The first part of this chapter outlines the small antenna type which is PCB antenna, chip antenna and whip antenna. To help characterize the antennas investigated, the fundamental antenna analysis parameters, such as bandwidth, return loss, radiation pattern, directivity, antenna efficiency, gain and polarization will be discussed. Besides, this chapter also introduces different type of PCB antenna such as IFA, microstrip patch, monopole, dipole and its operation. Various feeding techniques for antennas are also mentioned at this chapter. Last but not least, the matching method by using smith chart has been discussed.
SMALL ANTENNA CATEGORIES
An antenna is a transducer designed to transmit or receive electromagnetic waves. In other words, antennas function as convert electromagnetic waves into electrical currents and convert electrical currents into electromagnetic waves. For selecting an antenna, size, cost, and performance of antenna are the most important factors to consider. The three most popular and efficient small antenna types for short-range devices are whip antennas, chip antennas and PCB antennas.[4] Figure 2.: Small Antenna Types (a) Whip Antenna, (b) Chip Antenna (c) PCB Antenna shows three most popular small antenna types and Table 2.: Comparison of Small Antenna Types shows the comparison among them.
Whip Antennas
A whip antenna provides exceptional overall performance and stability, has an isotropic pattern, a wide bandwidth, it is low cost and it is easily designed. Since a full-wave or even a half-wave dipole whip is generally quite long, most whips are ¼ waves.
This simple and most effective small antenna is also called a quarter-wave monopole and is the most common antenna on today's portable devices. Since most devices have a circuit board anyway, using it for half of the antenna can make a lot of sense. Generally, this half of the antenna will be connected to ground and the transmitter or receiver will reference it accordingly.
Chip Antenna
If the board space for the antenna is limited, a chip antenna could be a good solution. This antenna type supports a small solution size even for frequencies below 1GHz. The trade-off compared to PCB antennas is that this solution will add materials and mounting cost. The typical cost of a chip antenna is between $0.10 and $1.00. Even if chip antenna manufacturers state that the antenna is matched to 50ohm for a certain frequency band, additional matching components are often required to obtain proper performance.
PCB Antenna
PCB antennas are made via photolithographic methods, with both the feeding structure and the antenna fabricated on a dielectric substrate. PCB antenna is not straightforward and usually requires a simulation tool to obtain an acceptable solution. In addition to deriving an optimum design, configuring such a tool to perform accurate simulations can be difficult and time consuming.
Various antenna types such as monopoles, dipoles antennas patch, spiral and printed F can be made. A PCB antenna is suitable for mobile applications or areas where internal antennas are required or where not much space or volume is available.
Figure 2.: Small Antenna Types (a) Whip Antenna, (b) Chip Antenna (c) PCB Antenna
Antenna type
Pros
Cons
Whip Antenna
•Good performance
•High cost
•Difficult to fit in many applications
Chip Antenna
•Small size
•Medium performance
•Medium cost
PCB Antenna
• Low cost
•Good performance is possible
•Small size is possible at high frequencies
•Difficult to design small and efficient antennas
•Potentially large size at low frequencies
Table 2.: Comparison of Small Antenna Types
BASIC ANTENNA THEORY
An antenna is a transducer designed to transmit or receive electromagnetic waves. When alternating electric current flows through a conductor, electric-E and magnetic-H fields are created around the conductor and both of these fields are mutually perpendicular oscillate fields travelling together in phase as shown in Figure 2.: Electromagnetic Wave consists of Right Angle Electric and Magnetic Fields. An antenna is form when the length of the conductor approaches a quarter of a wavelength (λ/4) at the frequency of the applied alternating current, most of the energy will escape in the form of electromagnetic radiation. This means that theoretically, any metallic structure can be used as an antenna. However, some structures are more efficient in radiating and receiving RF power than others.
