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![Geometry of the fourtear antenna presented in [94].](https://www.researchgate.net/publication/341809760/figure/fig5/AS:897782357450754@1591059402443/Geometry-of-the-fourtear-antenna-presented-in-94_Q320.jpg)
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Date of publication xxxx 00, 0000, date of current version xxxx 00, 0000.
Digital Object Identifier 10.1109/ACCESS.2017.DOI
A Survey on Antenna Designs for Breast
Cancer Detection Using Microwave
Imaging
HILAL M. EL MISILMANI1, (Member, IEEE), TAREK NAOUS1, (Student Member, IEEE),
SALWA K. AL KHATIB1, (Student Member, IEEE), and KARIM Y. KABALAN2(Member, IEEE)
1Electrical and Computer Engineering Department, Beirut Arab University, Debbieh, Lebanon (e-mails: hilal.elmisilmani@ieee.org, tareknaous@ieee.org,
salwa.alkhatib@ieee.org)
2Electrical and Computer Engineering Department, American University of Beirut, Beirut, Lebanon (e-mail: kabalan@aub.edu.lb)
Corresponding author: Hilal M. El Misilmani (e-mail: hilal.elmisilmani@ieee.org).
ABSTRACT With the prevalence of breast cancer among women and the shortcomings of conventional
techniques in detecting breast cancer at its early stages, microwave breast imaging has been an active area
of research and has gained momentum over the past few years, mainly due to the advantages and improved
detection rates it has to offer. To achieve this outcome, specifically designed antennas are needed to satisfy
the needs of such systems where an antenna array is typically used. These antennas need to comply with
several criteria to make them suitable for such applications, which most importantly include bandwidth, size,
design complexity, and cost of manufacturing. Many works in the literature proposed antennas designed to
meet these criteria, but no works have classified and evaluated these antennas for the use in microwave
breast imaging. This paper presents a comprehensive study of the different array configurations proposed
for microwave breast imaging, with a thorough investigation of the antenna elements proposed to be used
with these systems, classified per antenna type, and per the improvements that concern the operational
bandwidth, the size of the antenna, the radiation characteristics, and the techniques used to achieve the
improvement. At the end of the investigation, a qualitative evaluation of the antenna designs is presented,
providing a comparison between the investigated antennas, and determining whether a design is suitable
or not to be used in antenna arrays for microwave breast imaging, based on the performance of each. An
evaluation of the investigated arrays is also presented, where the advantages and limitations of each array
configuration are discussed.
INDEX TERMS Antenna Design, Antenna Arrays, Breast Cancer Detection, Microwave Breast Imaging,
Ultra Wideband Antennas (UWB)
I. INTRODUCTION
Breast cancer is the most common cancer among women
in developed and developing countries and it is the leading
cause of cancer-related deaths among women worldwide [1],
[2]. When accounting for both sexes, breast cancer is the
second most common cancer after lung cancer [3]. Cancer
screening is a practice that reduces disease-specific morbidity
and mortality since early detection has been shown to posi-
tively affect the outcome of the disease.
The first stage of breast cancer detection is the identifica-
tion of an abnormality in the breast tissue with either physical
examinations or imaging techniques such as mammography
and ultrasound [5]. However, mammograms are not always
accurate, as the rate of false negative mammograms has been
reported between 1977 and 1998 to be ranging from 4% to
34% [6]. Such results, which are due to the inherent limita-
tions of X-ray mammography, consequently lead to increas-
ing disease-specific morbidity and mortality as diagnosis and
treatment may be delayed due to the false sense of security
given to affected women [6]. This imaging technique is often
painful as it requires breast compression [7], and it exposes
the patient to ionizing radiation [8]. Ionizing radiation is
considered to be an associated risk with mammography [9].
It was also shown in [10], [11] that screening mammography
gives less sensitive results for women with radiographically
dense breast tissue.
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FIGURE 1. Comparison between X-ray image and microwave image for cancer patient. The lowest intensity values occur in the area of the tumor location given by
stronger attenuation of cancerous tissue [4].
FIGURE 2. Typical microwave breast imaging system.
Ultrasound imaging is frequently a follow-up to X-ray
mammograms. It helps with the evaluation of lumps that
are difficult to see on a mammogram, especially in dense
breasts. Similar to mammograms, ultrasound is a detection
technique that has inherent limitations. Though it is a painless
technique when compared to mammography, ultrasound falls
short when distinguishing between malignant and benign
tumors. It also has low resolution [12]. Thus, there exists
a need to explore other detection techniques that are both
efficient and accurate.
Microwave imaging [13] is developing as a promising
technology with various biomedical applications. In the case
of breast imaging, this technique entails transmitting short
pulses of low-power microwaves into the breast tissue. An-
tennas positioned around the breast are used to collect the
back-scattered energy, and the received signals are used to
produce a three-dimensional image of the scanned breast.
Fig. 1 shows a comparison between an X-ray image and a
microwave image for a cancer patient. The lowest intensity
values, in Fig. 1b, indicate the location of the tumor as a result
of the stronger attenuation caused by cancerous tissue.
A typical microwave breast imaging system is shown in
Fig. 2. Multiple antennas operating in the near-field region
are placed closely, in a specific array configuration, around
the breast of a patient. Transmitting antennas are sequentially
selected to illuminate the breast with microwave pulses.
Reflections and backscattered signals are then collected by
receiver antennas. The obtained data is then pre-processed
and used for image reconstruction.
Many reasons are behind the wide appeal of this new
detection technique; we present a selected few. Firstly, the
conductivity σand permittivity differ from normal human
breast tissues and malignant human breast tissues over the
range of microwave frequency [14], which can be used to
detect the presence of tumors. Second, it produces a high-
contrast three-dimensional image that is, in theory, equally
effective for dense breast tissues [5]. Third, it does not
require using ionizing radiation as in X-ray mammography,
and breast compression is avoided [15]. Lastly, breast energy
absorptions will be small due to the power levels adopted
and repetition periods, which makes it unlikely for patients
to suffer from potential health risks [16].
To this end, a plethora of antennas with design require-
ments tailored to this application have been proposed in the
literature. Specific microwave antennas have been designed
for transmitting and receiving short pulses where a scattering
map could be created to spot the cancerous tumor. Many
modifications and techniques have been introduced in the
literature to improve the antenna’s performance in terms of
bandwidth, size, and radiation characteristics.
We start our survey by providing a brief overview on
microwave breast imaging covering the different approaches
adopted in the literature, along with the image reconstruc-
tion algorithms used. The desired antenna characteristics to
satisfy the needs and challenges of such imaging systems
are then discussed. Next, we classify the available antenna
elements designs in the literature by type of enhancement
that pertain to bandwidth, miniaturization, and/or radiation
characteristics, and further by the antenna type and the
technique used. These antennas include Vivaldi antennas,
monopole antennas, bowtie antennas, in addition to fractal,
and horn antennas. After investigating the antenna elements,
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Sec. II
An Overview on Microwave
Breast Imaging
Image Reconstruction
Algorithms
Approaches in
Microwave Breast
Imaging
Sec. IV
Antennas Elements Proposed for Microwave Breast Imaging Systems
A) Enhanced Bandwidth B) Miniaturized Size C) Enhanced Radiation
Characteristics
1) Vivaldi Antenna
2) Monopole Antenna
3) Bowtie Antenna
4) Horn Antenna
1) Vivaldi Antenna
2) Monopole Antenna
3) Bowtie Antenna
4) Fractal Antenna
5) Horn Antenna
1) Vivaldi Antenna
2) Bowtie Antenna
Sec. V
Microwave Breast Imaging With Antenna Arrays
A) Planar Arrays
B) Enclosed Arrays
C) Hemispherical Arrays
Sec. VI
Qualitative Evaluation & Discussion
A) Antenna Elements
B) Antenna Array Configurations
Sec. III
Antenna Design
Considerations
Coupling Mediums
Attenuation Due to
Absorption
Sec. VII
Conclusion & Future Directions
FIGURE 3. Diagrammatic view of the organization of this survey.
the different antenna array configurations proposed in the
literature to be used with microwave breast imaging systems
are then investigated and classified as planar, enclosed, and
hemispherical arrays. After investigating the proposed an-
tenna designs, a qualitative evaluation of the antennas dis-
cussed that will highlight the advantages and shortcomings of
these antennas and determine which are more suitable to be
used in antenna array systems for microwave breast imaging
is presented. This is also followed by an evaluation of the
different arrays presented in the literature for microwave
breast imaging, which draws several conclusions about each
type of array configuration. Tables that summarize the differ-
ent investigated antennas and antenna arrays, and compare
their performance, are also given. We note here that several
surveys on microwave breast imaging exist in the literature
[12], [17], [18], but none investigates the large number of
antennas and antenna arrays provided here, with a thorough
investigation of the different enhancements and techniques
used to improve the design of each presented antenna.
The rest of this survey is organized as illustrated in Fig. 3,
and as follows: Section II presents an overview on the differ-
ent approaches in microwave breast imaging. In Section III,
the challenges that face the design of these antennas related
to coupling and absorption are provided. A thorough inves-
tigation of the antenna elements proposed in the literature
for microwave breast imaging is then presented in Section
IV, where the antennas are classified per improvement, the
antenna type, and the technique used. The complete antenna
system, as antenna arrays, are then presented in Section V
classified per antenna array configuration. A qualitative eval-
uation of the different antenna elements and antenna array
configurations is then presented in Section VI. Concluding
remarks follow in Section VII.
II. AN OVERVIEW ON MICROWAVE BREAST IMAGING
Over the past few years, reconstruction algorithms for mi-
crowave imaging have witnessed an important attention. The
interest in developing these near-field algorithms has been
motivated by the failure of diffraction-limit approximations
[19] as a foundation for image reconstruction, demanding
the need for adequate algorithms where the full-wave electro-
magnetic interactions in tissues would be properly accounted
for.
Previously, the imaging region took place in the far field,
which required the use of antennas with increased directivity
and gain while operating in the lower gigahertz frequency
range (1−3GHz). For this, waveguide antennas [20]
were employed as the radiators. This has been eliminated
with the introduction of near-field imaging, where different
transmit antenna configurations are considered. For instance,
a monopole antenna can be placed at the proximity of
the breast while successfully illuminating the whole area,
whereas the previously used waveguide antennas needed to
be placed farther away from the breast area, in order for the
main lobe of the antenna to provide adequate coverage. Ad-
ditionally, a monopole has a much smaller and more flexible
size, offering better economy-of-space than the waveguide
antenna and making it a more attractive choice when con-
sidering an array design. Therefore, near-field imaging has
surfaced as a promising approach for clinical settings.
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FIGURE 4. The different approaches and methods in microwave imaging.
Several approaches have been explored in the literature for
breast imaging at microwave frequencies, which are active,
passive, and hybrid approaches [21], shown in Fig. 4.
