique was verified by
calculation. Note that, in the simulated cases, HPBW changes
from 45 deg to 93 deg, depending on the beam direction.

3 (uncorrelated)-60 deg, 0 deg, and 45 deg
20 dB (each of the waves)

This is because the equivalent array works as a broad sidearray at ϕ = 0 deg and as an end-fire array at ϕ = ±90
deg. Second, the patterns by MoM and the equivalent model
are quite similar especially around the direction of the beam.
Therefore, the fundamental validity of the proposed equivalentmodel was confirmed. On the other hand, slight differences arefound at the sidelobes and the nulls. These may be caused bya model error of the equivalent model, such as positions of
the equivalent dipoles and/or their resonance amplitudes andphases.

C. MUSIC
Another capability of the proposed antenna, i.e., DOA
estimation, is presented here. As described in Section II, theequivalent model enables the steering vector to be defined, as
represented in (3). Using this steering vector, the conventional
reactance domain MUSIC algorithm [11] was applied to the
proposed antenna, and its performance was verified numerically. The simulation parameters are listed in Table III. Here,three uncorrelated waves are assumed, and the signal-to-noise
ratio (SNR) of each of the waves is given as 20 dB. Fig.
6 shows the simulation results of MUSIC. It was found that
DOAs can be estimated at a high resolution and with estimatederrors of less than 0.5 deg. Note that the proposed antenna
can estimate a maximum of five DOAs. As shown in [11],this maximum number is equal to the number of variable
reactance circuits. On the other hand, in a real dipole array ,the configuration of which corresponds to the equivalent modelof the proposed antenna, a maximum of only two DOAs canbe estimated. This is because the active element couples with
only the closest parasitic elements in such a real dipole array.
Thus, the proposed antenna has the advantage of the ability toestimate many more DOAs than the conventional dipole arrayin the conformal configuration.

ACCEPTED VERSION

0

5

-59.5

0.5

43.5

]B
d[
mu -10rtcepS -20
CI
SU
M-30

-90

[dB] 0

-60

-30
0
30
Angle [deg.]

60

jxj

Experiment
MoM

ExperimentMoM(a) 10 deg(b) 30 deg

TABLE IVI NCIDENT VALUES OF R EACTANCE C IRCUITS U SED FOR M EASUREDP ATTERNS .
Direction
10 deg30 degvi
RFig. 7. Equivalent circuit model of variable reactance circuit.

IV. E XPERIMENT
A. Prototype Antenna
A prototype antenna was constructed on a typical Teflon
substrate with a thickness H of 0.6 mm and a dielectric
constant ϵr of 2.6. Here, the configured parameters of the
prototype antenna are the same as those listed in Table I,
except for H = 0.6 mm. Copper was etched on both sidesof the substrate, and reactance circuits were mounted using
chip parts (varactors, capacitors, and resistors). The equivalentcircuit of the reactance circuits is shown in Fig. 7. The
main components of the circuit are a serial varactor and a
blocking capacitor. The varactor is DC biased through choke
resistors of 10 kΩ. The bias lines are drawn orthogonally tothe polarization in order not to affect the radiation pattern. The
feed point is connected to a parallel line , the characteristic
impedance of which is 200 Ω. The width of each line and the
distance between the centers of the two lines were chosen tobe 1.6 mm and 3.0 mm, respectively. The end of the parallel
line is connected to an LC balun, which transforms the 200Ω balanced line to a 50 Ω unbalanced line. The measured
insertion loss of the LC balun is 0.29 dB at 2.4 GHz. Fig. 8
shows the patterns that were measured in an anechoic chamberand calculated using MoM. Here, Fig. 8(a) and 8(b) represent
the beam patterns toward 10 deg and 30 deg, respectively. The
incident sets of reactance values are also listed in Table IV.

-5 -10 -15 -20

Fig. 8. Comparison between patterns measured in anechoic chamber andcalculated by MoM.

