A Novel Corner Etched Rectangular Shaped Ultrawideband Antenna Loaded with Truncated Ground Plane for Microwave Imaging

Microwave imaging has become a popular research issue in recent decades as a result of its several benefits over traditional imaging technologies. To analyze biological tissues in depth, a microwave imaging instrument is employed. The test determines the presence and location of morphological alterations in specific biological tissues. Ultrawideband (UWB) microwave imaging is a new technique that produces improved results while avoiding the use of ionizing radiation. Antennas play a critical part in these systems, and as a result, antenna optimization has become a hot topic due to the device's proximity to the human body. Recent research has revealed a number of initiatives to improve the electromagnetic sensors employed in these systems, whether as single or array components. In this article, the development of a compact ultrawideband (UWB) antenna functioning in the frequency range of 3.25–14.63 GHz is proposed. The reported antenna comprises of a rectangular patch and a modified ground plane "etched on an FR4" substrate excited by a 50 Ω feed line. The overall dimension of the antenna is 0.44λ0×0.38λ0×0.02λ0\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$0.44 {\lambda }_{0}\times 0.38 {\lambda }_{0}\times 0.02{\lambda }_{0}$$\end{document}. The proposed antenna has been fabricated on FR-4 substrate. Measured and simulated results are in good agreement. The proposed antenna is compact and displays good radiation characteristics along with good gain.


Introduction
In recent years, UWB technology has gained significant interest from researchers throughout the globe. Ultrawideband is a radio technology that can communicate over a vast chunk of the radio spectrum using very low energy levels. In February 2002, the Federal Communication Commission (FCC) released frequencies of 3.1 GHz to 10.6 GHz for commercial applications [1]. The design of a UWB antenna requires the antenna to be small 1 3 and compact, it should have wide bandwidth performance, the radiation pattern has to be Omnidirectional, and the antenna should be easy to fabricate [2,3].
The benefits of an ultra-wideband antenna are its low complexity and it is inexpensive to manufacture. UWB patch antenna [4][5][6][7][8] can be designed with various geometries such as circular, triangular, square, or rectangular. UWB technology can be optimized using various algorithms such as genetic algorithm [9][10][11] and optimization algorithm [12]. Antenna optimization creates advanced, complicated electromagnetic devices with competitive performance, serviceability, and cost. This requires choosing to compete for objective functions, design variables, parameters, and restrictions [12]. UWB technology is applied in several fields such as microwave imaging and radar systems. Microwave imaging utilizes EM waves to see through the inner structure of a tissue. It recognizes tumors based on dielectric properties, which are different compared to normal tissue [13,14]. X-rays, ultrasound, and MRI scans are used in the early detection of cancerous cells [15]. However, these methods pose some disadvantages. X-ray mammography is agonizing for patients, and long exposure to X-ray may cause healthy cells to become cancerous [16]. Another technique used to detect cancerous cells is ultrasound, but this technology is unable to detect deeply buried tumors and it is comparatively expensive [17]. To overcome these limitations, microwave imaging employs low ionizing radiation and is cost-effective compared to other methods [18]. Microwave imaging incorporates the use of UWB pulses which range from "low to high" frequencies, in order to generate images of human tissues [19]. The higher frequency band is used to create high-resolution images of the tissue, whereas the lower frequency band is used to detect deeply buried tumors [20]. A microwave imaging system has two parts, a "front end," which requires the antenna to be inexpensive and compact and it should operate efficiently, whereas the second part requires analyzing the performance of the antenna to detect the tumor and generate the image using the various processing techniques. The antenna needs to be compact because it is to be incorporated into portable microwave imaging systems. This paper overviews the design and optimization of a novel microwave antenna that must serve as an element in a sensor array for early detection. The proposed microstrip patch antenna in this work displays good UWB characteristics and also displays good gain characteristics. The antenna consists of a modified square patch and a ground plane built on an FR4 substrate. Parametric analysis is done on the proposed antenna to observe the effect of various design parameters on the antenna performance. Further, this paper also reports the current density, gain, and radiation pattern simulations.

