1. Introduction

The study of microwave imaging in the health sector is a major research topic among researchers because of its non-ionizing and non-invasive properties ^[1]. The cost-effective directional antenna is used in microwave imaging ^[2]. This antenna also provides low complexity, low spectral power density, and high data rates over the current medical imaging techniques, such as X-ray mammography, magnetic resonance imaging (MRI), echography, and PET scans. Other significant characteristics of the Vivaldi antennas are large bandwidth, low cross-polarization, and highly focused radiation in a particular direction ^[3]. In a microwave imaging system, the antenna requires a microwave transceiver and a signal processing unit to provide a non-destructive, balanced, and high-resolution image ^[4]. The term non-ionizing means that the radiating antenna does not produce ions or extract electrons from molecules ^[5]. Microwaves, ultraviolet, infrared, and laser light are called electromagnetic radiation ^[6]. For microwave imaging, the antenna needs to be an ultra-wideband (UWB) with a compact size. Generally, the bandwidth is greater than 0.25 GHz ^[7].

The Vivaldi antenna has the advantages of high gain, ultra-wideband, compact size, and ease to manufacture. This makes the UWB Vivaldi antenna one of the top choices for microwave imaging applications. In 1979, Gibson introduced the first Vivaldi aerial ^[8]. Theoretically, the Vivaldi antenna has an infinite bandwidth because of the family of end-fire traveling wave antennas. Generally, a large-sized Vivaldi antenna is required to perform well in the low-frequency band. According to the theory, the width of the Vivaldi antenna is at least one half-wavelength for successful radiation to occur ^[9-11]. Vivaldi antennas can be categorized as tapered slot, antipodal, and balanced antipodal Vivaldi antenna ^[12,13].

Some distinct features of the proposed UWB-slotted Vivaldi antenna can be presented as follows:

· The designed and analyzed Vivaldi antenna with slots and resonant cavities makes it compact in size (50${\times}$50${\times}$1.5 mm$^{3}$) without compromising the performance for microwave imaging applications.

· The proposed slotted Vivaldi antenna has an ultra-wide operating band ranging from 3.6 - 13.6 GHz.

· The antenna shows an excellent profile of reflection coefficient, VSWR, gain, and efficiency over the ultra-wide operating band.

· The recommended Vivaldi antenna is a strong candidate for microwave imaging applications after examining all the radiation properties.

The remainder of the paper is organized as follows. Some relevant research is briefly discussed in Section 2. Section 3 describes the structure of the proposed UWB compact Vivaldi Antenna with design evolution. Section 4 reports the performance evaluation and discusses the proposed compact Vivaldi antenna. Finally, some concluding remarks are reported in section 5.

2. Related Work

Although many related studies have been performed, there is scope for improving the performance, including size reduction. Previous studies used different techniques to obtain ultra-wide bandwidth and gain enhancement. A Vivaldi antenna with a dimension of 40.6${\times}$57${\times}$1.6 mm$^{3}$ was proposed for MI applications ^[3]. The antenna covered a 6 GHz working bandwidth with a low gain. A U-slotted 55${\times}$65${\times}$1.6 mm$^{3}$ Vivaldi antenna and confocal radar-based MI algorithm were proposed ^[4] for breast cancer detection. Compared to the conventional antenna, the U-shaped slot provided enhancements of 35.28% in return loss, 2.7% in bandwidth, and 10% in gain. A 95${\times}$130${\times}$0.813 mm$^{3}$ double-slotted Vivaldi antenna was proposed for microwave imaging with corrugation and director ^[14]. The double slot, corrugated slot, and semicircle director enhanced antenna gain. Finally, a maximum gain of 13.3 dB was obtained, and a higher frequency resulted in a higher gain. The authors in ^[15] proposed a radar-based method to enhance the properties of an Antipodal Vivaldi for MI applications. The desired performance was obtained by modifying the structure of the exponentially tapered slot and adding some corrugated slots in the flares. The volume of the antenna was 110${\times}$98${\times}$1.6 mm$^{3}$, and it covered a wide bandwidth of 1.45-9.82 GHz. A UWB periodic elliptical slot-based antipodal Vivaldi antenna was proposed for MI applications ^[16]. A triangular-shaped dielectric lens has been used to enhance the gain, radiation pattern, and directivity, where the pick gain was 10.6 dB and covered a bandwidth of 3-10 GHz. Another slotted UWB 42.8${\times}$57.3${\times}$1.6 mm$^{3}$ antipodal Vivaldi antenna for MI application was proposed ^[17], where the slots on the flares increased the maximum gain and directivity up to 73.65% with 7.64 dB and 8.92 dBi, respectively. A square-shaped dielectric lens was used before the antenna to enhance the gain and bandwidth of a UWB 75${\times}$140 mm$^{2}$ antipodal Vivaldi antenna. With a dielectric lens, the antenna covers a working bandwidth of 10.2 GHz ^[18]. A side-slotted UWB antenna was proposed for breast cancer detection based on the difference in dielectric constant between normal tissue and tumor tissue at microwave frequencies ^[19]. The antenna dimension (75${\times}$88${\times}$1.57 mm$^{3}$) is high and covers a frequency range of 1.8-2.4 GHz. The bandwidth was improved using corrugated slots on the antenna flares.

