INTRODUCTION:
The interest for high-speed point to point microwave link is very
significant in upcoming 5G technologies. As the current commercial
spectrum is getting increasingly crowded, mobile operators and ISP need
more and more bandwidth [1]. They are looking for new spectrum
allocation that will reduce their cost for wireless backhaul. In this
regard, mm-wave frequency range has become an excellent candidate for
this backhaul connection for the cellular tower that will be the
backbone of the 5G system [2]. As most of the devices are still
working on the sub 6 GHz range, this will be more resistant to
interference. The beam-width in this spectrum range is limited to just a
few degrees. The narrow beam has the benefit of not interfering with
co-located and densely populated links around it. The beam steering
characteristic of the arrays makes it suitable for maintaining good
coverage while also allowing the control on the direction of the main
beam by phasing the excitations in the linear array [3]. However,
this spectrum range has some drawbacks- it is subject to high
atmospheric attenuation, and the path loss is very high. Various types
of antennas have been reported for cellular towers and mobile devices
that are working in these bands [4]–[7]. However, these
antennas are not flexible in terms of directivity, bandwidth, and
radiation pattern.
To fulfill the demand of this technology, a double-sided bowtie (DSBT)
antenna operating in K and Ku band have been investigated in this
letter. This antenna consists of two homogenous perfect electric
conductors patched on both sides of the substrate, where the lower one
acts as a ground plane. There is an additional rectangular conductor in
the bottom that merges with the ground plane for mainly improving the
gain characteristics of the antenna. The structure is called the partial
ground plane. The study includes the development of DSBT antenna model,
antenna simulation, and analysis of results based on various outputs of
the simulation tool from ANSYS HFSS. Then, the procedure is extended to
design different array configurations for higher gain and beam steering
nature.
DESIGN AND ANALYSIS OF DSBT ANTENNA:
Single Element Design
The initial goal is to determine the dimensions of the DSBT antenna
model to obtain the optimal performance at 17.5 GHz. In this work,
parametric study for various dimensions is conducted to design the DSBT
antenna. The optimized design of the top and ground plane of this
antenna are shown in Figure 1(a) and 1(b), respectively. Later, this
design was fabricated using an LPKF milling machine. Figure 1(c) shows
the antenna prototype.
In order to choose the substrate, various features such as relative
permittivity, dielectric loss tangent, and thickness of the material are
considered. In the initial design, Rogers RO-3203 with properties of
relative permittivity 3.02, dielectric loss tangent 0.0016, and
thickness of 0.5 mm is chosen as the substrate after a thorough
investigation on some laminations with different thicknesses and values
of relative permittivity. However, TSM-DS3M, which has very similar
properties with RO-3203, was used for the final prototype. Better
antenna performance was obtained with this substrate.
There are occasions where a different ratio of width to length are
required because of space limitations, or to change the input impedance.
This version of the bowtie antenna is designed
by investigating the performance properties of the model. The patch
designs, having various width to length ratios, were optimized. The
values of the design parameters are given in Table 1.
Performance analysis of the single
antenna
In this section, at first, the effects of the partial ground plane on
the radiation pattern, impedance bandwidth, and gain have been
investigated. In the simulation results, it is seen that changing the
length of the partial ground (Lg) affects the gain value. From Figure 2,
it is seen that at 17.5 GHz, the maximum value of the gain is increasing
from 4.2 dB to 4.9 dB for the change of the partial ground from 0.7 mm
to 4.4 mm.
This change of the LG also has a very significant effect
on the radiation pattern and bandwidth. In Figures 3-7, the effects of
Lg parameter on the radiation pattern are shown. It is
seen that for a small value of the length of the partial ground plane,
the radiation pattern is almost omnidirectional (Figure 3 and Figure 4).
However, when the length of the partial ground is being increased
gradually, this omnidirectional characteristic is lost and instead, for
the larger partial ground plane, the antenna shows a directional pattern
(Figure 5, Figure 6, and Figure 7). The front to back ratio of this
directional antenna is higher than 10 dB.
Figure 8 shows the simulation results of the return loss (S11) for the
different length of the partial ground. The operating frequency shifts
between 15 to 20 GHz. The antenna has been designed initially for
operating at 17.5 GHz, and as illustrated, the covered bandwidth for
this frequency is 4.6 GHz. For the lower value of Lg, it
is seen that bandwidth is significantly higher. For instance, when the
length of the partial ground is 0.7 mm, the resonance frequency shifted
to 19.48 GHz, and the bandwidth is 8.7 GHz. When Lgvalue increases to 1.5mm, the center of the resonance goes to 19.58 GHz,
and the bandwidth becomes 15.18 GHz. Due to the operating limitations of
the network analyzer, the design with Lg=4.4mm has been
chosen, and this model was built using RO-3203 substrate. The fabricated
antenna was tested using an Agilent PNA- L N5230C network analyzer.
