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Millimeter-Wave Beam-Tilting Vivaldi Antenna with Gain Enhancement

using Multi-layer FSS

Article  in  IEEE Antennas and Wireless Propagation Letters · October 2018

DOI: 10.1109/LAWP.2018.2873113

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IEEE ANTENNAS AND WIRELESS PROPAGATION LETTERS, VOL. 17, NO. 12, DECEMBER 2018 2279

Millimeter-Wave Beam-Tilting Vivaldi Antenna With

Gain Enhancement Using Multilayer FSS
Mehri Borhani Kakhki , Mohamad Mantash , Senior Member, IEEE, Arun Kesavan , Muhammad M. Tahseen,
and Tayeb Ahmed Denidni, Senior Member, IEEE

Abstract—This letter presents a new design of tilted-beam an-

tenna with gain enhancement based on the multilayer frequency
selective surfaces (FSSs) for fifth-generation applications. A dual-
sided printed FSS element with two C-shaped resonators at the
top layer and a slotted circular patch at the bottom side, is pro-
posed. The FSS element holds a size of 0.46λ × 0.46λ (at 28 GHz).
A wideband Vivaldi antenna with an end-fire radiation is used to
excite the elements of the FSS layers. The effects of different sizes,
number, and the angular rotation of the FSS layers are employed
to achieve the best antenna performance in terms of beam-tilting,
realized gain, and reducing the sidelobe level (SLL). The best an-
tenna performance is achieved when two unequal-sized FSS layers
rotated to 45° and fixed under the Vivaldi. The proposed antenna
is fabricated and measured. The obtained measured results show a Fig. 1. Geometry of the top and side view of the planar Vivaldi antenna.
maximum beam tilt angle of 38°, realized gain of 9 dBi, and SLL at
–8 dB. The beam tilt angles are validated with the results obtained
using the Snell’s law in a multilayer environment and found a good in a 5 × 4 array fixed over an end-fire bow-tie antenna has been
agreement. used, which has resulted in a maximum beam tilt angle of 35°
Index Terms—Beam-tilting, frequency selective surface (FSS), in the H-plane (3.4–3.6 GHz). The drawback of this structure
gain enhancement, millimeter-wave antennas, sidelobe level (SLL) is its bulky framework. In [9], an array of high refractive-index
improvement. metamaterial unit-cell has been integrated on a bow-tie antenna
to redirect the main radiation beam of end fire antenna over
7.3–7.5 GHz. With this method, the beam tilting angle is
I. INTRODUCTION restricted to 17° at C-band. In addition, the antenna gain is
HE advancement of the fifth-generation (5G) wireless limited to 1.1–2.4 dBi. The antenna design presented in [9]
T network is in progress to support increasing demands
for low-latency, high-capacity, and ubiquitous mobile access,
is three-dimensional (3-D), which makes the fabrication and
measurements complex in terms of placing the slabs at accurate
positions and maintaining the right gap between them.
which will play a key role in connecting and enabling services
[1]. To boost data rates and available capacity in the spectral In this letter, new methods are described to control beam-
domain, there is a growing trend towards millimeter-wave fre- tilting, enhancing the gain, and reducing the sidelobe level (SLL)
quencies, and 28 GHz band is broadly considered as a candidate of the Vivaldi antenna at 28 GHz using a multilayer frequency
for 5G applications [2]. Utilizing mm-wave bands with massive selective surface (FSS) structure. In this approach, we have
beam tilting in access network systems is a main solution to started with a single-layer FSS consists of 3 × 3 elements,
overcome the huge traffic in mobile communication networks placed at the bottom of the Vivaldi antenna to deflect the beam.
[3]–[5]. Tilting the beam of the antenna can be accomplished The effects of the FSS layer size, number of layers, and angular
electronically by using agile elements, such as p-i-n diodes [6], rotation, on the antenna performance are evaluated. An antenna
radio frequency (RF) microelectromechanical system (MEMS) prototype with two unequal-size layers provides a measured
switches [7], or varactor diodes [8]. However, they suffer from beam tilt of 38° in the H-plane, a realized gain of 9 dBi, and an
gain drop when the beam is tilted or require a large number of SLL of –8 dB down than the peak.
switches per antenna [3], [4]. In the mechanically beam-tilting This letter is organized as follows. Section II introduces the
approach, the gain is not reduced as well as it has the advantage geometry of a Ka-band Vivaldi antenna. In Section III, a new
of having a wider scan angle. In [3], double G-shaped resonators FSS unit-cell is presented. Section IV discusses the performance
of the structure comprised of FSS layers and Vivaldi. The exper-
imental results are presented in Section V, while the conclusion
Manuscript received August 10, 2018; accepted September 18, 2018. Date is given in Section VI.
of publication October 1, 2018; date of current version November 29, 2018.
(Corresponding author: Mehri Borhani Kakhki.)
The authors are with the INRS-EMT, University of Quebec, Montreal, QC II. VIVALDI ANTENNA
H5A 1K6, Canada (e-mail:, mehri.borhani@emt.inrs.ca; mohamad.mantash@
emt.inrs.ca; arun.kesavan@emt.inrs.ca; citronian@gmail.com; denidni@
In this work, a planar Vivaldi antenna is designed to excite
emt.inrs.ca). the elements of FSS layers. Fig. 1 depicts the structure and the
Digital Object Identifier 10.1109/LAWP.2018.2873113 dimensions of the antenna. The main advantages of the Vivaldi

