Design and Comparison of Terahertz Graphene Antenna: Ordinary dipole, Fractal dipole, Spiral,
Bow-tie and Log-periodic
Biabanifard M1, Biabanifard S*2, Javad Hosseini S3 and Jahanshiri A3
1Department of Electrical Engineering, KN Toosi University of Technology, Iran
2Department of Engineering, Iran Analog Research Group, Iran
3Department of Engineering, Buin Zahra Technical University, Iran
Submission: June 07, 2018; Published: July 25, 2018
*Corresponding author: Sadegh Biabanifard, Department of Electrical Engineering, Iran Analog Research Group, Tehran, Iran, Email: [email protected]
How to cite this article: Biabanifard M, Biabanifard S, Javad Hosseini S, Jahanshiri A. Design and Comparison of Terahertz Graphene Antenna: Ordinary
dipole, Fractal dipole, Spiral, Bow-tie and Log-periodic. Eng Technol Open Acc. 2018; 2(2): 555584. 10.19080/ETOAJ.2018.02.555584
This paper investigated several configurations of terahertz graphene antenna. Five structures: Ordinary dipole, Fractal dipole, Spiral, Bow-tie and Log-periodic antenna have been investigated and graphene is the main substance while a thin layer of SiO2 as a bed was used. The fractal dipole and round spiral antenna in the terahertz band and comparison of these structures for the first time has been considered. Designed antenna are credited, exploiting proper full-wave numerical simulations while using time domain simulations. According to simulation results, designed round spiral and log-periodic antennas show adaptive behavior on wide range of frequencies and confirms wideband operation. Also, antenna is compared regarding on board sizes and the absorption cross section. The radiation efficiency is above 85% for all antennas. The results of this study can offer design insight and give vision to researchers, selecting appropriate structure with specific features and applications (Figure 1).
In recent years, Nano technology has provided novel solutions for engineering society. So, fabrication of electronic devices for specific application are realized and appropriately discussed. In any effort to increase sustainability and energy efficiency, neither productivity nor quality should suffer – quite the opposite should be the case. Increased sustainability and efficiency can only be achieved through the use of reliable and modern technology. Graphene based devices outperform silicon products, regarding higher operational frequency, design efficiency and even fewer losses. So, researchers hope, graphene
will pavethe way to new era of high frequency-high efficiency electronics and optical devices. As a two-dimensional substance, Graphene shows transparent structure with hexagonal shape, has attracted a lot of attention due to its excellent physical, electrical and optical properties . Starting with transparency, Graphene has constantly extended its range of properties in recent years. Nowadays this 2-D substance meets a growing range of requirements, from excellence electrical properties and simple control of charge careers to reliable frequency response. Extensive research and investigations have begun and continue to
design, description and implementation of graphene-based Nano devices. Long and highlighted list of these researches is existing.
For instance, ultra-high-speed transistor , transparent solar
cells , meta-materials  and graphene plasmonics [5-6] are
among most known fields. Regarding increasing demand for high
speed operation and powerful processing, the need of design and
fabrication of high frequency antennas is rising. On the other
hand, one of the functional features of graphene is the ability
to operate in high frequencies, which can be used in wireless
communications and terahertz band . Due to comparable
depth of penetration with wavelength and increasing losses, we
cannot increase operation frequency via dimension reduction.
Since in a few micro-meter structure, the microwave rules are
not entirely true in this area, so concept like PEC which exploits
in microwave analysis is not compatible in the new shrinked
domain, but still a half-wavelength antenna with 1 micro-meter
long works at frequency about 150THz. This phenomena is
justifiable, exploiting effective wavelength concept, which states
antenna design can be transferred to optical frequencies with
linear effective wavelength substituting . The planar antenna
which are placed on the substrate are mainly different from
microwave antenna, because they tend to radiate in the sub bed
medium and power division of each medium varies approximately
as . Basic research of planar structure of graphene antenna
are studied in [10,11]. Graphene-based Nano patch and different
shape of such antenna have been studied in [12,13] and 
respectively. These antennas radiate, harvesting the graphene
capability of supporting Surface Plasmon Polaritons (SPPs)
which is comprehensively presented in [15,16]. In addition,
propagation of SPP waves on doped graphene is analyzed,
studied and proved [17-19].
In this work, to exploit the fascinating potential of this
transparent material, five famous types of antenna are designed
and simulated which graphene is main substance in these
structures. We consider an ordinary dipole antenna with L as
length and was width, with two separate graphene arms, as
basic design for other antennas. All of presented configurations
lay on a thin layer of SiO2 as substrate. Return Loss, Bandwidth,
Absorption Cross Section, Directivity and Radiation Efficiency
are numerically simulated and analyzed. The remainder of this
paper is organized as follows. In section 2, the expression used
to model the electric conductivity of graphene is presented.
In section 3, the ordinary dipole antenna has been analyzed
and design of other configurations is obtained based on the
approximate length and exact width of ordinary dipole antenna.
In next sub-section, the fractal dipole antenna has been designed
and analyzed. The other sub-sections are design and analysis of
spiral, bow-tie and log-periodic structures. In the section 4, the
comparison of these structures has been presented and finally
the paper has been concluded.
