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IEEE TRANSACTIONS ON ANTENNAS AND PROPAGATION, VOL. 67, NO. 3, MARCH 2019
1957
Communication
Quarter-Wave Balun Fed Vivaldi Antenna Pair for
V2X Communication Measurement
Philip Ayiku Dzagbletey , Jae-Yeon Shim , and Jae-Young Chung
Abstract— A dual-polarized Vivaldi antenna pair has been designed for
use in a vehicle-to-anything (V2X) measurement facility. The operating
bandwidth is from 560 MHz to 7.7 GHz (173% fractional bandwidth),
covering all the existing V2X communication frequencies. The ultrawideband (UWB) property was achieved by integrating quarter-wave
baluns at the feeding point of two orthogonally crossed Vivaldi antennas. The measured port isolation between the two antennas is better
than −28 dB. The cross-polarization discrimination of better than 17.2 dB
and antenna gain of up to 9.2 dB have also been realized across the
whole bandwidth. Three of the designed antennas were mounted on a
vehicle testing facility and were used to measure the radiation patterns
of a roof-top shark antenna under four different V2X scenarios. That is,
vehicle-to-pedestrian, vehicle-to-vehicle, and vehicle-to-infrastructure and
vehicle-to-network. The results have also been presented in this
communication.
Index Terms— Antenna measurement, balun, dual polarization,
quarter-wave tapering, tapered slot antenna (TSA), vehicle-to-anything
(V2X) communication, Vivaldi antenna.
Fig. 1.
models.
V2X Measurement facility with three mounted Vivaldi antenna
I. I NTRODUCTION
Vehicle-to-anything (V2X) communication is one of the key technologies to realize the intelligent transport system. Car makers and
broadband network service providers are rigorously discussing the
implementations of V2X systems based on two strong candidates
of wireless communication standards: the IEEE 802.11p and the
cellular-based V2X (C-V2X). The former relies on direct communication between vehicles using the wireless access in vehicular
environmental (WAVE) protocol. On the other hand, C-V2X, which
is based on cellular telecommunications, achieves V2X via direct
device peering and/or through the cellular operators [1], [2].
WAVE under IEEE 802.11p is to operate within the Wi-Fi band
of 5.77–5.925 GHz, whereas C-V2X is to operate vis-à-vis legacy
supported third Generation Partnership Project cellular bands. These
include but are not limited to 3G, 4G long-term evolution (LTE), and
the tentative IMT2020 5G bands between 600 and 3500 MHz.
Due to the variety of operating bands and applications of V2X
systems, be it IEEE 802.11p or C-V2X, a feasible measurement
facility for testing the wireless performance of a vehicle within the
range of 600 MHz to 6 GHz is needed. Fig. 1 illustrates a prototype
of an inexpensive, simple to construct and rapid V2X communication measurement system. As can be seen, three identical antennas
are mounted on a gantry (or wall) to test the signal reception at
different grazing incidences of electromagnetic waves. With this,
an existing vehicle testing facility can be utilized for V2X testing
in the most economical manner. The mounted antennas should be
Manuscript received February 27, 2018; revised November 27, 2018;
accepted December 9, 2018. Date of publication January 21, 2019; date of
current version March 5, 2019. This work was supported by the Seoul National
University of Science and Technology through the Advanced Research Project.
(Corresponding author: Jae-Young Chung.)
The authors are with the Electrical and Information Department, Seoul
National University of Science and Technology, Seoul 01811, South Korea
(e-mail: jychung@seoultech.ac.kr).
Color versions of one or more of the figures in this communication are
available online at http://ieeexplore.ieee.org.
Digital Object Identifier 10.1109/TAP.2019.2893201
compact, lightweight, and inexpensive and, above all, should have
ultrawideband (UWB) radiation characteristics and dual-polarization
capability.
The use of quad-ridged horn antennas in [3]–[5] has been the traditional method of implementing dual-polarized wideband antennas.
Although these offer excellent performance across the bandwidth,
they tend to be bulky, heavy, and expensive to develop.