1536-1225 © 2018 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission.
See http://www.ieee.org/publications standards/publications/rights/index.html for more information.
2280 IEEE ANTENNAS AND WIRELESS PROPAGATION LETTERS, VOL. 17, NO. 12, DECEMBER 2018

Fig. 3. (a) Front and side schematic views of the reference antenna. (b) Sim-
Fig. 2. (a) Top view and (b) bottom view of the FSS unit-cell. (c) Effect of ulated radiation patterns of the proposed antenna in the H-plane at 28 GHz for
variation of r1 on the frequency response. The design parameters are: L1 = L2 the different mentioned states in this section.
= 0.2 mm, W1 = W2 = 1.5 mm, r1 = 0.83 mm, and r2 = 2.4 mm.

element are its end-fire radiation pattern and wider bandwidth

(BW), which make it suitable for beam-tilting applications to
achieve a higher beam tilt angle [3]. The top and bottom layers
of the feed antenna employ an exponential taper based on the
equations described in [10]. The investigated antenna has an
overall size of 27 mm × 25 mm × 0.508 mm (2.5λ × 2.3λ ×
0.047λ) and fabricated on both sides of a Roger 4003C substrate
with permittivity εr = 3.38, and loss tangent δ = 0.0027. The
Vivaldi antenna’s performance is optimized at Ka-band and pro-
vides a main beam direction at 91°, a realized gain of 5.5 dBi, Fig. 4. (a) Front and side schematic views. (b) Simulated radiation patterns
and an SLL of –7.6 dB. of the antenna with two unequal-size FSS with different rotation angles of FSS
layers at 28 GHz.

III. FSS UNIT-CELL

Vivaldi antenna is 0.2λ (at 28 GHz). The antenna performance
The schematic of the proposed FSS unit-cell is shown in obtained in the single-layer FSS environment is considered as
Fig. 2(a) and (b). The element is simulated with the CST mi- the reference. It is observed that in the reference antenna, the
crowave studio for a full-wave analysis and printed on the same main beam is tilted by 17° (from 91° to 108°), but it shows neg-
substrate used for the Vivaldi antenna. The dual-side printed ligible positive influence on the gain and SLL. To know more
single-layer FSS unit-cell comprises of two C-shaped resonators about the antenna behavior due to increasing the number of FSS
at the top layer while a slotted circular patch at the bottom side, is layers, a second FSS layer is added with the same size and gap
proposed for designing a periodic surface used for antenna beam of 0.2λ between the layers. From Fig. 3(b), it is visible that
tilting in the Ka-Band. The element scattering performance is the first state with two FSS layers increases the SLL by 3.1 dB
optimized in a way that when an incident wave spatially excites (from –5.7 to –2.6 dB) and does not make any positive effect on
the FSS elements, the surface operates in its transmission mode the antenna performance.
in the operating band while it exhibits reflection characteristics The effect of the rotation of the FSS layers in the first state is
out of the band. The unit-cell is compactly designed at 28 GHz evaluated in the state two. It exhibits that when the FSS layers
(0.46λ × 0.46λ) and provides 21.4% BW, which is higher than are rotated by 45°, a maximum gain of 7.2 and –2.3 dB SLL
BW of the reported unit-cell in [9] and [11]. are achieved. In addition, the main beam direction is located at
Different parametric studies were done to optimize the scat- 134°, which shows a good improvement compared to the first
tering characteristic of the unit-cell to operate in the desired state, but the obtained SLL is unacceptable and needs to be
frequency band. The effect of smaller circular patch of radius suppressed, which is discussed in the next state.
“r1” on the scattering performance is shown in Fig. 2(c). It is In the third state, the effect of the FSS layers with unequal
evident that the resonant frequency decreases when the width sizes, is evaluated. The size of the first FSS layer is decreased,
of the circular loop (r2–r1) is decreased. The conductors inside from 15 mm2 (3 × 3 unit-cells) to 10 mm2 (2 × 2 unit-cells),
and surrounding the loop act like a capacitor. These conductors and the results are presented in Fig. 3(b). The antenna provides
come closer and the equivalent capacitance is increased when a realized gain of 8.75 dBi, SLL of –7.9 dB, and a beam tilt
the width of the circular loop is decreased. Therefore, with the angle of 130°. In comparison with the second state, a gain en-
increase of capacitance, the resonant frequency shifts down. hancement of 1.55 dBi and an SLL improvement of 5.6 dB are
obtained when the size of the first FSS layer decreased by one
IV. VIVALDI ANTENNA WITH FSS LAYERS unit-cell. On the other hand, decreasing the size of the second
layer to 10 mm2 and keeping the first layer 15 mm2 show an
A four-step procedure is followed in the proposed antenna de- improvement in antenna characteristic but not as much as the
sign, to achieve the best antenna performance. Those steps are third state, so results are omitted for brevity.
named as: Reference antenna, first state, second state, and third
state, where the radiation patterns of these states are shown in
A. Effect of Rotation and Number of FSS Layers
Fig. 3(b), respectively. An FSS-based beam tilt antenna model
is demonstrated in Fig. 3(a), where a planar Vivaldi antenna To understand the effect of rotation and increasing the number
is explored as the feed to excite an FSS layer consists of of the FSS layers in the final design (Third state), parametric
3 × 3 proposed unit-cell. The gap between FSS layer and the studies were undertaken. The effect of different rotation angles
BORHANI KAKHKI et al.: MILLIMETER-WAVE BEAM-TILTING VIVALDI ANTENNA WITH GAIN ENHANCEMENT USING MULTILAYER FSS 2281

