29-01-2011, 12:04 PM
[attachment=8533]
Submitted to:
Mr. DEEPAK SOOD
[Submitted by:
PRADEEP(1407234)
SHRI KRISHAN DEV BHOOMI INSTITUTE OF ENGINEERING & TECHNOLOGY
History of Microstrip Patch Antenna
The rapid development of microstrip antenna technology began in the late 1970s. By the early 1980s basic microstrip antenna elements and arrays were fairly well established in terms of design and modeling, and workers were turning their attentions to improving antenna performance features (e.g. bandwidth), and to the increased application of the technology. One of these applications involved the use of microstrip antennas for integrated phased array systems, as the printed technology of microstrip antenna seemed perfectly suited to low-cost and high-density integration with active MIC or MMIC phase shifter and T/R circuitry.
The group at the University of Massachusetts (Dan Schaubert, Bob Jackson, Sigfrid Yngvesson) had received an Air Force contract to study this problem, in terms of design tradeoffs for various integrated phased array architectures, as well as theoretical modeling of large printed phased array antennas. The straightforward approach of building an integrated millimeter wave array (or subarray) using a single GaAs substrate layer had several drawbacks. First, there is generally not enough space on a single layer to hold antenna elements, active phase shifter and amplifier circuitry, bias lines, and RF feed lines. Second, the high permittivity of a semiconductor substrate such as GaAs was a poor choice for antenna bandwidth, since the bandwidth of a microstrip antenna is best for low dielectric constant substrates. And if substrate thickness is increased in an attempt to improve bandwidth, spurious feed radiation increases and surface wave power increases. This latter problem ultimately leads to scan blindness, whereby the antenna is unable to receive or transmit at a particular scan angle. Because of these and other issues, they were looking at the use of a variety of two or more layered substrates. One obvious possibility was to use two back to-back substrates with feed through pins. This would allow plenty of surface area, and had the critical advantage of allowing the use of GaAs (or similar) material for one substrate, with a low dielectric constant for the antenna elements. The main problem with this approach was that the large number of via holes presented fabrication problems in terms of yield and reliability. They had looked at the possibility of using a two sided-substrate with printed slot antennas fed with microstrip lines, but the bidirectionality of the radiating element was unacceptable.
At some point in the summer of 1984 they arrived at the idea of combining these two geometries, using a slot or aperture to couple a microstrip feed line to a resonant microstrip patch antenna.
After considering the application of small hole coupling theory to the fields of the microstrip line and the microstrip antenna, they designed a prototype element for testing. Their intuitive theory was very simple, but good enough to suggest that maximum coupling would occur when the feed line was centered across the aperture, with the aperture positioned below the center of the patch, and oriented to excite the magnetic field of the patch.
The first aperture coupled microstrip antenna was fabricated and tested by a graduate student, Allen Buck, on August 1, 1984, in the University of Massachusetts Antenna Lab. This antenna used 0.062” Duroid substrates with a circular coupling aperture, and operated at 2 GHz. As is the case with most original antenna developments, the prototype element was designed without any rigorous analysis or CAD - only an intuitive view of how the fields might possibly couple through a small aperture. They were pleasantly surprised to find that this first prototype worked almost perfectly – it was impedance matched, and the radiation patterns were good. Most importantly, the required coupling aperture was small enough so that the back radiation from the coupling aperture was much smaller than the forward radiation level.
The geometry of the basic aperture coupled patch antenna is described. The radiating microstrip patch element is etched on the top of the antenna substrate, and the microstrip feed line is etched on the bottom of the feed substrate. The thickness and dielectric constants of these two substrates may thus be chosen independently to optimize the distinct electrical functions of radiation and circuitry. Although the original prototype antenna used a circular coupling aperture, it was quickly realized that the use of a rectangular slot would improve the coupling, for a given aperture area, due to its increased magnetic polarizability. Most aperture coupled microstrip antennas now use rectangular slots, or variations thereof.
Aim and Objectives
Microstrip patch antenna used to send onboard parameters of article to the ground while under operating conditions. The aim of the thesis is to design and fabricate an probe-fedSquare Microstrip Patch Antenna and study the effect of antenna dimensions Length (L),and substrate parameters relative Dielectric constant (εr), substrate thickness (t) on theRadiation parameters of Bandwidth and Beam-width.