Description: C:\Users\Ngiap\Desktop\thesis photo\Capture.PNG
Figure 2.: Electromagnetic Wave consists of Right Angle Electric and Magnetic Fields
By applying Ampere's Law that illustrates that a current-carrying element or antenna creates a time-varying magnetic field which then creates a time-varying electric field and so forth to generate a free-space electromagnetic wave. Figure 2.: Ampere's Law Illustrates Current Induce Magnetic Field illustrates generation of electromagnetic wave by applying Ampere's Law.
Description: ampere law.JPG
Figure 2.: Ampere's Law Illustrates Current Induce Magnetic Field
On the other hand, the transmission lines are designed to transport RF power from antenna with as little radiation loss as possible to integrated circuit (IC) or vice versa because these structures are designed to contain the electromagnetic fields. To obtain any appreciable radiation from such a structure requires excessively high RF currents which causes low efficiency due to high losses. Likewise, the ability to introduce RF currents into the structure is of importance as described by the feed point impedance. If the feed point impedance is very high, low, and/or highly complex, it will be difficult to introduce RF current with good efficiency. [5]
ANTENNA PARAMETER
Maximum Power Transfer (VSWR)
According to Moritz Von Jacobi's, the maximum power transfer happens when the source resistance equals the load resistance, as shown in Equation 2.1.
For complex impedances, it is important that Z0 is properly matched to Za to make sure the maximum power delivered from a transmission line with impedance Z0 to an antenna with impedance Za without any reflected power. When Z0 is not properly matched to Za, the signal with amplitude Vin will be reflected and only a part of the incident wave will be transmitted to the antenna.
The complex reflection coefficient (Γ) is defined as the ratio of the reflected wave's amplitude to the amplitude of the incident wave. Γ can be calculated from the impedance of the transmission line and the impedance of the antenna, as shown in Equation 2.2.
The reflection coefficient is zero if the transmission line impedance is the complex conjugate of the antenna impedance. Thus if Z0 = Za the antenna is perfectly matched to the transmission line and all the applied power is delivered to the antenna.
Antenna matching typically uses both the Return Loss and the Voltage Standing Wave Ratio (VSWR) terminology. The VSWR is defined as the ratio between the voltage maximum and voltage minimum of the standing wave created by the mismatch at the load on a transmission line, refer to Equation 2.3.
Return Loss define as the loss of signal power resulting from the reflection caused at a discontinuity in a transmission line. This indicates how many decibels reflected wave power is below the incident wave. Refer to Equation 2.4.
In antenna design, VSWR and Return Loss are usually to define how well the antenna is matched.[4]
Bandwidth
In electrical systems, bandwidth is often defined in terms of its half-power, -3dB bandwidth. The half-power bandwidth is the range of frequencies around the resonant frequency at which the system is operating with least half of its peak power.
In antenna design, bandwidth is more often described in terms of the VSWR or the impedance bandwidth. The impedance bandwidth is usually specified as the range of frequencies over which the VSWR is less than 2 which translates to an 11% power loss. The percentage of bandwidth of an antenna can be calculated by:
where Δf = bandwidth
fc = center frequency
Q= quality factor
Input Impedance
The input impedance is the parameter, which describes the antenna input behavior as a circuit element. As usual in electronic circuit design it is important to match this input impedance (Z antenna) to a given source impedance. The maximum power delivered from the source to the antenna is given if the antenna input impedance is complex conjugate to the chip impedance.
Antenna Gain
Antenna gain is a measure of how well the antenna radiates the RF power in a given direction compared to a reference antenna such as a dipole or an isotropic radiator. The gain is usually measured in dB relative to a reference. Since the radiation intensity from a lossless isotropic antenna equals the power into the antenna divided by a solid angle of 4Ï€ steridians, we can write the following equation:
A negative number means that the antenna radiates less than the reference antenna and a positive number means that the antenna radiates more.
Directivity
The directive gain of the antenna is the ratio of the radiation intensity in a given direction over that of an isotropic source. If the antenna was to radiate in all directions (isotropic radiator) then its directivity would be unity. As an isotropic radiator cannot be realized practically, the most comparable antenna is a short dipole, which has a directivity of 1.5. Generally, most of the antenna has D > 1. If an antenna with directivity D >> 1 mean that it is directive antenna.