In the active approach, the breast is illuminated with mi-
crowave radiation by the means of several antennas. Trans-
mitted and reflected energy can then be collected and used
to form images by using imaging algorithms. The presence
of a tumor causes scattering of the incident waves due to
the change in electrical properties. Using the information
contained in the detected energies, microwave images can
be constructed [22]. These algorithms perform coherent ad-
dition of backscattered radar signals, that are received after
illumination of the breast with UWB pulses. They can be
divided into two categories: data independent (DI) and data
adaptive (DA) algorithms. DI algorithms perform the cohered
addition based on an assumed propagation model, while in
DA algorithms the propagation model is estimated from the
received signals after compensation factors are applied to
the channel model. Several algorithms have been proposed
in literature and evaluated in [23], including: Delay-And-
Sum (DAS) [24], Delay-Multiply-And-Sum (DMAS) [25],
Improved Delay-And-Sum (IDAS) [26], Coherence Factor
Based DAS (CF-DAS) [27], Channel Ranked DAS (CR-
DAS) [28] and Robust Capon Beamformer (RCB) [29]. In
this respect, there are two main approaches to create mi-
crowave images, which are transmission-reflection imaging
and reflection imaging.
In the transmission-reflection imaging, known as mi-
crowave tomography [30]–[33], a single antenna is used to
transmit microwave signals that travel through the breast and
are then received by several receiver antennas at the opposite
side of the breast, placed at equal distances from the surface
of the breast. This process is repeated for several positions of
the transmitting antenna. By using the incident and received
fields, the shape of the breast and the spatial distribution of
the permittivity can be obtained to create maps based on the
electrical properties of the breast. An example showing a
microwave tomographic permittivity image is shown in Fig.
(a) (b)
FIGURE 5. Microwave tomographic permittivity images at 1300 MHz: upper,
right breast; lower: left breast. (a) 2D images, (b) 3D images [34].
5. Through signal processing the measured data is then used
to create a map of the relative permittivity geometric distribu-
tions of the breast. Tumours usually reduce the strength of the
scattered signal, hence any observed areas of higher relative
permittivity and conductivity values corresponds to the pres-
ence of tumour cells. Solving inverse scattering problems, the
dielectric profile of the tissue under test is then deduced.
In reflection imaging, known as radar-based imaging or
confocal microwave imaging (CMI) [35]–[39], the reflections
are summed or synthetically focused to create the images,
using the time-delay from each antenna to a focal point. The
reflected waves are the only ones used to form the images.
The ideas adopted in this approach are mainly inspired by
radio detection and ranging (Radar) systems. Basically, an-
tennas are used to transmit microwave pulses or modulated
harmonic microwave signals. Reflections are then received
by the same transmitting antennas or by receiver antennas
positioned in different locations. This approach can be used
to localize tumors by using simpler algorithms that focus the
reflected energy from the breast, thus providing a complete
profile of the dielectric properties of the breast. An example
showing cross-sectional images of radar-based imaging is
shown in Fig. 6.
These two different approaches require different antenna
characteristics. Microwave tomography is considered as a
narrowband approach where antennas that operate in the
lower frequency range (1−6GHz) are needed, since high
frequency signals will have limited penetration in the breast
tissue, tumor, and skin. In radar-based imaging approach, the
antennas require a wideband performance to achieve high
resolution image reconstruction [41].
The passive approach, known as microwave radiometry,
has been a research subject for many years [42]–[44]. It
assumes that malicious tumors possess higher temperatures
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FIGURE 6. Cross-sectional images of restored breast model using
radar-based imaging, (a) x-y plane, z = 17 mm, (b) x-z plane, y = 70 mm, (c)
y-z plane, x = 60 mm, and (d) 3D image [40].
compared with healthy breast tissues. Based on this as-
sumption, the difference in temperature between the normal
and malignant tissues can be measured and used to draw
conclusion.
The hybrid approach is based on microwave-acoustic
imaging where microwave signals are used to selectively
heat tumors [45]. As a result of this heating, pressure-wave
signals are generated which can be measured by ultrasound
transducers. This collected data can then be useful in image
construction.
III. ANTENNA DESIGN CONSIDERATIONS
Ever since microwaves caught the interest of engineers de-
veloping biomedical imaging devices, there has been a huge
focus on finding suitable antennas that can satisfy the needs
of such imaging systems. By transmitting short pulses into
the breast area and based on the dielectric contrast between
malicious and healthy breast tissues, the differentiated scat-
tering can be obtained and used to construct images and
localize tumors.
The design of the antenna is critical to the overall mi-
crowave imaging system performance. For microwave breast
imaging systems, which are mainly an array of antennas,
the antenna element needs to exhibit a broadband behavior,
by being capable of radiating pulses over a wide range of
frequencies with high fidelity, while maintaining a fair value
of the gain. This has turned the attention to the usage of
antennas operating in the Ultra-Wideband (UWB) that have
been allocated a 7.5 GHz bandwidth (3.1 to 10.6 GHz) by
the Federal Communication Commission (FCC) for UWB
measurements, communications, and radar [46].
Also, the antenna needs to have a compact size so that it
would occupy a small area on the total surface area of the
breast. Complexity and cost of the design are also important
factors that needs to be considered. Ideal breast imaging tools
are labeled as cost effective and widely available, the fact
which demands the suitable antenna to be easy and cheap to
fabricate. A complex design might also be disadvantageous
for some image reconstruction algorithms that can find it
impractical to model the antenna.
As the antennas proposed for microwave imaging are de-
signed to be used in the near-field, as discussed in Section II,
this imposes several challenges on the design of the antennas.
In addition to the rapid decrease of the fields with distance,
and its effect on the absorption, several challenges arise that
need to be considered in the design of suitable antennas for
this application. For instance, as a result of working in the
near-field, the antennas need to be placed very close to the
breast, which could result in reflections that occur in the air-
skin interface. Another challenge is found in the properties
of the breast tissues that could result in the attenuation of
the field reflected to the antennas. The rest of this section
investigates these two challenges on the design of antennas
for microwave imaging.
A. COUPLING MEDIUM
One of the challenges that needs to be taken into considera-
tion in the design of antennas is the reflections that occur in
the air-skin interface that can have magnitudes higher than
the reflected tumor response. This issue can be alleviated by
immersing the antenna in a coupling medium that possess
dielectric characteristics similar to those of breast tissues.
This problem is mainly seen in the enclosed arrays as will
be discussed in Section V.
The coupling medium in its turn has its own effect on
the antenna performance and radiation characteristics. Since
the coupling medium will have a specific conductivity value,
the propagation constant kwill change in value taking into
account the permittivity of the coupling medium. The modi-
fied propagation constant can be written as in (1) [47], with
r=0
r−j00
rbeing the permittivity of the coupling medium.
If the coupling medium has no losses, 00
rwill be zero.
k0=ωpµ00(0
r−j00
r)(1)
This variation in the propagation constant inside the cou-
pling medium changes the value of the wavelength. This will
alter the current distribution of the antenna in the coupling
medium, and hence will affect its input impedance and ra-
diation characteristics. In addition, a lossy coupling medium
will also introduce additional loss to the decay factor between
source and the observation point. For this, the design of
antennas should take into account the input impedance of the
immersion coupling medium to properly match the antenna,
and avoid degrading the performance of the imaging system.
In addition, an understanding of the behavior of the proposed
antenna in different conductivity materials of different elec-
tric properties is important to predict the performance of the
antenna in the coupling medium.
B. ATTENUATION DUE TO ABSORPTION
Another challenge in the design of antennas for microwave
imaging is the attenuation due to the absorption by the breast
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tissues. Microwave breast imaging systems are greatly influ-
enced by the shape, size, and content tissues distribution of
the breast. The breast is an in-homogeneous organ that under-
goes anatomical transformation as a woman reaches puberty
until menopause and beyond. Having a protruding conical
form with a circular base, a mature breast is composed of
several underlying tissues. Lobes made of smaller lobules
of glandular tissues, where cancer tumors usually originate,
emanate from the nipple. These lobes are surrounded by the
adipose layer, which are the fatty tissues extended throughout
the breast, giving it its shape and size. The adipose layer is
finally topped by the skin layer as shown in the simplified
sketch of Fig. 7.
An important aspect to consider in designing microwave
imaging systems is the several parameters that affect the
transmission and reception of the microwave signals through
these tissues. Taking into account the electric properties of
the breast tissues, the design of the antenna has to consider
the propagation and penetration loss due to the biological
tissues. These tissues have varying conductivity and dielec-
tric constants that depend on the type of the biological tissue
(muscle, glandular tissues, fat, or skin), and the frequency of
operation, leading to complex simulation and testing proce-
dures. When breast tissues are exposed to microwaves, the
Specific Absorption Rate (SAR), measured in W/kg, is used
to obtain the power absorbed in a volume of the tissues [22].
The SAR can be obtained by averaging over a 1g of tissue in
the shape of a cube, using (2), where dW is the incremental
energy, dV is the volume element and ρis the density of
the volume. As per the Safety Standards [49], [50], the SAR
threshold level is taken to be 1.6 W/kg , and 2 W/kg in
Europe.
SAR =dW
ρdV (2)
In experimental microwave imaging research, testing must
take place on breast phantoms to obtain more realistic results.
These phantoms can be obtained by creating several mixtures
(a) (b)
FIGURE 7. Simplified breast model (a) female breast structure, (b) stacked
layers of the breast. [48].
FIGURE 8. Single-Pole Debye curve fits of data samples for dielectr ic
properties of normal and malignant breast tissues at radio and microwave
frequencies [58]
that are Oil-in-Gelatin-Based [51], Triton X-100-Based [52],
or other materials [53], [54].
In order to account for the differences in the electric
properties of free-space and those of the breast tissues, the
phantoms are required to mimic the breast tissue electric
properties. These properties have been a subject of research
for many years [14], [55]–[57], and they include the relative
permittivity rand the conductivity σ. Tissues with a higher
water content have a higher permittivity and conductivity
than tissues with low water content. It has been shown
that, at microwave frequencies, changes also occur to these
properties with the variation in frequency. As microwaves
travel through tissues with a higher water content such as skin
or tumors, more attenuation occurs compared to tissues with
lower water content due to the absorbed energy.
Most of the work done on finding the dielectric constants
of the different body tissues and organs are based on the
relaxation model, known as the Cole-Cole equation, which
reduces to Debye model, well known in measuring the di-
electric properties of normal and malignant breast tissues
[58]. Based on Debye model, Fig. 8, shows the dielectric
properties of normal and malignant breast tissues at radio
and microwave frequencies. In addition, Fig. 9 illustrates
these changes on oil, fat, skin, and tumor tissues from 0 to 7
GHz and shows that malignant tissues have higher dielectric
properties than healthy tissues [59].
