INTRODUCTION
Microwave Imaging (MI) based on Radar technique and Microwave tomography have been considered in cancer detection. Early detection of breast cancer is the most crucial aspect to discover. Ultrasound, X-ray mammogram, Magnetic Resonance Imaging (MRI) are some techniques applied to detect this cancer, but MI is a more encouraging technique, and others have a few restrictions. Since MI uses a Microstrip patch antenna, it is comfortable, nonionizing, low cost, and safe.
The working principle of the MI technique relay on the value of the dielectric constant between healthy and abnormal tumor tissues. Thus, the dielectric and conductivity of tissue are used for pathological recognition to differentiate defected and healthy tissue.
Electromagnetic waves are transmitted through the breast from the transmitting antenna, and scattered waves are received by receiving antenna, which is examined for cancer detection. Here, antenna selection becomes a major role as position, volume, and other parameters related to cancerous tissues are identified through the differences in a magnetic and electric field.
Sufficient work has been carried out for the detection of breast cancer using microwave imaging. A group of antennas called Wearable antennas have been proposed for this purpose. These antennas satisfy the necessities of all new requirements such as maintenances free, low cost, minimum weight. A structure of the MIMO antenna has been designed and examined through Microwave imaging.
A single layer of microstrip antenna has been modeled to get microwave imaging by emitting it into the breast tissue. In this article, the hexagonal patch microstrip antenna has been suggested to detect breast cancer tissue using microwave imaging and a simple 3D model of breast structure to represent breast cancer tissue.
Acceptable simulation results are produced by altering the ground plane on microstrip antenna patchenna operating at 2.45 GHz. The designed antenna is kept on the breast skin, and the electromagnetic field value's differences results as per simulation are examined through graphics.
MICROSTRIP PATCH ANTENNA
INTRODUCTION
In its most basic form, a Microstrip patch antenna consists of a radiating patch on one side of a dielectric substrate with a ground plane.
The patch is generally made of conducting material such as copper or gold and can take any shape. The radiating patch and the feed lines are usually photo etched on the dielectric substrate. To simplify analysis and performance prediction, the patch is generally square, rectangular, circular, triangular, elliptical, or other common shapes. For a rectangular patch, the length L of the patch is usually 0.3333𝜆0
The patch is selected to be very thin such that 𝜆 ≪λ0 (where t is the patch thickness). The height h of the dielectric substrate is usually 0.003λ0 ≤h ≤ 0.05λ0. Dielectric
constant of the substrate (ℇr) is typically in the range 2.2 ≤ ℇr≤ 12.
Common shapes of Microstrip Patch Elements
Microstrip patch antennas radiate primarily because of the fringing fields between the patch edge and the ground plane. For good antenna performance, a thick dielectric substrate having a low dielectric constant is
desirable since this provides better efficiency, larger bandwidth, and better radiation.
However, such a configuration leads to larger antenna size. To design a compact Microstrip patch antenna, higher dielectric constants must be used, which are less efficient and result in narrower bandwidth. Hence a compromise must be reached between antenna dimensions and antenna performance.
APPLICATION, ADVANTAGES, AND DISADVANTAGES
Application
Some of their major applications discussed are given below:
● Aerospace vehicles including high-performance missiles, spacecraft,
aircraft, Satellite.
● Microstrip patch antennas are increasing in popularity for use in wireless
applications due to their low-profile structure.
● Mobile radios, phones, pagers.
● Base station for personal communication.
● Large ground-based phased array antenna.
Advantages
Some of their principal advantages discussed are given below:
● Lightweight and low volume.
● Low profile planar configuration can be easily made conformal to
the host surface.
● Low fabrication cost, thus can be manufactured in large quantities.
● Supports both linear as well as circular polarization.
● It can be easily integrated with microwave integrated circuits (MICs).