Fig. 5. (a) Front and side schematic views of the antenna. (b) Simulated
radiation pattern of the proposed antenna with different number of FSS layers
at 28 GHz (The dimensions are in mm). Fig. 6. Simulated radiation patterns of the proposed antenna in the H-plane
with two FSS layers at 28 GHz as a function of parameter “d.”
TABLE I
EFFECT OF DIFFERENT ROTATION ANGLES OF FSS LAYERS ON THE ANTENNA
PERFORMANCE

Fig. 7. E-field distribution over the antenna at 28 GHz with two FSS layers.
TABLE II
EFFECT OF DIFFERENT FSS LAYERS ON THE ANTENNA PERFORMANCE WITH d
= 2 MM size FSS layers and 45° rotation to continue due to its best
performance.

B. Effect of the Gap Between the Layers

The effect of the gap between the layers was analyzed and
found a marginal effect on the antenna gain, but no effect on the
beam tilt angle when “d” changed between λ/4 to λ/5, as shown
in Fig. 6.

C. Field Distribution
on the FSS layers is presented in Fig. 4 and Table I. As can be The electrical field distribution is demonstrated in Fig. 7,
seen from Table I, for the presented structure with two unequal- which shows the modification of the E-field distribution in the
sized FSS, the best antenna performance is achieved for 45° presence of the FSS structure. It is clearly visible that when
rotation of FSS layers. the FSS layers are placed near the radiator, the E-field direction
Similarly, the effect of the number of FSS layers on the an- tilts toward the proposed structure and induces current in the
tenna performance, when the layers are rotated by 45° and the FSS unit-cells. This phenomenon makes it behave like a lens
first layer is smaller than the others, is also presented in Fig. 5 to tilt the antenna beam as well as improving the gain due to
and Table II. From the presented results in Table II, it is clear that constructive interference of the reflected field.
when the number of FSS layers is increased, a higher tilt angle is
produced. In fact, it demonstrates that adding the first layer has D. Beam-Tilting Antenna Theory Analysis
the most influence on the antenna characteristic improvement.
Compared to the Vivaldi antenna, the main beam is tilted by To validate the beam tilting mechanism, Snell’s law is applied
31° in the XoZ-plane, and a gain improvement of 3.34 dBi is on the boundary of the layers and the air interface [12]. Fig. 8
achieved. In addition, adding each extra layer mainly affects tilt shows the beam tilting mechanism of the presented antenna.
angle by an average enhancement of 8°. By increasing the num- According to Snell’s law
ber of layers more than four in the –Z-direction, we still observe nair . sin θ1 = nFSS . sin θ2 (1)
increasing in beam tilt direction by 8°, but the result shows gain
reduction and huge deterioration in terms of SLL. The results where nair and nFSS are the refractive indexes of air and the FSS
also depict reduction in the 3 dB BW with an increase in the unit-cell, respectively. As shown in Fig. 8(b), θ1 , θ2 , and θ3 are
number of the layers. the incident angles of the waves from air to the first FSS layer
Considering the investigated effects of the rotation angle and and from the first and second FSS layers to the air interface,
the number of FSS layers in this section, as a compromise respectively. It must be taken into account that the designed
solution, we thus considered the antenna with two unequal- unit-cell should work as a planar lens to enhance the gain of the
2282 IEEE ANTENNAS AND WIRELESS PROPAGATION LETTERS, VOL. 17, NO. 12, DECEMBER 2018

