Metamaterial Antenna for Microwave Imaging

Microwave imaging is a straightforward approach to detect scattering in a detected object. One application of microwave imaging is breast cancer detection, the most prevalent type of cancer in Indonesia and the leading cause of cancer-related deaths. The planar monopole antenna offers several advantages, including low bandwidth compared to microstrip antennas and a simple structure for easy and affordable fabrication. Metamaterial can be added to the antenna to operate within the ultra-wideband frequency range of 3,1-10,6 GHz. Metamaterial expands the bandwidth and offers advantages such as low fabrication costs and miniaturization potential. This study aims to design a planar monopole antenna with a hexagonal patch using metamaterial. The simulation results show a working frequency range of 3,0699 - 20,779 GHz, while the realized results are 2,6 - 11,3 GHz and 15,2 - 17 GHz. The simulated bandwidth value is 17,7091 GHz, whereas the realized bandwidth values are 8,7 GHz and 1,8 GHz. The return loss values are ≤ 10 dB and the VSWR values are ≤ 2. The gain at the frequency of 7,038 GHz is 1,963 dBi in simulation and 1,698 dBi in realization. Additionally, changes in electromagnetic parameters are observed when the antenna detects cancer.


Introduction
Wireless communication, from remote control to medical uses, has become indispensable in everyday life [1].Microwave imaging is a frequent approach for detecting scattering or distribution in things, such as breast cancer, Indonesia's most common cancer [2].Antennas are essential in wireless communication, and the planar monopole antenna is preferred for dealing with its limits [3].The Ultra-Wideband (UWB) frequency band, introduced by the Federal Communications Commission in 2002, has a wide bandwidth, a simple structure, and low cost [4], [5].Metamaterial approaches can be used to attain the needed UWB bandwidth or even wider bandwidth.Metamaterial is an artificial material with negative permittivity and permeability, giving it characteristics that aren't found in nature [6].The electromagnetic band gap (EBG) is one method that can produce metamaterials that can operate on numerous frequency bands while remaining cost-effective and offering promising data transmission speeds [7], [8].
Previous research on the electromagnetic band gap (EBG) for wireless communication in the human body has yielded promising results.The antenna in this study operates at 3,5 GHz, having a mushroomlike form for the unit cell and vias in the middle.The constructed antenna has an off-body return loss of -20,90 dB, a bandwidth of 77,60 MHz, an on-body return loss of -37,13 dB, and a bandwidth of 217,90 MHz [9].Research has also been done on ultra-wideband antennas for the early detection of breast cancer using a rectangular microstrip antenna with several rectangular slots.The antenna operates at 8,41 GHz to 10,29 GHz and has a directivity of 6,451 dBi.Simulations with breast tissue structures, both without and with cancer sizes, reveal that when cancer is present, the E/H-Field and current density values are more prominent than when there is no cancer [10].An antipodal Vivaldi antenna with metamaterial (MTM-AVA) has been designed and realized for UWB breast cancer detection.The metamaterial is applied to the antenna's front section, lowering the dimension by 33% to 80 x 61 mm and enhancing parameters such as return loss.The breast cancer detection experiment uses a phantom breast, and eight antennas are placed in an array surrounding the phantom.The measurement results show a difference between the presence and absence of cancer at  11 −  88 5.34 GHz [11].
This final project aims to design and realize a planar monopole antenna with a hexagonal patch and an electromagnetic band gap with a mushroom-like structure to detect breast cancer.The EBG structures are intended to enhance the bandwidth.The antenna will use FR-4 material and operate within the frequency range of 3,1 GHz to 10,6 GHz.The analysis will be conducted on changes in electromagnetic parameters, including E-Field, H-Field, and current density, when the antenna interacts with breast tissue structures, both with and without cancer.