Radiation Pattern
The radiation pattern is a mathematical function or a graphical representation of the radiation properties of the antenna as a function of space that is the antenna's pattern describes how the antenna radiates energy out into space. In most cases, the radiation pattern is determined in the far-field region and is represented as a function of the directional coordinates. Radiation properties include power flux density, radiation intensity, field strength, directivity, phase or polarization.
Radiation patterns describe in 3D pattern with two planar patterns, called the principal plane patterns. These principal plane patterns can be obtained by making two slices through the 3D pattern through the maximum value of the pattern or by direct measurement. It is these principal plane patterns that are commonly referred to as the antenna patterns.
The azimuth plane pattern is measured when the measurement is made traversing the entire XY plane around the antenna under test as shown in Figure 2.: Spherical Coordinate System for Radiation Patterns. The elevation plane is then a plane orthogonal to the XY plane, say the YZ plane (φ = 90°). The elevation plane pattern is made traversing the entire y-z plane around the antenna under test. θ is associated with elevation plane and φ with azimuth plane. The antenna patterns (azimuth and elevation plane patterns) are frequently shown as plots in polar coordinates. [4]
Description: C:\Users\Ngiap\Desktop\thesis photo\radation pattern.PNG
Figure 2.: Spherical Coordinate System for Radiation Patterns
Polarization
The polarization of an electromagnetic wave is defined as the orientation of the electric field vector. Electric field vector is perpendicular to both the direction of travel and the magnetic field vector. The polarization is described by the geometric figure traced by the electric field vector upon a stationary plane perpendicular to the direction of propagation, as the wave travels through that plane.
An electromagnetic wave is can be decompose into two orthogonal vectors, one parallel to positive x-axis and one parallel to positive y-axis as shown in Figure 2.: Polarization Coordinates.
Figure 2.: Polarization Coordinates
If the x- and y- component have equal magnitude and same phase (or are different by an integer multiple of π), the wave is said to be linearly polarized, as the electric field vector is always directed along a fixed line. Most terrestrial radio signals are linearly polarized with the electric field oriented either horizontally or vertically.
If the two components differ in phase, their sum describes an ellipse about z-axis. This is an elliptically polarized wave.
In the other extreme, when the two components are of equal magnitude and π/2 (or an odd multiple of π/2) out of phase, the ellipse will become circular. Thus linear and circular polarizations are the two special cases of elliptical polarization.
Circular polarization implies a "handedness". If at a fixed point in space, the electric field (and magnetic field) vectors rotate clockwise (counter clockwise) for an observer looking from the source toward the direction of wave propagation, the polarization is right-handed. One of example implements right hand circularly polarization is GPS satellite.
Axial Ratio (AR)
Axial Ratio is defined as the ratio of the major to minor axes of the polarization ellipse. AR is defined to be infinity for linear polarization and one (0dB) for a perfect circular polarization of either RHCP or LHCP. The formula for axial ratio is given in Equation 2.7.
where
EÏ´ and EÏ• are the linear field components in the Ï´and Ï• directions defined in spherical coordinates.
Beamwidth
Antenna bemwidth is defined as the angle between two half power points on the main beam. In case that we have a logarithm radiation power pattern in dB units, it means that we measure the angle between two 3dB points.
DIELECTRIC SUBSTRATE
In most cases, considerations in substrate characteristics involved the dielectric constant and loss tangent and their variation with temperature and frequency, dimensional stability with processing, homogeneity and isotropicity. In order to provide support and protection for the patch elements, the dielectric substrate must be strong and able to endure high temperature during soldering process and has high resistant towards chemicals that are used in fabrication process.
The surface of the substrate has to be smooth to reduce losses and adhere well to the metal used. Substrate thickness and permittivity determine the electrical characteristics of the antenna. Thicker substrate will increase the bandwidth but it will cause the surface waves to propagate and spurious coupling will happen. This problem can be reduced or avoided by using a suitably low permittivity substrate.
