The Modern Telecommunications Industries Engineering Essay

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In modern telecommunications industries, the present of wireless technologies have given birth to the Radio Frequency Identification (RFID) system. RFID enable identification, location and information exchange of distant via radio waves [1]. RFID become very vital in our daily life as it growing number of application in supply chain management, identification documents such as passport, and contactless payment such as "visa waves". The performance of antenna that consist in the RFID system become important to ensure the efficiency of communication between tag and reader. The antenna properties such as overall size of antenna, return loss, antenna gain, and detection range are the main factors to justify the antenna performance. This had initiated researchers to continue study in various type of antennas, one of which is microstrip patch antenna.

Microstrip patch antenna becoming popular in RFID system because of its several advantageous that allow us to made conformal into host object such as low profile, light weight and planar configuration.

The presents of UHF band for RFID systems again made the RFID systems particularly important in recent years. UHF RFID are able to provide high speed reading because of its higher data rate as compared to HF RFID, capable of multiple access, and long range detection. [1]

In RFID systems, miniaturization of microstrip antenna is one of directions for recent study of RFID antenna, several techniques had been introduced in reducing antenna size such as fractal-shape antennas. Since tag are usually attached in the object that is to be identified, so tag antenna need to be as small size as possile whereas the performance of antenna would not be compromised.

Objective

Objective of this project is to investigate, analyse, and study the performance of microstrip patch antenna. Thus, designed microstrip patch antenna in UHF band for RFID applications. Besides that, the parameters that affected the performance antenna are studied to achieved the objective.

Scope of work

Background research of microstrip patch antenna.

Background research of RFID systems.

Design microstrip patch antenna based on targeted RFID chip.

Study on techniques to reduce size of antenna.

Project timeline

An organized project management is essential for a project to be carried out efficiently. The timeline of Project Part 1 and Part 2 are summarized below:

Part 1:

Milestone

Week

1

2

3

4

5

6

7

8

9

10

11

12

13

14

Project Proposal

•

Background Research

•

•

•

Development

•

•

•

•

•

•

•

•

•

•

•

•

•

Project Documentation

•

•

Project Testing

•

•

Project Presentation

•

Part 2:

Milestone

Week

1

2

3

4

5

6

7

8

Finalized Part 1

•

Development

•

•

•

•

•

•

Project Documentation

•

•

Project Testing

•

•

•

Project Presentation

•

Disertion Overview

There are total 6 topics covered in this disertion:

Chapter 1: The first chapter would be the introduction to the project, explanation of objective, specified the scope of works, and project management.

Chapter 2: Study the background knowledge of microstrip patch antenna, description of antenna characteristic, literature review on RFID systems, and matching techniques.

Chapter 3: Briefly description of Ansoft High Frequency Structure Simulator (HFSS) that used in designing and simulating the antenna.

Chapter 4: Discussion on the antenna design and appoarch. This chapter discusses the basic design of microstrip patach antenna and presents miniaturization techniques for microstrip patch antenna.

Chapter 6: presents the simulation result and the discussion on the results obtained from the simulation.

Chapter 7: presents the conclusion of the project and suggestion for future work.

Chapter 2: Literature Review

2.1 Introduction to Antenna

In telecommunications network, antenna as the device to convert guided waves into free space waves at transmitting end, meanwhile convert free space wave into guided waves at the receiving end of the network. Antenna can be categorised as active and passive antenna. Active antenna comes with an amplifier that is attached very close to the antenna to minimize the transmission losses. Active antenna also known as non-reciprocal devices, whereas passive antennas are reciprocal. A antenna is said to be reciprocal device if and only if the antenna are passive and contains only isotropic materials. Meanwhile, reciprocal antenna also radiating power equally regardless of direction, hence made power losses are same in antenna between any two ports[2].

There are three major classes of passive antenna: printed antenna, wire antennas, and aperture antenna. These antennas are classified based on the geomatries and shape of the antenna. Wire antennas consists of long wire that is electricallly connected to devices in one end, whereas the other end are freely to the space It can be arranged in any shape depend on the availability of space, the most common wire antennas are dipole antenna, helix antenna and loop antenna.