Alternatively, dual-polarized Vivaldi antennas implemented with
two orthogonally crossed printed circuit boards (PCB) [6]–[8] offer
excellent radiation performances in a more economical way. However,
a significant drawback of Vivaldi antenna is the poor impedance
matching condition, low gains at lower frequencies and a midfrequency gain drop. The latter is present in the cross-shaped antennas
only. As reported in [9] that for longer wavelengths (<2 GHz),
the guided waves along the tapered edges reflect at the wider ends and
form standing-waves; making low-frequency impedance matching
difficult. This also results in the radiation pattern deformation and
the antenna gain reduction. To correct this challenge, Han et al. [7]
introduced corrugated ripples on the outer edges of the radiating arms
on two cross-shaped Vivaldi antennas. It achieved a 140% fractional
bandwidth (fBW) from 1.4 to 8.0 GHz. Also, Sonkki et al. [6]
developed a pair of cross-shaped Vivaldi antennas with a 166% fBW
at −10 dB S11 from 0.683 GHz up to 7.0 GHz. Using a traditional
tapered balun structure, it achieved the wideband results mainly due
to good optimization and a large antenna size of 220 mm × 240 mm.
This communication was set out to overcome the above-mentioned
challenge of the Vivaldi antenna by introducing a two-stage quarterwave balun at the feed, thus improving the low-frequency matching
condition without increasing the size arbitrarily. The macroscopic air
gaps between the cross-shaped Vivaldi antennas which also results in
gain drops at the mid-frequency was investigated and corrected with
silicone adhesive.
The proposed design was fabricated, measured, and implemented in a V2X test system to ascertain the results. An fBW
of 172.95% (0.56–7.72 GHz) is achieved. The realized gain of up
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1958
IEEE TRANSACTIONS ON ANTENNAS AND PROPAGATION, VOL. 67, NO. 3, MARCH 2019
Fig. 2.
Two-stage quarter-wave balun. (a) Detailed geometry of balun.
(b) Front/Back view of antenna.
to 9.2 dB is also achieved at a compact size of 187.5 mm × 190 mm.
These results are not only superior to the previously reported
cross-shaped Vivaldi antennas [6]–[8] but also provide alternative
approaches to improving the Vivaldi antenna’s shortcomings.
With preliminary simulation and some measured results of the
antenna discussed in a conference communication [10], this communication discusses details of the balun design and measured results.
Moreover, using the proposed model as a test antenna, the radiation pattern measurement results of the roof-top shark antenna are
obtained from the V2X testing facility (described in Fig. 1). These are
presented in Section IV to demonstrate the usefulness of the proposed
design.
II. A NTENNA D ESIGN
Fig. 3. (a) Real input impedance of balun by varying width w1 . (b) Imaginary
input impedance of balun by varying width w1 .
A. Design Procedure
The Vivaldi antenna or tapered slot antenna (TSA) has a hypothetically unlimited instantaneous frequency bandwidth with stable high
gains and linear polarization [9], [11]. Therefore, it is frequently used
in wideband communication systems and testing facilities. Due to its
planar structure, the antenna can be easily fabricated using the PCB
technology with various modifications [12].
The geometry and dimensions of the antenna are shown in the
inset of Fig. 1 [10]. The linear dual-polarization is achieved by
orthogonally integrating the two identical Antennas A and B; which
were fabricated on 0.8 mm thick FR-4 substrates. Two slits incisions
were made from either ends of each antenna toward the feeding
point to allow one antenna to be slid into the other. The exponentially tapered profiles (shaded region in Fig. 1) offer smooth
impedance transition from guided waves (on the radiators) to radiated
waves (in free space). The width of the tapered end defines the lowest
cutoff frequency of the antenna at the guided wavelength. This width
in Fig. 1 is 187.5 mm, corresponding to the guided wavelength
at 750 MHz when FR-4 substrate with the dielectric constant (εr )
of 4.4 is used. Thus, instead of the intended 600 MHz opening gap,
750 MHz was used. This reduction was made possible in part by
rigorous optimization of the radiator taper and the implementation of
the quarter-wave balun feedline. The high-frequency cutoff region is
also determined by the opposite end of the tapered structure, where
the antenna is fed. Vivaldi antenna baluns are, therefore, designed
with the mid-frequency wavelength; to ensure a balanced matching
condition across the bandwidth of operation. With these methods,
impedance performance at the lower frequency regions below 2 GHz
was greatly improved, hence, the size reduction.
