1 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 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.

1.1 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 mm2 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.

2 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.

Fig. 1
figure 1

Evolution stage of UWB antenna a Initial antenna design, b reduced ground plane, c modified rectangular patch, d proposed antenna

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The S11 plot of the evolution stages is depicted in Fig. 2. The rectangular microstrip patch antenna proposed is formed on an FR4 substrate with εr = 4.4. The dimension of the rectangular patch (in mm) is \(14.5\times 15\). The results obtained for Fig. 1a were substandard, and to achieve better S11 characteristics, the ground plane was modified. The dimensions of the modified ground plane (in mm) are \(12.5\times 30\). 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."

Fig. 2
figure 2

Reflection coefficient (S11) of the proposed antenna

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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.

Fig. 3
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Proposed antenna

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Table 1 Dimension of the proposed antenna
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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.

3 Design Analysis of the Ultrawideband Antenna

3.1 Analysis of Evolution Stages

In the initial design, the S11 value was significantly greater than −10 dB. Hence the ground plane was modified. As a result, the S11 value reduces. To enhance the result, a circular slot is etched on the patch. The corner cuts on the patch significantly bring the S11 value below −10 dB, which is depicted in the plot shown in Fig 4.

Fig. 4
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S11 for evolution stages of UWB Antenna

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3.2 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 S11 value decreases even more than the −10 dB mark.

Fig. 5
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Design of corner cut on the Rectangular patch

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Fig. 6
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S11 variation for various corner cuts on rectangular patch

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3.3 Effect of Truncation on the Ground Plane

Figure 7 illustrates the development stage in modifying the ground plane. The S11 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 S11 < −10 dB bandwidth and impedance matching. The S11 characteristics are plotted and are shown in Fig. 8.

Fig. 7
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Design for truncation on the ground plane

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Fig. 8
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Variation for various steps of truncation on the ground plane

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3.4 Parametric Analysis

3.4.1 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 S11 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, S11 values are away from the −10 dB mark, but the result is optimum when the radius is 2 mm.

Fig. 9
figure 9

Parametric Analysis for corner cuts on rectangular patch

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3.4.2 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.

Fig. 10
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Parametric Analysis for corner cuts on the ground plane

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3.4.3 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.

Fig. 11
figure 11

Parametric Analysis for the center circular cut on rectangular patch

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3.4.4 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 S11 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.

Fig. 12
figure 12

Parametric analysis for the ground slot

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3.4.5 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.

Fig. 13
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Parametric analysis for the width of transmission line

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3.5 Current Distribution

The current distribution is simulated and analyzed, as shown in Fig. 14, to realize the operation of the proposed UWB antenna. The resonance characteristics are analyzed at the 3.5, 5.2, 5.8, 8.0, 10.0, and 12.0 GHz frequencies. It is observed that at 5.2 GHz, the entire patch and the ground plane are conducting with a value of 1 Amp/m. The maximum current is located in the lower portion of the patch and the feed. Similar results are seen at the other frequencies. It can be observed from Fig. 14 that different parts of the antenna have different resonance paths for different operating modes.

Fig. 14
figure 14

Surface current distribution of the proposed antenna at a 3.5 GHz, b 5.2 GHz, c 5.8 GHz, d 8.0 GHz, e 10.0 GHz, f 12.0 GHz

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4 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.

Fig. 15
figure 15

Fabricated proposed UWB antenna a Top part, b bottom part

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4.1 Simulated and Measured S 11

The comparison of the models and observations of S11 is shown in Fig. 16. The ROHDE & SCHWARZ ZVL network analyzer was used to determine S11. At the S11 < −10 dB level, the simulated impedance bandwidth is 3.25–14.63 GHz. At the S11 < −10 dB level, the measured operating impedance bandwidth is 3.1–13.9 GHz. There is a little disparity between S11 simulations and measurements.. It's because to soldering, failing to account for the SMA connector during simulation, errors in the substrate's dielectric constant, and fabrication tolerances.

Fig. 16
figure 16

Simulated versus Measured S11

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4.2 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.

Fig. 17
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Gain of the antenna

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Fig. 18
figure 18

3D polar gain of the proposed antenna at a 3.5 GHz, b 5.2 GHz, c 5.8 GHz, d 8.0 GHz, e 10.0 GHz, f 12.0 GHz

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4.3 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.

Fig. 19
figure 19

Radiation Pattern at a 3.5 GHz, b 5.2 GHz, c 5.8 GHz, d 8.0 GHz, e 10.0 GHz, f 12.0 GHz

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A comparative analysis is done to know how improved the proposed antenna is compared to the current literature available, as shown in Table 2.

Table 2 Comparison of the proposed antenna with previous works
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5 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.