Figure 2.0: Helical antenna

Figure 2.1: Loop antenna

Figure 2.2: Dipole antenna

Aperture antennas are the antenna that have some sort of opening at one end like a open mouth which allows the electromagnetic waves propagate in and out through the opening for transmitting and receiving purposes. Horn antenna and slot antenna are two common aperture antenna.

Figure 2.3: Horn antenna

Printed antenna also known as microstrip antenna, the most common printed antenna are printed slot antenna and microstrip patch antenna. Microstrip patch antenna will be further discuss in this chapter.

2.2 Antenna Properties

2.2.1 Antenna Gain

Gain of antenna is measurement that combine antenna's directivity and efficiency. It is the ratio of the radiation intensity in the given direction to the radiation intensity of antenna that the obtained power accepted from antenna radiated isotropically. Gain measurement are excluded the losses due to polarisation missmatch and impedance missmatch. Gain and directivity is related by the efficiency of antenna, thus antenna's gain would be equal to antenna's directivity if there are no losses in antenna.[3]

Directivity, D = -----------------------------------------------------------(2.1)

Where, U = radiation intensity

= total radiated power

Efficiency, --------------------------------------------------------(2.2)

Where, = radiation resistance

= dielectric resistance

Gain, G = -----------------------------------------------------------------------------(2.3)

2.2.2 Reflection Coefficient

Reflection coefficient is a figure that indicated reflected wave's amplitude or intensity when the electromagnetic wave propagete throughout the medium that contain discontinuities. It is a ratio of reflected wave's amplitude to incident wave's amplitude[4].

-----------------------------------------------------------------------------------(2.4)

Where, is reflected wave in term of electric field strength

is incident wave in term of eletric field strength

---------------------------------------------------------------------------------(2.5)

Where, is impedance toward the source

is the impedance toward the load

----------------------------------------------------------------------------(2.6)

Where, VSWR is voltage standing wave ratio

2.2.3 Impedance

The characteristic impedance of transmission line and input impedance of antenna must be strongly matched in order to prevent large reflection of incident waves to be generated and scattered back to the energy source. Only identically matched of impedance between transmission line and antenna can achieve maximum power transfer, hence to maximize the overall system performance by reduce noise due to reflected waves.

From the equation 2.5, missmatch loss can be calculated as[5]

------------(2.7)

Where, is incident power

is reflected power

2.2.4 Voltage Standing Wave Ratio ( VSWR)

Voltage Standing wave ratio is described as a ratio of maximum voltage to minimum voltage in the transmission line. Voltage standing waves pattern generated due to impedance mismatch, when a incident wave reached the terminal, some of the incident waves will be reflected back as reflected wave. These reflected waves mixes with incident waves to form voltage standing waves.

The ideal cases for VSWR is 1:1, this mean no power is being reflected back to source and all energy is being absorped in input terminal. In real world, most of system required antenna with VSWR value from 1.2: 1 to 1.5:1 . VSWR can be improved by several ways, one of it is by implement impedance matching circuit, thus antenna input impedance can be closely to transmission line's characteristic impedance.[5]

From equation 2.6 at above, VSWR can rewrite as,

------------------------------------------------------------------(2.8)

2.2.5 Bandwidth

Antenna bandwidth refers to a range of frequencies which allow the antenna to operates with fulfill certain set of specification performance, hence the incident wave can radiate effectively. Switched circuit is needed for the antenna that have a broad operating frequency band in order to maintain the impedance matching thus, made the antenna conforms to a specified standard.

Figure 2.4: VSWR over Frequency

From figure 2, we are able to determine bandwidth by taking the value of VSWR that below 2. The first intersection point in the graph is determined as low frequency, meanwhile the second intersection point in the graph is determined as high frequency. Bandwidth percentage can be obtain by taking the difference between low and high frequency divided by resonant frequency.