In this research, the substrate using is FR-4 PCB fiber-glass resin material with dielectric constant εr = 4.4 and loss tangent tan δ = 0.002. This material is relatively low in cost for such low loss tangent. However, fiber-glass resin tends to be anisotropic. [6]
SMALL TYPES OF PCB ANTENNA
Dipole antenna
The dipole antenna can be realised by a short straight wire of finite length, which terminates at two points allowing charge to be collected. If an alternating current generator is connected to the centre of the wire dipole it can drive charge from one end to the other. The dipole antenna can be illustrated in Figure 2.: (a) A Two-Wire Line with an Open End (b) The Line with the Ends Turned Out to Form A Dipole(a).The line is driven by a signal source at its left and is open-circuited at its right end. The open-circuit causes a standing-wave pattern on the line, with zero current at the open end. At a given cross-section of the line, the currents are equal in magnitude and opposite in direction. Their radiated fields will likewise be equal in magnitude and opposite in direction at any point in space, providing the wire separation is tiny compared to wavelength. Therefore, the line produces no net radiation. Now suppose the ends of the wires are turned up and down, respectively, as shown in Figure 2.: (a) A Two-Wire Line with an Open End (b) The Line with the Ends Turned Out to Form A Dipole(b).The current distribution will have a sinusoidal shape with a constant minimum at the ends, and a maximum at the feeding point.
Figure 2.: (a) A Two-Wire Line with an Open End (b) The Line with the Ends Turned Out to Form A Dipole
The current distribution will have a sinusoidal shape as shown in Figure 2.: Current Distribution and Voltage Distribution with a constant minimum at the ends, and a maximum at the feeding point (the opposite for the voltage distribution).
Figure 2.: Current Distribution and Voltage Distribution
Design methodology for dipole antenna
Typically, the length of a half wave dipole, assuming that the conductor diameter is much less than the length of the antenna, is 95% of one-half wavelength measured in free space [7]. The free space wavelength is given by
where λ= free-space wavelength in meter
c=300x106m/s
f= operating frequency in hertz
Therefore, the length of a half-wave dipole is (in meter)
Monopole (λ/4) Antennas
A monopole antenna most commonly refers to a quarter-wavelength (λ/4). The antenna is constructed of conductive elements whose combined length is about quarter the wavelength at its intended frequency of operation. This is very popular due to its size since one antenna element is one λ/4 wavelength and the ground plane acts as the other λ/4 wavelength which produces an effective λ/2 antenna. Therefore, for monopole antenna designs the performance of the antenna is dependent on the ground size, refer to Figure 2.: Monopole Antenna Utilizing Ground Plane as an Effective λ/4 Antenna Element, all small antennas are derivatives of a simple dipole where one element is folded into the ground and serves as the second radiator.
Description: C:\Users\Ngiap\Desktop\thesis photo\monopole.PNG
Figure 2.: Monopole Antenna Utilizing Ground Plane as an Effective λ/4 Antenna Element
Microstrip patch antenna
In its simplest form, microstrip antenna is a dielectric substrate panel sandwiched in between two conductors. The lower conductor is called ground plane and the upper conductor is known as patch. Microstrip antenna is commonly used at frequencies from 1 to 100GHz and at frequencies below ultra-high frequency, UHF microstrip patch become exceptionally large. Illustrated in Figure 2.: Microstrip Antenna is the simplest structure of a rectangular microstrip patch antenna.
Description: C:\Users\yngoh\Desktop\microstrip.JPG
Figure 2.: Microstrip Antenna
These antennas are low profile, conformable to planar and nonplanar surfaces, simple and inexpensive to manufacture using modern printed-circuit technology, mechanically robust when mounted on rigid surfaces, compatible with MMIC designs, and when the particular patch shape and mode are selected, they are very versatile in terms of resonant frequency, polarization, pattern, and impedance. [8]
Major operational disadvantages of microstrip antennas are their low efficiency, low power, high Q (sometimes in excess of 100), poor polarization purity, poor scan performance, spurious feed radiation and very narrow frequency bandwidth, which is typically only a fraction of a percent or at most a few per cent.
