DUAL FREQUENCY 24 GHZ TSHAPED AND 52 GHZ INVERTED LSHAPED MONOPOLE ANTENNA FOR WLAN APPLICATIONS

SV:Nguyễn Xuân Kim Cương MSSV:060534D Page 5 1.2 Plane Wave Properties Many solutions to Maxwell’s equations exist and all of these solutions represent fields which could actually be pr

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DUAL FREQUENCY 2.4 GHZ T-SHAPED AND 5.2 GHZ

INVERTED L-SHAPED

MONOPOLE ANTENNA FOR

WLAN APPLICATIONS

Adviser Dr YIH-CHIEN CHEN

Student NGUYEN XUAN KIM CUONG

ID Student : 060534D

Class : 06DD2D

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SV:Nguyễn Xuân Kim Cương MSSV:060534D

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Contents

Chapter1: PROPERTIES OF ELECTROMAGNETIC 4

1.1 Maxwell’ Equation 4

1.2 Plane Wave Properties 5

1.3 Diffraction 5

1.4 Field Relationships 6

1.5 Poynting Vector 6

1.6 Phase Velocity 7

1.7 Polarisation States 7

1.8 Lossy Media 8

Chapter 2: PROPAGATION MECHANISMS 10

2.1 Lossless Media 10

2.2 Rough Surface Scattering 13

Chapter 3:ANTENNA FUNDAMENTALS 15

3.1 Necessary Conditions for Radiation 15

3.2 Near-Field and Far-Field Regions 16

3.3 Far-Field Radiation from Wires 17

3.4 Radiation Pattern 18

3.5 Radiation Resistance and Efficiency 19

3.6 Power Gain 20

3.7 Bandwidth 21

3.8 Directivity 23

Chapter 4: PRACTICAL ANTENNA 24

4.1 Dipole Structure 24

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4.2 Current Distribution 25

4.3 Reﬂector Antennas 25

4.4 Horn Antennas 26

4.5 Loop Antennas 27

4.6 Helical Antennas 28

4.7 Patch Antennas 28

Chapter 5: THE MICROSTRIP ANTENNA DESIGN 31

5.1 Historical Development 31

5.2 Basic Microstrip Line 31

5.3 Microstrip Field Radiation 32

5.4 Substrate Materials 33

5.5 Basic Microstrip Antenna 35

5.6 Basic configuration of Microstrip Antenna 36

5.7 Advantages vs Disadvantages of Microstrip Antennas 38

5.8 Applications 39

5.9 Types of Microstrip Antennas 39

5.10 Microstrip Traveling-Wave Antennas 40

5.11 Microstrip Slot Antennas 42

Chapter 6: PRODUCTION PROCESS 45

6.1 Production Overview 45

6.2 Production Details 45

6.3 Improving Antenna 49

6.4 Measurement 49

6.5 Antenna 50

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CHAPTER 7: THE FINAL REPORT 51

7.1 Introduction 51

7.2 Antenna Design Structure: 51

7.3 Simulations 52

7.4 Experience Design 53

7.5 Measurement Result: 53

7.6 Radiation Patterns: 53

7.6.1 The Simulated Radiation Patterns In The X-Y, X-Z, Y-Z Planes at 2.4 GHz 53

7.6.2 The Simulated Radiation Patterns In The X-Y, X-Z, Y-Z Planes at 5.2 GHz 55

7.7 Conclusion: 57

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or a free charge The H ﬁeld is measured in amperes per metre and is generated by either a time-varying electric ﬁeld or a current Maxwell’s four equations can be summarised in words as

An electric field is produced by a time-varying magnetic field

A magnetic field is produced by a time-varying electric field or by a current Electric field lines may either start and end on charges; or are continuous Magnetic field lines are continuous

The first two equations, Maxwell’s curl equations, constain constants of proportionality which dictate the strengths of the fields These are the permeability

of the medium µ in the henrys per metre and permittivity of the medium Ɛ in the farads per metre They are normally expressed relative to the value of free space:

Ɛ=ƐrƐ0 µ0=4π x 10-7 Hm-1 µ=µrµ0 Ɛ0=8.854x 10-12 Fm-1

Ɛr, µr =1 in the free space Free space strictly indicates a vacuum, but the same value can be used as good approximations in dry air at typical temperatures and pressures