Related Work
This literature reviews various UWB antennas designed to be used in microwave imaging systems. In the work proposed by Zhang et al. [21], the designed microstrip antenna had low gain, which was less than 5 dB and had low radiation performance, In order to overcome these limitations, several methods were proposed by researchers but it would increase the complexity in designing the proposed antenna for this work. Similarly, Abak et al. [22], designed a cavity back Vivaldi antenna, which resulted in a compact-sized antenna but displayed low gain characteristics. In order to improve gain characteristics, He et al. [23] proposed to fabricate the antenna with planar directors. However, in doing so, it increased the size of the antenna significantly. M. T Islam et al. [24] proposed a low-cost UWB antenna, but its dimensions were comparatively larger dimension 51x42 mm 2 and also displayed low bandwidth. The antenna designs in [25][26][27] have limitations in size, gain, and bandwidth and the proposed antenna in this report aims to overcome these limitations. A rectangular microstrip patch antenna is developed to function in the UWB frequency range. The detailed analysis of UWB is discussed in the coming sections.

Design of Ultrawideband Antenna
The primary purpose of this work is to develop an inexpensive antenna that operates in the UWB frequency range with optimal performance to be used for microwave imaging applications. To obtain this objective, the evolution of the design stages is depicted in Fig. 1.
The S 11 plot of the evolution stages is depicted in Fig. 2. The rectangular microstrip patch antenna proposed is formed on an FR4 substrate with ε r = 4.4. The dimension of the rectangular patch (in mm) is 14.5 × 15 . The results obtained for Fig. 1a were substandard, and to achieve better S 11 characteristics, the ground plane was modified. The dimensions of the modified ground plane (in mm) are 12.5 × 30 . To improve the "impedance matching" of the antenna, circular corner cuts are made on the top portion of the ground as illustrated in Fig. 1c and to the top and bottom sections of the "rectangular patch" as shown in Fig. 1d. The corner cuts at the lower end of the rectangular patch increase the space between the ground and patch, which tunes the capacitive coupling between them; further adding corner cuts to the top portion of the patch tunes the inductive coupling capacity of the antenna. It cancels out the capacitive coupling between the patch and the ground resulting in "resistive input impedance." In contrast, the slots in the ground plane neutralize the "capacitive" effects by the inductive nature of the patch to get purely resistive input impedance." This proposed antenna depicted in Fig. 3 exhibits good UWB characteristics with S11 < −10 dB and displays good bandwidth characteristics ranging from 3.25 to 14.63 GHz, as represented in Fig. 2. The dimensions for the "proposed antenna" are given in Table 1.
Where Lsub, Wsub, Hsub are the substrate's length, width and height, respectively, L,W, are the length and width of the radiating patch. Lf, Wf are the length and width of the feedline. Ra, Rb, Rc, and Rd are the radius of the corner cuts on the radiating patch    whereas Re, Rf the radius of the corner cuts on the truncated ground plane. Wx, L1, and L2 are the width and height of truncation.

Analysis of Evolution Stages
In the initial design, the S 11 value was significantly greater than −10 dB. Hence the ground plane was modified. As a result, the S 11 value reduces. To enhance the result, a circular slot is etched on the patch. The corner cuts on the patch significantly bring the S 11 value below −10 dB, which is depicted in the plot shown in Fig 4.

Effect of Corner Cuts on Rectangular Patch
To analyze the effect of corner cuts on the rectangular patch, each corner cut was simulated individually. The addition of each corner cut is illustrated in Fig. 5. It can be noticed from Fig. 6 that with the addition of each cut, the S 11 value decreases even more than the −10 dB mark. Figure 7 illustrates the development stage in modifying the ground plane. The S 11 characteristics were plotted for each stage of development. It was observed that the addition of corner cuts and modification to the ground plane increased the space between the rectangular patch and the ground, thereby canceling out the inductive and capacitive effects. The proposed antenna displays good S 11 < −10 dB bandwidth and impedance matching. The S 11 characteristics are plotted and are shown in Fig. 8.

Effect of Corner Cuts on Rectangular Patch
To analyze the effects of corner cuts on the rectangular patch, the addition of each corner cut was simulated and analyzed. The S 11 characteristics are represented in Fig. 9. It can be  observed that when the radius is 1.5 mm, the bandwidth of the antenna is reduced. For a radius of 2.5 mm, S 11 values are away from the −10 dB mark, but the result is optimum when the radius is 2 mm.