Bandwidth, --------------------------------------------------(2.9)

Percentage bandwidth, ------------------------(2.10)

Where, is high frequency

is low frequency

is resonant frequency

2.2.6 Polarisation

The polarisation of electromagnetic waves can be described as the orientation of E-field vector contains in electromagnetic waves when it is being radiated. Polarisation can be classified as linear, circular, and elliptically polarisation. Linear polarisation can be devided into two categories which are vertically polarised and horizontally polarised. Vertically polarisation are usually in low frequency antenna because of ground effect and physically construction method, whereas horizontally polarisation generally in high frequency antenna.

Elliptically polarisation is described as E-field contains electromagnetic traces ellipse in space as function of time. E-field in electromagnetic traced in circle, it it said to be circular polarisation. Circular polarisation can be classified as clockwise (CW) and counter clock-wise (CCW). Clockwise orientation of electric field is refers to right-hand polarisation, whereas counter clock-wise is knowns as left-hand polarisation.

The instantaneous field of plane wave travelling in the negative z-direction can be written as

--------------------------------------------------(2.11)

The instantaneous components are related to its complex counterparts by

---------------------------------------------------(2.12)

-------------------------------------------------(2.13)

Where and are respectively the maximum magnitude of the x and y components. For the wave to have linear polarisation, the time-phase difference between the two components must be

, ------------------------------(2.14)

Circular polarisation can be achieved only when the magnitude of the two components are the same and the time-phase difference between them is odd multiple of . That is,

----------------------------------------------------(2.15)

When,

----------------------(2.16)

If the direction of wave propagation is reversed, (ie + Z direction) the phase for CW and CCW must be interchanged.

Elliptical polarization can be attained only when the time-phase difference between the two components is odd multiples of and their magnitudes are not the same or when the time-phase difference between the two components is not equal to multiples of . That is,

----------------------------------------------------(2.17)

When,

Or

-------------------------------------------------(2.18)

2.2.7 Radiation pattern and Half- power beanwidth

Radiation patterns are graphical depiction of the direction of energy radiated that usually presented in 360 degree polar forms and it is presented on relative power dB scale. The radiation pattern of antenna shows how well the relative field strength being transmited or received by an antenna. Antenna do not radiated as much energy as it received by the input connector, which means the total energy radiate to its input connector is not equal to energy radiated by antenna.

Figure 2.5: Radiation Pattern

The side lobs and back lobes are unwanted properties for radiation patterns, the radiated or received energies from side lobes and back lobes are from the undesire direction, only the energy from main lob are useful, hence the energies from side and back lobes are wasted and reduce the antenna performance. Besides that energies received from side and back lobes may caused interference, meanwhile energies radiated through side and back lobes may caused interference to other systems as well. In real world, side and back lobes cannot be eliminated completely[6].

The radiation pattern can be expressed in a mathematical equation as:

----------------------------------------------------------------(2.19)

E-plane with

-----------------------------------------------------------(2.20)

H-plane with

Half Power beamwidth can be determined using the following equation:

----------------------------------------------------------(2.21)

-------------------------------------------------------------(2.22)

2.3 Introduction to RFID System

2.3.1 What is RFID?

RFID stands for Radio Frequency Identification, it is a technology that use radiowave to comminicate between transponder (tag) and interrogators ( reader), by allowing reader to extract the data stored in the tag, thus to identify the object based on the information from transponder that integrated in the object or person, and all these are done automatically with human involvement.

RFID systems consists of three basics components, there are antenna, transponder and interrogator. Transponder is data-carrying device which consists of antenna for receiving and transmitting RF signal and an integrated circuit for storing and signal processing such as modulating and demodulating RF signal. Generally, transponder can be classifiied into 3 categories; passive, active, and battery assisted passive (BAP)[7].

Meanwhile, interrogator is a data-capturing device that allow data from transponder to be collected. The integorator consists of three components, there are antenna, control unit and radio frequency module that consists transmitter and receiver. Interrogators can classified to two types; read only interrogator or read and write interrogators. Reader also can be categorised as fixed RFID reader or mobile RFID reader. Fixed RFID reader is the reader that in stationary position, where as mobile RFID reader is the reader that can be mobilised and created larger interrogation zone.