The conductivity of graphene, an allotrope of carbon in
the form of a two-dimensional material, includes two parts,
interband and intraband transitions. Since we are working on
lower terahertz frequencies, intraband transition is considered
solely  which is modeled using Kubo formula . So, we
assume that in low frequencies in terahertz band, intraband is
just represented graphene conductivity. By harvesting random
phase approximate method, surface conductivity of graphene
with the time harmonic dependency of exp (jωt) can be descript
in local form :
Where e is the electron charge, kB is the Boltzman constant,
ℏ is the reduced Planck’s constant, τ is the transport relaxation
time, T is the temperature and μc is the chemical potential. In this
work, we use T=300K (ambient temperature) and τ =1ps. These
values are considered to as real as possible graphene parameters.
The real and imaginary parts of intraband conductivity are
depicted in Figure 2.
The chemical potential of graphene can be controlled
using gate voltage. In all sections, μc is considered equal to
0.3eV. To developing a reliable library to simulate graphene, a
combination of Drude model and surface conductivity can be
used to determine the plasma frequency as follows:
A detailed description is represented in Appendix A.
Five configurations that mentioned above is depicted in
Figure 3. In this section, these structures will be analyzed and
simulated at low frequency of terahertz band. Graphene with a
thickness of 0.34nm is used in these structures and the results,
exploiting time domain simulations has been achieved using CST
Studio ver. 2016.
To avoid conjugate impedance matching, a half wavelength
dipole antenna can be fabricated, resonates and shows pure real
impedance. Antenna theory states that a normal half-wavelength
dipole antenna possess 73+i42.5 ohm as impedance. If we
assume the length of antenna about 0.48λ, bite smaller than
λ/2, imaginary part will be set to zero. Usually dipole antenna is
fabricated using conductive wires, but as we use graphene plates,
design is similar to micro-strip antenna, where graphene plates
plays roles of conducting wires. There is a gap between graphene
plates, which antenna feeds through this gap. The feed can be a
THz continuous –wave (CW) photo mixer placed in the middle
of the patch. In order to model photo mixer [21,22], a current
source is used to simulate antenna. To loss reduction, matching
between supply and antenna is very significant. The geometrical
shape of planar ordinary dipole is shown in Figure 3(a). As small
as possible, if targeted gap changes, matched impedance changes
accordingly. Regarding fabrication considerations, we assume
gap width 5μm. Also designed antenna possess L=225μm and
W =11μm. A standard principle states that an antenna can be
designed wideband if occupied volume increased . Based on
our results, this is true while variations of bandwidth were small.
The return loss for different width values are shown in Figure 4.
For W=11μm, directivity is obtained about 2.51dBi and
radiation efficiency is achieved about 90%. These values are
relatively constant for different width. In addition, bandwidth
is obtained equal to 0.201THz. With shrinking ordinary dipole
width, resonant frequency increase and antenna operates in
second resonant frequency. This is clearly shown in Figure 4.
For design of directive antenna, we can add a ground plate to
bottom of substrate and be sure that the thickness of substrate is
selected properly. This guarantees that the reflected waves from
ground plate adds in-phase with the radiated waves.
One of the nature inspired methods for bandwidth boosting,
is fractal antennas design. Its features include low area, but
unlimited circumference. Here we construct the proposed fractal
dipole antenna by dividing each ordinary dipole antenna arm
into three equal parts and creating a Koch curve with a 60° angle.
The geometrical structure is depicted in Figure 3(b) while length
of this antenna can be described by equation (3).
Where n is number of iterations and h states the initial length
value. Regarding to the antenna width of 11μm and initial length
value of 220μm, high numbers of repetitions cannot be achieved,
and the shape of the antenna is achieved using single repetition.
By repeating on the ordinary dipole antenna, it was observed
that the resonant frequency replaced, and this replacement
was considerably tending to higher frequencies while has more
bandwidth, and the resonant frequency for the fractalized
ordinary dipole antenna was discussed, about 1.6terahertz was
obtained and depicted in Figure 5.
By increasing the length of the antenna, the resonant
frequency can be reduced. Designed fractal antenna has 390μm
length to resonate at the frequency of designed ordinary dipole.
The fractal dipole shape for planar antenna creates sharp points
and sharp points, which does not work well for distribution of
the current on the antenna surface. To solve this problem, we can
use the technique of rounding the sharp points, which results in
rounding is the frequency shift, changes of matched impedance
and radiation at higher frequencies.
Spiral antennas with circular structure whose obvious
characteristic are independent of frequency, due to their circular
structure, usually have high bandwidth and circular polarization.
The geometry of these antennas is shown in Figure 3(c). The
important point in designing these antennas is attention to the
relation D=λ /π, where D is the diameter of the large circle of the
antenna and λ is the desired resonant wavelength.