Effect of Corner Cuts on the Ground Plane
Optimum results were obtained for a radius of 2.5 mm on the corner cuts. Both the corner cuts on the ground plane were varied. They were simulated and analyzed for values of 2 mm, 2.5 mm, and 3 mm. The results obtained were plotted on the graph, and the S11 characteristics displayed optimal results at a 2.5 mm radius, as shown in Fig. 10.

Effect of Center Circular Cut on Rectangular Patch
To increase the space between the ground and the rectangular patch to cancel out the capacitive and inductive effects to produce pure resistive input, the circular cut at the center of the circle (Re) was increased from 1 to 2 mm in increments of 0.5 mm. It is  observed that the optimal results were obtained when Re equals 1.5 mm. The plot is depicted in Fig. 11.

Effect of Center Ground Cut
The rectangular cut in the center of the ground plane was simulated and analyzed at 2.5 mm, 3 mm, and 3.5 mm. For all three values, the S 11 value was significantly less than −10 dB, but the best results were seen for a width of 3 mm, which can be observed in Fig. 12.

Effect of the Width of Transmission Line
The width of the transmission line was simulated and analyzed at 2.5 mm, 2.85 mm, and 3 mm. Better results were obtained for a width of 2.5 mm but at the cost of bandwidth. Hence, for optimal results, 2.85 mm is selected as the width of the transmission line. The result can be observed in Fig. 13.

Current Distribution
The current distribution is simulated and analyzed, as shown in Fig. 14

Result of UWB Antenna
The suggested antenna was designed and manufactured using the FEM approach on the HFSSv.13.0 simulator. The suggested UWB antenna is shown in Fig. 15.

Simulated and Measured S 11
The comparison of the models and observations of S 11 is shown in Fig. 16. The ROHDE & SCHWARZ ZVL network analyzer was used to determine S 11 . At the S 11 < −10 dB level, the simulated impedance bandwidth is 3.25-14.63 GHz. At the S 11 < −10 dB level, the measured operating impedance bandwidth is 3.1-13.9 GHz. There is a little disparity between S 11 simulations and measurements.. It's because to soldering, failing to account

Gain
The "gain of the Ultrawideband antenna is shown in Fig. 17. It is observed that the gain ranges from 2.98 to 9.68 dB. Under simulation, the antenna displays a minimum gain of 2.98 dB at 3.5 Hz and 9.68 dB at 14 GHz. Figure 18 shows the 3D polar gain of proposed antenna.

Radiation Pattern
The radiation pattern for the antenna is illustrated in Fig. 19 for different frequencies. From  Fig. 19, it is observed at 3.5 GHz, the pattern is directional. The radiation pattern for 5.2 and 5.8 GHz is directional. At 8 GHz, the pattern observed is bi-directional). The reported antenna exhibits a "bidirectional pattern in the E-plane" and an "omnidirectional pattern in the H-plane" at frequencies below 5.8 GHz. At frequencies greater than 5.8 GHz, the radiation pattern distorts. These distortions can be attributed to higher-order modes. A comparative analysis is done to know how improved the proposed antenna is compared to the current literature available, as shown in Table 2.

Conclusion
In order to facilitate microwave imaging, a rectangular microstrip patch antenna designed to operate in the UWB frequency band has been developed. Improvements are made to the antenna's ground and patch to boost its performance. The antenna also displays good frequency bandwidth from 3.25 to 14.63 GHz. Parametric analysis on the antenna shows that a slight change in the dimensions can affect the UWB characteristics. It shows a good gain from 2.98 to 9.68 dB. The proposed antenna is compact and displays good radiation characteristics while being cost-effective.   [30] 0.17 0 × 0.15 0 × 0.008 0 2.7-13.8 11.1 [31] 0.54 0 × 0.54 0 × 0.02 0 3.4-9.0 5.6 [32] 0.99 0 × 0.99 0 × 0.02 0 3.1-9.6 6.5 [33] 0 He has published more than 115 papers in reputed web of science (SCI) and scopus indexed journal and conferences and also has filled two Indian patents. He has more than 1000 citations with an h-index of 19 and i-10 index of 28. Presently he is guiding 4 regular PhD students and 5 part-time PhD students and is heading the antenna research group at MIT, Manipal. In 2020 he has received best Ph.D. thesis award from Board of IT Education (BITES), Karnataka. He is on the board of reviewers of journals, like the IEEE TRANSACTIONS ON ANTENNAS AND PROPAGATION, IEEE ANTENNAS AND