B. Two-Stage Quarter-Wave Balun Feeding
The Vivaldi guides the traveling wave along its balanced structure
and thus requires a balanced-to-unbalanced (balun) transformer when
fed by an unbalanced coaxial cable. Fig. 2 shows the implemented
two-stage quarter-wave balun structure at the Vivaldi feed. The rightangled balun feedline at the backside of the substrate, couples with
the radiators at the narrow slot of width w1 just above the circular
cavity. The purpose of employing the quarter-wave balun instead of
a widely used tapered balun [6], [12] is to improve the bandwidth.
The impedance transition stages of the quarter-wave balun are
from the 50 input (length l1 ) through to the open circuit (OC)
radial stub. The slot impedance of 138 is calculated at the
center frequency ( f C ) of 3.3 GHz for the slot width w1 . This
width is kept very narrow to achieve better impedance matching
conditions at higher frequencies (>3 GHz). However, at narrower w1 ,
higher capacitances are introduced into the balun, which can be
suppressed by extending l4 and/or r1 (i.e., increasing the inductance
of the resonator). Nevertheless, this is nearly impossible due to the
orthogonal cross-integration limitations of the Vivaldi; as it can cause
galvanic contact between the two antennas. Another challenge is
that the impedance of the feedline at the coupling point must be
high enough to properly couple with the high slot impedance. This
requires a very thin microstrip feedline width at this point; which is
also limited by fabrication precision and accuracy. The slot width is,
therefore, widened to reduce the high capacitances and high slot
impedance. The increment of w1 results in mid-frequency gain drops
which have been discussed in Section III of this communication.
Due to these constraints, the size of cross-shaped Vivaldi antennas is often enlarged beyond the cutoff frequency to maintain the
wideband impedance matching and compensate for gain drops. These
limitations only leave room for rigorous feed design and optimization.
The two-stage quarter-wave balun was designed using the
Klopfenstein taper approach in [13] and [14]. A full-wave EM simulation software, Ansys HFSS [15], was used to optimize the balun
geometry. The optimization was performed by observing the input
impedances and S11 while varying w1 , r2 , and l2 among other
parameters.
Fig. 3(a) and (b) shows the real and imaginary input impedances
when w1 is varied. In this simulation, the OC radial stub is replaced
by a lumped port to optimize the performance of the slot width alone.
The lower frequency band (<1 GHz) hardly changes by varying
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IEEE TRANSACTIONS ON ANTENNAS AND PROPAGATION
1959
TABLE I
O PTIMIZED BALUN PARAMETERS
Fig. 4.
Input impedance of the balun by varying r2 .
Fig. 5.
S11 by varying length l2 ≈ l3 .
w1 for both real and imaginary components. At higher frequencies,
however, the real input impedance, which properly characterizes the
slot, is sensitive to w1 . A large fluctuation is observed in both
the real and imaginary curves at around 6.0 GHz. This is due to
the unmatched OC port used, which defines the high-frequency cutoff
region. This cutoff is usually extended with the radial stub technique.
Although w1 = 1 mm in Fig. 3 is better for 50 matching; it might
cause galvanic contact with the second antenna due to the 0.8 mm
substrate thickness. It was, therefore, optimized to 1.65 mm, which
is more twice the substrate thickness.