R

Cv[dB] 0-5 -10 -15 -20

90

Fig. 6. Calculated MUSIC spectrum for a set of reactance values. DOAs =-60 deg, 0 deg, and 45 deg. SNR = 20 dB for each signal.

Cb

30 deg10 degx1-51.8 Ω
-3.7 Ω

x2-2.1 Ω-21.4 Ω

x3
-3.4 Ω-37.3 Ω

x4
-56.1 Ω
-1.8 Ω

x5-83.3 Ω
-1.6 Ω

Here, in order to obtain these values, MC simulations wereconducted in the same way as in Section III. Based on theseresults, the measured patterns and simulated patterns were
found to be quite similar.
B. Mounted Antenna
The prototype antenna was mounted at the upper centerregion of the windshield, as shown in Fig. 1(c). The distance
between the metal-top roof and the nearest side of the prototype antenna was set to be 40 mm. The angle between thewindshield and the horizontal plane was approximately 30 deg.
Fig. 9 shows the beam patterns of the mounted antenna towardϕ’ = 10 deg and ϕ’ = 30 deg in the coordinate system shownin Fig. 1(c). In each of the cases, three patterns at every 10
degrees of elevation angle from θ’ = 90 deg to θ’ = 70 deg
are plotted. Here, the incident reactance values are the sameas the cases shown in Fig. 8. The VSWR with the 10-deg

φ’ = 30 deg

φ’ = 10 deg[dBd]dBd] 1050-5 -10Elevation
θ’ = 90deg
θ’ = 80deg
θ’ = 70deg(a) ϕ’ = 10 deg[dBd]dBd] 1050-5 -10Elevation
θ’ = 90deg
θ’ = 80deg
θ’ = 70deg(b) ϕ’ = 30 deg

Fig. 9. Measured radiation patterns of mounted RSRAA in the ϕ’ plane.

ACCEPTED VERSION

and 30-deg beams are 1.8 and 1.9, respectively. The measured
results indicate that effective beam patterns were obtained
while keeping the return loss quite low. Note that the VSWR
changes depending on the values of reactance circuits, and that
the VSWR can be suppressed by choosing appropriate values.
The ability to suppress the VSWR depends on the variable
range of reactance circuits, as well as the beamforming ability.
In addition, radiation patterns in Fig. 9 are somewhat noisy,
primarily because of the reflection from the vehicle body.
In general, radiation patterns at 2.4 GHz are more strongly
influenced than at the VHF band when antennas are mounted
on vehicles.

6

Multi-layered films
RF Connecter

Balanced linemm
05
LNA Filter Balun

4mm
m
m
2

C. Antenna Module
We present a brief description of a practical antenna modulethat has been designed from a practical point of view. Fig.
10(a) shows a configuration of the proposed antenna module,
and an example of the antenna mounted on a windshield isshown in Fig. 10(b). The overall size including a voltage bias
circuit is 220 mm × 50 mm. The antenna is composed ofcopper-etched multi-layered films. The fed ring wire , as wellas the other two ring wires, are situated on the independentfilms. The films are made of a transparent polyethylene
terephthalate (PET) substrate , the thickness H of which is
125 µm. As for the reactance circuits, chip parts are mounted
using conductive paste. Each of the varactors in the reactancecircuit is connected to bias lines with a width of 20 µm,
and is supplied with control voltage. Then, the feed point
is connected to a RF connecter through a RF circuit that issurrounded by a metal box. The RF circuit in the metal box
is composed of an LC balun, a filter, and an LNA, where the
LNA is prepared for the case in which the antenna is used asa part of receiving system. The filter is situated such that the
LNA is not saturated. At 2.4 GHz, the insertion loss of the
LC balun and the filter are 0.29 dB and 0.46 dB, respectively,
and the gain of the LNA is 6.7 dB.