Figure 2.5: Basic Configuration of RFID System

Type of Transponder

2.3.2.1 Passive RFID Transponder

Passive tag does not require internal or external power source to provoke communication between tag and reader. When entering the interrogation zone created by reader, the tag derive all the power from the induce current from the E-field.

Advantages of Passive Transponder:

Tag function without battery, hence these tags have longer life time.

Inexpensive to manufacture

Small in size.

Disadvantages of Passive Transponder:

Tag can be only at very short distance

May not possible to include sensors that can use electricity for power.

2.3.2.1Active RFID Transponder

Active RFID Transponder uses internal battery to power the tag circuit. It can transmit signals once an radiowave sent by reader has been successfully identified. Active Transponder also uses its baterry to broadcast radiowave to reader.

Advantages of Active Transponder:

Improve the utility of device as it can be read from a far distance.

Can integrate other sensors that require electricity for power.

Disadvantages of Active Transponder:

Limited lifetime of tag due to internal battery flat.

Expensive

Physically larger that limit its application

Require mantainance

2.3.2.3 Baterry Assisted Passive (BAP) Tansponder

Baterry assisted passive transponder also know as semi-passive tranponder require an external source to wake up the comminucation between interrogator, but it has significant higher forward link capability providing greater range.

Advantages of BAP Transponder:

Greater detection range due to internal power source

Not an intentional radiator which will generate eletromagnetic interference

Higher success rate.

Disadvantages of BAP Transponder:

Limited life time

Require mantainance if battery is flat.

Physically large

Figure 2.6b: Interrogator (tag) Figure 2.6b: Transponder (Tag)

How RFID works?

RFID operates starting with interrogator that antenna to transmit modulated RF signal, the m,odulated signal contain a series of data commands to the transponder. The integrated circuit in transponder received power and respond by varying its input impedance and thus modulating the backscattered signal that carry identification information to interrogator[7]. The interrogator and then covert signal received from transponder into digital signal and sents to computer for data processing.

RFID systems are able to operate in several frequencies band such as high frequncy(HF) and Ultra High Frequency (UHF). According to ISO and some other international organization frequency for UHF RFID is in between 850 MHz and 950 MHz. However, UHF RFID operates at different frequencies around the world. 915 MHz is a frequency for UHF RFID in United States, while, 867MHz is frequency for Europe and 960 MHz is frequency for Japan[8].

Microstrip Patch Antenna

2.4.1 Introduction to Microstrip Patch Antenna

The concept of microstrip radiator antenna was first proposed by Deschamp in 1953[9], and the first microstrip antenna was fabricated after 20 years the concept had been proposed [9]. Microstrip patch antenna consists of 3 basic components, there are patch, ground and substrate. The radiating patch is on one side of dielectric substrate, meanwhile the ground is place on the other side of dielectric, these means the patch and the ground of microstrip patch antenna is separated by dielectric substrate. The patch is generating made of good conductor materials such as copper and it can be in any geometric shapes, but in this project rectagular shape is used because of its simplicity of the analysis and performance prediction. The patches are generally photo etched on the dielectric substrate.

Figure 2.7: Basic Microstrip Patch Antenna

In rectangular microstrip patch antenna, the value of length (L) is normally in between 0.3333 to 0.5, where is wavelength in the free-space. Meanwhile, the height of the dielectric substrate is usually and the relative permeability is typically in range . Patch thickness, should be very such that .

Width of microstrip patch antenna can be calculated using:

-----------------------------------------------------------------(2.23)

Where, is operating frequency

is relative permeability of dielectric substrate

is speed of light

Length of the microstrip patch antenna can be calculated using:

----------------------------------------------------------------(2.24)

Where, the efficient dielectric constant,

----------------------------------------------(2.25)

The effective patch length ( is calculated using,

--------------------(2.26)

Besides rectangular shape, there are some other common shape like square, dipole, circular, triangular, circular ring and elliptical as shown in figure 2.8. Microstrip patch antenna radiate primarily because of the fringing fields between the patch edge and the ground plane[9]. The thick dielectric substrate with low dielectric constant is better as it give better efficiency, larger bandwidth and better radiation. In other words, the size of the antenna will be larger, in order to produce compact antenna higher dielectric constant which reduce antenna's efficiency and caused narrower bandwidth are used.