Fig. 4 also shows the absolute input impedance by varying r2 ,
the radius of the radial stub. The real/imaginary components are not
shown since the optimized stub effectively removes the reactance
components in the feedline; hence, making the real components
similar to the absolute value. The increase in impedance is clearly
observed as r2 increases at the lower frequency of 500 MHz. The
radial stub at certain lengths reaches antiresonance as its dominant
inductive property reverts to capacitive. This change makes the stub
an OC and is effective for canceling the remaining currents in the
feedline. At r2 = 6.7 mm, the frequency resonance is seen to be
closer to the 50 line; indicating good matching. Also, the cutoff
region is extended beyond 6 GHz by the stub; as the fluctuations
observed around 6 GHz in Fig. 3 are reduced.
Fig. 5 shows the final S11 optimization by varying l2 and l3
simultaneously. As can be seen, it greatly affects the impedance
matching characteristics across the bandwidth and dominantly around
the mid-frequency region. l2 was, therefore, set at 11.68 mm ≈
12 mm, which shows the best result. This confirms the impedance
transition with two quarter-wavelength stages for λg /4 = 11.68 mm
at f C = 3.3 GHz. Their (l2 and l3 ) equivalent widths were set
to 0.75 and 0.3 mm corresponding to ∼73 and ∼106 . The
optimized values are summarized in Table I.
III. A NTENNA FABRICATION AND M EASUREMENT
A. Cross-Integration and Mid-Frequency Gain Drop
Fig. 6 shows the fabricated and assembled antennas. The frontside
of Antenna A with the longitudinal slit (S1) is shown in Fig. 6(a). The
Fig. 6.
Fabricated Vivaldi antenna pair. (a) Front-side of Antenna A.
(b) Back-side of Antenna B. (c) Assembled antenna with silicone
adhesive (left inset) and soldering (right inset).
inset shows the circular cavity and the thin microstrip line connecting
the two radiator halves. The backside of the second Antenna B is
shown in Fig. 6(b) with the quarter-wave balun and second slit (S2).
The two antennas are slid into each other with the base of Antenna 2
soldered as shown in the inset of Fig. 6(c) (right). The soldering
connects the separating halves of Antenna B. To ensure rigidity and
measurement stability, the two antennas are encased in a transparent
holder. The case did not, however, affect the electrical performance
of the antenna.
During fabrication and measurement process, the cross-shaped
Vivaldi antenna had a structural limitation which reduced the midfrequency gain values. As discussed in Section II and from Fig. 2,
the slit (S1) in Antenna A was optimized to 1.65 mm to ensure good
impedance matching with the balun, prevent galvanic contact and
minimize high capacitance in the balun. However, with a substrate
thickness of 0.8 mm, an air gap of 0.85 mm is created when both
antennas are cross-shaped. The air gap tends to introduce more
resonant interferences between the antennas at the middle frequency
which results in gain losses.
References [6] and [7] among other cross-shaped Vivaldi references
also reported gain drops between 1.5 and 2.5 dB at their midfrequencies; making the midfrequency gain drop an inherent drawback in the
cross-shaped Vivaldi antenna. Most researchers use larger radiator
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1960
IEEE TRANSACTIONS ON ANTENNAS AND PROPAGATION, VOL. 67, NO. 3, MARCH 2019
Fig. 8.
Simulated and measured S11 and port isolation.
Fig. 9.
Simulated and measured realized gain.
Fig. 7. Comparison of antenna with (w.) and without (w/t.) silicone adhesive
and styrofoam sealant. (a) Measured gain (port 1). (b) Measured S11.
sizes to compensate for the gain drop. From Fig. 7(a), two resonant
dips of up to 3 dB are observed at around 3.0 and 5.8 GHz for the
antenna with air gaps. These dips are, however, nonexistent in the
single antenna measurement and less pronounced in the simulation
results. The gain drop is not suitable for an antenna in a UWB test
system.
The authors, therefore, experimented with various novel
approaches to mitigate the gain drop problem. These included
sealing the air gaps with dissolved styrofoam or with silicone
adhesive, having a low dielectric permittivity. Alternatively,
the crossed Vivaldi structure can be constructed using 3-D printing.