Ultra-Wideband
Ultra-Wideband (UWB) is a wireless technology used for military purposes, medical engineering, and radar systems [1].UWB transmits information in short durations, occupying multiple frequency spectra with low power and wide bandwidth.The Federal Communications Commission regulates UWB communication from 3,1 GHz to 10,6 GHz, with a maximum emission power limit of 41,3 dBm/MHz [12].UWB offers a high data rate, low cost, wide operating frequency range, low power requirements, and the ability to eliminate electromagnetic interference [13].Planar monopole antennas are suitable for UWB communication due to their simple shape, lightweight design, and cost-effective fabrication [5].The planar monopole antenna has an incomplete ground plane, while the microstrip antenna has a complete ground plane.A lower frequency value can be obtained using this method [14]: L represents the height of the patch, r is the effective radius of the antenna with a cylindrical monopole equivalence, p is the distance between the patch and the ground plane, H is the length of the patch side, k is the practical permittivity value, and   is the relative permittivity.The values of L, r, and k can be obtained using the following equations [14]:

Metamaterial
Metamaterial can be defined as an artificial material or homogenous electromagnetic structure designed to exhibit unusual properties not found in nature [15].Various types of metamaterials have been introduced in research, depending on the exhibited Metamaterials are artificial materials with unique properties, such as negative permittivity and permeability, high impedance surfaces, left-handed materials, magneto materials, negative refractive index materials, soft and hard surfaces, and artificial magnetic conductors.These materials offer numerous advantages, such as multiple working frequencies, antenna miniaturization, low manufacturing costs, and high data rates, making them increasingly used in technology development and research [7].

Electromagnetic Band Gap (EBG)
EBG structures are artificial objects that prevent or assist electromagnetic wave propagation in specific frequency bands.They consist of dielectric materials and metal as conductors and enhance antenna performance.EBG structures can be divided into three-dimensional volumetric structures, twodimensional planar surfaces, and one-dimensional transmission lines [7].The mushroom-like structure, resembling a mushroom, has vias connecting it to the ground plane, creating inductance (L) and capacitance (C) as we see in Figure 1, where inductance is created due to the current flowing through the vias, and capacitance arises from the effects caused by the distance between adjacent different cells [16].

Microwave Imaging
Microwave imaging is a technique to locate scattering regions in detected objects [1].It has the potential for breast cancer detection due to its short wavelengths and increased operating frequencies [10].
Research on microwave signals for cancer detection is ongoing, as they are cost-effective and do not involve ionizing radiation.Antennas are crucial components in microwave imaging systems, and studies include observing changes in electromagnetic parameters and analysing  11 values [11].

VSWR VSWR or Voltage Standing
Wave Ratio is a parameter that indicates how much power is reflected back by an antenna as a function of the reflection coefficient.VSWR is the ratio of the maximum voltage to the minimum voltage in a standing wave.VSWR has values ranging from 1 to infinity, but a good VSWR value for an antenna is between 1 and 2. If the VSWR value is larger than 2, the antenna can be considered poor, and it should be repaired promptly because it indicates that too much power is being reflected by the antenna [17] 3. Antenna Design and Simulation

Antenna Specifications and Material Selection
The proposed Ultra-Wideband (UWB) planar monopole antenna with and without metamaterial is designed to cover multiple frequencies and exhibit good characteristics, particularly with metamaterial addition.The specifications for both simulated and designed antennas include: The following are the material specifications that will be used for the design and realization of the planar monopole antenna with metamaterial for microwave imaging: Table 2. Material Specifications.

Design and Simulation of Monopole Planar Antenna with EBG
The conventional antenna is optimized by adding the Electromagnetic Band Gap (EBG) method with a mushroom-like structure, increasing its bandwidth.The EBG is added to the front, near the microstrip feedline, and is hexagonal, matching the patch shape.The dimensions and shape of the modified antenna are shown in Table 3 and Figure 2.  1,963 dBi The gain of the planar monopole antenna with the addition of EBG is determined from the most optimum return loss value.The antenna has the most optimum return loss value at a frequency of 7,038 GHz with a value of -32,676719 dB.

