1. Introduction
In recent decades, microwave imaging (MWI) has been extensively researched, finding applications across a wide range of technical fields, including ground penetrating radar [
1], through-wall detection [
2], remote sensing [
3], and biomedical imaging [
4]. Among these applications, microwave imaging’s utility in biomedical applications stands out due to its favored performance compared to other imaging modalities like X-ray, computed tomography (CT), magnetic resonance imaging (MRI), etc. [
5]. In particular, MWI systems are characterized by the use of non-ionizing radiation, are cost-effective, portable, and capable of real-time imaging. At the core of the MWI system lies the critical role of the used antennas, which determine the extent of electromagnetic wave penetration into biological tissues [
6]. Additionally, employing multiple antennas in an array configuration proves to be advantageous, enhancing available data and consequently refining imaging resolution [
7]. This multi-static arrangement accelerates measurement speed by avoiding the mechanical movement of a mono-static configuration.
Compact sensors are essential in biomedical imaging systems due to spatial constraints within the detection area. Notably, electromagnetic waves experience significant attenuation as they penetrate into biological tissues with increasing frequency [
8]. Consequently, the design of compact antennas becomes crucial to maintain good performances at low frequencies.
Vivaldi antenna is a promising candidate for biomedical applications due to its end-fire radiation pattern, its compact aperture dimension (thickness × width), and its capability of being used within an array configuration [
9]. The wide bandwidth of the Vivaldi antenna provides more freedom when it comes to multi-frequency system design. However, the lowest frequency of operation is directly related to the Vivaldi antenna dimension (length × width), so miniaturization techniques should be applied to allow operations at lower frequencies without increasing the overall size. Scaling down the antenna’s physical size without degradation of its performance is a challenge that researchers have confronted over the years. The relation between the antenna’s physical dimension and quality factor Q was early discussed by Wheeler [
10], Chu [
11] and Hansen [
12]: the shrink of the antenna’s physical size always comes together with the reduction in its bandwidth and Q factor.
A first technique for antenna miniaturization is to fit the limited space with larger radiating structures [
13], using, e.g., meander-line design [
14] or fractal structures [
15]. It is well known that increasing the surface current path of an antenna is an efficient way to reduce antenna size [
16]. Promising studies such as [
17,
18,
19,
20,
21] demonstrated the possibility of reducing the Vivaldi antenna’s physical dimensions by using either circular or corrugated slots on the antenna radiator. A second technique is to use metamaterials [
22] as, e.g., split-resonant-ring (SRR), and add resonances to the antenna. The third technique is to incorporate reactive loads into the antenna transmission line structure [
13]. This introduces time delay and slows down the electromagnetic wave propagation, hence making the transmission line electrically longer. However, due to the limited Q factor of the reactive load, the antenna benefit of dimension reduction always comes with efficiency losses [
23].
The goal of this study was to show the design approach of a miniaturized Vivaldi antenna for biomedical applications. The concerned frequency range was between 500 MHz and 5 GHz, typical of MWI systems [
24,
25]. To show the influence of the design parameters on the antenna’s dimensions and performances, a class of antenna miniaturization techniques was investigated. Firstly, the reduction in the electromagnetic wavelength was explored, which was achieved by employing an antenna substrate material with a high relative permittivity (
) and situating the antenna within a coupling medium (
). The results in [
26] showed that using a high-permittivity substrate and a high-permittivity coupling medium efficiently reduced the antenna’s width to 0.4
. However, it proved challenging to further reduce the antenna’s physical size by modifying the antenna structure. Accordingly, in this study, further reduction in the antenna’s dimensions was looked for by extending the current path of the antenna etching slots on both sides of the radiator. Different antenna slot configurations were studied. In particular, the influence of different slot lengths, positions, inclination angles, and numbers of slots on the antenna bandwidth were studied. Among the study designs, a four-slot Vivaldi antenna was considered the optimal choice for MWI applications. The proposed antenna was fabricated, and its performances were experimentally characterized inside the coupling medium and in an array configuration.
