Ultra-Wideband Pentagonal Fractal Antenna with Stable Radiation Characteristics for Microwave Imaging Applications

: For microwave imaging applications, a design for an ultra-wideband (UWB) fractal antenna is presented. The antenna design is composed of a pentagonal fractal patch radiator fed by a modiﬁed co-planar waveguide (CPW) plane. It is built on a low-loss Rogers RT/Duroid 5880 dielectric substrate with a dimensions of 24 × 30 × 0.787 mm 3 . According to the measurements, the designed antenna offers a fractional bandwidth of 123.56% ranging from 3 GHz to 12.7 GHz. In addition, a maximum gain of 3.6 dBi is achieved at 8.5 GHz. From the results, it is also observed that the proposed antenna structure attains constant radiation characteristics in the operating bandwidth, which is useful for microwave imaging applications. The time domain analysis of the proposed design is also performed, and it is observed that the designed antenna offers a group delay of ≤ 1.5 ns, which ensures minimum pulse distortion.


Introduction
Near-field microwave imaging has gotten a lot of attention in recent years, especially for detecting and localizing cancer tissues in the human body [1]. Microwave imaging offers promising solutions for a number of biological applications [2] when compared to existing approaches such as ultrasound [3] and mammography [4]. In this technique, short pulses of low power are emitted by antennas towards the human body. The backscattered radiation is then collected by the antennas and processed to form an image. One of the examples is shown in Figure 1. From the figure, one can observe that multiple antennas operating in the near field are placed around the breast. The antennas forming the array are sequentially selected to transmit pulses into the breast. Then, the backscattered signals are collected by the receiving antennas. Finally, a suitable signal processing technique is applied to backscattered signals for image reconstruction. Microwave imaging antennas should have a wide bandwidth and be able to provide constant radiation performance. Furthermore, the antenna's time domain performance should be sufficient to allow transmitted pulses to pass through human tissues with minimal signal distortion. Ultra-wideband (UWB) technology may be useful in meeting these criteria as the pulse duration in UWB systems is often on the scale of a few nanoseconds, resulting in a bandwidth of a few gigahertz. In addition, the antenna size should be small enough so that it can easily be integrated into microwave imaging systems. To accommodate this requirement, researchers presented a variety of antenna designs. From the presented designs, planar antennas received a lot of attention due to their small size and light weight.
For UWB microwave imaging systems, the authors in [6] presented a modified square monopole antenna design. In the ground plane, a pair of loop sleeves was designed, which tend to achieve an impedance bandwidth in the frequency range of 3.19-11.03 GHz. Although the designed antenna offers a wide impedance bandwidth, its efficiency is decreasing with an increase in frequency. For body-centric imaging applications, Danjuma et al. [7] designed a co-planar waveguide (CPW)-fed quasi-cross slot-based circular monopole antenna. The presented antenna can operate at frequencies ranging from 3 GHz to more than 11 GHz. A Hibiscus petal-like patch antenna was presented in [8] for microwave imaging systems. To obtain an impedance bandwidth in the frequency range of 3.04-11 GHz, a tapered microstrip feed line and a partial trapezoid ground plane were used. The performance of the antenna was evaluated in the time domain and a high value of group delay was observed for the side-by-side configuration, which is not useful for microwave imaging applications. In [9], an octagonal planar antenna was designed to detect malignant tissues in the human body. To achieve UWB response, the radiating element was fed by a 50 Ω microstrip feeding line and a step impedance transformer. For improved radiation characteristics, a circular slot including a symmetrical metal cross was designed in the octagonal radiator. From the presented configuration, the authors achieved an impedance bandwidth of 12 GHz from 3 GHz to 15 GHz.