Figure 2.8: Common Microstrip Patch Antenna Shape

Advantages and Disadvantages of Microstrip Patch Antenna

Microstrip patch antenna gain popularity in recent telecommunication and wireless application among all the antennas because of its low-profile structure. Microstrip patch antenna are used as communication antennas on misile because of its conformability and thin. Microstrip patch antenna also used in statellite communication, it also used in handheld wireless device, therefore it is extremely compatible to embed. Below are the advantages discussed by Kumar and Ray [9] are given below:

Light weight and low volume

.

Low profile planar configuration which can be easily made conformal to host surface.

Low fabrication cost, hence can be manufactured in large quantities.

Support both, linear and circular polarization.

Can be easily integrated with microwave integrated circuits (MICs).

Capable of dual and triple frequency operation.

Mechanically robust when mounted on rigid surfaces.

However, microstrip patch antenna also some limitations compared to conventional microwave antenna, there are:

Narrow bandwidth.

Low efficiency.

Low gain.

Extraneous radiation from feed and junction.

Poor end fire radiator except tapered slot antenna.

Low power handling capacity.

Surface wave excitation.

Microstrip patch antenna have a very high antenna quality factor, Q which means it have low efficiency and narrow bandwidth[9]. Q can be reduce by increasing the thinkness of dielectric substrate, and this can caused degradation of the antenna characteristic because of increasing fraction of the total power delivered by the source goes into a surface wave, the surface wave scattered at the dielectric bends thus it is consider as an unwanted power loss. The effect of the surface wave can be reduced by use of photnic bad gap structures.

Feeding Mechanism

There are 2 categories of feed techniques for microstrip patch antenna which are contacting and non-contacting. From the word contacting we know that the feeding element such as microstrip line have physical contact to the patch, the RF power is then fed directly to the radiating patch. Meanwhile, non-contacting techniques is using eletromagnetic field coupling to deliver power from microstrip line to radiating patch. There are 4 commons feed mechanism : microstrip line feeding, coaxial probe feeding, aperture coupling and proximity coupling. Microstrip line and coaxial probe feeding are contacting schemes, while aperture and proximity coupling are non-contacting schemes.

Coaxial probe

Coaxial probe using the inner of the coaxial connector extends through the dielectric and is soldered to the radiating patch, and the ground plane is connected with outer layer conductor from same coaxial cable. Figure 2.9 illustrated the coaxial probe feeding techniques.

Figure 2.9: Coxial probe feeding

The main advantages of this techniques is that the feed can be placed at any desired location within the radiating patch as impedance matching technique thus, no external circuit need to be constructed for impedance matching purpose. This feed method is easy to fabricate and has low spurious radiation. But is has several disadvantages. First, coaxial feeding compromises its reliability as it need large number of solder joint. Second, thicker substrate is needed to increase bandwidth of the patch antenna and therfore requires longer coxial probe. This make the input impedance more inductive and caused matching difficuities. These problems can be solved by non-contacting feeding schemes[9].

Microstrip line feeding

Microstrip line feeding is using a conductive strip to connect directly to the radiating patch. This technique is appears to be easier way to fabricate because the microstrip line can be considered as an extension from the radiating patch and thus, it can be fabricate simultaneously. The conducting strip have smaller width as compared to patch, and it provided planar structure as both of the feed line and patch are ecthed on same substrate[9].

Figure 2.10: Microstrip line feeding

Aperture coupled feeding

Aperture coupled feeding uses two substrates seperated by a common ground plane. A microstrip feed line on the lower substrate is electromagnetically coupled to the patch through the slot aperture in the common ground.