Disadvantages
On the other hand, Microstrip antennas also feature some disadvantages
compared to conventional antennas:
● Narrow bandwidth
● Low efficiency
● Low gain
● Extra radiation from feeds and junction
FEEDING TECHNIQUES
Microstrip patch antennas can be fed by a variety of methods. These methods can be classified into two categories- contacting and non-contacting. The RF power is fed directly to the radiating patch using a connecting element such as a microstrip line in the contacting method.
In the non-contacting scheme, electromagnetic field coupling transfers power between the microstrip line and the radiating patch. The four most popular feed techniques used are the microstrip line, coaxial probe (both contacting schemes), aperture coupling, and proximity coupling (both non-contacting schemes).
Microstrip (Offset Microstrip) Line Feed
In this type of feed technique, a conducting strip is connected directly to the edge of the microstrip patch. The conducting strip is smaller in width as compared to the patch. This kind of feed arrangement has the advantage of being etched on the same substrate to provide a planar structure.
Coaxial Feed
The Coaxial feed or probe feed is one of the most common techniques for feeding microstrip patch antennas. The inner conductor of the coaxial connector extends through the dielectric and is soldered to the radiating patch, while the outer conductor is connected to the ground plane.
The main advantage of this feeding scheme is that the feed can be placed at any desired position inside the patch to obtain impedance matching.
This feed method is easy to fabricate and has low spurious radiation effects. However, its major disadvantage is that it provides narrow bandwidth and is difficult to model since a hole must be drilled into the substrate. Also, the increased probe length of thicker substrates makes the input impedance more inductive, leading to matching problems.
Using a thick dielectric substrate to improve the bandwidth, the microstrip line feed and the coaxial feed suffer from numerous disadvantages such as spurious feed radiation and matching problems.
Aperture Coupled Feed
In aperture coupling, the radiating microstrip patch element is etched on the top of the antenna substrate. The microstrip feed line is etched on the bottom of the feed substrate to obtain aperture coupling.
SUBSTRATE MATERIAL
In today’s market, there are a lot of different PCB substrate products. Unfortunately, there is not only one product that can cover all applications. It all depends on the application itself. Even though your application is simple, it is still difficult to meet all requirements.
To design a Microstrip Patch Antenna, the following are the requirements:
● Type of substrate
● Resonant frequency
● Substrate thickness
Though all the above are the requirements for antenna fabrication, the right substrate selection is a must based on cost, efficiency, and size for best results. A lot of work has been done on different substrates. Substrates use in microstrip patch antenna varies from 2.2≤Ɛ≤12.
Lower the permittivity of dielectric material larger the size of the antenna, but it achieves better efficiency and larger bandwidth. The Ɛr is limited by radiofrequency or microwave circuits connected to antennas. When substrates of higher dielectric constants are used, then the performance result degrades.
FR4 gives better performance in terms of gain, directivity, and bandwidth. The dielectric substrates used are Bakelite, FR4 Glass Epoxy, RO4003, Taconic TLC, and RT Duroid.
Dielectric constant
The dielectric constant (“DK” or “relative static permittivity”) is the ratio between the stored amount of electrical energy in material and that stored by a vacuum (which is in definition 1). It is also a measure of the degree to which an electromagnetic wave is slowed down as it travels through the insulating material. Dielectrics are, i.e., used in capacitors to store more electrical charge than vacuum.
Loss tangent
The loss tangent (also called “tan δ,” “DF,” “low loss,” “dissipation factor”) is a measure of how much of the electromagnetic field traveling through a dielectric is absorbed or lost in the dielectric, usually through heat.
As the dielectric in a substrate is similar to the dielectric in a capacitor, the loss tangent can best be described as the loss through an equivalent series resistor (ESR) inside a capacitor. A small ESR describes a good capacitor with low loss.
Thickness
The thinner the substrate is, the less loss, but the less power you can send through it because the transmission line must be thinner to keep the same impedance. With a thicker substrate, you need a wider strip line to keep the same impedance.