IV. ANTENNA ELEMENTS PROPOSED FOR
MICROWAVE BREAST IMAGING SYSTEMS
In this section, the different antennas proposed in the liter-
ature to be used as antenna elements for microwave breast
imaging systems are investigated. These antennas mainly in-
clude Vivaldi antennas, monopole antennas, bowtie antennas,
and fractal antennas, in addition to horn antennas. As the aim
in antenna design works is to optimize antennas for a spe-
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(a)
(b)
FIGURE 9. Variations of the (a) Relative Permittivity and (b) Conductivity of
Oil, Fat, Skin, and Tumor Tissues with the Change in Frequency over the
Range of 0−7GHz [59].
cific application and desired characteristics, the investigated
antennas in this section are classified as per the enhancement
done in each, including: enhanced bandwidth, miniaturized
size, and enhanced radiation characteristics. In each of these
classes, the antennas are further grouped based on their type,
and then based on the technique used in each to achieve this
enhancement.
A. ENHANCED BANDWIDTH
1) Vivaldi Antenna
Due to their broadband behavior, Vivaldi antennas (VA), also
known as exponentially tapered slot antennas, have been also
the subject of interest for several years [60], [61].
a: Corrugations
Adding corrugations to the conventional Vivaldi structure can
help achieve a broader bandwidth and higher gain. A Vivaldi
antenna with improved corrugated edges was presented in
[62] that has an operational bandwidth of 1−3GHz and
a return loss of less than −10 dB. This structure showed
an improved radiation pattern as a result of the corruga-
tions introduced at the edges of the Vivaldi substrate. The
improvement in the radiation pattern was achieved by using
two corrugation lengths, 16 mm and 15 mm, with a constant
corrugation width of 1.2 mm. In [63], the lengths of the
corrugations in a Vivaldi antenna were independently opti-
mized to reach a broader input impedance bandwidth than in
[62] of 1.96 −8.61 GHz. The distance Lfbetween the first
slot and the feed of the Vivaldi antenna, the distance Wgap
between each slot, and the width Wsof each slot were also
optimized to 20.2 mm, 1.07 mm, and 1.13 mm, respectively.
It was shown that increasing the number of corrugations
and varying their lengths between 5−25 mm resulted in
a wider bandwidth and higher gain when compared with a
generic Vivaldi of the same size. Furthermore, corrugation
at the flares and grating elements in the area of tapered
slot were used in a Vivaldi antenna in [64], where an even
wider impedance bandwidth from 2.9 GHz to more than 12
GHz was measured. The radiating structure of the Vivaldi
antenna has a circular cavity on one end of the exponential
tapered slot, which acts as an open circuit that minimizes
the reflections from microstrip line. As a result, the proposed
antennas showed a stable unidirectional radiation in E-plane
and H-plane.
b: Balanced Antipodal Vivaldi Antenna
Another modification to the conventional Vivaldi antenna
structure is found in [65] that presented a balanced antipo-
dal Vivaldi antenna (BAVA). The BAVA is composed of
three copper layers. The central layer acts as the conductor,
while the external layers are considered as the ground plane.
Substrate layers are placed on the sides of the antenna,
and between the three copper layers as a separation. This
geometry of the antenna helps balancing the dielectric load
between the ground planes and the conductor. A narrower
beamwidth of the antenna was obtained by introducing a
director element in the antenna’s structure resulting in a
BAVA with Dielectric Director (BAVA-D). This improved di-
rectivity allowed for having more back-scattered energy from
the breast which translates into better quality imaging. Both
antennas are capable of operating over a broad frequency
range of 2.4 to 18 GHz. Nevertheless, it was shown in [66],
based on experiments, that BAVA-D antennas outperform
BAVA antennas in tumor response localization.
c: Double Exponentially Tapered Slots
A Double Exponentially Tapered Slot Antenna (DETSA) that
operated across the whole UWB has been presented in [67].
The proposed design is based on the Vivaldi structure with
modifications to the slot-line conductors where the outside
edge is tapered. The DETSA was built on a flexible liquid
crystal polymer (LCP) organic material, allowing the antenna
to be folded and used in a wearable device. A high an-
tenna gain was observed after simulations and measurements,
where 7 dBi was obtained for lower frequencies and 12
dBi for the higher frequencies. However, the antenna has
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a large size of 136.2×66.4mm2. In [68], it was shown
that the performance characteristics of the DETSA, such
as the directivity and electrical length, can be improved by
modifying the outer edge parameters.
d: Resistive Loading
Resistive loading is another technique that has shown its
ability in enhancing the bandwidth of Vivaldi antennas. By
modeling the equivalent circuit of the design and analyzing
its properties, the bandwidth of a Vivaldi antenna of a size
of 62 ×70 ×0.5mm3was improved in [69], by adding
a resistive load on the antenna and a short circuit pin. The
resulting design was able to operate in the frequency range
of 1 to 20 GHz and has an antenna gain that varies between
0.9 and 7.8 dBi over the frequency range.
e: Circular Slotted Arm Structure
A slot-loaded Vivaldi antenna with a circular shape has been
presented in [70]. Each arm of the conventional design is
replaced with a circular-shaped load which allows to im-
prove the antenna bandwidth while maintaining a compact
geometry. The lower frequencies can be extended by adding
properly optimized slots to the circular arms. These slots
also improve the antenna’s radiation characteristics, such as
its radiation pattern and directivity. The proposed antenna
operates in the frequency range of 2 to 50 GHz and it is
characterized by a suppressed sidelobe compared with a
circular design without slots, and a higher antenna gain that
varies between 3 and 12 dBi.
f: Fractal Leaf Arm Structure
A Fern Fractal Leaf structure of the Antipodal Vivaldi An-
tenna (AVA) was presented in [71] which offers improve-
ments in the lower impedance bandwidth. The proposed
design operates in the bandwidth of 1.3 to 20 GHz and has
a size of 50.8×62 mm2. Simulations were performed to
evaluate the S11 parameter of the antenna, which was then
compared to the measured results of the fabricated design.
The simulated and measured results showed great agreement
where the value of the reflection coefficient remained below
−10 dB across the whole operational bandwidth.
2) Monopole Antenna
Monopole antennas have been widely used in microwave
breast imaging research. Their different shapes can be op-
timized and modified to achieve a wide bandwidth [72], [73].
For example, a square monopole structure has been desirable
due to its ease of manufacturing and ability to support a wide
range of frequencies. A square monopole antenna presented
in [74] had a size as small as 10 ×10 mm2while operating
in the frequency range of 4 to 9 GHz.
a: Ground Slot and Slit Geometries
The operational bandwidth of the square monopole structure
can be significantly increased by performing some modi-
fications to the ground plane that provide adjustments to
FIGURE 10. Monopole antenna: (a) side view, (b) top layer, and (c) bottom
layer [77].
the electromagnetic coupling effects between the radiating
patch and the ground plane, without enlarging the size of the
antenna. Such a case has been reported in [75] where an open
circuit structure slit was cut into the ground plane of a square
monopole of size 12 ×18 mm2. In addition, the insertion of
aπ-shaped parasitic structure was done to the same ground
plane. These modifications allowed a 130% increase in the
operational bandwidth of the antenna, by extending the upper
frequency from 10.45 to 14.72 GHz and the lower frequency
from 3.12 to 2.19 GHz. Simulations and measurements of
the proposed designed validated the ability of the antenna
to operate efficiently across the whole bandwidth with two
resonances observed at 11.5 and 14.2 GHz.
Another approach is to insert a T-shaped notch into the
ground plane of a square monopole antenna, which can
provide a usable fractional bandwidth that exceeds 120%, as
presented in [76]. The proposed antenna operates in the range
of 3.12 to 12.74 GHz and has a compact size of 12 ×18
mm2. Similarly, three T-shaped slots were inserted into the
ground of a square monopole in [77] where one slot was
placed between two rotated slots, as shown in Fig. 10. This
design operates in a wider range of 2.96 to 15.8 GHz with a
maximum gain measured to be 6.1 dBi. Another type of slots
is found in [78] where E-shaped slots were used. In [79] and
[80], L-shaped slits were embedded into the ground plane,
concluding with similar results. A hybrid design that incor-
porates two different types of slot geometries is found in [81],
where E-shaped slots and T-shaped strips were introduced
into the ground plane of the same conventional monopole,
allowing an 86% increase in the radiation efficiency over a
larger bandwidth. A pair of loop sleeves were also introduced
to the ground plane of a square monopole in [82], allowing
the antenna to operate in the range of 3.19 to 11.03 GHz
while maintaining a size of 20 ×12 mm2.
An optimized coplanar waveguide-fed triangular planar
monopole antenna was presented in [83], where rectangular
notches were introduced in the ground planes to increase the
impedance bandwidth to 3.05 −12.85 GHz. The rectangular
notch of dimensions L= 4 mm and W= 1.25 mm was
introduced at the upper corner of each of the two finite ground
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planes near the radiating patch.
In [84], the operational bandwidth of two elliptical anten-
nas was improved to span across the range of 3 to 20 GHz
with the usage of ground slots. In the first design, which is
of size 30 ×28 ×1.6mm3, two slots of 1 mm width and 9
mm length have been engraved on the back of the antenna
with an orientation angle of 45 degrees. The second design is
similar to the first, with the difference of a bigger slot length
of 14 mm and an orientation angle of 90 degrees, in addition
to a reduced substrate width of 24 mm. Simulations showed
antenna gain values that range between 7.4 to 8.84 dBi and
7.15 to 9.08 dBi for the first and second design respectively.
b: Semicircular Base
A shortened planar hexagonal antenna for UWB applications
with an improved impedance bandwidth was presented in
[85]. A semicircular base of the upper and lower side of the
hexagonal radiator was utilized to increase the impedance
bandwidth to 1.24 −20 GHz. This resulted in a higher
impedance bandwidth when compared with other reported
planar hexagonal antennas. Similarly, smooth transitions be-
tween resonant frequencies was ensured by tapering the
base of the radiating patch to a semi-circle in [86], where
the design of a coplanar waveguide-fed (CPW-fed) square
monopole antenna that operates in the frequency range of 3.4
to 9.9 GHz was presented. Simulated and measured results
showed good agreement, where the antenna was able to effi-
ciently provide a stable pattern across the desired bandwidth
while achieving a good impedance match in the coupling
medium.
c: Octagonal Shape
A wider operation frequency range was provided in
[87], where a Modified Octagonal Shape Ultra-Wideband
Monopole Microstrip Antenna (MOSUMMA) was pre-
sented. The proposed antenna operates in the range of 3 to
15 GHz while having a size of 27 ×29 ×1.6mm3. This
design is based on the basic rectangular monopole structure
but features and etched out a circle in the radiating patch, a
patch with a plus symbol inside the circle, and step cuts in the
rectangular shape’s front face. These modifications allowed
to significantly increase the operational bandwidth which was
validated by simulations and measurements.