Fig. 8. (a) Refractive-index of the proposed FSS unit-cell as a function of

frequency. (b) Beam tilting mechanism of the structure.

Fig. 9. (a) Photograph of the fabricated antenna in front, rear, and side views.
Fig. 10. Simulated and measured normalized radiation patterns of the pro-
(b) Measured S11 of the antenna with and without FSS layers.
posed antenna in the H-plane. (a) Without FSS layers at 28 GHz and with FSS
layers at (b) 26 GHz, (c) 28 GHz, and (d) 29 GHz.
TABLE III
EFFECTS OF FSS LAYERS ON THE ANTENNA PERFORMANCE AT 28 GHZ TABLE IV
COMPARISON WITH PREVIOUSLY PUBLISHED WORKS

feed antenna. The extracted refractive index of the unit-cell as a

function of frequency, shown in Fig. 8(a), confirms this unit-cell
provides 1.9 refractive index at 28 GHz.
It can be seen from Fig. 8(b) that the
 emitted wave from the
radiator travels along the length R = (L2 + d2 ) and it is clear
that θ1 is decided by L and d. According to the value of the nFSS
demonstrated in Fig. 8(a), the average of refractive indexes for summarized in Table IV. The proposed work exhibits a better
more than one FSS layer, and the final optimized parameters of performance in terms of gain, SLL, and higher beam tilt angle
d = 2 mm and L = 16 mm, the reflected angle calculated by (1) rather than [9] and [11]. In addition, in [13], four vertical layers
at 28 GHz is found 31.4° and 38.3° for one and two FSS layers, metamaterial slabs are used to deflect the beam, which makes
respectively, which agree with the simulated beam tilt angles. the antenna complex and 3-D, while the presented structure in
this work shows less complexity in terms of fabrication and
V. EXPERIMENTAL RESULTS measurements.
The final proposed structure discussed in Section IV con-
VI. CONCLUSION
sists of the Vivaldi antenna as feed and two FSS layers was
fabricated for measurement as shown in Fig. 9(a). The FSS This letter has presented a new beam-tilting and gain en-
layers are fixed under the Vivaldi antenna using plastic screws hancement design using multilayer FSSs for 5G applications.
to maintain required gap between the antenna and FSS layers. A wideband Vivaldi antenna with end-fire radiation has been
The measured reflection coefficients of the antenna with and used to excite the elements of the FSS layers. An FSS unit-cell
without FSS layers are shown in Fig. 9(b). The simulated and with C-shaped strips at the top and a ring slot at the bottom
measured normalized radiation patterns in 26, 28, and 29 GHz has been proposed. Different sizes of the FSS layers have been
in the H-plane are shown in Fig. 10. The proposed FSS-based employed for a single- and multilayer environment to achieve
double layer antenna provides a measured beam tilt angle of 30° the best antenna performance in terms of beam tilting, real-
(26 GHz), 38° (28 GHz), and 36° (29 GHz), and the measured ized gain, and reducing the SLL. The effects of FSS layer size,
gains of 8.52, 9, and 7.3 dBi, respectively. The results presented number of layers, and the angular rotation on the antenna per-
in Fig. 10 for 28 GHz are summarized in Table III. The antenna formance has been evaluated. The best antenna performance has
performance shows a good agreement with the simulations. The been achieved when two unequal-sized FSS layers are used and
disagreement between simulation and measurement results is rotated to 45°. The proposed antenna has been fabricated and
probably due to fabrication tolerance and bending of the sub- measured. The measured results have shown a maximum beam
strate layers during measurements. tilt angle of 38°, a realized gain of 9 dBi, and an SLL of al-
The performance of the proposed antenna is compared with most –8 dB. The antenna has a compact size and can be a good
the formerly published works in the literature, and results are candidate for the 5G wireless networks.
BORHANI KAKHKI et al.: MILLIMETER-WAVE BEAM-TILTING VIVALDI ANTENNA WITH GAIN ENHANCEMENT USING MULTILAYER FSS 2283

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