Methods of Analysis for Patch Antennas
The most popular models for analysis of microstrip patch antennas are the transmission line model and cavity model. The transmission line model is the simplest of all and it gives good physical insight but it is less accurate. The cavity model is more accurate and gives good physical insight but is complex in nature.
Transmission Line Model
This model represents the microstrip antenna by two slots of width W and height h, separated by a transmission line of length L. The microstrip is essentially a non-homogeneous line of two dielectrics, typically the substrate and air. A typical microstrip line is shown in Figure 2. (a) Microstrip line (b) Electric field lines(a) while the electric field lines associated with it are shown in Figure 2. (a) Microstrip line (b) Electric field lines(b).
C:\Users\Ngiap\Desktop\New folder\photo\t.PNG
Figure 2. (a) Microstrip line (b) Electric field lines
As seen from Figure 2. (a) Microstrip line (b) Electric field lines(b), most of the electric field lines reside in the substrate while some electric field lines exist 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.
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. (a) Microstrip line (b) Electric field lines above. The expression for is given by [8]as:
where denotes effective dielectric constant, stands for dielectric constant of substrate, h represents height of dielectric substrate, and w identifies width of the patch.
Figure 2.: Transmission Line Model for Patch Antenna shows the transmission line model for patch antenna, where Figure 2.: Transmission Line Model for Patch Antenna(a) is the patch antenna, Figure 2.: Transmission Line Model for Patch Antenna(b) is the top view and Figure 2.: Transmission Line Model for Patch Antenna(c) is the side view of the antenna.
Description: C:\Users\Ngiap\Desktop\New folder\photo\transmission line model.PNG
Figure 2.: Transmission Line Model for Patch Antenna
In order to operate in the fundamental TM10 mode, the length of the patch must be slightly less than λ/2, where λ is the wavelength in the dielectric medium and is equal to , where is the free space wavelength. The TM10 model implies that the field varies one λ/2cycle along the length and there is no variation along the width of the patch. In Figure 2.: Transmission Line Model for Patch Antenna(b) shown above, the microstrip patch antenna is represented by two slots, separated by a transmission line of length L and open circuited at both the ends. Along the width of the patch, the voltage is maximum and current is minimum due to the open ends. The fields at the edges can be resolved into normal and tangential components with respect to the ground plane.
It is seen from Figure 2.: Transmission Line Model for Patch Antenna (c) 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 λ/2 long and hence they cancel each other in the broadside direction. The tangential components (seen in Figure 2.: Transmission Line Model for Patch Antenna (c)), 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 λ/2 apart 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 ΔL, which is given empirically by [8] as
The length of the patch now becomes
For a given resonance frequency fo, the effective length is given by as
For efficient radiation, the width W is given as
Cavity Model
Although the transmission line model discussed in the previous section is easy to use, it has some inherent disadvantages. Specifically, it is useful for patches of rectangular design and it ignores field variations along the radiating edges. These disadvantages can be overcome by using the cavity model. A brief overview of this model is given below.
In this model, the interior region of the dielectric substrate is modeled as a cavity bounded by electric walls on the top and bottom. The basis of this assumption is the following observations for thin substrates (λ<<h):
• Since the substrate is thin, the fields in the interior region do not vary much in the z direction, i.e. normal to the patch.
• The electric field is z directed only, and the magnetic field has only the transverse components Hx and Hy in the region bounded by the patch metallization and the ground plane. This observation provides for the electric walls at the top and the bottom.
Description: C:\Users\Ngiap\Desktop\New folder\photo\cavity model.PNG
Figure 2.: Charge Distribution and Current Density Creation on the Microstrip Patch
Consider Figure 2.: Charge Distribution and Current Density Creation on the Microstrip Patch shown above. When the microstrip patch is provided power, a charge distribution is seen on the upper and lower surfaces of the patch and at the bottom of the ground plane. This charge distribution is controlled by two mechanisms an attractive mechanism and a repulsive mechanism[9].