The performance of the antenna decays as the antenna size decreases. Therefore, reducing the size without compromising the antenna performance is challenging. The main focus is to design such a compact size antenna with acceptable performance for microwave imaging applications. After introducing tapered slots and resonant cavities on the flares and radial stub at the feeder, the gain, directivity, bandwidth, VSWR, and efficiency were enhanced compared to a simple Vivaldi antenna. All the techniques above help achieve the large working band of 10 GHz, ranging from 3.6-13.6 GHz, maintaining a compact size, which is the main achievement.

3. Structure of the Proposed UWB Slotted Vivaldi Antenna

This section describes step by step the design process of a microstrip line feed tapered slotted and cavity resonator-based UWB Vivaldi antenna for microwave imaging applications. The entire design layout has been narrated in three steps, namely a simple Vivaldi antenna (step-1), a slotted Vivaldi antenna (step-2), and a resonant cavity-based slotted Vivaldi with a radial stub (step-3). The substrate material, dielectric constant, tangent loss, and substrate thickness were FR4 (lossy), 4.6, 0.02, and 1.5 mm, respectively. The area (L${\times}$W) of the Vivaldi antenna was 50${\times}$50 mm$^{2}$. In the first step, a simple Vivaldi antenna was designed. In the second step, two exponential tapered slots were introduced uniformly on both flares, and a resonant cavity was designed. The microstrip line feed was used in step-1 and step-2. In the final step, two resonant cavities were used with the tapered slots, and a radial stub was used with the microstrip line feed. After optimizing the design parameters of the antenna, the desired outcome was obtained. In this design, resonant cavities, tapered slots, and radial stub were used, which helped obtain good radiation properties with an ultra-wideband and high gain. The position of the microstrip line feed plays a vital role in obtaining good radiation properties for microwave imaging applications.

3.1 Simple Vivaldi Antenna (Step-1)

First, a model of a simple microstrip line feed Vivaldi antenna was produced using computer simulation technology (CST), as shown in Fig. 1(a). Copper was used for all the metallic layers whose thickness was 0.035 mm. A microstrip line feeding technique was used to connect the two exponential tapered flares. The following formulae (1-3) can be used to define the exponential equation for the proposed Vivaldi antenna ^[8].

(1)

$\begin{aligned} X&=C_{1}e^{rT}+C_{2}\end{aligned} $

(2)

$\begin{aligned} C_{1}&=\frac{X_{2}-X_{1}}{e^{r{T_{2}}}-e^{r{T_{1}}}}\end{aligned} $

(3)

$\begin{aligned} C_{2}&=\frac{X_{2}e^{r{T_{2}}}-X_{1}e^{r{T_{1}}}}{e^{r{T_{2}}}-e^{r{T_{1}}}}\end{aligned} $

The taper rate r (0.13) was obtained from the above equation, indicating the beam width of the antenna. The bandwidth was enhanced by lengthening the taper. Here, the points $(X_{1},\,\,T_{1})$ and $(X_{2},\,\,T_{2})$ are the endpoints of the flare. Table 1 lists the parameters of the designed simple Vivaldi antenna.