Antenna Simulation with Breast Tissue Modelling
The antenna simulation with EBG addition analyses differences in electromagnetic parameters to determine cancer presence in breast tissue.Breast tissue modelling is divided into two categories: breast tissue without cancer and breast tissue with cancer.The values of electrical parameters and dimensions are shown in Table 5 and Figure 3.The antenna simulation will be conducted on breast tissue without cancer and with the presence of cancer.The distance between the antenna and the breast tissue modelling is 3 mm.The simulation will be performed with 3 mm, 4 mm, and 5 mm tumour sizes at frequencies ranging from 3,1 GHz to 20 GHz.The antenna modelling with breast tissue without cancer and with cancer can be seen in Figure 4.The electromagnetic parameters observed to distinguish the two breast tissue models are the E-Field, H-Field, and current density.The simulation results of the E-Field can be seen in Figure 5.
Figure 5 shows that the E-Field experiment examines multiple frequencies to vary wave density.When waves encounter obstacles like tumours, they experience different phases, forming a sinusoidal wave in different frequency ranges.The highest peak occurs at 20 GHz frequency.Factors like phase and frequency influence sinusoidal waves.Figure 5 shows that high frequencies cause amplitude to approach maximum, resulting in a phase close to 0. Larger tumour sizes increase the absolute percentage of the E-Field.Simulation results for the H-field with tumour sizes of 3mm, 4mm, and 5mm are shown in Table 6, Table 7, and Table 8.Tables 6-8 reveal higher H-Field values in breast tissue with cancer than those without cancer due to larger antenna reflected and received magnetic fields.The difference in H-field values between breast tissue without cancer and cancer at each frequency is not significantly different.The last parameter to be examined is the current density.The simulation results for current density can be seen in Figure 6. Figure 6 shows that frequency or cancer size does not significantly affect current density.Current density, representing electric current on a conductor's surface, is closely related to Ampere's Law and surface area, as per Ampere's Law and Maxwell's Law: In equation 7, there is a magnetic field (B) on the left and an electric field (E) on the right.Symbol (∇ ×) represents the curl operator and pronounced "del cross".The curl of the magnetic field ((∇ × ) is the electric field that partial derivative with respect to time (

𝜕𝐸 𝜕𝑡
).  0 is the permeability of free space, officially referred to as the magnetic constant. 0 is the permittivity of free space, officially referred to as the electric constant.J represents the current density in the dielectric material, which can be divided into three components: free current density, bound current density, and polarization current density.
represents free current density, M is magnetic polarization or magnetization, and P is electric polarization.The formula shows that frequency doesn't influence the electric current, yielding negligible effects on the current density.Furthermore, the current density is influenced mainly by the material utilized in the antenna rather than the surrounding material.Consequently, the size of the tumour doesn't have a significant impact on the current density.

Antenna Realization
The antenna design and simulation are completed, and the next step is to implement the design as a monopole planar antenna with EBG.The antenna will be made of FR-4 material and copper, with dimensions shown in Figure 7.

Antenna Realization
A comparison of specifications, simulation results, and measurement results of realized antenna fabrication and measurement can be seen in Table 9 and Table 10.

Conclusion
The monopole planar antenna with EBG addition for microwave imaging, particularly for breast cancer detection, has been designed and realized using FR-4 substrate and copper materials.The antenna meets initial design specifications, with VSWR ≤ 2, return loss ≤ -10, and bandwidth ≥ 7.5 GHz.It is classified as an ultra-wideband antenna, increasing bandwidth value by 3,6436 GHz.The difference between E-Field values and absolute percentage in breast tissue becomes more significant as tumour size increases.The E-Field results show a sinusoidal waveform due to different frequencies, and the H-field values in breast tissue with tumours are larger than those without tumours.The antenna's frequency does not affect electric current, and tumours size does not significantly influence current density.The monopole planar antenna with metamaterial works well but may not be suitable for cancer detection due to its wide frequency range.

Figure 2 .
Figure 2. Antenna with EBG Addition -Front and Back View.

Table 5 . 5 Figure 3 .
Figure 3. Breast Tissue Modelling.The antenna simulation will be conducted on breast tissue without cancer and with the presence of cancer.The distance between the antenna and the breast tissue modelling is 3 mm.The simulation will be performed with 3 mm, 4 mm, and 5 mm tumour sizes at frequencies ranging from 3,1 GHz to 20 GHz.The antenna modelling with breast tissue without cancer and with cancer can be seen in Figure4.

Figure 5 :
Figure 5: Absolute Percentage E-Field Graph.The absolute percentage in the graph shows the differences in the E-Field values with and without cancer.The absolute percentage value is determined using the equation:

Figure 7 .
Realization of the Antenna -(a) Front and (b) Back View.

Table 3 .
Dimensions of Antenna with EBG.The simulation results of the planar monopole antenna with the addition of EBG can be seen in the table below:

Table 4 .
Simulation Results of Antenna with EBG.

Table 6 .
H-Field Results on 3 mm Radius Cancer.

Table 8 .
H-Field Results on 5 mm Radius Cancer.

Table 9 .
Comparison of Antenna Circuit Parameters: Simulation Results and Measurements.

Table 10 .
Comparison of Antenna Radiation Parameters: Simulation Results and Measurements.