The attractive mechanism is between the opposite charges on the bottom side of the patch and the ground plane, which helps in keeping the charge concentration intact at the bottom of the patch. The repulsive mechanism is between the like charges on the bottom surface of the patch, which causes pushing of some charges from the bottom, to the top of the patch. As a result of this charge movement, currents flow at the top and bottom surfaces of the patch.
The cavity model assumes that the height to width ratio (i.e. height of substrate and width of the patch) is very small and as a result of this the attractive mechanism dominates and causes most of the charge concentration and the current to be below the patch surface. Much less current would flow on the top surface of the patch and as the height to width ratio further decreases, the current on the top surface of the patch would be almost equal to zero, which would not allow the creation of any tangential magnetic field components to the patch edges. Hence, the four sidewalls could be modeled as perfectly magnetic conducting surfaces. This implies that the magnetic fields and the electric field distribution beneath the patch would not be disturbed. However, in practice, a finite width to height ratio would be there and this would not make the tangential magnetic fields to be completely zero, but they being very small, the side walls could be approximated to be perfectly magnetic conducting.
Since the walls of the cavity, as well as the material within the cavity are lossless, the cavity would not radiate and its input impedance would be purely reactive. Hence, in order to account for radiation and a loss mechanism, one must introduce a radiation resistance Rr and a loss resistance RL. A lossy cavity would now represent an antenna and the loss is taken into account by the effective loss tangent δeff given by
where QT denotes the total antenna quality factor and has been expressed in the following form:
In Equation (2.16), the Qd represents the quality factor of the dielectric and is given as
where ωr denotes the angular resonant frequency, WT stands for the total energy stored in the patch at resonance, Pd represents the dielectric loss, and tanδ is the loss tangent of the dielectric.
The Qc represents the quality factor for radiation and is given as
where Pc is the conductor loss, Δ is the skin depth of the conductor, and h is the height of the substrate.
The Qr represents the quality factor for radiation and is given as
Where Pr is the power radiated from the patch.
Substituting Equations (2.16), (2.17), (2.18) and (2.19) into Equation (2.15), we get
Thus, Equation (2.20) describes the total effective loss tangent for the microstrip patch antenna.
Inverted F Antennas (IFA)
Description: C:\Users\Ngiap\Desktop\thesis photo\ifa.PNG
Figure 2.: Inverted-F Antenna
The Inverted-F antenna (IFA) as shown in Figure 2.: Inverted-F Antenna is most common used in printed trace on a PCB that is essentially a quarter-wave vertical antenna, but that has been bent horizontally in order to be parallel with the substrate's copper ground pour, and then feed at an appropriate point that will supply a good input match. The antenna and ground combination will behave as an asymmetric dipole, the differences in current distribution on the two-dipole arms being responsible for some distortion of the radiation pattern.
In general, the required PCB ground plane length is roughly one quarter (λ/4) of the operating wavelength. If the ground plane is much longer than λ/4, the radiation patterns will become increasingly multi-lobed. On the other hand, if the ground plane is significantly smaller than λ/4, then tuning becomes increasingly difficult and the overall performance degrades. The optimum location of the IFA in order to achieve an omni-directional far-field pattern and 50ohm impedance matching was found to be close to the edge of the Printed Circuit Board. [10]
IFA is an excellent choice for small, low-profile wireless designs, and is not as adversely affected by tiny, poorly shaped ground-planes as that of the monopole above. The IFA also supplies decent efficiency, is of a compact geometry, and has a relatively omni-directional radiation pattern. IFA antennas do have somewhat of a narrower bandwidth than the average monopole.
FEEDING METHODS
Matching is usually required between the antenna and the feed line, because antenna input impedances differ from customary 50ohm line impedance. An appropriately selected port location will provide matching between the antenna and its feed line. They four most popular are the microstrip line, coaxial probe, aperture coupling, and proximity coupling.