The paper’s structure is as follows:
Section 2 presents the study methodology. In
Section 3, the basic antenna structure is given. In
Section 4, a range of slot configurations are discussed, focusing on their potential to enhance antenna matching at lower frequencies. This section also investigates the mutual coupling effect of the antennas when arranged in an array configuration. Moreover, it examines the E-field distribution of the antenna and its performance in the presence of a phantom. The comparison of the antenna’s simulation and measurement results are reported in this section as well. Finally,
Section 5 presents the paper’s conclusion.
2. Methodology
The antenna was designed and optimized with CST Studio Suite R2021x software (Dassault Systèmes SE, Vélizy-Villacoublay, France). The basic antenna geometry shown in
Figure 1 was first optimized by using the parameter sweep function on the following parameters:
,
,
,
,
,
,
,
, looking for the widest bandwidth. Then, the insertion of slots to allow reducing antenna’s dimensions while keeping its matching in the desired frequency band was considered. Since the interest was in investigating the influence of slot parameters on the antenna performances, antenna optimization was performed as a mix of physical considerations and software sweep analysis.
To study the impact of antenna slot configurations on the antenna impedance, the following slot configurations were investigated: one-slot, two-slot, three-slot, and four-slot configurations. In particular, studies on different slots lengths and inclination angles were performed.
Ultimately, the best-performing antenna was fabricated. It was realized by using a single-layered substrate TF1020 (TaiZhou WangLing, Taizhou, China. Thickness
mm,
). The experimental set-up for the antenna measurement consisted of an antenna connected with a coaxial cable and immersed inside a tank (dimensions: 200 mm × 200 mm × 200 mm) filled with the coupling medium. The coupling medium (
,
S/m @ 915 MHz) was made by following the recipe from [
26]; it is a time-stable low-loss liquid medium consisting of distilled water, sunflower oil, guar gum, and dishwashing detergent. The measurement of the antenna scattering parameters inside the coupling medium was made by using the P5024A Vector Network Analyzer (Keysight Technologies, Santa Rosa, CA, USA).
5. Conclusions
This paper presents the design steps for a slot-loaded Vivaldi antenna tailored for biomedical microwave imaging applications.
While Vivaldi antennas and the use of slots to reduce their dimensions are well known in the literature, the novelty of this study lies in the analysis of the influence of the different slot parameters (location, inclination angle, length, and number) on the achieved performances. Additionally, a physical interpretation of the achieved results is provided. This systematic approach and detailed analysis of different slot configurations to optimize antenna performances offer initial guidelines for an optimal location of the slots on the antenna radiator that can be applied for designs conceived for different applications. Indeed, several parameters should be optimized so that there is a great chance to obtain sub-optimal designs from software optimizations. Furthermore, the investigation combines several strategies, besides the slots, including the use of a high-permittivity substrate material with , and placing the antenna within a coupling medium with . High-permittivity materials shorten the electromagnetic wavelength, enabling the antenna to operate at the desired frequency range with a smaller physical footprint. Additionally, the coupling liquid allows matching with the biological tissue, reducing electromagnetic field reflections. With a focus on the lowest working frequency and widest bandwidth, the optimal design is identified.
In the final phase, the antenna is fabricated and characterized for performance within the coupling medium. The results from both simulation and measurement exhibit agreement affirming the effectiveness and reliability of the proposed design. Notably, measurements demonstrate that the slot-loaded design substantially mitigated mutual coupling effects when the antenna was placed in an array configuration, enhancing the antenna’s suitability for its intended imaging application. Additionally, the final prototype shows the minimum dimensions with respect to other designs proposed in the literature (
Table 8).
To conclude, this work contributes insights into the development of slot-loaded Vivaldi antennas for biomedical microwave imaging, offering a promising solution for enhanced imaging capabilities in the relevant frequency range. The main limitation of the proposed antenna is that it must operate within a coupling medium, which reduces its suitability for wearability in biomedical applications. Future works include realizing the experimental set-up of an imaging system with the proposed antenna.