Overview of Microstrip Antenna
A microstrip antenna is characterized by its Length, Width, Input impedance, and Gain and radiation patterns. Various parameters of the microstrip antenna and its design considerations were discussed in the subsequent chapters. The A microstrip antenna consists of conducting patch on a ground plane separated by dielectric substrate. This concept was undeveloped until the revolution in electronic circuit miniaturization and large-scale integration in 1970. After that many authors have described the radiation from the ground plane by a dielectric substrate for different configurations. The early work of Munson on micro strip antennas for use as a low profile flush mounted antennas on rockets and missiles showed that this was a practical concept for use in many antenna system problems. Various mathematical models were developed for this antenna and its applications were extended to many other fields. The number of papers, articles published in the journals for the last ten years, on these antennas shows the importance gained by them. The micro strip antennas are the present day antenna designer’s choice. Low dielectric constant substrates are generally preferred for maximum radiation. The conducting patch can take any shape but rectangular and circular configurations are the most length of the antenna is nearly half wavelength in the dielectric; it is a very critical parameter, which governs the resonant frequency of the antenna. There are no hard and fast rules to find the width of the patch.
Waves on Microstrip
The mechanisms of transmission and radiation in a microstrip can be understood by considering a point current source (Hertz dipole) located on top of the grounded dielectric substrate (fig. 1.1) This source radiateselectromagnetic waves. Depending on thedirection toward which waves are transmitted, they fall within three distinct categories,each of which exhibits different behaviors.
Surface Waves
The waves transmitted slightly downward, having elevation angles θ between π/2and π - arcsin (1/√εr), meet the ground plane, which reflects them, and then meet the dielectric-to-air boundary, which also reflects them (total reflection condition). The magnitude of the field amplitudes builds up for some particular incidence angles that leads to the excitation of a discrete set of surface wave modes; which are similar to the modes in metallic waveguide. The fields remain mostly trapped within the dielectric, decaying exponentially above the interface . The vector α, pointing upward, indicates the direction of largest attenuation. The wave propagates horizontally along β, with little absorption in good quality dielectric. With two directions of α and β orthogonal to each other, the wave is anon-uniform plane wave. Surface waves spread out in cylindrical fashion around the excitation point, with field amplitudes decreasing with distance ®, say1/r, more slowly than space waves. The same guiding mechanism provides propagation within optical fibers . Surface waves take up some part of the signal’s energy, which does not reach the intended user. The signal’s amplitude is thus reduced, contributing to an apparent attenuation or a decrease in antenna efficiency. Additionally, surface waves also introduce spurious coupling between different circuit or antenna elements. This effect severely degrades the performance of microstrip filters because the parasitic interaction reduces the isolation in the stop bands .In large periodic phased arrays, the effect of surface wave coupling becomes particularly obnoxious, and the array can neither transmit nor receive when it is pointed at some particular directions (blind spots). This is due to a resonance phenomenon, when the surface waves excite in synchronism the Floquet modes of the periodic structure. Surface waves reaching the outer boundaries of an open microstrip structure are reflected and diffracted by the edges. The diffracted waves provide an additional contribution to radiation, degrading the antenna pattern by raising the side lobe and the cross polarization levels. Surface wave effects are mostly negative, for circuits and for antennas, so their excitation should be suppressed if possible.
Leaky Waves
Waves directed more sharply downward, with θ angles between π – arc sin (1/√εr) and π, are also reflected by the ground plane but only partially by the dielectric-to-air boundary. They progressively leak from the substrate into the air (Fig 1.3), hence their name laky waves, and eventually contribute to radiation. The leaky waves are also non uniform plane waves for which the attenuation direction α points downward, which may appear to be rather odd; the amplitude of the waves increases as one moves away from the dielectric surface. This apparent paradox is easily understood by looking at the figure 1.3; actually, the field amplitude increases as one move away from the substrate because the wave radiates from a point where the signal amplitude is larger. Since the structure is finite, this apparent divergent behaviour can only exist locally, and the wave vanishes abruptly as one crosses the trajectory of the first ray in the figure. In more complex structures made with several layers of different dielectrics, leaky waves can be used to increase the apparent antenna size and thus provide a larger gain .This occurs for favourable stacking arrangements and at a particular frequency. Conversely, leaky waves are not excited in some other multilayer structures.