There were differences between simulated and measured results.
The reason is accounted for ED copper cladding type, which is not very
suitable for high-frequency operations like Ku band applications.
Therefore, the substrate is changed to TSM-DS3M with properties of
relative permittivity 2.94±0.05, dielectric loss tangent 0.0011, and a
thickness of 0.51 mm for the fabrication purpose. This ultra-low profile
(ULP) copper is also electrodeposited, but has a smoother surface,
resulting in lower insertion loss at high frequency, where the skin
effect is the dominant factor in the loss equation. As seen in Figure 9,
the measured return loss is compatible with the simulation. The
bandwidth becomes slightly narrower than the simulated result, and there
is a small dip at 15.8 GHz. One possible reason can be soldering effect
that might bring some differences.
DESIGN AND ANALYSIS OF 1X4 ELEMENTS ARRAY:
1x4 Elements Array
Design
To improve the overall system performance of the antenna, various array
configurations have been investigated. First, 1x4 element array has been
designed with a single power excitation for each radiating element. The
distance between the radiating structures is chosen as 13.35mm with an
electrical distance of 0.75λ at 17.5GHz. Later, the top four radiating
elements are connected to a corporate-fed network, which is basically a
1x4 power divider. This feed network has a single input port and
multiple feed lines that constitute the output ports. This configuration
is shown in Figure 10. All the bowties in the ground plane are
terminated in the partial ground plane. The parameters for feed networks
are designed and optimized in HFSS circuit to make it work as a 50-ohm
transmission line at 17.5 GHz, and then push excitation of this power
divider is used to empower each element of the array. The values of the
parameters are listed in Table 2. Figure 11 shows the geometric
structures of the array along with the fabricated antenna.
Performance Analysis of 1x4 Elements
Array
The S-parameters of the individual fed 1x4 element array is shown in
Figure 12. It can be seen that the bandwidth of the four-elements array
covers from 14.9 to 19.4 GHz. It shows that the mutual coupling between
the antenna element is lower than -20 dB. It implies that there is not
much interference among the array elements.
The electric field distribution of the corporate fed array in Figure 13
also shows that the individual radiating elements are not affecting each
other significantly.
In Figure 14, the return loss for both simulated and fabricated 1x4
element corporate-fed array is given. In simulation, the antenna array
works in the band from 15 to 19.2 GHz and in measured result, it covers
the frequency from 12.49 to 18 GHz. It is seen that the resonance
frequency slightly moves from 17.5 to 17.28GHz. This may happen because
of the extended ground plane. The bandwidth is very close to the single
element antenna.
Figure 15 shows that for both individual and corporate fed antenna
array, the peak realized gain is around 10dB in the entire operating
frequency. At center frequency 17.5 GHz, the peak realized gain is 11.59
dB for the individual fed array. On the other hand, for corporate fed,
it is 10.34 dB at 17.5 GHz.
- DESIGN AND ANALYSIS OF 1X8 ELEMENTS ARRAY:
- 1x8 Elements Array
Design
At first, an array has been designed with an individual power excitation
for eight radiating elements that are patched in parallel to cover 17.5
GHz operating frequency. In this linear phased array, the distance
between the radiating structures is optimized as 13.35mm with an
electrical size of 0.75λ at 17.5GHz. Following this design, a corporate
feed network is designed for empowering the whole array with a single
port. The configuration of the 1x8 elements corporate fed array is shown
in Figure 16.
Performance Analysis of 1x8 Elements
Array
Figure 17 shows the return loss and mutual coupling for the 1x8 element
individual fed array. The return loss is 21.7574dB at 17.5 GHz and the
mutual coupling is less than -20dB that means there is a good isolation
between the array elements. The operating frequency of the antenna array
with |s11| < -10dB is 15.2
to 19.3 GHz.
Figure 18 depicts the variations of peak realized gain in the operating
frequency range for both individual and corporate fed antenna array. In
the operation region, the peak gain is stable, and the average value is
more than 12dB.
The designed antenna array shows a very good beam steering
characteristic. For 0º, progressive phase shift of ±80º and ±160º,
Figure 19 depicts the illustration for the beam steering phenomena of
the array respectively. The main beam is scanning up to a maximum of
±77° based on the progressive phase shift. The 3D directional beam is
also shown in this figure for 0º phase shift. Table 3 shows the proposed
arrays in comparison with antenna arrays from the literature. It is
observed that the proposed designs achieve high gain values within a
broad bandwidth. Also, the array has low mutual coupling in a compact
size.