3) Bowtie Antenna
a: Shortening Strips
In [88], shortening strips were placed on the upper side of
the substrate of a bowtie antenna with meandering lines,
as shown in Fig. 11. The capacitive nature of the double-
layered antenna with meandered lines is compensated with
this modification, which also provided additional resonances.
These strips allowed to enhance the bandwidth of the antenna
to operate in the range of 0.85 to 3 GHz while maintaining
the same size. This was validated through simulations and
measurements.
FIGURE 11. Presented bowtie antenna in [88].
FIGURE 12. Wide-Slot antenna with a microstrip fork feed presented in [90].
4) Fractal Antennas
a: Fork Feeding
Using a microstrip feed line with a fork-like tuning stub
is a technique that has proven its ability in enhancing the
bandwidth of wide slot antennas [89]. By properly selecting
the parameters of the fork-feed, coupling can be controlled
between the microstrip line and the wide-slot, thus providing
enhancement in the bandwidth. In [90], a wide-slot UWB
antenna, shown in Fig. 12, of dimensions of 14 ×13 ×1.25
mm3, was designed using the fork feed structure which
enhanced radiation efficiency at higher frequencies. Results
showed great success where the system was able to detect
tumors, possessing a diameter of 7 mm, that may be located
anywhere inside the breast and up to a distance of 4 mm from
the skin. Similarly, in [91], a good matching performance was
achieved for a microstrip tapered slot antenna from 2 to 8
GHz by using the fork feeding line.
b: Resistive Loading
Resistive coating was placed on top of the substrate in the
design of a miniaturized microstrip “Dark Eyes” antenna [92]
that operates in the frequency range of 2.7 to 9.7 GHz. The
added resistive layers allow the antenna to achieve a broad-
band behavior. Simulations and analysis of the antenna’s
return loss curve validated the ability of this design to operate
in the desired bandwidth. The conceptual design of this an-
tenna was later developed in [93] with the purpose of further
VOLUME 4, 2016 9
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FIGURE 13. Geometry of the fourtear antenna presented in [94].
miniaturizing the design and offering improvements in the
lower operation frequency of the antenna. This resulted in
a traveling wave tapered and loaded antenna (TWTLA) that
operates in the frequency range of 1.9 to 35 GHz, offering an
ultra-broadband operation.
c: Fourtear Structure
A Fourtear antenna, shown in Fig. 13, operating in the 2−10
GHz range, has been presented in [94]. The effect of different
antenna polarizations were shown to affect the performance
of artificial neural networks in the statistical detection of
breast cancer tumors. The proposed antenna was chosen due
to its broadband characteristics. By using identical arms, the
antenna was capable of operating with dual linear polariza-
tions and was designed using two different layers of substrate
made of silicon and benzo-cyclobutene (BCB) of dimensions
of 15 ×15 mm2. Simulations on a breast and tumor model
were performed. Results highlighted the effect of the excited
polarization and tumor orientation on the prediction success
of the neural network, and showed that higher accuracy
predictions may be achieved when the tumor’s main axis and
the polarization are parallel.
d: Hibiscus Petal Structure
A hibiscus petal shape of the radiating patch was shown to
offer a wide bandwidth and a stable radiation pattern in [95],
where the design of a microstrip antenna with a hibiscus
petal pattern patch structure operating in the frequency range
of 3.1 to 10.6 GHz was presented. This special structure is
fed by a microstrip line, with the whole radiating patch built
using RT/Duroid 5870 substrate with a dielectric constant of
2.33. Simulations were performed to evaluate the reflection
coefficient of the antenna, which showed two resonance
peeks at 4.5 GHz and 8.5 GHz with the whole S11 curve
being below −10 dB for the desired bandwidth. These results
agree well with the measured ones after fabrication of the
antenna.
5) Horn Antennas
A few works in the literature have also introduced the usage
of horn antennas for microwave breast imaging. Such types
of antennas are attractive due to their ability to support
a wide bandwidth of operation [96]. Ridges can be also
applied to horn antenna to enhance the operation bandwidth
by extending the lower cut-off frequency of the waveguide. In
[97], a double-ridged horn antenna was designed to operate
in the frequency range of 1 to 6 GHz and was compared
with the performance of a Vivaldi antenna in measuring
reflection from a breast phantom that was used for image
formation later on. It was shown that the double-ridged horn
provided a higher value of tumor to fibroglandular ratio for
all reconstructed images. Similarly, in [98], a pyramidal horn
antenna was designed with one ridge on the lower plate
and the other ridge replaced by a curved metallic launching
plane on the upper plate. The curvature and shape of the
plate was optimized to minimize reflections and provide good
impedance matching with a coaxial feed. The obtained design
was able to operate over the range of 1 to 11 GHz with a
fidelity value that exceeds 0.92.
Table. 1 summarizes the different investigated antennas
with bandwidth enhancement. These papers are further com-
pared based on their performance in Table. 2.
B. MINIATURIZED SIZE
1) Vivaldi Antenna
a: Antipodal Design
The antipodal vivaldi antenna (AVA) is known to achieve
a wider bandwidth and a smaller size compared with the
conventional Vivaldi antenna. An AVA that operates over
the UWB range of 3.1 to 10.6 GHz has been presented
in [99]. The proposed design was fabricated using Rogers
RT6010LM substrate and has a compact size of 52 ×52
mm2. Results validated the capability of this design in being
used for high resolution microwave imaging, where a peak
gain of 10.8 dBi was measured, along with a front-to-back
ratio of above 16 dB. These results came in close agreement
with simulations, demonstrating the ability of the antenna to
provide distortionless support for narrow pulses. Unwanted
backward radiations of the AVA design were minimized in
[100] by the use of resistive layers that can be placed behind
the radiation layers of the antenna structure. A similar design
is found in [101] where a tapered slot antenna in the form of
an AVA was presented. The proposed design is also compact,
having a size of 50 ×50 mm2and operates across the whole
UWB range. Compared with [99], it has a lower antenna gain
that varies between 3.5 and 9.4 dBi.
b: Corrugations
Corrugations may also be used to keep the size of the AVA
compact. In [102], two steps were taken to miniaturize a
microstrip-fed antipodal tapered-slot antenna to about 20%
of the same antenna designed using the traditional approach.
First, the microstrip feeder was directly connected to the ra-
diator after it was curved away from the edge of the structure.
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TABLE 1. Investigated antennas and techniques for bandwidth enhancement.
Antenna Type Technique Reference Design Type
Vivaldi Antenna
Corrugations
[62] Uniformly Corrugated Vivaldi
[63] Non-Uniformly Corrugated Vivaldi
[64] Corrugated Vivaldi with Cavity Opening
Balanced Design [65] - [66] Balanced Antipodal
Vivaldi Antenna (BAVA)
Double Exponentially
Tapered Slots (DETSA) [67] - [68] DETSA
Resistive Loading [69] Resistively Loaded Vivaldi Antenna
Circular Slotted
Arm Structure [70] Slot Loaded Vivaldi Antenna
Fractal Leaf
Arm Structure [71] Fern Fractal Leaf AVA
Monopole Antenna
Ground Slot and Slit Geometries
[75] Square Monopole with Slits
and π-shaped Parasitic Structure
[76] Square Monopole with T-shaped Notch
[77] Square Monopole with T-shaped Slots
[78] Square Monopole with E-shaped Slots
[79] Square Monopole with L-shaped Slits
[80] Rectangular Monopole
with L-shaped Slots
[81] Square Monopole with E-shaped
and T-shaped Slits
[82] Monopole Antenna with
Loop-Sleeve Ground Structure
[83] Ground-Notched Triangular Monopole
[84] Ground-Slotted Elliptical Monopoles
Semicircular Base [85] Shorted Planar Hexagonal Antenna
[86] CPW-fed Square Monopole
Octagonal Shape [87] Modified Octagonal Shape
Monopole Antenna (MOSUMMA)
Bowtie Antenna Shortening Strips [88] Bowtie Antenna with Straight Strip Line
Fractal Antenna
Microstrip Fork Feeding [90] Wide-Slot Antenna
[91] Tapered Microstrip Slot Antenna
Resistive Loading [92] Microstrip Dark Eyes Antenna
[93] Traveling Wave Tapered
and Loaded Antenna (TWTLA)
Fourtear Structure [94] Fourtear Antenna
Hibiscus Petal Structure [95] Hibiscus Petal Pattern Patch Antenna
Horn Antenna Ridges [97] Double Ridged Horn Antenna
[98] Ridged Pyramidal Horn Antenna
Then, symmetrical corrugations of depth varying from 1 mm
for inner corrugations to 3 mm for outer corrugations were
introduced in the radiator and the ground plane. The final
design has dimensions of 25 ×30 mm2, compared with the
original dimensions of 60 ×60 mm2. Similarly, in [103], an
AVA operating across the whole UWB range was presented.
Corrugations with a depth of 4 mm, a length of 1 mm, and a
spacing value of 0.5 mm, were also used to give the antenna
a compact size of 22 ×40 mm2.
c: Substrate With a High Dielectric Constant
When dealing with microstrip antennas, choosing a substrate
with a high dielectric constant, if available, is considered
the simplest solution to reduce the size of the antenna. As
the permittivity of the substrate increases, the speed of wave
propagation in the medium decreases, and hence the wave-
length also decreases with the frequency remaining constant,
as in (3),
λd=λ0
√r
(3)
where λdis the wavelength in the dielectric substrate,
λ0is the wavelength corresponding to the frequency of
operation, and ris the dielectric constant of the substrate.
As a result, the required length of the antenna decreases, and
a miniaturized design can be achieved. This is exemplified in
[104] where the substrate TCream E-37 with a high dielectric
constant of 37 was chosen to miniaturize the design of an
AVA. The proposed antenna is of size 30 ×36 mm2and
operates in the frequency range of 0.5 to 3 GHz.
2) Bowtie Antenna
a: Cross Design
A compact size of the bowtie antenna can be obtained by
using the cross design introduced in [105] where two crossed
bowtie antennas of size 62.5×62.5mm2were presented.
To block any waves from radiating away from the breast,
an octagonal cavity was placed behind the bowtie elements
and was attached using a metal flange. Dielectric filling of a
fat-like substance was placed inside the cavity to provide a
better matching with the breast. The fabricated prototype of
this design showed its ability of detecting elliptical tumors
VOLUME 4, 2016 11
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TABLE 2. Performance of the investigated antennas with bandwidth enhancement.