Guided Waves
When realizing printed circuits, one locally adds a metal layer on top of thesubstrate, which modifies the geometry, introducing an additional reflecting boundary.
Waves directed into the dielectric located under the upper conductor bounce back and forth on the metal boundaries, which form a parallel plate waveguide. The waves in the metallic guide can only exist for some Particular values of the angle of incidence, forming a discrete set of waveguide modes. The guided waves provide the normal operation of all transmission lines and circuits,in which the electromagnetic fields are mostly concentrated in the volume below the upper conductor. On the other hand, this build up of electromagnetic energy is not favourable for patch antennas, which behave like resonators with a limited frequency bandwidth.
Parameters for Designing Antenna Bandwidth
How can your cell phone and your television work at the same time? Both use antennas to receive information from electromagnetic waves, so why isn't there a problem?
The answer goes back to the fundamental secret of the universe. No matter what information you want to send, that waveform can be represented as the sum of a range of frequencies. By the use of modulation (which in a nutshell shifts the frequency range of the waveform to be sent to a higher frequency band), the waveforms can be relocated to separate frequency bands.
As an example, cell phones that use the PCS (Personal Communications Service) band have their signals shifted to 1850-1900 MHz. Television is broadcast primarily at 54-216 MHz. FM radio operates between 87.5-108 MHz.
The set of all frequencies is referred to as "the spectrum". Cell phone companies have to pay big money to get access to part of the spectrum. For instance, AT&T has to bid on a slice of the spectrum with the FCC, for the "right" to transmit information within that band. The transmission of EM energy is greatly regulated. When AT&T is sold a slice of the spectrum, they can not transmit energy at any other band (technically, the amount transmitted must be below some threshold in adjacent bands)
The Bandwidth of a signal is the difference between the signals high and low frequencies. For instance, a signal transmitting between 40 and 50 MHz has a bandwidth of 10 MHz.
We'll wrap up with a table of frequency bands along with the corresponding wavelengths. From the table, we see that VHF is in the range 30-300 MHz (30 Million-300 Million cycles per second). At the very least then, if someone says they need a "VHF antenna", you should now understand that the antenna should transmit or receive electromagnetic waves that have a frequency of 30-300 MHz.
RADIATION PATTERN
A radiation pattern defines the variation of the power radiated by an antenna as a function of the direction away from the antenna. This power variation as a function of the arrival angle is observed in the far field.
As an example, consider the 3-dimensional radiation pattern in Figure 1, plotted in decibels (dB) .
DIRECTIVITY
Directivity is a fundamental antenna parameter. It is a measure of how 'directional' an antenna's radiation pattern is. An antenna that radiates equally in all directions would have effectively zero directionality, and the directivity of this type of antenna would be 1 (or 0 dB).
An antenna's normalized radiation pattern can be written as a function in spherical coordinates
MICROSTRIP PATCH ANTENNA
Microstrip antennas are attractive due to their light weight, conformability and lowcost. These antennas can be integrated with printed strip-line feed networks and active devices. This is a relatively new area of antenna engineering. The radiation properties of micro strip structures have been known since the mid 1950’s. The application of this type of antennas started in early 1970’s when conformal antennas were required for missiles. Rectangular and circular micro strip resonant patches have been used extensively in a variety of array configurations. A major contributing factor for recent advances of microstrip antennas is the current revolution in electronic circuit miniaturization brought about by developments in large scale integration. As conventional antennas are often bulky and costly part of an electronic system, micro strip antennas based on photolithographic technology are seen as an engineering breakthrough.
Introduction
In its most fundamental form, a Microstrip Patch antenna consists of a radiatingpatch on one side of a dielectric substrate which has a ground plane on the other side asshown in Figure below The patch is generally made of conducting material such as copper or gold and can take any possible shape. The radiating patch and the feed lines are usually photo etched on the dielectric substrate.
ADVANTAGES
Microstrip patch antennas are increasing in popularity for use in wireless
applications due to their low-profile structure. Therefore they are extremely compatible for embedded antennas in handheld wireless devices such as cellular phones, pagers etc... The telemetry and communication antennas on missiles need to be thin and conformal and are often in the form of Microstrip patch antennas. Another area where they have been used successfully is in Satellite communication. Some of their principal advantages 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.
• Supports both, linear as well as circular polarization.
• Can be easily integrated with microwave integrated circuits (MICs).
• Capable of dual and triple frequency operations.