Lee et al. [10] introduced a UWB-resistive dipole antenna for microwave imaging applications. They demonstrated that the designed dipole antenna offers minimal internal reflections compared to other UWB antenna structures. They employed chip resistors on both arms of the dipole antenna according to the technique presented in [11], and observed a UWB response ranging from 1 GHz to 10 GHz. However, this kind of configuration led to a large antenna size and was complex in nature. A square fractal monopole antenna with a slotted partial ground plane was presented in [12] for imaging purposes. The designed antenna-measured results demonstrate that it offers a fractional bandwidth (FBW) of 117.88% from 3.1 GHz to 12 GHz. A modified square monopole antenna design was presented in [13] for medical applications. With the use of modified square patch and inverted U-shaped parasitic elements, the authors achieved a 5.2 GHz impedance bandwidth from 3.8 GHz to 9 GHz. Another square monopole antenna with a T-slot-loaded ground plane was designed in [14] for UWB microwave imaging applications. In addition to the T-slot, a parasitic element with a π shape was placed beneath the microstrip line to achieve a UWB response in the frequency range of 2.91-14.72 GHz.
Ojaroudi and Ghadimi [15] utilized the same technique as presented in [14] to design a multi-resonance UWB antenna for microwave imaging applications. In this design, an E-shaped slot was etched in the ground plane, and an E-shaped parasitic structure was designed behind the microstrip feeding line. The presented configuration extends the upper-frequency limit of the antenna from 10.3 GHz to 15 GHz. The designs presented in [14,15] were able to provide a UWB response, but their radiation characteristics were not stable over the operating bandwidth. In [16], three T-shaped slots were etched in the ground plane to achieve resonance ranging from 2.96 GHz to 15.8 GHz. Lasemi and Atlasbaf [17] designed compact UWB fractal antennas for breast tumor detection. The Sierpinski gasket configuration was used for one antenna, and a ring-inscribed hexagonal fractal antenna was used for the other. According to the measurements, the designed antennas operate well from 2.95 GHz to 12 GHz. However, the time domain performance of the presented designs reveals that the transmission phase of the antennas was not stable over the operating bandwidth, which ultimately led to pulse distortion.
A negative index mematerial-based planar antenna was designed in [18] for microwave imaging applications. To achieve a compact size, the edges of the triangular patch were loaded with four left-handed metamaterial unit cells. To achieve negative permittivity and permeability, each metamaterial unit cell was composed of a modified split-ring resonator (SRR) and a capacitance-loaded strip. Measurement results show that the presented antenna operates well from 3.4 GHz to 12.5 GHz and offers a maximum gain of 5.16 dBi at 10.15 GHz. The same technique was utilized in [19] to design a compact planar metamaterial antenna. In this design, a trapezoidal patch radiator was used, which was fed through a stepped tapered microstrip feed line. The presented antenna was able to operate in the frequency range of 4.23-14 GHz. In [20], a wearable textile antenna with a full ground plane was designed for breast cancer detection. Photonic bandgap (PBG) and substrate-integrated waveguide (SIW) technology were utilized for antenna design. The antenna was fed using a grounded CPW (GCPW) through a SIW transition. The results demonstrate that the antenna offered an impedance bandwidth of 21 GHz from 7 GHz to 28 GHz. The presented antenna operates at higher frequencies that are not useful for biomedical applications. In addition, the use of PBG and SIW technologies offers complexities in the fabrication process.
In this article, a CPW-fed pentagonal fractal antenna is designed for near-field microwave imaging applications, especially for breast cancer detection. The results demonstrate that the antenna ranges from 3 GHz to 12.7 GHz, with a peak gain of 3.6 dBi at 8.5 GHz. Furthermore, from the time domain analysis, it is noted that a group delay for both face-to-face and side-by-side configurations is ≤ 1.5 ns, ensuring minimum pulse distortion, which is a fundamental criterion of near-field microwave imaging systems. To further demonstrate the suitability of the proposed antenna, a comparison is presented among proposed and previously published UWB antennas meant for microwave imaging applications (see Table 1).