Ref. Dimensions (mm) Dielectric Material Dielectric
Constant
Frequency
Band (GHz)
Results Obtained
Through
Simulations Measurements
[62] 260x360 FR4 4.5 1-3 X
[63] 50x62x1.52 Taconic RF-35 3.5 1.96-8.61 X X
[64] 50x50 FR4 4.4 2.9-12 X X
[65] - [66] 80x44x9.2 RT/duroid 6002 2.94 2.8-18 X X
[67] - [68] 136.2x66.4 Liquid Crystal
Polymer (LCP) 3.1 3.1-10.6 X X
[69] 62x70x0.5 FR4 4.4 1-20 X X
[70] N/A Rogers Duroid 5880 2.2 2-50 X X
[71] 50.8x62 FR4 4.4 1.3-20 X X
[74] 10x10 Rogers RO3010 10.2 4-9 X X
[75] 12x18 FR4 4.4 2.91-14.72 X X
[76] 12x18 FR4 4.4 3.12–12.73 X X
[77] 12x18x0.8 FR4 4.4 2.96-15.8 X X
[78] 12x18x1.6 FR4 N/A 2.9-15 X X
[79] 12x18x1.6 FR4 N/A 2.95-14.27 X X
[80] 32x35 FR4 4.6 3.1-10.6 X X
[81] 12x18 FR4 4.4 2.97-12.83 X X
[82] 20x25 FR4 4.3 3.19-11.03 X X
[83] 24x31 FR4 4.4 3.05-12.85 X X
[84] 30x28x1.6 FR4 3.9 3-20 X
30x28x1.6 FR4 3.9 3-20 X
[85] N/A N/A N/A 1.24-20 X X
[86] 30x26 FR4 4.4 3.4-9.9 X X
[87] 27x29x1.6 FR4 4.4 3-15 X X
[88] 30x30 Rogers 6010 10.2 0.85-3 X X
[90] 14x13x1.25 Rogers Duroid 6010 10.2 3-10 X X
[91] 19x19
Rogers Duroid 6010
(Top Layer)
& Rogers Duroid 5880
(Bottom Layer)
10.2
(Top Layer)
& 2.2
(Bottom Layer)
2-8 X X
[92] 22.25x20x1.3 Rogers Duroid 6010 10.2 2.7-9.7 X
[93] 18x14 Rogers Duroid 6010 10.2 1.9-35 X
[94] 15x15 Silicon &
Benzocyclobutene 2.33 2-10 X
[95] 31x31 RT/Duroid 5870 2.33 3.1-10.6 X X
with a length of 1.5 cm up to a depth of 1 cm.
b: Meandering Lines
Miniaturization of the bowtie antenna can also be achieved by
introducing meandered lines within a double-layer structure.
This technique was validated in [106], where a compact
double-layer bowtie antenna operating in the frequency range
of 0.5 to 2 GHz has been presented. The proposed antenna
was miniaturized to achieve total dimensions of 30 ×30
mm2and was able to operate for the same frequency range
of the conventional bowtie structure with a larger size of
50 ×50 mm2. The antenna was fabricated using Rogers
RT 6010 substrate with a relative permittivity of 10.2 and
was immersed in a liquid phantom for measurements. Good
agreement was observed between the measured and simu-
lated reflection coefficient results.
Table. 3 summarizes the different investigated antennas
with miniaturized size. These papers are further compared
based on their performance in Table. 4.
C. ENHANCED RADIATION CHARACTERISTICS
1) Vivaldi Antenna
a: Side Slots
Applying different shapes of slots to the sides of the con-
ventional and antipodal Vivaldi antennas is a technique that
can provide several enhancements in the radiation charac-
teristics of these antennas. Six side slots were applied to
the regular Vivaldi antenna in [107], where the final design
provided a pattern with better directivity compared with the
conventional design, as well as a higher antenna gain. These
improvements can be offered merely by introducing the side
slots and would not affect the size of the antenna, which is
desired to be as miniaturized as possible. The final design
was able to operate in the frequency range of 1.54 to 7
GHz with a radiation efficiency of 92%. Hemi-cylindrical
slots were introduced to a Vivaldi antenna in [108], allow-
ing to improve the antenna gain, front-to-back ratio (FBR),
impedance matching, and cross-polarization discrimination
(XPD). The proposed design was able to operate in the range
of 2.9 to more than 9 GHz with a peak gain of 9.5 dBi while
maintaining a size of 49 ×48.5×0.8mm3.
In [109], an improvement of the gain of an antipodal
linearly tapered slot antenna (ALTSA) with unequal half-
circular slots embedded on both edge-sides was presented.
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TABLE 3. Investigated antennas and miniaturization techniques.
Antenna Type Technique Reference Design Type
Vivaldi Antenna
Antipodal Design
[99] Antipodal Vivaldi Antenna (AVA)
[100] Tapered Slot AVA
[101] Tapered Slot AVA
Corrugations [102] Corrugated Tapered Slot AVA
[103] Corrugated Tapered Slot AVA
Substrate With a High
Dielectric Constant [104] AVA
Bowtie Antenna Cross Design [105] Crossed Bowtie Antenna
Meandering Lines [106] Double-Layered Bowtie Antenna
TABLE 4. Performance of the investigated miniaturized antennas.
Ref. Dimensions (mm) Dielectric Material Dielectric
Constant
Frequency
Band (GHz)
Results Obtained
Through
Simulations Measurements
[99] 52x52 Rogers RT6010LM 10.2 3.1-10.6 X X
[100] 50x50 Rogers RO4003C 3.38 3.1-10.6 X X
[101] 59.6x59.9 Rogers RT6010LM 10.2 3.1-10.6 X X
[102] 25x30 Rogers RT6010 10.2 1.8-10.8 X X
[103] 22x40 Rogers RT6010 10.2 3.1-10.6 X
[104] 30x36 TCream E-37 37 0.5-3 X
[105] 62.5x62.5 N/A N/A 2-4 X X
[106] 30x30 Rogers RT6010 10.2 0.5-2 X X
This antenna, which operates at 3.1 GHz to more than 10.6
GHz, showed narrow 3 dB beamwidths since the slots can
change the edge surface current. Its gain was improved by up
to 3.5 dB when compared with a reference ALTSA.
In [110], periodic slit edges were applied to the conven-
tional AVA design with a trapezoid-shaped dielectric lens to
improve its radiation characteristics, by extending the lower
frequency range while also increasing the antenna gain. In
later work [111], the same group further improved their
proposed design by using rectangular slits resulting in a final
design that provided a gain larger than 10 dBi across the
whole operating range of 10 to 50 GHz, with size reduction of
40% reaching final dimensions of 30×55 mm2. An elliptical-
shaped dielectric lens was also applied in this design, offering
an improved front-to-back ratio, elevated gain values at high
frequencies, lower sidelobes in the radiation pattern, as well
as lower cross-polarization levels. Both designs are shown in
Fig. 14.
Regular slot edges (RSE) were introduced to the design
of an AVA in [112], allowing to extend the lower frequency
of the by 9% without altering the size of the antenna. The
resulting RSE AVA was shown to have a return loss below
-10 dB from 4 to 30 GHz. The problem of distortions that
occur in the radiation pattern of the antenna and the decrease
in the antenna gain at higher frequencies were reduced by the
introduction of a curved lens that can offer gain enhancement
and stability in the radiation pattern. This was validated by
simulations and measurements which showed an increase in
the antenna gain by a maximum of 7 dB at 23 GHz and 1
dB at 13 GHz. Exponential slot edges (ESE) were applied
to an AVA in [113], giving the antenna a palm tree structure
that allows the extension of the low-end bandwidth, reducing
the side lobe levels (SLL) and back lobe level, in addition to
(a)
(b)
FIGURE 14. Modifications of Vivaldi antenna: (a) original design [110], (b)
modified design, presented in [111].
increasing the antenna gain in the main lobe. Compared to a
regular AVA, the ESE-AVA was able to reach a gain of 8.3
dBi and a SLL of −15 dB in an operational bandwidth from
5.6 to 11 GHz. Similar ESEs were applied to the design of a
BAVA in [114] with a curving dielectric director, allowing to
improve its gain to a value of 12.6 dBi. More improvements
were obtained in [115], where a tapered slot edge (TSE)
design was used in an AVA resulting in a wider bandwidth.
The TSE-AVA operates from 2.4 to more than 14 GHz with
a peak gain of 10 dBi at 7 GHz while maintaining a compact
size of 48 ×60 mm2.
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b: Dielectric Lens
To improve the radiation pattern and cross-polarization of a
conventional BAVA, where a tilted beam is usually obtained
at higher frequencies, a dielectric lens (DL) is introduced in
[116] to the front of the antenna’s aperture. The DL helps in
increasing the path length and retarding the passing wave,
thus causing it to collimate. This offers a more directive
pattern and a higher antenna gain. The proposed DL-BAVA
was capable of operating in the frequency range of 3 to 18
GHz with a better return loss value compared with the regular
BAVA. Additionally, a bore-sight gain higher than 10 dBi
was achieved, which is an improvement on the conventional
BAVA where the gain does not exceed 10 dBi. Modifications
to the shape of the substrate were proposed in [117] to act
as a DL, where similar results in the radiation pattern can be
obtained without the need to incorporate a separate lens to
the antenna.
c: Reflector Structure
The antenna gain of conventional Vivaldi antennas was im-
proved by the cavity-backed design introduced in [118],
where a carefully-designed reflector structure was incorpo-
rated with the antenna. The cavity-backed Vivaldi antenna
(CBVA) was designed to operate in the frequency range
of 2.5 to 8.5 GHz. By measurements and simulations, the
CBVA proved to have multiple improvements in the radiation
characteristics compared with a regular Vivaldi, most notably
by an increase of 2.1 dBi in the peak antenna gain and an 8.5
dB decrease in the front-to-back ratio.
Table. 5 summarizes the different investigated antennas
with bandwidth enhancement. These papers are further com-
pared based on their performance in Table. 6.
2) Monopole Antenna
a: Reflector Structure
An elliptical shape was chosen in [41] to offer a compro-
mise between the wide bandwidth provided by the rectan-
gular monopole and the pattern stability found in circular
monopoles. The elliptical monopole is of size 24 ×33.5
mm2and operates in the frequency range of 1 to 4 GHz.
To improve the directional properties, a reflector was used
and positioned behind the antenna, resulting in reduced back-
radiations at the expense of a larger antenna size of 60 ×60
mm2and a slightly reduced impedance match. However, the
improved transmission response was proven in both simu-
lated and measured results that showed a S21 curve above
−60 dB from 1 to 4 GHz.
b: Parabolic Shaped Ground
In [119], a directional monopole antenna with an improved
directivity and a stable directional radiation pattern was
presented by introducing a modified ground plane with a
parabolic shape. The axis of the parabolic shape was ex-
tended throughout the direction of the substrate diagonal
and parabolic-shaped slots were inserted at the corners of
the ground plane to reach an improved directivity of 5-15
degrees. It was shown through both simulations and measure-
ments that the antennas had a stable radiation pattern and in
the bandwidth of 4 to 9 GHz an improved gain of 1.3−3.1
dB.