220 mm Film(thickness 125µm)

Metal box (RF circuit)

VCC lines(width 20µm)

Variable reactance circuits
(a)

(b)

D. Discussion
This paper has focused on an RSRAA with three ringelements. However, the element number should be adjusted
depending on the requirements of the application. For thecase in which a narrower beam is required, for example, alarger number of ring elements is effective. This is because theeffective length of the antenna can be increased while keeping
mutual couplings among the equivalent dipoles. On the other
hand, the number of reactance circuits also goes up, resulting
in a more complex system. This design property has a tradeoff relationship. Furthermore, in addition to the number ofring elements, there is no limit for the relationship among thefeed and the reactance circuits. The position of the feed canbe replaced by the position of one of the reactance circuits. It
is important to adjust the parameters so that the performancemeets the requirements of the target system.
In addition, the directivity function (1) is derived on thebasis of the assumption that the circuit length of each of
the ring elements is one wavelength. Thus, it is clear thatthe concept of the beamforming does not work when theFig. 10. Photograph of an antenna module. (a) configuration of antenna
module and (b) mounted antenna module on a windshield.

operating frequency is out of the allowable range. A detailed
consideration of the bandwidth is beyond the scope of thepresent paper.
V. C ONCLUSION
A reactively steered ring antenna array and its beamforming concept have been proposed for forthcoming automotive
applications. The antenna features a conformal and simple
configuration composed of only one RF port with multiple
variable reactance circuits. Strong mutual couplings among
the feed and the reactance circuits have been realized, which
contribute to beamforming performance. An equivalent dipole
model was presented for simple calculation of directivity,
which was verified through computer simulation. An equivalent model also enabled the conventional MUSIC algorithm

ACCEPTED VERSION

7

to be applied to the proposed antenna. Representative beam
patterns of the prototype antenna were measured for the caseof ϕ = 10 deg and 30 deg in the horizontal plane. Here, thevalidity was confirmed in comparison with simulation results.
Furthermore, the beam patterns of the antenna mounted on
the windshield of an automobile were measured, and were
found to be formed effectively. Finally, brief comments on theantenna module and its settings were described. The proposed
antenna will be utilized for future vehicle communications
related to beamforming applications.

[10] J. Lu, D. Ireland, and R. Schlub, ”Dielectric embedded ESPAR (DBESPAR) antenna array for wireless communication,” IEEE Trans. Antennas Propag., vol.53, no.8, pp.2437-2443, Aug. 2005.
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Dec. 2004.
[12] S. Sugiura and H. Iizuka, ”Study of reactively steered ring antenna arraysuitable for automobiles,” in IEEE Int. Antennas and Propag. Symp. Dig.,
Albuquerque, NM, July 2006, pp.2365-2368.
[13] K. Sawaya, ”Review of research and development on linear antennas,”
IEICE Trans. Commun., vol.E86-B, no.3, pp.892-899, March 2003.

ACKNOWLEDGMENT
The authors would like to express their sincere gratitude to
Dr. K. Nishikawa of Toyota Central Research and Development Laboratories for his continuous interest in this study.
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Shinya Sugiura was born in Nagoya, Japan, in 1979. He received the B.S. and
M.S. degrees in aeronautics and astronautics from Kyoto University, Kyoto,
Japan, in 2002 and 2004, respectively.
Since 2004, he has been with Toyota Central Research and Development
Laboratories, Inc., Aichi, Japan. His research interests include smart antenna
systems and mobile communications. Mr. Sugiura is a member of the Instituteof Electronics and Communication Engineers in Japan.

Hideo Iizuka received the B.S. and M.S. degrees in electrical engineeringfrom Saitama University, Saitama, Japan, in 1995 and 1997, respectively.
He joined Toyota Central Research & Development Laboratories, Incorporated, Aichi, Japan, in 1997. From 2001 to 2002, he was a Visiting Scholar at
the School of Electronic, Electrical and Computer Engineering, Universityof Birmingham, Birmingham, U.K. He has been engaged in research on
millimeter-wave antennas, digital terrestrial antennas and active antennas forautomobile applications.
Mr. Iizuka received the IEICE Young Engineering Award in 2001.