Figure 2.21: Aperture coupled feeding

The coupling aperture is usually centered under the patch, this symmetrical configuration caused lower cross-polarisation. The amount of coupling is depend on the shape, size and location of the aperture. Spurious radiation is minimised because the feed line and the patch are seperated by ground plane. High dielectric materials is used for bottom substrate while, the low dielectric constant is used for the top substrate to maximised the radiation from the patch. Difficulties to fabricate multiple layers antenna, which also increases antenna thickness and narrow bandwidth are the disadvantages

Proximity Coupled feeding

Proximity coupling feeding are electromagnetically coupling to the radiating patch just like aperture coupling.The difference between proximity coupling and aperture coupling technique is that proximity coupling do not have a slot aperture. The main advantages of proximity coupling feeding is that it eliminates spurious feed radiation and provide very high bandwidth, due to overall increase in the thickness of the microstrip patch antenna[10]

Figure 2.22: proximity coupled feeding

Impedance matching can be done by adjusting the length of the feed line and the wifth-to-line ratio of the patch. Difficulties to fabricate multiple layers antenna, which also increases antenna thickness is the disadvantages of it.

Table 2.1: The characteristic of different feed techniques.

Characteristic

Microstrip line feeding

Coaxial feed

Aperture coupled feed

Proximity coupled feed

Spurious feed radiation

More

More

Less

Minimum

Reliability

Better

Poor due to soldering

Good

Good

Ease of fabrication

Easy

Soldering and drilling needed

Alignment required

Alignment required

Impedance matching

Easy

Easy

Easy

Easy

Bandwidth

2-5%

2-5%

2-5%

13%

2.4.4 Method of Analysis

2.3.4.1 Transmission line model

This model represents the microstrip antenna by two slots of width W and height , separated by a transmission line of length L. The microstrip is essentially nonhomogeneous line of two dielectric, typicall the substrate and air[10].

Figure 2.23: Microstrip line

Figure 2.24: Electric field lines

From figure 2.24, most of the electric field lines reside in the substrate and some parts of line are in the air. As a result, this transmission line cannot support pure transverse electric-magnetic (TEM) mode of transmission, since the phase velocities would be different in the air and the substrate. Instead, the dominant mode of propagation would be the quasi-TEM mode. Hence, an effective dielectric constant must be obtained in order to account for the fringing and the wave propagation in the line[10]. The value of is slightly less than because the fringing fields around the periphery of the patch are not confined in the dielectric substrate but are also spread in the air as shown in Figure 2.23 above. The expression for is given by Balanis as[9]:

----------------------------------------------(2.25)

Where, = Effective dielectric constant

= Dielectric constant of substrate

= Height of dielectric substrate

= Width of the patch

Consider figure 2.25 below shows that antenna with length L, and width W resting on a substrate of height h. the co-ordinate axis is selected such that the length is along the x-direction and the height is along the z-direction[10].

Figure 2.25: Microstrip Patch Antenna

In order to operate in the fundamental mode, the length of the patch must be slightly less than where is the wavelength in the dielectric medium and is equal to where is free space wavelength. The mode implies that the field varies one cycle along the length, and there is no variation along the width of patch. Figure 2.26 shown, the microstrip patch antenna is represented by two slots, seperated by a transmission line of length L and open circuited at both the ends. The fields at the edges can be resolves into normal and tangential components with respect to the ground plane [10].

Figure 2.26: Top view of the antenna

Figure 2.27: Side view of antenna

It is seen from Figure 2.27 that the normal components of the electric field at the two edges along the width are in opposite directions and thus out of phase since the patch is 2λlong and hence they cancel each other in the broadside direction. The tangential components, which are in phase, means that the resulting fields combine to give maximum radiated field normal to the surface of the structure. Hence the edges along the width can be represented as two radiating slots, which are 2λapart and excited in phase and radiating in the half space above the ground plane. The fringing fields along the width can be modeled as radiating slots and electrically the patch of the microstrip antenna looks greater than its physical dimensions. The dimensions of the patch along its length have now been extended on each end by a distance Δ L

-----------------(2.26)

The effective length of the patch

Chapter 3: Simulation Software

3.1

Chapter 4: Design and Appoarch

Miniaturization of Microstrip Patch antenna

Chapter 5: Result and Discussion

Chapter 6: Conclusion and Recommendation

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