The latter is, however, complex and not practical for this design. The
left-inset photograph in Fig. 6(c) shows the antenna with silicone
adhesive (silicone 1035, εr = 2.8), used to seal the air gaps. Minimal
amounts were carefully applied at the intersecting regions without
touching the radiators. The same sealing technique was applied with
the styrofoam paste made by dissolving a piece of styrofoam in pure
acetone. After about a 12 h waiting time, the paste hardens and is
secure.
Fig. 7 shows the measured results of the Vivaldi antenna with and
without the silicone and styrofoam paste. Silicone 1055, which is
widely used for bonding circuit components, offered better performance with no air gaps after sealing. styrofoam, on the other hand,
still had some trapped air which resulted in a poorer performance
compared with the silicone.
Gain improvement of about 2.1 dBi was recorded for the styrofoam
paste model in Fig. 7(a). The best performance was observed with
the silicone sealant. It achieved gain improvement of about 2.8 dBi.
Similar results are observed in the S11 of Fig. 7(b) with the styrofoam
and silicone adhesives performing better than the air-gap model.
As noted, the silicone sealant with a lower dielectric constant did not
Fig. 10.
Simulated and measured XPD.
alter the antenna’s intrinsic parameters as the results (both S11 and
far-field patterns) were similar to the simulated values.
B. Measured Result Discussion
Fig. 8 shows the simulated and measured S11, S22, and S21.
For the measured results, an fBW of 172.95% is recorded at
−10 dB S11 (560 MHz to 7.72 GHz) for Antenna A (Port 1). Also,
the port isolation (S21) of better than −28 dB is observed. A similar
S11 is obtained from Antenna B (Port 2), where a 173.5% fBW at
−10 dB S22 (530 MHz to 7.48 GHz) is obtained. Also, at −8 dB,
a 173% and 175.3% fBW for S11 and S22 are observed, respectively.
The measured results agreed well with the simulation results.
The fabricated antenna was brought into an anechoic chamber for
the measurements of radiation characteristics. The gradual change
in gain levels across the bandwidth is a common phenomenon with
traveling wave structures, that is, their gains are proportional to the
antenna size and the rate at which the energy dissipates as it travels
along the tapered structure. Fig. 9 shows the realized gains ranging
from 1.2 to 9.2 dBi across the bandwidth. Although the structural
minimization also affected the gain performance, the quarter-wave
balun enabled the minimal gain increment of 1.2 dBi at the lowest
frequency. Fig. 10 shows the cross-polarization discrimination (XPD)
which is better than 17.2 dB for both ports, when measured. Although
there is a change in the XPD of the simulated and measured results,
17.2 dB XPD is acceptable for a test antenna. Normalized radiation
patterns in decibels are shown in Fig. 11. Similar radiation patterns
between the ports 1 and 2 antennas are observed. The WAVE
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IEEE TRANSACTIONS ON ANTENNAS AND PROPAGATION
1961
Fig. 11. Normalized radiation patterns of measured co-polarization (Cpol.) and cross polarization (Xpol.). (a) 0.7 GHz. (b) 1.0 GHz. (c) 3.0 GHz. (d) 5.8 GHz.
TABLE II
C OMPARISON OF P ROPOSED M ODEL W ITH S IMILAR D ESIGNS
frequency of 5.8 GHz is also shown to have very good performance.
An average 3 dB beamwidth ranging between 77.2° and 45.3° for
port 1 and 88.7°–37.9° for port 2 is also observed across the
bandwidth.
Table II summarizes the performance of the proposed antenna and
compares it with other reports on the cross-shaped Vivaldi antenna
design. Evidently, the proposed design has better results compared
with the existing structures with respect to structural compactness,
bandwidth enhancement and gain stability.
IV. V2X S YSTEM
With the required antenna performance for the V2X system
achieved, this section discusses the measurement results in the
V2X test facility.