3) Bowtie Antenna
a: Resistive and Capacitive Loading
To minimize the late ringing time of the bowtie antenna,
which are caused by internal reflections, resistive loadings
may be used to achieve consistent pulse radiation. This
loading may be optimized for minimal end reflections [120],
[126]. Such a design has been reported in [121], where a
resistively loaded bowtie antenna of dimensions of 70.3×37
mm2operating in the frequency range of 3.3 to 10 GHz
was presented. The resistive loading consists of six resistors
optimized using a genetic optimization algorithm. To validate
this design, simulations of the near field distributions at 4,
6, and 8 GHz were performed and showed good agreement
with the measured results. A similar study is found in [122]
where a combination of resistive and capacitive loadings was
applied to the design of a bowtie antenna. A constant resistive
load was maintained along the antenna with a capacitive
load that increases linearly towards the ends of the antenna.
This combined loading scheme improved the pulse radiation
by 54% and maintained a late ringing time below −40 dB
compared with a similar bowtie antenna without loading.
b: Slotline
Internal reflections of the antenna may also be reduced by
introducing a slot to the structure, such as done in [123],
where a slotline bowtie hybrid antenna (SBH) that operates in
the frequency range of 1 to 10 GHz has been presented. The
design was fabricated and tested on breast phantom material
to evaluate the effect of the tumor depth on the reflection
coefficient. The presented results showed that the amplitude
of the reflections decreased by approximately 10 dB after an
increase of 1 cm in the depth of the tumor.
4) Horn Antennas
a: Dielectric Shielding
A dielectric shield was used in [127] to enclose a TEM
horn antenna, and hence block any possible electromagnetic
interference that may occur from its surroundings. The pro-
posed horn operates in the UWB range and possesses better
radiation efficiency compared with regular designs, which
was validated through simulations by a lower reflection
coefficient value across the whole bandwidth. Similarly, in
[124], [125], a TEM horn was placed in a solid dielectric
medium. Copper sheets were placed on the top, bottom, and
sides of the antenna resulting in a better radiation efficiency,
blocked interference, improved coupling efficiency, and good
impedance matching. Apertures were also introduced in the
top sheet of the shield, providing a uniform field distribution.
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TABLE 5. Investigated antennas and techniques for radiation characteristics enhancement.
Antenna Type Technique Reference Design Type
Vivaldi Antenna
Side Slots
[107] Side Slotted Vivaldi Antenna
[108] Hemi-cylindrical Slotted Vivaldi
[109] ALTSA
[111] AVA with Rectangular Slits
[110] AVA with Periodic Slit Edge
[112] AVA with Regular Slot Edges
(RSE-AVA)
[113] AVA with Exponential Slot Edges
(ESE-AVA)
[115] AVA with Tapered Slot Edge
(TSE-AVA)
[114] ESE-BAVA
Dielectric Lens [116] DL-BAVA
[117] Modified BAVA
Reflector Structure [118] Cavity-Backed Vivaldi Antenna
Monopole Antenna Parabolic-Shaped Ground [119] Monopole Antenna with
Parabolic-Shaped Ground Plane
Reflector Structure [41] Elliptical Monopole with Reflector
Bowtie Antenna Resistive and Capacitive Loading
[120] Modified Wire Bowtie Antenna
[121] Resistively Loaded Bowtie Antenna
[122] RC-Loaded Bowtie Antenna
Slotline [123] Slotline Bowtie Hybrid Antenna
Horn Antenna Dielectric Shielding [91] TEM Horn Antenna
[124] - [125] TEM Horn Antenna
TABLE 6. Performance of the investigated antennas with enhanced radiation characteristics.
Ref. Dimensions (mm) Dielectric Material Dielectric
Constant
Frequency
Band (GHz)
Results Obtained
Through
Simulations Measurements
[107] 88x75 Rogers RT/duroid 5870 2.33 1.54-7 X X
[108] 49x48.5x0.8 FR4 4.3 2.9-9 X X
[109] 52x145 RT/Duroid 5880 2.2 3.1-10.6 X X
[111] 30×55×0.508 Rogers RO4003C 3.38 5-50 X X
[110] 40x90x0.508 Rogers RO4003C 3.38 3.4-40 X X
[112] 66.4x50 N/A 4.5 4-30 X X
[113] 36.3x59.81 Rogers RO3206 6.15 5.6-11 X X
[115] 48x60 FR4 4.6 2.5-14 X X
[114] 70x140x3 ARLON Diclad880 2.2 1.5-15 X X
[116] 96x50x3.15 Arlon AD255 2.55 3-18 X X
[117] 105x56 N/A 2.5 6-20 X
[118] 63x51x1 FR4 4.4 2.5-8.5 X X
[119] 50x50 FR4 4.4 4-9 X X
[41] 24x33.5 Taconic CER-10 9.8 1-4 X X
[120] 23x7 FR4 4.4 0.42-1.67 X X
[121] 70.3x37 N/A N/A 3.3-10 X X
[122] N/A N/A N/A 0.5-5.1 X X
[123] N/A Rogers RT6010LM 10.2 2.5-9 X X
Table. 5 summarizes the different investigated antennas
with enhanced radiation characteristics. These papers are
further compared based on their performance in Table. 6.
V. ANTENNA ARRAYS PROPOSED FOR MICROWAVE
BREAST IMAGING
After investigating the antenna designs proposed for mi-
crowave breast imaging, the different antenna arrays found
in the literature are presented in this section. We classified
these arrays based on their configuration or structure which
include planar, enclosed structures, and hemispherical. The
performance of the investigated arrays is summarized at the
end of this section in Table. 7, where they can be compared
according to some of their major characteristics including:
the methods adopted to obtain measurements, which include
monostatic, bistatic, and multistatic methods, the array oper-
ating frequency band, the design of the antenna elements, and
their total number in the investigated array.
A. PLANAR ARRAYS
The earliest antenna array proposed for microwave breast
imaging is a 2×2bowtie planar antenna with resistive
profiles, presented for breast cancer detection in [128]. The
antennas were printed on a lossy substrate that can help in
decreasing the mutual coupling and increasing the operat-
ing bandwidth. Scanning of the breast required rotating the
antenna to increase the amount of information received and
improve the accuracy of the detection process. The array has
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(a) (b)
FIGURE 15. Proposed Planar array: (a) back side, (b) front side, presented in
[130].
total dimensions of 94 ×49 mm2, with each element fed
by a coaxial transmission line. The antenna has not been
fabricated, and the study investigated the simulated results.
In [129], two antenna arrays designed for microwave
breast imaging have been proposed. The same antenna design
has been used for both arrays. Each array consists of two
antennas, and the two arrays differ according to the polariza-
tion. To study the effect of received signal polarization on the
detection accuracy, two different configurations have been
studied, one with co-polarization, and the second with cross-
polarization. The antenna element is a slot patch, operating
from 3.1 to 10 GHz. It was shown that the co-polarized
configuration performs better in detecting tumors away from
the chest wall, whereas the cross-polarized configuration is
preferred in detecting tumors close to the chest wall. The
antennas have been tested in two different measurements
setups, one with pork fat, and the second with pork muscle,
both with a phantom mimicking the tumor.
In [130], an UWB planar 4×4array, shown in Fig. 15,
has been also presented for microwave breast imaging. Each
antenna element is made of a monopole slot antenna with a
miniaturized size. Duroid RT with a relative permittivity of
10.2 has been used as a substrate. The antenna dimensions
have been optimized using a stack of dielectric materials of
40 mm thickness placed below the antenna that can mimic
the skin and adipose. The whole antenna array operated
from 3.5 to 15 GHz. Homogenous and inhomogeneous breast
phantoms along with a glandular phantom have been used
in the measurements and in the 3D confocal imaging. It has
been shown that the proposed system was able to detect a
tumor of 10 ×10 ×1mm3size with a 10 mm separation
distance.
In [131], an integrated system proposed for microwave
imaging system has been presented. A planar antenna array
formed by a pair of patch antennas has been used with
CMOS technology. The antenna array operated from 2 to
16 GHz using a combined circular and rectangular shape in
the patches. Roger RO4003c with a permittivity of 3.55 has
been used as a substrate. It was shown through experiments
with breast and tumor phantoms that the proposed system
can detect tumors with a resolution of 3 mm. The paper also
discussed the procedure done in implementing the CMOS at
the receiver side.
B. ENCLOSED ARRAYS
The second type of arrays is that formed by an enclosed array.
In [132], an enclosed array in a cubical structure has been
presented. The array was made of miniaturized multiband
patch antennas, surrounded by a conducting enclosure. The
enclosed configuration has an elliptical shape opening in the
top panel that permits the patient to extend her breast through
the opening, as shown in Fig. 16a. A dielectric material of
relative permittivity of 6.15 was used to coat the four lateral
panels, having eight slot antenna elements on each panel,
arranged in an optimized configuration to reduce the mutual
coupling, with a total of 32 antennas. The slot in each element
was used to decrease the operating frequency to a lower range
from 0.5 to 3.5 GHz with a miniaturized size. Each patch
antenna has a total size of 30 ×28 mm2, spaced horizontally
and vertically by 48 mm from the nearest antenna element.
An immersion medium has been also used to fill the interior
volume of the array, with dielectric properties that match
the coupling safflower oil properties usually used in such
systems. The whole system operated at 1.60, 2.20, and 3.02
GHz. The validation of the proposed system has been done
through simulations.
In a continuation of their work in [132], the same group
presented in [133] an improved design of the antenna patches
to be used in their enclosed structure, as shown in Fig. 16b.
The improved antenna element has a miniaturized size rela-
tive to its resonance frequencies operating at 1.36, 1.74, and
3.03 GHz. The improvement has been made through adding
additional slots in the patch antenna design. The images
constructed using the whole array system were compared
to those resulting from simulated array measurements using
a realistic breast phantom, where good agreement has been
seen.
The enclosed configuration of [132] has been also pre-
sented in [134] to protect the imaging system from external
interference, but with a single operating frequency rather than
the dual band operation presented in [132]. In [135], the
array has the shape of a 12 sided polygon, with each panel
containing three bow-tie antennas, in a total of 36 antennas.
The enclosed system is also filled with a coupling liquid.
It was shown, through experiments and simulations, that
images can be successfully formed in the cavity geometry
using the numerical characterization of the incident fields
and, and the link between the incident fields and the inverse
scattering algorithm resulting from the vector Green’s func-
tion formulation. In addition, the location and permittivity of
the object tested can be also detected.
C. HEMISPHERICAL ARRAYS
One of the earliest hemispherical array systems, have been
investigated with real experimental data assessment in [53],
[137], [139]. In [137], an UWB conformal hemispherical
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(a) (b)
FIGURE 16. Enclosed array of 32 antennas: (a) original array [132], (b) improved antenna element design [133].
(a) (b)
FIGURE 17. Cavity backed aperture stacked patch antenna: (a) original array [136], (b) improved antenna element design [137].
array designed for breast cancer detection has been presented.
The antenna used as the element array is a modified version
of the cavity backed aperture stacked patch antenna presented
in [136], both shown in Fig. 17.
