Dielectric Properties

CHAPTER 2

LITERATURE REVIEW

2.1 INTRODUCTION

The rest of this chapter is concerned with the review of related literature and studies, the conceptual model of the study, the research hypothesis, and the definition of terms used in the study.This research study cited technical paper, journal or book which are relevant to the research. After this research on the journal, the improvement the usability ofopen probe and the determine the complex permittivity of the fruits based on S-parameter.

2.2 OPEN ENDED COAXIAL METHOD

Firstly, this method is non-destructive method. The dielectric properties of materials are characteristics that determine their interaction with electromagnetic energy. An open-ended coaxial probe has been used as an electromagnetic sensor for various industrial and scientific applications. Most of these applications based on the principal that the signal reflected by the coaxial line which carry the desired information about the material. At radio and microwave frequencies, the dielectric properties determine the rate of dielectric heating that takes place when materials are subjected to intense radio-frequency (RF) or microwave electric field[4]. Fruit is a major agricultural product in many growing regions. Sensing fruit quality rapidly and reliably, especially nondestructively, can be helpful in production, harvesting, and processing of such crops. The dielectric properties of fruits have been investigated in several studies. The differences in permittivity values between skin and pulp of several fruits have been noticed. It will help my research on determining the fruit maturity by not make the harmful to the fruits[1].It only connected to the fruits by touch only the outer skin. This open probe is dual probe, its content two ports which is port 1 and port 2. Usually the research that figure out from the other literature review and journal shows the dielectric spectroscopy or dielectric properties open ended probe that only use one single probe or one port only. When the research has been made, there is the almost similarity to the method that will be develop. Research on the fruits, there are many type of fruits that many researcher make it as a sample to their study.

The dielectric properties of most materials vary with several factors. In agricultural products, the amount of water in the materials is generally a dominant factor. The dielectric properties also depend on frequency of applied electric field, temperature, density, structure and chemical composition of materials.[5] Increasing consumer demand for high-quality fruit has led to the development of optical, acoustic and mechanical sensors that determine its quality. According to Shewfelt, the internal characteristics, which are perceivable by the senses of taste, smell, and touch (mouthfeel), are the ones that will determine the decision to repurchase that product. About the interaction between food and electromagnetic energy at low frequencies, much less is known. At high frequencies, the electric properties of most basic interest are the dielectric properties that affect energy coupling and distribution within the product, which includes the product attenuation constant determining voltage and power penetration depths (Dp) within the product and therefore the temperature at a specific depth[6]. The main advantages of high frequency methods consist of reducing process time, offering more uniform heating patterns and improving product quality in selected applications.

Exact knowledge of the penetration depth can have significant ramifications in certain applications. For instance, Meaney have utilized a small diameter probe to acquire superficial measurements. The former of these two developed their own technique for determining an effective penetration depth, which is similar in some respects to that described in this paper. This effort was primarily geared towards differentiating the properties of skin and subcutaneous fat. It did indicate that the penetration depth was primarily a function of the probe diameter[7].

2.3 SCATTERING PARAMETER

S-parameters describe the input-output relationship between ports (or terminals) in an electrical system. For instance, if there are 2 ports (intelligently called Port 1 and Port 2), then S12 represents the power transferred from Port 2 to Port 1. S21 represents the power transferred from Port 1 to Port 2[8]. In general, SNM represents the power transferred from Port M to Port N in a multi-port network.

A port can be loosely defined as any place where it can deliver voltage and current. So, if the communication system with two radios (radio 1 and radio 2), then the radio terminals (which deliver power to the two antennas) would be the two ports. S11 then would be the reflected power radio 1 is trying to deliver to antenna 1. S22 would be the reflected power radio 2 is attempting to deliver to antenna 2. And S12 is the power from radio 2 that is delivered through antenna 1 to radio 1. Note that in general S-parameters are a function of frequency.

As an example, consider the following two-port network:

Figure ‎2.3.1: Two ports of antenna

In the above Figure 2.3.1, S21 represents the power received at antenna 2 relative to the power input to antenna 1. For instance, S21=0 dB implies that all the power delivered to antenna 1 ends up at the terminals of antenna 2. If S21=-10 dB, then if 1 Watt (or 0 dB) is delivered to antenna 1, then -10 dB (0.1 Watts) of power is received at antenna 2.

If an amplifier exists in the circuitry, then S21 can show gain (i.e. S21 > 0 dB). This means that for 1 W of power delivered to Port 1, more than 1 W of power is received at Port 2.

Figure ‎2.3.2: Graphical view of S- parameter

In practice, the most commonly quoted parameter in regards to antennas is S11. S11 represents how much power is reflected from the antenna, and hence is known as the reflection coefficient (sometimes written as gamma: R or return loss. If S11=0 dB, then all the power is reflected from the antenna and nothing is radiated. If S11=-10 dB, this implies that if 3 dB of power is delivered to the antenna, -7 dB is the reflected power. The remainder of the power was "accepted by" or delivered to the antenna. This accepted power is either radiated or absorbed as losses within the antenna. Since antennas are typically designed to be low loss, ideally the majority of the power delivered to the antenna is radiated. See also VSWR, which is directly related to S11.

As an example, consider the plot of S11 in the following figure:

Figure ‎2.3.3: Scattering parameter graph of S11

The above Figure 2.3.2 would typically be measured using a Vector Network Analyzer (VNA), which can plot S11. The above figure implies that the antenna radiates best at 2.5 GHz, where S11=-10 dB. Further, at 1.5 GHz the antenna will radiate virtually nothing, as S11 is close to 0 dB (so all the power is reflected)[9]. The antenna bandwidth can also be determined from the above figure. If the bandwidth is defined as the frequency range where S11 is to be less than -6 dB, then the bandwidth would be roughly 1 GHz, with 3 GHz the high end and 2 GHz the low end of the frequency band.