This will give a higher Q in the copper = more power through it, while the disadvantage will be more weight and higher radiated power from the transmission line, which we want as little as possible. As an alternative, you could use a substrate with a lower dielectric constant. That way, you can increase the microstrip line (transmission line) = higher Q without increasing the thickness of the substrate.
The thickness also depends on your application. For instance: a designer wants to design a thin mobile phone because that attracts certain customers. This means that there is use for a thinner substrate, and you have to cater to it.
Material Selection
FR-4 (Flame Retardant-4) FR-4 substrate is a very common and by far the most used substrate in the consumer electronics market as it has a good quality-to-price ratio. It is mostly used where cost is more efficient than performance. FR-4 is a standard with many different distributors making many different FR-4 quality and property boards.
It is made of woven fiberglass with an epoxy resin binder (binds the copper clad to the dielectric substrate) that is flame resistant. The dielectric constant goes down the more the FR-4 PCB is reinforced with epoxy resin instead of fiberglass, as this is not determined as a standardized parameter. 100% epoxy resin boards dielectric constant is 3.4 @ 1MHz. The FR-4 changes its dielectric constant along its area, making it too unstable to mass-produce precise antennas.
Also, the FR-4 has a higher loss at frequencies over 3GHz, because of the sensitivity of the cheap substrate. Other products are therefore recommended to perform better than FR-4 in RF applications. In the cell phone industry, companies use higher quality FR-4 substrates because it is more cost-efficient, but from only one manufacturer to be sure of the quality and properties when mass-producing. The performance is typically around -13 dBm.
Rogers
Rogers Corporation is a company that specializes in high-frequency PCB and makes some of the best on the market in the low-cost low-loss substrate area. They match the loss tangent to other high-end (not low-loss, low-cost) substrate material products.
They usually have a slightly higher dielectric constant but cost around 3-4 times less than the other substrate products. Rogers also makes low-cost types with higher dielectric constants to be used for other applications such as high power.
Bandwidth
It is defined as “The range of usable frequencies within which the performance of the antenna, concerning some characteristic, conforms to a specified standard.” The bandwidth can also be defined as the range of
frequencies on either side of the center frequency where the antenna characteristics like input impedance, radiation pattern, beamwidth, polarization, sidelobe level or gain, are close to those values which have been obtained at the center frequency.
The antenna's bandwidth is the number of Hz for which the antennas will exhibit a VSWR less than 2:1. The bandwidth can also be described in terms of the percentage of the center frequency of the band.
CONCLUSION
A Microstrip Patch Antenna is designed to detect (tumor cells) breast cancer detection, thus reducing hospitalization terms improving the patient’s quality of life. The proposed work is to design and develop of wearable textile antenna for breast cancer detection.
The antenna structure operating at 2.45 GHz is simulated with a basic 3D breast structure. Different antenna designs are evaluated by modifying the ground plane and slotting on the microstrip patch. This study proves that antenna guarantees a decrease in the results of the electric fields, magnetic fields, and current density inside healthy tissue with a malignat tumor in the breast compared with a breast without a tumor. It fulfills the requirement given to detect the tumor cells up to a few millimeter range.
Thus an antenna to detect breast cancer detection is made possible by SAR analysis. SAR is higher for Tumor cells. Hence they can be easily differentiated. An antenna is designed using FR-4 substrate in a 2.4GHz frequency range. It can be said that this work obtains better results when compared to works in literature. Depending on simulation results and graphical observation, the antenna structure provides the best detection for breast cancer.
We have developed a simplified antenna array to enhance better imaging for tumor detection compared with other works. The simulation results of the antenna array show a good impedance matching with low mutual coupling and high radiation pattern.
This array proved to be efficient and easily made for Microwave Breast Imaging (MBI) compared with other imaging techniques. The antenna exhibited a good directional radiation pattern with acceptable gain.
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