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1.2 Plane Wave Properties

Many solutions to Maxwell’s equations exist and all of these solutions represent fields which could actually be produced in practice However, they can all

be represented as a sum of plane waves, which represent the simplest possible time varying solution

Figure shows a plane wave, propagating parallel to the z-axis at time t = 0

The electric and magnetic fields are perpendicular to each other and to the direction of propagation of the wave; the direction of propagation is along the z axis; the vector in this direction is the propagation vector or Poynting vector The two fields are in phase at any point in time or in space Their magnitude is constant in the xy plane, and a surface of constant phase (a wavefront) forms a plane parallel to the xy plane, hence the term plane wave The oscillating electric field produces a magnetic field, which itself oscillates to recreate an electric field and so on, in accordance with Maxwell’s curl equations This interplay between the two fields stores energy and hence carries power along the Poynting vector Variation, or modulation, of the properties of the wave (amplitude, frequency or phase) then allows information to be carried in the wave between its source and destination, which is the central aim of a wireless commu- nication system

1.3 Diffraction

The geometrical optics field, accurate for many problems where the path from transmitter to receiver is not blocked However, such a description leads to entirely incorrect predictions when considering fields in the shadow region behind an

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obstruction, since it predicts that no field whatsoever exists in the shadow region as shown in Figure 3.11 This suggests that there is an infinitely sharp transition from the shadow region to the illuminated region outside In practice, however, shadows are never completely sharp, and some energy does propagate into the shadow region This effect is diffraction and can most easily be understood by using Huygen’s principle

1 Each element of a wave front at a point in time may be regarded as the centre of a secondary disturbance, which gives rise to spherical wavelets

2 The position of the wave front at any later time is the envelope of all such wavelets

1.5 Poynting Vector

The Poynting vector S, measured in watts per square metre, describes the magnitude and direction of the power flow carried by the wave per square metre of area parallel to the xy plane, i.e the power density of the wave Its instantaneous

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1.6 Phase Velocity

The velocity of a point of constant phase on the wave, the phase velocity v

at which wave fronts advance in the S direction, is given by

to the ground (E field horizontal) primarily generates waves that are horizontally polarised

The waves described so far have been linearly polarised, since the electric

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field vector has a single direction along the whole of the propagation axis If two plane waves of equal amplitude and orthogonal polarisation are combined with a 90 phase difference, the resulting wave will be circularly polarised (CP), in that the motion of the electric field vector will describe a circle centred on the propagation vector The field vector will rotate by 360 for every wavelength travelled Circularly polarised waves are most commonly used in satellite communications, since they can be generated and received using antennas which are oriented in any direction around their axis without loss of power They may be generated as either right-hand circularly polarised (RHCP) or left-hand circularly polarised (LHCP); RHCP describes a wave with the electric ﬁeld vector rotating clockwise when looking in the direction of propagation In the most general case, the component waves could be of unequal amplitudes or at a phase angle other than 90 The result

is an elliptically polarised wave, where the electric ﬁeld vector still rotates at the same rate but varies in amplitude with time, thereby describing an ellipse In this case, the wave is characterised by the ratio between the maximum and minimum values of the instantaneous electric ﬁeld, known as the axial ratio, AR,

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signiﬁcant conductivity, the amplitude of the wave diminishes with distance travelled through the medium as energy is removed from the wave and converted to heat

In consequence, the ﬁeld strength (both electric and magnetic) diminishes exponentially as the wave travels through the medium The distance through which the wave travels before its ﬁeld strength reduces to e- 1 = 0.368 = 36.8% of its initial value is its skin depth , which is given by

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The angle of the reﬂected ray is related to the incidence angle as follows:

Equation is Snell’s law of reflection, which may be used to find the point of reflection given by any pair of source (transmitter) and field (receiver) points as shown in the following figure

This law is one consequence of a deeper truth, Fermat’s principle, which states that every ray path represents an extremum (maximum or minimum) of the total electrical length kd of the ray, usually a minimum In Figure, the actual ray path

is simply the path which minimises the distance (d1 + d2 ), because the wave number is the same for the whole ray

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Fermat’s principle can also be used to ﬁnd the path of the refracted ray In this case, the wave

number in the two media is different, so the quantity which is minimised is (k1d1+

k2dt ), where dt is the distance from the point of reﬂection to the ﬁeld point in medium