2.4 FREE SPACE METHOD

Free-space measurement techniques provide a method for determining the permittivity and Permeability of a material under test. These methods are distinct from others in that they contactless; the material under test does not make direct contact with any active component of the measurement setup, such as a coaxial probe or a waveguide segment.

The active portion of a free-space setup is used for generating a plane-wave and measuring it after it has interacted with the material under test, either reflection from or transmission through the sample. Several methods have been used to generate a plane-wave for the purpose of material characterization.

The developments of microwave and millimetre-wave technologies in the 20th and early 21st centuries have enabled measurements at progressively higher frequencies. Over time, the accuracy and bandwidth of measurements have been pushed higher as the technology has been refined. A review of key works related to free-space measurements follows, which serves to reinforce the validity of the free-space measurement technique. In the 1940s, microwave free-space experiments had reached up to 50 GHz in a few cases. The measurements were largely focused on developing an understanding of wave propagation in the Earth’s atmosphere for communications and radar applications, in part due to the onset ofWorld War II. Thus, much of the research on free-space measurement and characterization of dielectrics was performed on water in liquid and gaseous states, to better understand its effects on free-space wave propagation.

This experimental setup was notable in that its horn antennas incorporated dielectric lenses made of polystyrene, which were used to reduce the divergence of the microwave beam. Despite this, it was noted by the author that diffraction effects about the sample were significant enough to deviate significantly from the ideal plane-wave condition. These diffraction effects were reduced by orienting the polarization of the wave perpendicular to the plane of incidence.

Nicolson, Ross and Weir’s published results were band-limited due to the nature of their experimental setups and were not performed in free-space. In their works, samples were placed inside a waveguide structure, which limits the usable frequency range for measurements and requires that samples be precisely machined to fit. A technique was proposed by A. L. Cullen 1987 that allowed for the measurement of much broader frequency ranges by measuring the sample in free-space. This eliminated the need for waveguides and allowed for samples to be measured nondestructively, without altering them to fit precisely within a waveguide. The researchers in presented results for quartz, Teflon and PVC and concluded that free-space measurements were accurate and that the results were comparable to waveguide-based techniques[10]. They also found that the spot-focus microwave beam (approximately 1 wavelength in footprint) did not suffer significant diffraction effects for samples larger than 3 beam-widths. They noted that the iterative solution method used could result in ambiguous solutions for samples thicker than a quarter-wavelength in the medium, unless two samples of differing samples were measured.

In 1990, Ghodgaonkar et al. expanded on their measurement system, widening the bandwidth up to 40 GHz and modifying their technique to work with transmission-mode measurements[11]. Results were presented from 8.6 GHz to 13.4 GHz for Teflon and borosilicate glass, reporting less than 5% error at mid-band for both complex permittivity and complex permeability using the technique on samples approximately one quarter-wavelength thick. Inaccuracy was noted with low-loss materials, with the authors suggesting reflection measurements to determine this accurately for nonmagnetic materials. For thin, flexible samples, the technique of sandwiching a sample between quartz plates was used to prevent the samples from sagging and thus deflecting the microwave beam. The thickness of the quartz plates was λm / 2 at mid-band, where λm is the wavelength inside the quartz plates. To compensate for their presence, the technique of de-embedding was employed, which is discussed in further detail in the section I.B Microwave and Millimeter-wave Measurements, De-embedding.

2.5 SOFTWARE

2.5.1 Computer Simulation Technology (CST) Microwave Studio

CST Microwave Studio provides a wide variety of different solvers and results. Its contain a lot of results and many parameters, however, in this chapter its only concentrates on S-parameters. The results will be obtained by two alternative techniques. One is from the Waveguide and Cavity filter and the other one is Coaxial (TEM) connectors. One simulation will be performed in time domain on a hexahedral mesh and the other in frequency domain on a tetrahedral mesh. Each port that is defined will simulate an infinitely long waveguide (here a coaxial cable) that is connected to the structure at the port’s plane. Waveguide ports are the most accurate way to calculate the S-parameters of filters and should therefore be used in this case. Since a waveguide port is based on the two dimensional mode patterns in the waveguide’s cross-section, the port must be defined large enough to entirely cover these mode fields. In the case of a coaxial cable, the port has to completely cover the coaxial cable’s substrate.

By choosing the electromagnetic properties and appropriate thickness for each layer, it is possible to synthesize composite bi-layer materials with novel electromagnetic properties otherwise not found in a single material[12]. a waveguide measurement method is

presented to estimate the complex permittivity εr of each layer of a composite bi-layer material. This method requires associating an optimization program with dynamic electromagnetic analysis. The electromagnetic analysis is based on the use of transmission. rectangular waveguide loaded with bi-layer dielectric sample presented in Figure 1.

Figure ‎2.5.1: Rectangular waveguide loaded with bi-layer dielectric material

Figure 2.5.1 shows two open-ended coaxial lines terminated in flanges are in contact with the MUT. The MUT has a homogeneous and isotropic complex permittivity ε and permeability μ. The flanges and the MUT are assumed to be infinite in the radial direction. The region between flanges filled by the MUT forms a radial waveguide excited at its centre by the probe fields. Coaxial lines are filled with air and operate at frequencies that only fundamental TEM mode can propagate through the lines[2].