Microstrip Feed Line
The microstrip feed line is a conducting strip, usually of much smaller width compared to the patch as shown in Figure 2.: Microstrip Line Feeding . The microstrip-line feed is easy to fabricate, simple to match by controlling the inset position and rather simple to model. [8]
Description: C:\Users\Ngiap\Desktop\New folder\photo\mircostrip feed.PNG
Figure 2.: Microstrip Line Feeding
Coaxial-Line Feeds
Coaxial-line feeds, where the inner conductor of the coax is attached to the radiation patch while the outer conductor is connected to the ground plane, are also widely used. The coaxial probe feed is also easy to fabricate and match, and it has low spurious radiation. However, it also has narrow bandwidth and it is more difficult to model, especially for thick substrates (h > 0.02λ). For single coaxial cable like the one shown in cross section Figure 2.: Cross Section Coaxial Cable, the impedance can be determine by using following equation[7]:
Where Z0 = characteristic impendence of the line
D = inside diameter of the outer conductor
d = diameter of the inner conductor
= relative permittivity of the dielectric, compare with that of free space ( is often called the dielectric constant)
Figure 2.: Cross Section Coaxial Cable
The advantages of this method are that the probe location can electively excite additional modes and it can be used with plated vias for multilayer circuits. Figure 2.: Coaxial Line Feeding show the coaxial line feeding techniques.
Description: C:\Users\Ngiap\Desktop\New folder\photo\COAXIAL feed.png
Figure 2.: Coaxial Line Feeding
Aperture coupling
Aperture Coupling is the most difficult of all four to fabricate and it also has narrow bandwidth. The RF energy from the feed line is coupled to the radiating element through a common aperture in the form of a rectangular slot. It mainly consists of two substrates separated by a ground plane. Top substrate is for the radiating element and the bottom substrate is for the feed-line. A slot is made in the ground plane to provide coupling between the feed line and patch. For the sake of maximum coupling the slot is usually placed at the center and it is perpendicular to the feed line, as a result the patch and the slot may share common center. The length of the slot should be kept larger than the width of the slot. The diagrammatic setup for aperture coupling is shown in Figure 2.: Aperture Coupled Feed.
Description: C:\Users\Ngiap\Desktop\New folder\photo\feed.PNG
Figure 2.: Aperture Coupled Feed
This scheme has the advantage of isolating the feeding network from the radiating patch element. It also overcomes the limitation on substrate thickness imposed by the feed inductance of a coaxial probe, so that thicker substrates and hence higher bandwidths can be obtained but it suffers from high back radiation. [9]
Proximity Coupled Feed
Proximity-coupled microstrip antenna is also known as non-contacting feed and also called the electromagnetic coupling scheme. As shown in Figure 2.: Proximity Coupled Feed for Patch Antenna, two dielectric substrates are used such that the feed line is between the two substrates and the radiating patch is on top of the upper substrate.
The main advantage of this feed technique is that it eliminates spurious feed radiation and provides higher bandwidth in comparison to the other feeding techniques (as high as 13%), due to overall increase in the thickness of the microstrip patch antenna. This scheme also provides choices between two different dielectric media, one for the patch and one for the feed line to optimize the individual performances.
Description: C:\Users\Ngiap\Desktop\New folder\photo\feed pro.PNG
Figure 2.: Proximity Coupled Feed for Patch Antenna
Matching can be achieved by controlling the length of the feed line and the width-to-line ratio of the patch. The major disadvantage of this feed scheme is that it is difficult to fabricate because of the two dielectric layers which need proper alignment. Also, there is an increase in the overall thickness of the antenna.
Table 2.: Comparison between Different Feed Techniques for Patch Antennas[9] summarizes the characteristics of the different feed techniques of patch antennas.
Characteristics
Microstrip Line Feed
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 required
Alignment required
Alignment required
Impedance matching
Easy
Easy
Easy
Easy
Bandwidth (achieve with impedance matching)
2-5%
2-5%
2-5%
13%
Table 2.: Comparison between Different Feed Techniques for Patch Antennas[9]
ANTENNA IMPEDANCE MATCHING
Antenna matching is critical in order to get the lowest VSWR, i.e. the best amount of energy transmitted to the antenna. A matching circuit for antenna normally is only build by inductors or capacitors. In antenna designs, one should never use resistors or other lossy components. Because the whole purpose of an antenna in a cellular device is to transmit or receive power, so the efficiency of the antenna is the most critical parameter. Whenever a resistor is added to a matching circuit, the efficiency always drops and that is not desired. Several techniques are available for antenna matching. The easiest method is the Smith's chart visualization.