Fig. 1. Design evolution of the Vivaldi antenna.

Table 1. Parameters for Simple Vivaldi Antenna (Step-1).

Parameters	Values (mm)
Length of the antenna (L)	50
Width of the antenna (W)	50
Height of the substrate (h)	1.5
Thickness of the metallic layer (t)	0.035
Taper length of the antenna (T1L)	30
Slot length of the antenna (S1)	15
Microstrip feed length (ML)	29.9
Microstrip feed width (MW)	2.794
Tapper rate (r)	0.13
Throat width (s)	0.4
Outer mouth opening (M1)	39.6

3.3 Proposed Vivaldi Antenna (Step-3)

In this last design step, two circular-shaped resonant cavities were used at the edge of the tapered slots, and a radial stub was added to the microstrip line feeder. Figs. 1(d) and (e) shows the top and back views of the proposed antenna structure with circular-shaped resonant cavities and a radial stub. The radius of the two circular slots, R$_{2}$ and R$_{3}$, was 4 mm. The radius of the radial stub was R$_{4}$= 2.5 mm. The angle of the radial stub is $\boldsymbol{\alpha}$ = 90$^{\circ}$. A pair of identical tapered slots and three resonant cavities were used in the design to enhance the antenna radiation pattern. After adding the radial stub to the microstrip feed line, the position of the feed line was optimized. The antenna covered the UWB and increased the antenna efficiency and gain. The width of the microstrip feed line was set to 2.794 mm. Fig. 2 shows the 3D view of the proposed UWB Vivaldi antenna. Table 3 lists all the new design parameters of the final design.

Fig. 2. 3D view of the proposed UWB Vivaldi antenna.

Table 3. Additional Parameters for Proposed UWB Vivaldi Antenna (Step-3).

Parameters	Values
Radius of the resonant cavities (R2 and R3)	4 mm
Radius of the radial stub (R4)	2.5 mm
Angle of the radial stub (α)	90°

4. Performance Evaluation of the Proposed UWB Antenna

The performance of the suggested UWB Vivaldi antenna was estimated using CST microwave studio. This section analyzes many radiation factors, including reflection coefficient, VSWR, gain, directivity, and radiation efficiency.

4.1 Reflection Coefficient

The motive was to use this antenna for MI applications. Hence, a UWB operating frequency range is needed. Several techniques were applied to the proposed Vivaldi antenna to achieve acceptable performance for the intended MI applications. All the parameters of the designed antenna for MI applications have been identified within a frequency range of 3 GHz to 14 GHz. Fig. 3 shows the return loss curve of steps 1, 2, and 3. The simple Vivaldi antenna of step-1 covered the bandwidth of 4.7 GHz over the range of 7.2-11.9 GHz, with a return loss of ${-}$25 dB. The antenna in step-2, with two tapered slots and a resonant cavity, covered the working bandwidth of 2.3 GHz over the range of 5.2-7.5 GHz, with a return loss of ${-}$17 dB. A working bandwidth of 10 GHz over the range of 3.6-13.6 GHz, with a reflection coefficient of -39 dB, was obtained using another two resonant cavities and a radial stub and optimizing the position of the microstrip line feed. In the final step, the bandwidth was increased by 112.8% compared to the 1$^{\mathrm{st}}$ step.

Fig. 3. Reflection coefficient for the different design states.

4.2 Voltage Standing Wave Ratio (VSWR)

Fig. 4 shows the VSWR curve at the different design states. Our proposed UWB compact slotted Vivaldi antenna shows the VSWR of 1.001, whereas in step 1 and step 2, the antenna shows a VSWR of 1.1 and 1.25. The slotted Vivaldi antenna performed well when the VSWR was close to 1.

Fig. 4. VSWR for the different design states.