• Mechanically robust when mounted on rigid surfaces.
DISADVANTAGES
Microstrip patch antennas suffer from more drawbacks as compared to conventional antennas. Some of their major disadvantages aregiven below:
• Narrow bandwidth.
• Low efficiency.
• Low Gain.
• Extraneous radiation from feeds and junctions.
• Poor end fire radiator except tapered slot antennas.
• Low power handling capacity.
• Surface wave excitation.
Microstrip patch antennas have a very high antenna quality factor (Q). It represents the losses associated with the antenna where a large Q leads to narrow bandwidth and low efficiency. Q can be reduced by increasing the thickness of the dielectric substrate. But as the thickness increases, an increasing fraction of the total power delivered by the source goes into a surface wave. This surface wave contribution can be counted as an unwanted power loss since it is ultimately scattered at the dielectric bends and causes degradation of the antenna characteristics. Other problems such as lower gain and lower power handling capacity can be overcome by using an arrayconfiguration for the elements
Feed Techniques
Microstrip patch antennas can be fed by a variety of methods. These methods can be classified into two categories- contacting and non-contacting. In the contacting method, the RF power is fed directly to the radiating patch using a connecting element such as a microstrip line. In the non-contacting scheme, electromagnetic field coupling is done to transfer power between the microstrip line and the radiating patch. The four most popular feed techniques used are the microstrip line, coaxial probe (both contacting schemes), aperture coupling and proximity coupling (both non-contacting schemes).
Microstrip Line Feed
In this type of feed technique, a conducting strip is connected directly to the edge of the Microstrip patch as shown in Figure 2.3. The conducting strip is smaller in width as compared to the patch and this kind of feed arrangement has the advantage that the feed can be etched on the same substrate to provide a planar structure.
Methods of Analysis
The preferred models for the analysis of Microstrip patch antennas are the transmission line model, cavity model, and full wave model (which include primarily integral equations/Moment Method). 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. The full wave models are extremely accurate, versatile and can treat single elements, finite and infinite arrays, stacked elements, arbitrary shaped elements and coupling. These give less insight as compared to the two models mentioned above and are far more complex in nature.
Different Types of Circularly Polarized Antennas.
Generally antenna radiates an elliptical polarization, which is defined by three parameters: axial ratio, tilt angle and sense of rotation. When the axial ratio is infinite or zero, the polarization becomes linear with the tilt angle defining the orientation. The quality of linear polarization is usually indicated by the level of the cross polarization. For the unity axial ratio, a perfect circular polarization results and the tilt angle is not applicable. In general the axial ratio is used to specify the quality of circularly polarized waves. Antennas produce circularly polarized waves when two orthogonal field components with equal amplitude but in phase quadrature are radiated. Various antennas are capable of satisfying these requirements. They can be classified as a resonator and traveling-wave types. A resonator-type antenna consists of a single patch antenna that is capable of simultaneously supporting two orthogonal modes in phase quadrature or an array of linearly polarized resonating patches with proper orientation and phasing. A traveling-wave type of antenna is usually constructed from a microstrip transmission line. It generates circular polarization by radiating orthogonal components with appropriate phasing along discontinuities is the travelling-wave line.
Microstrip Patch Antennas
A microstrip antenna is a resonator type antenna. It is usually designed for single mode operation that radiates mainly linear polarization. For a circular polarization radiation, a patch must support orthogonal fields of equal magnitude but in-phase quadrature. This requirement can be accomplished by single patch with proper excitations or by an array of patches with an appropriate arrangement and phasing.
Circularly Polarized Patch
A microstrip patch is one of the most widely used radiators for circular polarization. Figure 3.1shows some patches, including square, circular, pentagonal, equilateral triangular, ring, and elliptical shapes which are capable of circular polarization operation. However square and circular patches are widely utilized in practice. A single patch antenna can be made to radiate circular polarization if two orthogonal patch modes are simultaneously excited with equal amplitude and out of phase with sign determining the sense of rotation. Two types of feeding schemes can accomplish the task as given in figure 3.2. The first type is a dual-orthogonal feed, which employs an external power divider network. The other is a single point for which an external power divider is not required.