A. System Setup
At an indoor C-V2X test facility, the antennas are configured as
seen in Figs. 1 and 12. The latter figure shows a partial photograph
of the measurement facility with the three-antenna array. Antennas (Ant.) 1, 2, and 3 are mounted on a gantry at 50°, 60°, and 70°
grazing incident angles. These are represented as channels 1, 2, and 3
in the radiation patterns of Fig. 13. Each antenna is encased in a
special radome to protect it and reduce its sensitivity. This is to mimic
unfavorable outdoor conditions at the interface of the transceiver
antennas for worst case scenarios. The shark antenna used for the
test is also shown in the inset of Fig. 12. This is a proprietary
MIMO transceiver shark antenna mounted on a vehicular platform
and operating within the standard cellular frequency band.
Due to system limitations at the measurement facility, the
IEEE 802.11p standard which includes the WAVE frequency band
was not tested. The antenna, however, is designed to operate in
these bands as well; as seen from the measured radiation pattern
in Fig. 11(c) and (d). Therefore, the following bands were under test
for C-V2X in the facility. GSM at 850–1900 MHz with related PCS
and DCS systems, 7002100 MHz for UMTS-based communication
Fig. 12. V2X measurement chamber with three mounted Vivaldi antennas.
Inset: rooftop mounted shark antenna.
variants and 600–3500 MHz for 4G-LTE systems. At these cellular
enabled frequency bands, the mounted TSAs served as an ideal
transceiver in a stationary scenario using a proprietary cellular network configuration. Channel 1 represented vehicle-to-infrastructure
and vehicle-to-network systems as these have ideally similar interface
structures. The antenna is placed at a 50° inclined angle from the
normal of the shark antenna. Vehicle-to-vehicle is channel 2° at
60 from the vehicle and channel 3° at 70 is vehicle-to-pedestrian
scenario.
B. Measurement Results
Fig. 13 shows the received gains of the shark antenna at certain
frequency bands with the Vivaldi antennas. Fig. 13(a) shows 915 MHz
for the 900 MHz GSM band (15 MHz bandgap required). The results
show a fairly good isolation between the horizontal and vertical
patterns; representing the two linear polarizations of the Vivaldi
antenna. The 900 MHz GSM band has receiver gain (Rx. G) of up to
5 dBi across all three channels. Similarly, Fig. 13(c) at 787 MHz LTE
frequency-division multiplexed (FDD) has a relatively good polarization isolation with Rx. G of up to 3.3 dBi. The 2100 UMTS band,
recorded at 2110 MHz in Fig. 13(b), has Rx. G of up to 7 dBi but
with a much irregular receive pattern in channel 2 as the horizontal
polarization is seen to be stronger than expected. The LTE timedivision multiplexed (TDD) patterns are also observed in Fig. 13(d)
at 1930 MHz with similar characteristics as the 2100 UMTS.
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IEEE TRANSACTIONS ON ANTENNAS AND PROPAGATION, VOL. 67, NO. 3, MARCH 2019
Fig. 13. Measured radiation patterns by the Vivaldi mounted antennas. (a) GSM 900 MHz band. (b) UMTS/WCDMA band 2100 MHz. (c) FDD LTE
700 MHz. (d) TDD LTE 2100 MHz.
In all, the received gains across the three scenarios proved stable
and efficient for the measurement. The overall effect of the quarterwave balun is seen as being superior in impedance matching as
compared with the traditional approach; as it offered the opportunity
to measure antennas at these frequencies in a cost-effective manner.
V. C ONCLUSION
A compact C-V2X Vivaldi test antenna has been proposed with
up to 172.95% fBW from 560 MHz to 7.72 GHz. A two-stage balun
design has been put forward in the Vivaldi antenna design to improve
a known drawback of the antenna: low-frequency input impedance
matching. A simple silicone adhesive sealing technique has also been
investigated to correct the mid-frequency gain drop in cross-shaped
Vivaldi antennas. This structure also achieved good gains in the lowfrequency region of the antenna. Although more can be done in later
studies to improve the low-frequency gains, the antenna has sufficient
gain stability for measuring UWB V2X communication standards in
a controlled environment. Measurement results from the antenna in
the V2X test facility have also been presented with stable results.
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