2

Equation is consistent with the observation that the phase velocity of the wave in the medium with higher permittivity and permeability (the denser medium) is reduced, causing the transmitted wave to bend toward the surface normal This change in velocity can be expressed in terms of the refractive index, n, which is the radio of the free space phrase velocity, c, to the phrase velocity in the medium

Thus Snell’s law of refraction can be expressed as

Note that the frequency of the wave is unchanged following reﬂection and transmission; instead, the ratio v=2πf is maintained everywhere For example, a wave within a dense medium will have a smaller phase velocity and longer wavelength than that in free space

In addition to the change of direction, the interaction between the wave and the boundary also causes the energy in the incident wave to be split between the

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reflected and transmitted waves The amplitudes of the reflected and transmitted waves are given relative to the incident wave amplitude by the Fresnel [fra - nel] reflection and transmission coefficients, which arise from the solution of Maxwell’s equations at the boundary These express the ratio of the transmitted and reflected electric fields to the incident electric field The coefficients are different for cases when the electric field is parallel and normal to the scattering plane which are denoted

by subscripts || and ﬩, respectively The reflection coefficients are denoted by R and the transmission coefficients by T The coefficients depend on the impedances of the media and on the angles,

where Z1 and Z2 are the wave impedances of medium 1 and medium 2, respectively and the E fields are defined in the directions shown in the following figure The total reflected electric field is therefore given by

where a|| and a﬩ are unit vectors parallel and normal to the scattering plane, respectively, and the incident electric ﬁeld is permitted to take any polarisation state expressed as

In lossy media, Snell’s law of refraction no longer holds in its standard, form because the phase velocity of the transmitted wave (and the attenuation constant) depends on the incidence angle as well as on the constitutive parameters If a wave is incident from a dielectric onto a conductor, increasing the conductivity causes the refraction angle t to decrease towards zero whereas the attenuation constant increases, so the penetration of the wave into the conductor decreases See [Balanis, 89] for more details

Snell’s law of reﬂection still holds in lossy media, however and the Fresnel coefﬁcients can still be applied, the wave impedance is:

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2.2 Rough Surface Scattering

The reflection processes discussed so far have been applicable to smooth surfaces only; this is termed specular reflection When the surface is made progressively rougher, the reflected wave becomes scattered from a large number of positions on the surface, broadening the scattered energy This reduces the energy in the specular direction and increases the energy radiated in other directions The degree of scattering depends on the angle of incidence and on the roughness of the surface in comparison to the wavelength The apparent roughness of the surface is reduced as the incidence angle comes closer to grazing incidence (θi=90o) and as the wavelength is made larger

If a surface is to be considered smooth, then waves reﬂected from the surface must be only very slightly shifted in phase with respect to each other If there is a height difference between two points on the surface, then waves reflected from those points will have a relative

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Phrase difference of

A reasonable criterion for considering a surface smooth is if this phase shift

is less than 90 , which leads to the Rayleigh criterion,

This is illustrated in Figure For accurate work, it is suggested that surfaces should only be considered smooth if the roughness is less than one-quarter of the value indicated by the Rayleigh criterion

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Chapter 3:ANTENNA FUNDAMENTALS

Most fundamentally, an antenna is a way of converting the guided waves present in a waveguide, feeder cable or transmission line into radiating waves travelling in free space, or vice versa Figure shows how the fields trapped in the transmission line travel in one dimension towards the antenna, which converts them into radiating waves, carrying power away from the transmitter in three dimensions into free space

The art of antenna design to ensure this process takes place as efficiently as possible, with the antenna radiating as much power for the transmitter into useful direction, particularly the direction of the intended receiver, as can practically be achieved

3.1 Necessary Conditions for Radiation

A question then arises as to what distinguishes the current in an antenna from the current in a guided wave structure As Figure (a) shows, and as a direct consequence of Maxwell’s equations, a group of charges in uniform motion (or stationary charges) do not produce radiation In Figure (b)–(d), however, radiation does occur, because the velocity of the charges is changing in time In Figure (b) the charges are reaching the end of the wire and reversing direction, producing radiation

In Figure (c) the speed of the charges remains constant, but their direction is changing, thereby creating radiation Finally, in Figure (d), the charges are

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oscillating in periodic motion, causing a continuous stream of radiation This is the usual practical case, where the periodic motion is excited by a sinusoidal transmitter Antennas can therefore be seen as devices which cause charges to be accelerated in ways which produce radiation with desired characteristics Similarly, rapid changes

of direction in structures which are designed to guide waves may produce undesired radiation, as is the case when a printed circuit track carrying high-frequency currents changes direction over a short distance