Dual-Orthogonal Fed circularly Polarized Patch
The fundamental configurations of a dual-orthogonal fed circularly polarized patch using an external power divider is shown in figure 3.3. The patch is usually square or circular. The dual-orthogonal feeds excite two orthogonal modes with equal amplitude but inphase quadrature. Several power divider circuits that have been successfully employed for CP generation include the quadrature hybrid, the ring hybrid, the Wilkinson power divider, and the T-junction power splitter. The quadrature hybrid splits the input into two outputs with equal magnitude but 900 out of phase. Other types of dividers, however, need a quarter-wavelength line in one of the output arms to produce a 900 phase shift at the two feeds. Consequently, the quadrature hybrid provides a broader axial ratio bandwidth. These splitters can be easily constructed from various planar transmission lines.
COMPACT BROADBAND MICROSTRIP ANTENNAS
With a size reduction at a fixed operating frequency, the impedance bandwidth of a microstrip antenna is usually decreased. To obtain an enhanced impedance bandwidth, one can simply increase the antenna’s substrate thickness to compensate for the decreased electrical thickness of the substrate due to the lowered operating frequency, or one can use a meandering ground plane (Figure 1.7) or a slotted ground plane (Figure 1.8). These design methods lower the quality factor of compact microstrip antennas and result in an enhanced impedance bandwidth.
By embedding suitable slots in a radiating patch, compact operation with an enhanced impedance bandwidth can be obtained.Atypical design is shown in Figure.
However, the obtained impedance bandwidth for such a design is usually about equal to or less than 2.0 times that of the corresponding conventional microstrip antenna.
Antenna using chip resistor
To achieve a much greater impedance bandwidth with a reduction in antenna size, one can use compact designs with chip-resistor loading or stacked shorted patches. The former design is achieved by replacing the shorting pin in a shorted patch antenna with a chip resistor of low resistance (generally 1 ohm)
In this case, with the same antenna parameters, the obtained antenna size reduction can be greater than for the design using chip-resistor loading. Moreover, the obtained impedance bandwidth can be increased by a factor of six compared to a design using shorting-pin loading. For an FR4 substrate of thickness 1.6 mm and relative permittivity 4.4, the impedance bandwidth can reach 10% in L-band operation [26]. However, due to the introduced ohmic loss of the chip-resistor loading, the antenna gain is decreased, and is estimated to be about 2 dBi, compared to a shorted patch antenna with a shorting pin. For the latter design with stacked shorted patches, an impedance bandwidth of greater than 10% can be obtained. For this design, of course, the total antenna volume or height is increased.
COMPACT DUAL-FREQUENCY MICROSTRIP ANTENNAS
Compact microstrip antennas with dual-frequency operation have attracted much attention. The two operating frequencies can have the same polarization planes or orthogonal polarization planes. One of the reported compact dual-frequency designs with the same polarization planes uses the first two operating frequencies of shorted microstrip antennas with a shorting pin, and the obtained frequency ratios between the two operating frequencies have been reported to be about
FULLY FABRICATED MICROSTRIP PATCH ANTENNA
The following figures 5.1 (a),(b) and show the resultant fabricated microstrip patch antenna. The figures shown are front, side and back view of antenna. This patch radiates at a frequency of 8.5 GHz.
Antenna Measurement
If a transmission line propagating energy is left open at the end, there will be radiating from this end. In case of a rectangular waveguide this antenna presents a mismatch of about 2:1 and it radiates in many directions. The match will improve if the open waveguide is horn shape.
The radiation pattern of an antenna is a diagram of field strength or more often the power intensity as a function of the aspect angle at a constant distance from the radiating antenna. An antenna pattern is presented as a two dimensional pattern in one or several planes. An antenna pattern is consists of several labels, the main lobe, side lobe, back bone lobe . The major power is concentrated in the main lobe as low as possible. The power intensity at the main lobe compared to the power intensity achieved from an imaginary omni-directional antenna (radiating equally in all directions) with the same power fed to the antenna is defined as gin of the antenna.
Conclusion and Future Scope
The design of slotted square patch antenna for circular polarization and dual bandwidth operated has been completed using HFSS software. The simulation gave results good enough to satisfy our requirements to fabricate it on hardware which can be used wherever needed. The investigation has been limited mostly to theoretical studies and simulations due to lack of fabrication facilities. Detailed experimental studies can be taken up at a later stage to fabricate the antenna. Before going for fabrication we can optimize the parameters of antenna using one of the soft computing techniques known as Particle Swarm Optimization(PSO).