4.3 Gain and Directivity

Fig. 5 shows the gain vs. frequency curve for different design states, and Fig. 6 shows both the gain and directivity of the proposed UWB Vivaldi antenna with respect to frequency. The peak gain of the final state was 7.35 dB, whereas in steps 1, and step 2, the peak gains were 5.6 dB, and 6.8 dB, respectively. The peak directivity was above 8 dBi, and in the entire bandwidth, the directivity was above 4.5 dBi.

Fig. 5. Gain for the different design states.

Fig. 6. Gain and directivity of the proposed UWB Vivaldi antenna.

4.4 Radiation Efficiency

The proposed microstrip line fed UWB slotted Vivaldi antenna with a resonant cavity, and radial stub showed a peak radiation efficiency of 82%. In the other two steps, the peak radiation efficiency was 80% (step-1) and 79% (step-2). Fig. 7 compares the radiation efficiency curves for all three states. Therefore, the radiation efficiency was also improved in the final step. Table 4 lists the performance parameters in different states of antenna design. Table 5 presents a comparison chart with several recently released works. The proposed UWB slotted Vivaldi antenna had a compact volume, good gain, good efficiency, and high bandwidth for the intended MI applications. The proposed slotted compact Vivaldi antenna was also very cost-effective because it was made of low-cost FR4 and a readily available substrate material.

Fig. 7. Efficiency for the different design states.

Table 4. Performances Comparison Among the Different Design Steps.

Parameters Name	Step-1	Step-2	Step-3
Operating frequency (GHz)	7.2-11.9	5.2-7.5	3.6-13.6
BW (GHz)	4.7	2.3	10
Reflection coefficient (dB)	-25	-17	-39
VSWR	1.1	1.25	1.001
Maximum gain (dB)	5.6	6.8	7.35
Maximum efficiency (%)	80	79	82

Table 5. Comparison Table.

Index	Reference Number									This work
	[3]	[4]	[14]	[15]	[16]	[17]	[18]	[19]	[20]
Year	2018	2020	2021	2021	2019	2019	2019	2018	2019
Size (L×W×h) mm3	57×40.6 ×1.6	65×55 ×1.6	130×95 ×0.813	98× 110× 1.6	66×60.75 ×1.57	57.3× 42.8× 1.6	140×75 ×1.6	88×75 ×1.57	30×30 ×0.8	50×50 ×1.5
Substrate material	FR4	FR4	Rogers RO003c	FR4	FR4	FR4	FR4	Rogers RT	FR4	FR4
Operating Frequency Range (GHz)	3–9	2.213–7.187	3.1–10.6	1.45–9.82	3–10	3.6–10	1.8–12	1.8–2.4	3.1–10.6	3.6–13.6
Return loss (dB)	≈ -33	-51.73	-36.39	≈ -27	≈ -33	-58.29	≈ -49	≈ -29	-60.10	-39
Gain (dB)	>2	7.36 Max.	13.3 Max.	6.61	10.6 Max.	7.64 Max.	13.01 Max.	4.5	3.504	7.35 Max.
-10 dB BW (GHz)	6	4.974	7.5	8.37	7	6.4	10.2	0.6	7.5	10
Application	MI, Breast cancer	MI, Brain cancer	MI	MI	MI	MI	MI	MI, Breast cancer	MI	MI

*MI = Microwave Imaging

5. Conclusion

This paper described a UWB 50${\times}$50${\times}$1.5 mm$^{3}$ slotted Vivaldi antenna for microwave imaging applications. Several applied methods including radial stubs, resonant cavities, and tapered slots were broken down into three steps to improve the performance of the Vivaldi antenna. The width of the fed line was chosen to correspond to the 50 ${\Omega}$ input impedance. The substrate was made of FR4 (lossy), which has a tangent loss, dielectric constant, and thickness of 0.02, 4.6, and 1.5 mm, respectively. With a good return loss of ${-}$39 dB and VSWR of 1.001, the suggested antenna spanned an extensive working bandwidth of 10 GHz, covering the 3.6-13.6 GHz range. The proposed Vivaldi antenna had a peak gain of more than 7 dB. As a result, the suggested Vivaldi antenna is a strong contender for microwave imaging applications.