A modified version of the antenna presented was used in
[137] in a conformal hemispherical array, shown in Fig. 18a,
rather than the typical planar array. The antenna element was
made of two stacked patches, previously presented by the au-
thors in [139] with the first patch being a typical rectangular
patch antenna. Both patches were separated from the ground
by stacked substrates: one with a relative permittivity of 2.2,
and the second with relative permittivity of 10.2. A cavity
backed aperture, with optimized dimensions of 23 ×29 mm2
with 17 mm long, was also used to protect the antenna from
the surroundings, lined with a back radiation absorber. To
decrease the mutual coupling between the adjacent elements,
a short metallic screen was included on the front face of the
antenna, which was matched between 4.5 and 10 GHz. The
antenna has been fabricated and tested, with 20 female volun-
teered in the fabrication process to detect the needed size of
the total array to fit different breast sizes. The antennas were
immersed in a matching medium to attenuate the reflections
from the breast skin. The measurements needed around 3
minutes to complete 120 independent measurements. Spher-
ical tumors with diameter of 4 and 6 mm have been detected
within a breast phantom of a relative permittivity of 9.8.
A later work by the same group has been presented in [138]
with 2:1 low dielectric contrast between tumor and normal
tissues. The improved array was made from wide-slot UWB
antennas [90], previously investigated in Section IV-A4a, and
shown in Fig. 12. Since the total dimensions of this antenna
was smaller than the one in [137], 31 elements were used
here instead of the 16 elements in the previous design, as
shown in Fig. 18b. As in [137], each antenna element was
fed through coaxial cables. Comparing the 2D and 3D images
of the newly designed system with the previously presented
one, the 31 system array showed better results with higher
accuracy and less clutter. The authors also compared having
only 16 elements with their previously presented array of
16 elements. The results also showed better performance.
Nevertheless, this system also needed to be filled with an
immersion matching medium to attenuate the reflections
from the breast skin.
In [140], a hemispherical UWB antenna array of 16 minia-
turized bowtie antennas has been proposed for microwave
imaging. The system operates from 1.2 to 7 GHz, with the
two arms of the bowtie antenna short connected to miniatur-
ize its size relative to its operating frequency range. The total
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(a)
(b)
FIGURE 18. Hemispherical constructed array: (a) original array [137], (b)
modified array [138].
dimensions of each antenna element was 15 ×15 mm2. The
antenna has been fabricated and tested, and then used in an
array of hemispherical configuration to test the performance
of the proposed system. Two different experiments have been
conducted with breast phantoms, with one and two embedded
tumors. It was shown multiple tumors, in both horizontal and
vertical planes, can be detected by the proposed systems. The
tumors had the sizes of 10 and 15 mm respectively, located at
a depth of 18 and 30 mm inside the breast phantom.
To remedy some of the drawbacks of the arrays presented
for microwave imaging related to the use of the immersion
liquid, the bulkiness of some arrays, and the need to rotate
either the antenna or the breast in most of these systems, a
flexible array for microwave breast cancer detection has been
presented by the same group in [141], [142], shown in Fig.
19.
In [141], single and dual-polarization flexible UWB an-
tenna arrays have been presented in [141], with the ar-
ray configuration shown in Fig. 19a. Kapton polyimide of
relativity permittivity of 3.5, known for its flexibility and
(a)
(b)
FIGURE 19. Flexible antenna array: (a) original array configuration [141], (b)
wearable prototype [142].
biocompatibility, was used in the design and fabrication of
antennas, which makes them easier to be used with wearable
applications. Each antenna array is made of two flexible 4×4
antenna elements, covered with a superstrate similar to the
substrate for full biocompatibility. In order to be able to fit
this number of antennas, the antenna elements in both arrays
have been miniaturized to total dimensions of 20 ×20 mm2.
The antenna element in the single polarization antenna array
was a monopole antenna, and that of the dual-polarization
antenna array is a spiral antenna. The two designs operate in
the 2−4GHz band. The designed antennas have been also
fabricated and tested using a breast phantom, and the results
showed good agreement with the simulated ones, with good
impedance matching. It has been also shown that, using a
reflector, the penetration of the electromagnetic waves inside
the body can be also improved. Also, the maximum power
that can be used to transmit the waves from these antenna as
per the ASAR was also calculated and found to be 4 mW and
3.1 mW, for the single and dual-polarization antenna arrays
presented. The coupling between the different elements was
not discussed in the paper.
In a continuation for the work done in [141], the same
group tested their designed antennas as a wearable system
in the form of a bra on a healthy volunteer over a period of
28 days in [142], and compared their wearable system to the
typical table-based one in [143]. The wearable prototype is
shown in Fig. 19b. It was shown that the wearable system
improved the quality of collected data, with a direct contact
to the skin, without the need for an immersion gel as in the
table-based systems. It was also a cost-effective system with
a smaller footprint.
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TABLE 7. Investigated antenna arrays with their major characteristics.
Ref. Array Configuration Measurements Operating Band Antenna Design Nb of
Antennas
Image Reconstruction
Algorithm
[128] Planar Monostatic 1.6–11.4 GHz Resistively Loaded
Bowtie 4 N/A
[129] Planar Monostatic 3.1–10 GHz Slot patch 2 N/A
[130] Planar Monostatic 3.5–15 GHz Slot Monopole 16 N/A
[131] Planar Monostatic
or Bistatic 2–16 GHz Monopole 2 DAS
[132] Enclosed Cubical Multistatic 0.5–3 GHz Slot patch 32 Distorted Born
Iterative Method (DBIM)
[133] Enclosed Cubical Multistatic 0.5–3 GHz Slot patch 32 N/A
[135] Enclosed 12-sided cavity Multistatic Design 1: 2.75 GHz
Design 2: 915 MHz Bowtie 36 Born Iterative Method
(BIM)
[137], [139] Hemispherical Multistatic 4.5–10 GHz Cavity backed aperture
stacked patch antenna
16 Modified DAS
[138] 31 Modified DAS
[140] Hemispherical Multistatic 1.2–7 GHz Modified Bowtie 16 IDAS
[141], [142] Hemispherical Multistatic 2–4 GHz Monopole & Spiral 16 DAS
VI. QUALITATIVE EVALUATION & DISCUSSION
A. ANTENNA ELEMENTS
The different investigated papers, presented in IV, are com-
pared based on their bandwidth, size, design complexity, and
cost of manufacturing, in Table. 8. Assuming the desired
characteristics is to have a wide frequency of operation that
covers the whole UWB range from 3.1 to 10.6 GHz, a
compact size that does not exceed 50 ×50 mm2, a simple
design procedure and structure, and a low manufacturing
cost, the following remarks can be concluded:
– Vivaldi antenna designs presented in [63], [67], [69]–
[71] provide a wide bandwidth but suffer from a large
antenna size and complex design procedure, making
them unattractive for array systems. Among the inves-
tigated Vivaldi antennas, the antipodal Vivaldi antenna
designs in [99]–[101] and side-slotted AVA designs in
[110], [111] are found suitable for microwave imaging
array systems, as they offer a smaller size compared to
the other Vivaldi antennas while exhibiting a broadband
behavior.
– Monopole antenna design presented in [75]–[84] satisfy
the assumed design criteria by simply performing small
modifications into the ground plane, making them also
suitable for usage in arrays. Other monopole designs,
such as [87], [119], are found not suitable due to their
design complexity and large size.
– Bowtie antenna designs presented in [88], [105], [106],
[120]–[123] provide a narrower band that do not span
the whole UWB range, for this they were found not
suitable for microwave imaging array systems.
– Other microstrip antenna, such as the fourtear [94] and
wide-slot [90], are found suitable, while [92], [93], [95]
are not due to their design complexity.
– Horn antennas presented in [97], [98], [124], [125],
[127] offer a wide bandwidth, but they suffer from a
large volume and have a higher manufacturing cost than
microstrip antennas, making them unattractive choices
for microwave imaging array systems.
As a conclusion, among all investigated antennas, the
antipodal Vivaldi antenna designs in [99]–[101], [110], [111]
and the ground-slotted monopoles in [75]–[84] stand out as
the most suitable and attractive antennas for being used in
antenna arrays for microwave breast imaging. The presented
antipodal Vivaldi antenna designs typically operate from 3
to 10.6 GHz, and the presented ground-slotted monopoles
operate from 3 to 14 GHz. Although both of these modified
versions of the Vivaldi antenna and monopole antenna offer
a wide bandwidth that covers the UWB range, these AVA
design has a larger size when compared with the monopoles.
The investigated AVA sizes range from 50 ×50 mm2to
60 ×60 mm2, while monopoles can achieve a size as small
as 12 ×18 mm2while also operating in the UWB range.
B. ANTENNA ARRAY CONFIGURATIONS
The different investigated papers, presented in V, are com-
pared based on their design complexity, flexibility, biocom-
patibility, and cost, in Table. 9. The following remarks are
found.
In conventional microwave breast imaging, the antennas
are either moved with respect to the scanned breast, or the
breast is moved with respect to the antennas. These systems
are mainly known as static systems, which require scanning
of the antenna locations to reach the optimized position
for better accuracy. In contrast, multistatic systems rarely
require this scanning, due to the larger number of collected
signals using well-distributed antennas, but rely on the array
configuration.
Typically, the array configurations found in the literature
can be planar, enclosed, or hemispherical arrays. The planar
arrays [128]–[131] are easier to design and fabricate, and
have the advantage of simple and fast enhancement in their
configuration if more antennas are required to be added, or
even if it is needed to vary the separating distance between
the antenna elements. However, for optimal microwave il-
lumination, these antennas typically require to be held per-
pendicular to the tangent of the breast surface. In addition,
the location of the antennas might require scanning for the
optimized position for more accurate measurements. For this,
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TABLE 8. A taxonomy of research that proposes antennas for microwave breast imaging.