From Table 1, it can be noted that all the antennas have fractional bandwidth >100%. The designs presented in [9,10,20] have maximum and approximately equal fractional bandwidth, but their electrical dimensions are large in comparison to the proposed design. The rest of the antennas exhibit low fractional bandwidth compared to the proposed antenna design. Furthermore, the proposed fractal antenna offers a high impedance bandwidth compared to the designs presented in [6][7][8]10,12,17,18].  Figure 2 illustrates the geometry of the proposed UWB antenna. The proposed design is composed of a modified CPW-fed pentagonal fractal radiator, printed on a low-loss Rogers RT/Duroid 5880 dielectric substrate with a relative permittivity (ε r ) of 2.2. The thickness of the dielectric substrate for the antenna design is chosen to be 0.787 mm. The overall dimensions of the antenna are 24 × 30 mm 2 . From Figure 2, one can note that a trapezoidal ground plane is used for better impedance matching in the band of interest. Furthermore, the CPW feed technique is utilized because it is easy to design and fabricate and it provides low losses [21]. The rest of the design parameters' values are as follows: W S = 24 mm, L S = 30 mm, W f = 2.5 mm, L f = 12 mm, t = 1.5 mm, g = 0.3 mm, L g = 5.5 mm, W g = 4.2 mm, R b = 3 mm, R e = 1 mm, S 1 = 9.6 mm, S 2 = 7.6 mm, S 3 = 5.4 mm, S 4 = 3 mm, R 1 = 7 mm, R 2 = 5 mm, and R 3 = 2.8 mm. The design steps of the proposed CPW-fed pentagonal fractal antenna are shown in Figure 3a-d. In the first step, a CPW-fed circle-inscribed pentagonal radiator is designed, as shown in Figure 3a. The pentagonal radiator's dimensions are calculated using the following expression [22].

Pentagonal Fractal Antenna Design
where A denotes the radius of the of the pentagonal radiator, which is equal to 9.6 mm. The parameters f l and k in Equation (1) correspond to the lowest operating frequency of the antenna and the effective relative permittivity (ε re f f ) of the dielectric substrate, respectively. The antenna design shown in Figure 3a is simulated in the CST Microwave Studio and the respective reflection coefficient (S 11 ) is given in Figure 4. From the figure, it is observed that the design shown in Figure 3a provides a dual-band response at 4 GHz and around 12 GHz. In the second step, another circle-inscribed pentagonal patch is added in the Step-1 design, as shown in Figure 3b. This modification tends to excite higher order modes and ultimately leads to a UWB response in the band of interest (see Figure 4, Step-2). For improved matching, another pentagonal radiator is added in the Step-2 design, as depicted in Figure 3c. This design offers improved impedance matching in the band of interest, as shown in Figure 4. To further optimize the response, another pentagonal patch is designed with the Step-3 design, as shown in Figure 3d. In this step, the ground plane shape is changed to a trapezoidal shape (see Figure 3d). This modification led to a UWB response in the frequency range of 3.38-12.89 GHz, as shown in Figure 4. The simulated surface current distribution plots for the proposed antenna are shown in Figure 5. The surface current is plotted for three different frequencies, i.e., 4 GHz, 8 GHz, and 12 GHz. It is evident from the figure that at 4 GHz, the dense current is distributed on the feed line, on the edges of the ground plane, and in the lower portion of the patch radiator, as shown in Figure 5a. For higher frequencies, the current is uniformly distributed on the surface of the radiator and modified the CPW ground plane (see Figure 5b  A parametric study was performed to understand the behavior of the proposed UWB antenna by varying the gap between the microstrip feeding line and the ground plane, denoted as g, and the gap between the ground plane and the main radiator, denoted as t. From Figure 6a, it is observed that when the value of g is increased from 0.3 mm to 0.9 mm, the bandwidth of the antenna decreases. The maximum impedance matching is obtained for g = 0.3 mm (see Figure 6a). On the other hand, when the value of t is changed from 0.5 mm to 1.5 mm, the impedance bandwidth of the antenna improves, as shown in Figure 6b. For t >1.5 mm, the bandwidth of the antenna is reduced (see Figure 6b).