3.2 Near-Field and Far-Field Regions

Close to an antenna, the field patterns change very rapidly with distance and include both radiating energy and reactive energy, which oscillates towards and away from the antenna, appearing as a reactance which only stores, but does not dissipate, energy Further away, the reactive fields are negligible and only the radiating energy is present, resulting in a variation of power with direction which is independent of distance These regions are conventionally divided at a radius R given by

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In sitting an antenna, it is particularly important to keep other objects out of the near field, as they will couple with the currents in the antenna and change them, which in turn may greatly alter the designed radiation and impedance characteristics

3.3 Far-Field Radiation from Wires

Many antenna types are composed only of wires with currents flowing on them In this section, we illustrate how the radiation from an antenna in the far field may be calculated from a knowledge of the current distribution on the wires

In Figure , an appropriate coordinate system is defined It is usually most convenient to work in spherical coordinates (r; θ ;φ) rather than Cartesian coordinates, with the antenna under analysis placed at or near the origin Often the z-

axis is taken to be the vertical direction and the x–y plane is horizontal, in which case denotes the azimuth angle

The simplest wire antenna is a Hertzian dipole or infinitesimal dipole, which is a piece of straight wire whose length L and diameter are both very much less than one wavelength, carrying a current I(0) which is uniform along its length, surrounded by free space If this dipole is placed along the z-axis at the origin, then, in accordance with Maxwell’s equations, it radiates fields which are given as follows Note that a phase term ejωt has been dropped from these equations for simplicity, and all of the fields are actually varying sinusoidally in

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at this distance and in any direction is

The radiation intensity is then

3.5 Radiation Resistance and Efficiency

The equivalent circuit of a transmitter and its associated antenna is shown in Figure The resistive part of the antenna impedance is split into two parts, a radiation resistance Rr and a loss resistance Rl The power dissipated in the radiation resistance is the power actually radiated by antenna, and the loss resistance is power lost within the antenna itself This may be due to losses in either the conducting or the dielectric parts of the antenna

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than 2:1, whichever is smaller The bandwidth is usually given as a percentage of the nominal operating frequency The radiation pattern of an antenna may change dramatically outside its speciﬁed operating bandwidth

This pattern is plotted as a surface in Figure with a cutaway portion to reveal the detail A radiation pattern plot for a generic directional antenna , illustrating the main lobe, which includes the direction of maximum radiation (sometimes called the bore- sight direction), a back lobe of radiation diametrically opposite the main lobe and several side lobes separated by nulls where no radiation occurs The Hertzian dipole has nulls along the z-axis

Some common parameters used to compare radiation patterns are deﬁned as follows:

The half-power beamwidth (HPBW), or commonly the beamwidth, is the angle subtended by the half-power points of the main lobe The pattern of the Hertzian dipole falls by one- half at θ =π/4 and θ=3/4, so its half-power beamwidth is =2 ¼ 90

The front-back ratio is the ratio between the peak amplitudes of the main and back lobes, usually expressed in decibels

The sidelobe level is the amplitude of the biggest sidelobe, usually expressed

in decibels relative to the peak of the main lobe

Two special cases of radiation patterns are often referred to The ﬁrst is the isotropic antenna, hypothetical antenna which radiates power equally in all directions This cannot be achieved in practice, but acts as a useful point of comparison More practical is the omnidirectional antenna, whose radiation pattern is constant in, say, the horizontal plane but may vary vertically The Hertzian dipole is thus clearly omnidirectional in the x–y plane as illustrated in Figure

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3.8 Directivity

The directivity D of an antenna, a function of direction, is deﬁned by

Sometimes directivity is speciﬁed without referring to a direction In this case the term

‘directivity’ implies the maximum value D(θ,ᵩ)=Dmax It is also common to express the directivity in decibels The use of the isotropic antenna as a reference in the second line is then emphasised by giving the directivity units of dBi:

In the case of the Hertzian dipole, the directivity can be shown to be D=3/2,

or approximately 1.8 dBi [Kraus, 01]

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in opposite directions in the two wires, so the radiation from the two elements is almost exactly cancelled, yielding no far-ﬁeld radiation

