Micro strip Antenna

Micro strip Antenna
A
Project Report
Submitted
in partial fulfillment
for the award of the Degree of
Master of Technology
in Department of Digital Communication Engineering



Submitted By:
Rakesh Kumar Bazad
 


ACKNOWLEDGEMENT

It is great opportunity to express my sincere thanks to all who have contributed to do this project through their support, encouragement and guidance.






Rakesh Kumar Bazad

Abstract


A novel antenna configuration comprised of two circular
microstrip antennas (CMAs) resonating in the TMtt and TM2, modes,
producing radiation characteristics suitable for a mobile telephone handset,
is presented. The antennas operating at the same frequency are
placed back to back with a separation comparable to the thickness of a
typical handset. The radiation pattern consists of a region of reduced
radiation intensity, which minimizes the radiation hazards to the user
 
INDEX

1. INTRODUCTION
1.1 Introduction
1.2 History






1. Introduction
1.1 Introduction :
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.

1.2 History :
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.

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.


















 
2. Waves on Micro strip Antenna

2.1 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.

2.2 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 , 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, 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.





2.3 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.





3.Application

1. Single element
2. Array
3. GPS
 
4. Advantage of Micro strip Antenna
• 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.
• Low profile (can even be “conformal”).
• Easy to fabricate (use etching and phototlithography).
• Easy to feed (coaxial cable, microstrip line, etc.) .
• Easy to use in an array or incorporate with othe microstrip circuit elements.
• Patterns are somewhat hemispherical, with a moderate directivity (about 6-8 dB is typical).


 
5.Disadvantage of Microstrip Antenna
• Low 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.




6.Reactangular Micro strip Antenna

6.1 Introduction of Patch Antenna :
A patch antenna is a narrowband, wide-beam antenna fabricated by etching the antenna element pattern in metal trace bonded to an insulating dielectric substrate, such as a printed circuit board, with a continuous metal layer bonded to the opposite side of the substrate which forms a ground plane. Common microstrip antenna shapes are square, rectangular, circular and elliptical, but any continuous shape is possible. Some patch antennas do not use a dielectric substrate and instead made of a metal patch mounted above a ground plane using dielectric spacers; the resulting structure is less rugged but has a widerbandwidth. Because such antennas have a very low profile, are mechanically rugged and can be shaped to conform to the curving skin of a vehicle, they are often mounted on the exterior of aircraft and spacecraft, or are incorporated into mobile radiocommunications devices.

The most commonly employed microstrip antenna is a rectangular patch. The rectangular patch antenna is approximately a one-half wavelength long section of rectangular microstrip transmission line. When air is the antenna substrate, the length of the rectangular microstrip antenna is approximately one-half of a free-space wavelength. As the antenna is loaded with a dielectric as its substrate, the length of the antenna decreases as the relative dielectric constant of the substrate increases. The resonant length of the antenna is slightly shorter because of the extended electric "fringing fields" which increase the electrical length of the antenna slightly. An early model of the microstrip antenna is a section of microstrip transmission line with equivalent loads on either end to represent the radiation loss.



6.2 Basic Principle of Micro Strip Antenna:
The patch acts approximately as a resonant cavity (short circuit walls on top and bottom, open-circuit walls on the sides).

In a cavity, only certain modes are allowed to exist, at different resonant frequencies.

If the antenna is excited at a resonant frequency, a strong field is set up inside the cavity, and a strong current on the (bottom) surface of the patch.
This produces significant radiation (a good antenna).
7. 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).

7.1 Microstrip Line Feed :
In this type of feed technique, a conducting strip is connected directly to the edge of the Microstrip patch. 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.
7.2 Methods of Analysis :
The preferred models for the analysis of Microstrip patch antennas are the transmission line model, cavity model, and full wave 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. 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.



 
8. Parameters
8.1 Gain :
The gain of a rectangular microstrip patch antenna with air dielectric can be very roughly estimated as follows. Since the length of the patch, half a wavelength, is about the same as the length of a resonant dipole, we get about 2 dB of gain from the directivity relative to the vertical axis of the patch. If the patch is square, the pattern in the horizontal plane will be directional, somewhat as if the patch were a pair of dipoles separated by a half-wave; this counts for about another 2-3 dB. Finally, the addition of the ground plane cuts off most or all radiation behind the antenna, reducing the power averaged over all directions by a factor of 2 (and thus increasing the gain by 3 dB).
8.2 Impedance Bandwidth :
The impedance bandwidth of a patch antenna is strongly influenced by the spacing between the patch and the ground plane. As the patch is moved closer to the ground plane, less energy is radiated and more energy is stored in the patch capacitance and inductance: that is, the quality factor Q of the antenna increases. A very rough estimate of the bandwidth is:

where d is the height of the patch above the ground plane, W is the width (typically a half-wavelength), is the impedance of free space, and is the radiation resistance of the antenna. The fractional bandwidth of a patch antenna is linear in the height of the antenna. The impedance of free space is approximately 377 ohms, so for the typical radiation resistance of about 150 ohms, a simplified expression can be obtained

For a square patch at 900 MHz, W will be around 16 cm. A height d of 1.6 cm will provide a fractional bandwidth of around 1.2(1.6/16) ≈ 12%, which gives a Bandwidth of 108MHz at the center frequency.

8.3 Resonance Frequency :
The resonance frequency is controlled by the patch length L and the substrate permittivity.

8.4 Resonant Input Resistance:

The resonant input resistance is almost independent of the substrate thickness h. The resonant input resistance is proportional to εr . The resonant input resistance is directly controlled by the location of the fed point. (maximum at edges x = 0 or x = L, zero at center of patch.
 
9. Circular Polarization

9.1 Three main techniques:
1) Single feed with “nearly degenerate” eigenmodes.
2) Dual feed with delay line or 90o hybrid phase shifter.
3) Synchronous subarray technique.


9.2 Feeding Methods :

1. Coaxial Feeds
2. Inset Feeds
3. Aperture Couple Feed
 
10.Refrence

1.www.google.com
2.www.yahoo.com

Posted in: Antenna