Eight-Element Microstrip Series Feed Antennas with Air Vias For 5G
Applications
A. MENAM AL AZZAWI, MOHAMAD KAMAL A. RAHIM, OSMAN AYOP
Advanced RF & Microwave Research Group (ARFMRG),
Faculty of Electrical Engineering,
University Technology Malaysia (UTM), 81310,
UTMJB Johor,
MALAYSIA
Abstract: - This study presents the design and fabrication of a novel series feed array antenna structure
consisting of eight elements, each integrated with 0.5mm air vias and separated by 0.9mm distances. The
research investigates the impact of air vias on millimeter wave 5G applications, utilizing Roger 5880 substrate
(εr = 2.2, loss tangent = 0.0009, thickness = 0.508mm) for resonance within the 26-28 GHz range. The
inclusion of air vias demonstrates a reduction in interference, evidenced by changes in current density, surface
current distribution, and VSWR. The antennas, configured with 20 x 60 mm substrates and λ/2 element
spacing, exhibit improved return loss (32 dB to 38 dB) and closer alignment with desired VSWR values upon
integrating air vias. The study also reveals heightened surface current and density at the radiating element edges
due to the vias' dielectric properties. Computational simulations utilizing CST studio validate the structural
designs.
Key-Words: - CST, Millimetre wave,5G, series feed antenna, Array antenna, Air vias. Automotive Radar.
Received: August 6, 2022. Revised: September 21, 2023. Accepted: November 13, 2023. Published: December 31, 2023.
1 Introduction
An array antenna with air vias in the millimeter
wave communication system, especially in 5G
applications, is very efficient. This integration is
crucial in high-speed data transmission and
increasing the system capacity, [1]. Generally, fifth-
generation devices operate with 24GHz frequency
and above. These antennas meet desirable
requirements for fulfilling fifth-generation systems.
Array antennas consist of a group arranged
differently depending on the suitable shape to
generate one beam and attached to the beamformer
circuit to steer it, [2]. Different array structures,
such as antenna gain evaluation, beamforming
capabilities, and spatial resolution, can achieve
various benefits. These advantages prove
instrumental in addressing the challenges associated
with 5G mm-wave systems, particularly in
achieving high data rates and minimizing signal
attenuation. In the context of array antenna design,
air visas play an acritical role. These apertures (air
vias) within the antenna structure facilitate the
propagation of electromagnetic waves by enabling
the passage of electromagnetic wave energy. By
incorporating air vias, the array antenna minimizes
signal losses and maximizes overall performance,
[3].
The array antennas and air vias in 5G mm-wave
systems effectively enhance the path loss and signal
attenuation encountered when operating in the mm-
Wave frequency range. It is essential Due to the
susceptibility problem of short mm-wave
wavelengths to atmospheric absorption and
obstruction caused by environmental objects, [4],
[5]. Integrating series feed array antennas with air
vias combat these challenges by enabling highly
efficient beamforming and enhancing the antenna
gain, compensating for signal losses. The precise
arrangement of the air vias at the radiation element's
edges is designed to optimize the radiation patterns
and the directivity. Phased arrays and beamforming
algorithms are advanced techniques for achieving
electronic beam steering and allowing the signal to
seek specific users or coverage areas, [6].
In conclusion, the integration of the array
antennas and air vias assumes a critical significance
within 5G mm-wave systems, facilitating the
realization of high-rate data, efficient network
capacity, and signal coverage. By effectively
contending with the concerns inherent to mm-wave
signal propagation and investigating the current
density, surface current distribution, and voltage
standing wave ratio (VSWR), these antenna
configurations conduce to the elevated efficiency
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DOI: 10.37394/23204.2023.22.20
A. Menam Al Azzawi,
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and dependability of the overarching 5G
communication infrastructure.
2 Series Feed Antennas Design
The design process began with a regular microstrip
patch antenna, which had specific dimensions for
the width (wp) and length (Lp) as in Equations (1-
5). However, it was found that this initial design
needed more bandwidth and gain. A series feed was
developed by adding more elements to the antenna.
These additional elements were placed at a
separation distance of half a wavelength (lambda)
between each element. This modification was made
to improve the antenna's performance in terms of
bandwidth and gain, [7].

󰇛󰇜
(1)


(2)
 

󰇣
󰇤
(3)
 󰇛󰇜󰇡
󰇢
󰇛󰇜󰇡
󰇢 (4)
  (5)
Fig. 1: Reflection coefficient (S11) of one to eight
series feed array antenna
Series feed array antennas are a type of antenna
where each element is connected in series with the
same feed to form a sequential chain in Figure 1.
This configuration is commonly used in various
applications such as wireless communication
systems and radar systems, [8]. The theory behind
the series feed array antennas involves the
understanding of radiation patterns, spacing between
the elements, and impedance matching. Designing
the series feed array antennas involves many steps
starting with single elements and calculations of
suitable dimensions to determine crucial parameters,
[9], [10], [11]. Figure 2(a), (b) shows the fabricated
eight elements series feed array antenna without
SIW, and general equivalent circuit respectively.