If a short section of length L/2 at the end of the transmission line is bent outwards, it forms a dipole perpendicular to the original line and of length L (lower half of Figure) The currents on the bent section are now in the same direction, and radiation occurs Although this radiation does change the current distribution slightly, the general shape of the current distribution remains the same and a sinusoidal approximation may be used to analyse the resulting radiation pattern

Some qualitative results may be deduced before the full analysis:

As the dipole is rotationally symmetric around its axis, it must be omnidirectional, whatever the current distribution

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In a plane through the transmission line and perpendicular to the plane of the page in In Figure, the distance from the arms of the dipole to all points is equal Hence the radiation contributions from all parts of the dipole will add in phase and a lobe will always be produced

The current always points directly towards or away from all points on the axis of the dipole, so no radiation is produced and a null appears at all such points

where I(0) is the current at the feed point, assuming that the dipole is aligned with the z-axis and centred on the origin This distribution is shown in Figure for various dipole lengths

It turns out that these results are exact if the wire forming the dipole is inﬁnitesimally thin, and they are good approximations if the wire thickness is

small compared with its length

Input Impedance

A thin, lossless dipole, exactly half a wavelength long, has an input impedance Za = 73 + j42,5Ω It is desirable to make it exactly resonant, which is usually achieved in practice by reducing its length to around 0:48l, depending slightly on the exact conductor radius and on the size of the feed gap This also reduces the radiation resistance

4.3 Reﬂector Antennas

Reﬂector antennas rely on the application of image theory, which may be described as follows If an antenna carrying a current is placed adjacent to a perfectly conducting plane, the ground plane, then the combined system has the same ﬁelds above the plane as if an image of the antenna were present below the plane The image carries a current of equal magnitude to the real antenna but in the opposite

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direction and is located an equal distance from the plane as the real antenna but on the other side This statement is a consequence of Snell’s law of reflection, given the Fresnel reflection coefficients for a perfect conductor

4.4 Horn Antennas

The horn antenna is a natural evolution of the idea that any antenna represents a region of transition between guided and propagating Horn antennas are highly suitable for frequencies (typically several gigahertz and above) where waveguides are the standard feed method, as they consist essentially of a waveguide whose end walls are ﬂared outwards to form a megaphone-like structure In the case illustrated, the aperture is maintained as a rectangle, but circular and elliptical versions are also possible The dimensions of the aperture are chosen to select an appropriate resonant mode, giving rise to a controlled ﬁeld distribution over the aperture The best patterns (narrow main lobe, low side lobes) are produced by making the length of the horn large compared to the aperture

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width, but this must be chosen as a compromise with the overall volume occupied A common application of horn antennas is as the feed element for parabolic dish antennas in satellite systems

4.5 Loop Antennas

The loop antenna is a simple loop of wire of radius a; it is small enough in comparison to a wavelength that the current I can be assumed constant around its circumference The resulting radiation pattern is

Note that this has exactly the shape of the Hertzian dipole pattern in Eq, except that the electric and magnetic ﬁelds are reversed in their roles As with the Hertzian dipole, the loop is relatively inefﬁcient In practice, loops are usually applied

as compact receiving antennas, e.g in pagers The loop need not be circular, with approximately the same ﬁelds being produced provided the area enclosed by the loop is held constant

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4.6 Helical Antennas

The helical antenna can be considered as a vertical array of loops, at least for the case when the diameter of the helix is small compared to a wavelength The result is normal mode radiation with higher gain than a single loop, providing an omnidirectional antenna with compact size and reasonable efﬁciency, but rather narrow bandwidth It is commonly used for hand-portable mobile applications where

it is more desirable to reduce the length of the antenna below that of a quarter-wave monopole

In the case where the diameter is around one wavelength or greater, the mode

of radiation changes completely to the axial mode

4.7 Patch Antennas

Patch antennas, as seen in Figure, are based upon printed circuit technology to create flat radiating structures on top of dielectric, ground-plane-backed substrates The appeal of such structures is in allowing compact antennas with low manufacturing cost and high reliability It is in practice difficult to achieve this at the same time as acceptably high bandwidth and efficiency Nevertheless, improvements

in the properties of the dielectric materials and in design techniques have led to enormous growth in their popularity and there are now a large number of commercial applications Many shapes of patch are possible, with varying applications, but the most popular are rectangular (pictured), circular and thin strips (i.e printed dipoles)

In the rectangular patch, the length L is typically up to half of the free space wavelength The incident wave fed into the feed line sets up a strong resonance within

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