Smith Chart
Smith Charts were originally developed around 1940 by Phillip Smith. The Smith chart is one of the most useful graphical tools for high frequency circuit applications. The chart provides a clever way to visualize complex functions and it continues to endure popularity, decades after its original conception. The goal of the Smith chart is to identify all possible impedances on the domain of existence of the reflection coefficient.
Description: F:\s.chart re and im.JPG
Figure 2.: Family of Curves of Constant rL and xL
Figure 2.: Family of Curves of Constant rL and xL shows four curves of constant rL, which are rL = 0, 0.5, 1 and 2 respectively. The rL =0 curve is superimposed on the =1 circle shown in the Figure 2.: Family of Curves of Constant rL and xL(a), which represents situations when the load is lossless and formed by only the reactance component. In Figure 2.: Family of Curves of Constant rL and xL(b), as well as the =1 circle, there are seven constant XL line/curves, where XL 0, +0.5,-0.5, +1,-1, +2 and -2 respectively. When XL =0, ZL is always a real value, thus is also a real value, and the corresponding constant XL trace superimposes on the real axis in the plane.
Another importance parameter for impendence matching is admittance, gL. The curves of the admittance Smith Chart can be obtained by simply rotating the Impedance Smith Chart by 180o as shown in Figure 2.: Family of Curves of Constant gL and rL.
Description: F:\s.chart g and r.JPG
Figure 2.: Family of Curves of Constant gL and rL
Figure 2.: Family of Curves of Constant gL and rL show that two sets of circles instead of four sets of curves in a standard Smith Chart. All circles crossing the leftmost point are fixed gL curves, and all circles crossing the rightmost point are equal rL curves.
In all the case of the mobile device, the target impedance is normally 50ohm. The 50ohm is an industry standard for most of the mobile device. The main reason of 50ohm as target impedance is 50ohms having great compromise between power handling and low loss.[11]
Matching with Lumped Element
There are several ways to tune an antenna to achieve better performance. But sometime even if the antenna resonates at the correct frequency it might not be well matched to the correct impedance. Dependent of the antenna type there are several possibilities to obtain optimum impedance at the correct frequency. Size of ground plane, distance from antenna to ground plane, dimensions of antenna elements, feed point, and plastic casing are factors that can affect the impedance. Thus by varying these factors it might be possible to improve the impedance match of the antenna. If varying these factors is not possible or if the performance still needs to be improved, discreet components could be used to optimize the impedance. Capacitors and inductors in series or parallel can be used to match the antenna to the desired impedance. Resistors should avoid to improving efficiency of antenna. Figure 2.: Serial and Parallel Capacitor and Inductors in Smith Chart and Figure 2.: Four Possible Connecting Methods of Matching Components shows how inductors and capacitors can be used to change the impedance.
Figure 2.: Serial and Parallel Capacitor and Inductors in Smith Chart
Figure 2.: Four Possible Connecting Methods of Matching Components
When a series inductor is connected to the antenna, which is shown as Figure 2.: Four Possible Connecting Methods of Matching Components(a), the combination impedance of the antenna and the series inductor at the output will move in direction a.
When a series capacitor is connected to the antenna, which is shown as Figure 2.: Four Possible Connecting Methods of Matching Components(b), the combination impedance of the antenna and the serial capacitor at the output will move in direction b.
When a shunt inductor is connected to the antenna, which is shown as Figure 2.: Four Possible Connecting Methods of Matching Components(c), the combination impedance of the antenna and the shunt inductor at the output will move in direction c.
When a shunt capacitor is connected to the antenna, which is shown as Figure 2.: Four Possible Connecting Methods of Matching Components(d), the combination impedance of the antenna and the shunt capacitor at the output will move in direction d.