Ref. Proposed Design Wide
Bandwidth
Compact
Size
Simple
Design
Low
Cost
Suitability for
Array Systems
[62] Uniformly Corrugated Vivaldi × × X X ×
[63] Non-Uniformly Corrugated Vivaldi ×X×X×
[64] Corrugated Vivaldi with Cavity Opening X X ×X×
[65] - [66] BAVA X×X X ×
[67] - [68] DETSA X×X X ×
[69] Resistively Loaded Vivaldi Antenna X× × X×
[70] Slot Loaded Vivaldi Antenna X× × X×
[71] Fern Fractal Leaf AVA X× × X×
[99] AVA X X X X X
[100] Tapered Slot AVA X X X X X
[101] Tapered Slot AVA X X X X X
[102] Corrugated Tapered Slot AVA X X ×X×
[103] Corrugated Tapered Slot AVA X X ×X×
[104] AVA ×X X X ×
[107] Side Slotted Vivaldi Antenna × × × X×
[108] Hemi-cylindrical Slotted Vivaldi × × × X×
[109] ALTSA X× × X×
[111] AVA with Rectangular Slits X X ×X X
[110] AVA with Periodic Slit Edge X X ×X X
[112] RSE-AVA X× × X×
[113] ESE-AVA ×X X X ×
[115] TSE-AVA X X ×X×
[114] ESE-BAVA X× × X×
[116] DL-BAVA X× × X×
[117] Modified BAVA X× × X×
[118] Cavity-Backed Vivaldi Antenna × × X X ×
[74] Square Monopole Antenna ×X X X ×
[75] Square Monopole with Slits
and π-shaped Parasitic Structure X X X X X
[76] Square Monopole with T-shaped Notch X X X X X
[77] Square Monopole with T-shaped Slots X X X X X
[78] Square Monopole with E-shaped Slots X X X X X
[79] Square Monopole with L-shaped Slits X X X X X
[80] Rectangular Monopole with L-shaped Slots X X X X X
[81] Square Monopole with E-shaped
and T-shaped Slits X X X X X
[82] Monopole with Loop-Sleeve
Ground Structure X X X X X
[83] Ground-Notched Triangular Monopole X X X X X
[84]
Ground-Slotted Elliptical Monopole
with Oriented Slots X X X X X
Ground-Slotted Elliptical Monopole
with Vertical Slots X X X X X
[85] Shorted Planar Hexagonal Antenna X X X X X
[86] CPW-fed Square Monopole ×X×X×
[87] MOSUMMA X X ×X×
[119] Monopole with Parabolic-Shaped
Ground Plane × × × X×
[41] Elliptical Monopole Antenna ×X X X ×
[105] Crossed Bowtie Antenna × × X X ×
[106] Double-Layered Bowtie Antenna ×X X X ×
[88] Bowtie Antenna with Straight Strip Line ×X X X ×
[120] Modified Wire Bowtie Antenna ×X×X×
[121] Resistively Loaded Bowtie Antenna X× × X×
[122] RC-Loaded Bowtie Antenna × × × X×
[123] Slotline Bowtie Hybrid Antenna × × X X ×
[90] Wide-Slot Antenna X X X X X
[91] Tapered Slot Antenna ×X X X ×
[92] Microstrip Dark Eyes Antenna X X ×X×
[93] TWTLA X X ×X×
[94] Fourtear Antenna X X X X X
[95] Hibiscus Petal Pattern Patch Antenna X X ×X×
[97] Double Ridged Horn Antenna × × × × ×
[98] Ridged Pyramidal Horn Antenna X× × × ×
[127] TEM Horn Antenna X× × × ×
[124] - [125] TEM Horn Antenna X× × × ×
20 VOLUME 4, 2016
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Author et al.: Preparation of Papers for IEEE TRANSACTIONS and JOURNALS
TABLE 9. A taxonomy of research that uses antenna arrays for microwave breast imaging
Paper Wide Bandwdith Simple Design Low Cost Flexibility Biocompatibility
[128] X×X× ×
[129] X X X × ×
[130] X X X × ×
[131] X X X × ×
[135] ×X X × ×
[132] X X X × ×
[133] X X X × ×
[137], [139] X× × × ×
[138] X X X × ×
[141], [142] X X X X X
[140] X X X × ×
enclosed arrays [132], [133], [135] and hemispherical arrays
[53], [137]–[142] have been proposed in the last years. These
systems have the advantage of fixing the antenna array and
inserting the breast inside the antenna array, and use mul-
tistatic measurements. However, as a result of the enclosed
structure of these configurations, reverberating reflections
arise between the walls and the breast skin.
To reduce the reflections found in the enclosed arrays con-
figurations, an immersion medium, typically a gel is usually
used to fill the array. This immersion medium, however,
might attenuate the received signal from the breast under
investigation. In addition, the position of the breast with
respect to the array wall might be difficult to estimate with
the presence of this medium. As for the accuracy, this type
of configuration might be affected by the different breast
sizes, and hence different distance separating the antennas
from the breast. Flexible wearable antennas, proposed in the
latest research [141], [142], can solve the different drawbacks
seen in the configurations provided. For instance, they can be
fixed on the breast with no immersion medium. As such, the
distance separating the antennas and the breast is reduced.
The need to scan the optimized position of the antennas
is no longer required since the antennas are placed on the
breast at different positions using multistatic measurements,
which reduces the error associated with antenna position. In
addition, the discomfort that could arise with the scanned
patient from the gel is eliminated. To further enhance the
flexibility and simplicity of this system, the antenna arrays
can be located on a bra rather than the breast skin, which can
also result ease the imaging process for the patient.
In their turn, these flexible antennas require to be fabri-
cated from biocompatible material. They also need to have
high bandwidth, in addition to a light weight to avoid any
distress. The bra itself can have some limitations related to
the breast size, and the total number of antennas that can be
placed on it. For this, special care should be also placed on
the design of the antennas.
Concerning the antennas, it is seen that the performance
of the microwave breast imaging system is highly affected
by the design of the antenna element and its structure. In
fact, a superior performance in terms of image quality is
realized with the arrays of larger number of antennas, with
an optimized antenna design. For this, a suitable antenna for
such arrays is required to have a small size, wide impedance
BW, high efficiency to couple the needed power to the
breast, and must be simple to fabricate, cost effective, and
biocompatible.
As a conclusion, for a simplified design, with the flexibility
to add more antennas if needed, while accepting possible
antenna scanning for the optimized location, planar arrays
can be used. If the optimized array configuration and antenna
design are found, with no need to add more antennas, flexible
antennas that can be placed on a bra are best to be used. This
can avoid the discomfort of the immersion gel, the bulkiness
of the hemispherical and enclosed fixed arrays, and the high
cost of fabrication. Further improvements can be made to the
design in [141], [142] by designing a flexible bra for different
breast sizes, or several bras for different sizes. In addition, the
performance could be further enhanced if more antennas are
added. For this, a miniaturized antenna size is needed to fit
large number of antennas on the breast surface.
VII. CONCLUSION
Microwave imaging is taking an important interest as a non-
invasive, highly sensitive, and a safer diagnostic technique,
with a low cost and low illumination power levels. It is
highly promising for breast imaging for cancer detection,
not requiring ionizing radiation nor breast compression. The
antenna system contained in this technology functions as the
main block responsible for transmitting and receiving the
reflections from the breast, needed for image reconstruction.
This paper serves as a comprehensive survey of antennas
proposed for microwave imaging. The different antenna array
configurations, in addition to the different antenna elements
designs and enhancements, presented in the literature for
breast imaging, have been investigated. An evaluation of
all of the investigated designs has been then presented with
concluding remarks. It was found that the antipodal Vivaldi
antenna (AVA) and the monopole antennas, with slots or slits
in their ground plane, are promising as antenna elements to
be used in an array of antennas placed in a hemispherical
configuration. Further improvements to the optimized array
designs of the literature can be made by designing flexible
bras for different breast sizes, where miniaturized antennas
can be used to fit a large number of antennas on the bra, and
hence further enhance the performance and patient experi-
VOLUME 4, 2016 21
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Author et al.: Preparation of Papers for IEEE TRANSACTIONS and JOURNALS
ence.
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This work is licensed under a Creative Commons Attribution 4.0 License. For more information, see https://creativecommons.org/licenses/by/4.0/.
This article has been accepted for publication in a future issue of this journal, but has not been fully edited. Content may change prior to final publication. Citation information: DOI
10.1109/ACCESS.2020.2999053, IEEE Access
Author et al.: Preparation of Papers for IEEE TRANSACTIONS and JOURNALS
HILAL M. EL MISILMANI (S’06–M’15) was
born in Beirut, Lebanon in 1987. He received
the B.E degree in communications and elec-
tronics engineering from Beirut Arab Univer-
sity, Debbieh, Lebanon, in 2010 and the M.E
and Ph.D. degrees in Electrical and Computer
Engineering from the American University of
Beirut, Beirut, Lebanon, in 2012 and 2015 respec-
tively.
From Aug. 2011 to Sept. 2012, he was a
Telecommunications Engineer with Dar Al-Handasah Consultants (Shair
and Partners). From Sept. 2012 to Aug. 2014, he was a Researcher with
Beirut Research and Innovation Center. From Sept. 2014 to May 2015 he
was a Lecturer with the American University of Beirut. From June 2015
to Sept 2016, he was a Research Associate with the American University
of Beirut. Since Sept. 2015, he has been an Assistant Professor with the
Electrical and Computer Engineering Department, Beirut Arab University,
Debbieh, Lebanon. He is the author of more than 20 papers. His research
interests include the design of high power microwave antennas, slotted
waveguide antennas and Vlasov antennas, the design and applications of
antenna arrays, antennas for biomedical applications, and machine learning
in antenna design.
Dr. El Misilmani was a recipient of several scholarships, Rafic Hariri
Foundation Scholarship from Sept. 2005 to June 2010, the Association of
Specialization and Scientific Guidance (SSG) Scholarship from Feb 2006
to June 2010, the Lebanese Association for Scientific Research (LASeR)
scholarship from Sept. 2013 to May 2015, and the National Council for
Scientific Research (CNRS) doctoral scholarship award from 2013 to May
2015.
TAREK NAOUS (S’16) is currently pursuing the
B.E. degree in Communications and Electronics
Engineering at Beirut Arab University (BAU). He
is an undergraduate research assistant at the Radio
Frequency and Antenna Design research team at
BAU and has served as the IEEE student branch
chairperson at the faculty of engineering for two
years. Mr. Naous has previously authored a survey
paper on the use of machine learning in antenna
design and currently has several manuscripts un-
der revision. His research interests include machine learning, and machine
learning in communications.
SALWA K. AL KHATIB (S’17) is currently pur-
suing a B.E in Computer Engineering at Beirut
Arab University (BAU). She is an undergraduate
research assistant at the Radio Frequency & An-
tenna Design research team at BAU and has been
serving as the IEEE student branch chairperson
since Sept. 2019 at the faculty of engineering. Her
research interests include machine learning and its
general applications, and intelligent transportation
systems.
KARIM KABALAN was born in Jbeil, Lebanon.
He received the B.S. degree in Physics from the
Lebanese University in 1979, and the M.S. and
Ph.D. degrees in Electrical and Computer Engi-
neering from Syracuse University, in 1983 and
1985, respectively.
During the 1986 fall semester, he was a visit-
ing assistant professor of Electrical and Computer
Engineering at Syracuse University. Currently, he
is a Professor of Electrical and Computer Engi-
neering with the Electrical and Computer Engineering Department, Faculty
of Engineering and Architecture, American University of Beirut. He is the
author of 2 copyrighted software, 6 book chapters, more than 100 journal
papers, and more than 124 conference papers. His research interest includes
Electromagnetic and Radio Frequency, microstrip antenna design by using
sophisticated patch element and array theoretical modelling techniques,
cognitive radio antenna, and MIMO antenna systems.
26 VOLUME 4, 2016