Fabricated Prototype and Measurements
To verify the simulation results, the designed antenna was fabricated using the miling process. The prototype of the fabricated antenna is shown in Figure 7a. The S 11 of the proposed antenna was measured by Rohde Schwarz (R&S) using the vector network analyzer (VNA) ZVA-67. The simulated and measured S 11 characteristics are shown in Figure 7b. The antenna has a computed impedance bandwidth of 9.51 GHz in the frequency range of 3.38-12.89 GHz and a measured impedance bandwidth of 9.7 GHz in the frequency range of 3-12.7 GHz (see Figure 7b). Some discrepancies are observed between the simulated and measured results, which could arise due to fabrication tolerances, SMA connector losses, and the scattering measurement environment. The simulated and measured radiation characteristics of the proposed fractal antenna are shown in Figure 8. For both the YZ and XZ planes, radiation characteristics are observed at three different frequencies: 4 GHz, 8 GHz, and 12 GHz. From the figure, it is observed that the designed antenna exhibits bi-directional radiation properties for the YZ plane and omnidirectional characteristics for the XZ-plane. It is also observed from Figure 8 that the radiation patterns are almost constant for the entire operating bandwidth, which is a useful property for microwave imaging applications.  Figure 9 shows the realized gain and efficiency of the proposed antenna. The peak gain of the antenna is noted to be 3.6 dBi at 8.5 GHz, while the minimum gain is 0.6 dBi at 3 GHz, as shown in Figure 9. Furthermore, the efficiency of the antenna is noted to be >80% for the entire operating bandwidth (see Figure 9).

Time Domain Analysis
For near-field microwave imaging applications, it is necessary to assess the performance of the antenna in the time domain [23]. For this purpose, two identical antennas are placed 30 cm apart from each other in two different configurations, i.e., face to face and side by side, respectively, as shown in Figure 10a,b, in which both the antennas work in the transceiver mode. To excite the antennas, a Gaussian pulse, centered at 8 GHz with a frequency band of 1-15 GHz, is utilized. Figure 11a shows the normalized amplitudes of both input and output signals for both configurations. The cross-correlation between transmitted and received signals is calculated using Equation (3), which is known as the fidelity factor (FF) [24].
where Y t (t) and Y r (t) represent transmitted and received signals, respectively; and τ represents the group delay. The FF values for face-to-face and side-by-side configurations are noted to be 96.85% and 88.52%, respectively. The high value of FF for the face-to-face configuration shows that the transmitted signal will be less distorted.
The group delay for both the configurations, shown in Figure 10, is presented in Figure 11b. The group delay is defined as the negative rate of change of the transfer function phase, φ(ω) with respect to frequency. The group delay quantifies the transition time taken by the signal to travel through a device [23]. Mathematically, it can be calculated as: For UWB applications, |τ| ≤ 2 ns is desirable to ensure linearity of the phase in the far-field region. From Figure 11b, one can observe that the |τ| value is ≤ 1.5 ns for the entire frequency range [25].

Conclusions
A compact UWB planar antenna is reported for near-field microwave imaging applications. The radiator geometry consists of a modified CPW-fed pentagonal fractal structure. To improve the impedance matching, a trapezoidal-shaped ground plane is utilized. The proposed design has an impedance bandwidth of 9.7 GHz, ranging from 3 GHz to 12.7 GHz, and offers a fractional bandwidth of 123.56%. It is also observed from the results that the proposed antenna exhibits constant radiation characteristics in the operating bandwidth, which is useful for near-field microwave imaging. The performance of the proposed antenna is also assessed in the time domain, and acceptable characteristics are observed.