The spacing between adjacent antenna elements
within the array can be determined using Equation
(6).
(a)
(b)
Fig. 2: (a) Fabricated microstrip Series feed antenna
resonating at 26-28GHz (b) Equivalent circuit
󰇛󰇜 (6)
Where S is the space between elements, θ
denotes the desired beam steering angle and
represent the wavelength at 26GHz. The
characteristic impedance of the feed network can be
computed utilizing the ensuing Equation (7)
  (7)
where (Zin) denotes the input impedance of each
antenna element, (Zout) represents the output
impedance of the preceding element or the feed
network. To ensure equitable power is distributed
among the eight elements, the power division factor
is calculated by employing Equation (8).

(8)
Where N shows the total number of antenna
elements in the array, eight elements in this work,
phase shifters can accomplish beam steering or
beamforming within a series feed array. The
requisite phase shift can be determined through
Equation (9)
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 󰇛󰇜
(9)
Where d clarifies the element's distance from the
reference element, θ denotes the desired beam
steering angle, and λ represents the wavelength of
the operating frequency.
The previous equations serve as an initial
foundation for series feed antenna design. However,
it is essential to acknowledge that the design process
can include another additional complexity,
necessitating considerations such as impedance
matching, radiation pattern synthesis, and mutual
coupling between radiating elements.
3 Series Feed Antennas with Air Vias
Integrating series feed array antennas with air vias
necessitates a comprehensive consideration of their
impact on various aspects such as radiation pattern,
crosstalk, impedance, and bandwidth, [12], [13].
The existence of air vias can introduce impedance
variations, which can affect the efficiency of the
antenna system and lead to signal reflections.
Furthermore, the small distances between these vias
to radiating elements can significantly change the
radiation pattern lobes, potentially resulting in beam
squint, sidelobe distortion, or reduced gain, [14]. In
addition, the coupling between adjacent elements
can give rise to crosstalk, reducing the isolation and
degradation of antenna performance. It is important
to acknowledge that air vias also impose limitations
on the bandwidth of the array antenna system due to
impedance variations and coupling effects, [15].
(a)
(b)
(c)
Fig. 3: (a) Series feed antennas without air vias (b)
with air vias (c) close look of air vias
Designing air vias with antennas involves
considering their placement, dimensions, and impact
on the antenna's performance. When integrating air
vias with antennas, it is essential to consider their
location, as shown in Figure 3 (a), (b), (c). Vias
should be positioned away from the antenna's
radiating elements to minimize potential
interference. Placing vias in the ground plane or
near the edges of the PCB can help maintain the
antenna's radiation pattern and minimize coupling
[16]. Moreover, the dimensions of the air vias are
crucial to minimize their impact on the antenna's
performance. These dimensions as in Table 1 show
that a smaller diameter helps reduce the impact on
the RF current flow. Generally, vias with diameters
around 0.3-0.4 times the wavelength of the highest
frequency of interest are recommended.
Table 1. The dimensions of the Series Feed
Antennas design structure. (All dimensions are in
mm units)
Dimensions
Value(mm)
Explanations
W
4.60
Substrate width
L
3.45
Substrate length
Wp
4.60
Patch antenna width
Lp
3.45
Patch antenna length
S
2.60
Distance between antenna
elements
f
7.48
The First feed of series feed
antennas
V
0.5
Air vias diameter
P
0.9
Space between air vias
λ
7.49
Wavelength at (27GHz)
εr
2.2
Dielectric constant
h
0.508
Substrate thickness
4 Results and Discussions
The proposed structural configuration has been
emulated utilizing CST software, with performance
evaluation conducted by assessing various
parameters, including return loss, gain, bandwidth,
radiation pattern, and current density. Figure 5 (a),
(b) visually represents the physically realized eight-
element front and back structures, with a 5-cent coin
included for scale comparison. Concurrently, Figure
6 (a), (b) illustrates the reflection coefficient and
compares the simulated and fabricated outcomes,
elucidating the bandwidth characteristics. In
contrast, the effect of air vias on series feed
antennas was studied before fabrication. Table 2
illustrates the improvements in parameters for series
feed array antennas following the addition of air
vias. Figure 4 shows how gain increased until the
peak and dropped by adding more elements. By
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adding the air vias at the eight elements, the gain
has enhanced again at the eight elements. These
enhancements are significant and contribute to the
overall performance of the antennas. The air visa
improves the Reflection coefficient S11, increasing
the bandwidth, surface current, and current density.
These improvements are achieved by minimizing
the surface wave losses and optimizing the radiation
efficiency.
Table 2. Series feed antennas with and without an
air vias at (26GHz)
No vias
With Vias
22
35
4.58
4.97
98.2
112
67.5
71.4
Fig. 4: Gain in dB for (1 to 8) elements and the eight
series feed antennas with Air vias
Through rigorous analysis, it is evident that the
results obtained from the fabricated structure closely
align with the initially designed parameters.