The outer radius of antenna determines the lowest frequency
of operation and usually approximated to occur when the wavelength is equal to the circumferences of largest circle in
And the highest frequency in the round spiral antenna’s
operating band occurs when the innermost radius of the spiral
is equal to λ /4. The highest frequency can be determined from
the inner radius :
Designed antenna has D =145μm and the width of the
graphene sheets is 11μm. The simulation results show that this
structure exhibits good frequency independent behavior, which
results is high bandwidth.
Another frequency independent configuration which the
basic feature of their structures is dependence on the bow and
not the antenna’s length, is the bow-tie antenna. Easy design and
broadband impedance are other features of this configuration.
The structure of this terahertz graphene antenna is slightly
different from its microwave model and its geometric shape is
shown in Figure 3(d).
The results show that the increase in the angle θ leads to
that the behavior independent of frequency of this configuration
more apparent. Bow-tie structure is simulated with angles of 3,
15 and 45 degrees, W=11μm, L=225μm and the return loss is
shown in Figure 6.
By circling the corners of the structure of this antenna, it can
also improve the current distribution and thus achieve better
The geometric structure of the log-periodic tooth planar
antenna is shown in Figure 3(e). This structure is chosen in such
a way that electrical properties are repeated with wavelength
logarithms and the teeth of this antenna make it suitable for the
distribution of current. If the values of β1 and β2 are chosen such
that β1+β2=90˚, the antenna becomes self-complementary .
The ratio of the circles of the log-periodic tooth planar antenna is
a constant number that gives the structure period:
In the designed antenna, the physical parameters appeared
in Figure 3(e) are optimized as follows:
τ =1.96, Rn=169.3μm, β1=60˚, β2=30˚
The antenna radiates when the length of each arc An, is equal
to λeff/2 which length of each arcs can be calculated as below:
The return loss of the five antennas discussed is shown in
Figure 7 and the designed frequency is 0.834THz. Spiral, bowtie
and log-periodic antennas as expected, shown independent
frequency behavior which can be seen well in Figure 7. The
bandwidth of the ordinary dipole antenna is 0.2 terahertz. By
changing the geometric structure of the antenna, the bandwidth
has reached to 2.7terahertz, which corresponds to the logperiodic
In order to study the performance of these terahertz
graphene antenna, it is interesting to investigate the absorption
cross section of these graphene patches with plane wave normal
incident as shown in Figure 7. The meaning of high absorption
is that the excitation of SPPs on the antenna surface is good and
it does not necessarily, but it can radiate at these frequencies.
The calculated absorption cross section of five configurations is
depicted in Figure 8.
In lower frequencies of terahertz band, the absorption
cross section of fractal dipole antenna is low, therefor the
characteristics of the fractal dipole antenna at designed
frequency compare to the ordinary dipole expected that it will
not improve, but at higher frequencies, about 3.8THz, where the
absorption cross section of fractalized dipole is much better than
the ordinary dipole, the fractalized antenna features are expected
to be improved. This analysis can also be done for the spiral
antenna at designed frequency and expected that this structure
has better parameters at higher frequencies. A remarkable point
in comparing these configurations is the significant peak of
absorption cross section in the log-periodic structure and the
fractalized dipole antenna at the frequencies of about 1THz and
3.8THz respectively. This fact indicates the better excitation of
the SPPs on the surface of these configurations.
The highest directivity between these five famous structures
is 3.08dBi, which is related to the log-periodic antenna. As shown
in Figure 9, all antennas except the spiral have null at angles of 0
and 180 degrees. The antennas discussed, do not have a narrow
beam, so the concept of reconfigurable for these graphene
antennas which can be obtained by applying the gate voltage,
does not make sense. The polarization of the spiral antenna is
also elliptic, while four other antennas have linear polarization.
Regarding to comparison of board sizes, the log-periodic
antenna occupies the largest size of the board which is
approximately 6 times larger than ordinary dipole and after
that is the bow-tie, fractalized dipole, spiral and ordinary dipole
According to results, the log-periodic antenna has better
features in terms of directivity and bandwidth compared to the
spiral antenna and less matched impedance, but in contrast, it has
more occupied volumes. Comparison of these five configurations
with consideration of important parameters is presented in
Five different types of antenna were studied, analyzed and
simulated. Ordinary dipole, fractalized dipole, spiral, bow-tie and
log-periodic configurations are compared in terms of bandwidth,
absorption cross section, directivity and finally board sizes.
It was shown that by decreasing the width of the ordinary
dipole antenna, the resonant frequency shifted towards higher
frequencies and the antenna radiates at its second frequency
resonance. It has been presented that fractalized ordinary dipole
antenna has the frequency shift to higher frequencies and in
lower frequencies of terahertz band, the SPPs cannot excite well
in this configuration. It was observed that at lower frequencies
of the terahertz band, as the bow angle increases from zero to 45
degrees, the behavior of independent of frequency becomes more
apparent. The designed spiral and log-periodic antennas shown
a good bandwidth and among these antennas, the log-periodic
configuration has the best features in terms of bandwidth,
absorption cross section and directivity. The calculated radiation
efficiency for ordinary dipole, fractal dipole, spiral, bow-tie and
log-periodic structures was 90%, 89%, 98%, 96% and 86%