Specifically, the reflection coefficient S11 registers
at approximately 30 dB for the fabricated structure,
whereas it reaches approximately 35 dB for the
designed structure. Moreover, the bandwidth
examination reveals a value of approximately 3.9
dB, with some marginal shifts in frequency
coverage, while both configurations effectively
encompass the desired frequency range of 26/28
GHz.
(a)
(b)
Fig. 5: Eight elements’ Series feed array antennas
(a) Front (b) Back
(a)
(b)
Fig. 6: (a) Reflection coefficient of fabricated Series
Feed Antennas with and without air vias (b) VNA
used to measure the performance of the antenna
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Figure 7 (a), (b) depicts the ultimate proposed
structure's 2D and 3D radiation pattern with surface
current. This assessment was carried out at a
frequency of 26 GHz, with varying Phi (Φ) values
set at 0 degrees and 90 degrees. The radiation
pattern offers valuable insights into the directional
characteristics and intensity distribution of the
electromagnetic waves emanating from the
structure.
(a)
(b)
Fig. 7: (a) 2D radiation Phi=0 and 90 at 26GHz (b)
3D radiation pattern with surface current
Figure 7 (b) shows that 0.5mm air vias exerts
0.9 mm distance separation within antennas, exerts a
discernible impact on surface currents, and gives
112 A/M at 26GHz and 96.5 A/M at 28GHz. Thus,
engendering noteworthy alterations in radiation
patterns, impedance characteristics, and interference
phenomena. Their presence facilitates the precise
modulation of electromagnetic fields, thereby
enabling the refinement of beamforming capabilities
and the mitigation of undesired radiation.
Appropriately engineered costless 0.5mm air vias
substantively augment antenna performance by
endowing it with the capacity to be finely tuned to
optimize wireless communication parameters. As in
Table 3, this structure has been compared with up-
to-date references to prove this procedure's benefits.
Table 3. The performance of a final structure with a
comparison with other references
Ref
Variable
This
work
(14)
(15)
(16)
Frequency /GHz
26/28
28
76.5
79
Return Loss/(dB)
>-34
>-15
>-35
>-35
Bandwidth (dB)
3.9
1.3
3
4.75
Gain /(dB)
14.7/15.7
10.2
13
13.38
SLL /(dB)
-9.2/-7.6
-15
-18
-14
5 Conclusion
In conclusion, the series-fed array antennas exhibit
significant parameter enhancements, featuring
0.5mm diameter air vias and a 0.9mm inter-via
separation distance. At the specific frequencies of
26GHz and 28GHz, these antennas manifest notable
gains of 14.7 dB and 15.7 dB, respectively.
Furthermore, their bandwidth is estimated to be
approximately 4.58 dB before attaching the air vias
and it became close to 4.97 dB, and they
demonstrate a substantial reflection coefficient of
approximately -34 dB. These outcomes underscore
the commendable performance of these antennas in
terms of both gain and bandwidth. It is essential to
mention that our design has been successfully
fabricated and has achieved results almost
similar to those of the originally designed work.
Moreover, it is crucial to highlight that this
work has a comparative analysis with other
high-impact published works in the field. This
assessment indicates that this series-fed array
antennas with the air vias design achieve
competitive results when juxtaposed with
existing research. This comparison underscores
the innovation and efficacy of the proposed
design, further solidifying its position as a
noteworthy contribution to the realm of high-
frequency communication systems and wireless
technologies. In the future, this structure has the
potential to be expanded to achieve (8x8) array
antennas. This expansion would allow it to meet
the Massive MIMO requirements that require a
minimum of 64 radiating elements.
Acknowledgement:
The authors express their gratitude to the Ministry
of Higher Education of Malaysia (MOHE), the
School of Postgraduate Studies (SPS), the Research
Management Centre, the Advanced RF and
Microwave Research Group, and the Faculty of
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Electrical Engineering at the University Technology
Malaysia (UTM), Johor Bahru. This
acknowledgment is extended for the invaluable
support for the research conducted under Grant
numbers FRGS/1/2021/TKO/UTM/01/7, 09G19,
and 09G24.
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Vehicular Technology, vol. 72, no. 4, pp.
4805-4814, April 2023.
Contribution of Individual Authors to the
Creation of a Scientific Article (Ghostwriting
Policy)
The authors equally contributed to the present
research, at all stages from the formulation of the
problem to the final findings and solution.
Sources of Funding for Research Presented in a
Scientific Article or Scientific Article Itself
This work is fully funded by Advanced RF and
Microwave Research Group, Faculty of Electrical
Engineering and University Technology Malaysia
(UTM).
Conflict of Interest
The authors have no conflicts of interest to declare.
Creative Commons Attribution License 4.0
(Attribution 4.0 International, CC BY 4.0)
This article is published under the terms of the
Creative Commons Attribution License 4.0
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