A SEMINAR REPORTONMICROSTRIP PATCH ANTENNA

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ANAND

MECE-163-2K10

INTRODUCTION to ANTENNA

Antennas are used to transmit and receive electromagnetic energy to serve the purposes of communications or detection.

Radiation from a dipole made of conductor wire originates from acceleration of electrons along the wire. As electrons are accelerated or decelerated, their kinetic energy is transferred into electromagnetic energy.

The electric field, magnetic field, and their associated electron and current distribution move along the transmission line which feeds the dipole.

MICROSTRIP PATCH ANTENNA

In its most basic form, a Microstrip patch antenna consists of a radiating patch on one side of a dielectric substrate which has a ground plane on the other side as shown in Figure.

Microstrip patch antennas radiate primarily because of the fringing fields between the patch edge and the ground plane.

For good antenna performance, a thick dielectric substrate having a low dielectric constant is desirable since this provides better efficiency, larger bandwidth and better radiation.

Common shapes of microstrip patch elements

Fundamental Parameters Performance of antennas can be evaluated based on the radiation

pattern or relevant parameters like power flux density, radiation intensity, and polarization.

Directivity & Gain: The directivity, D, is used to characterize the relative radiation intensity in a specific direction with respect to the average radiation intensity over all directions. In contrast, the gain of an antenna, G, is defined based on the power delivered to the antenna.

Bandwidth: Bandwidth of an antenna is the range of frequency within which its performance of certain characteristics(input impedance, radiation pattern, beam width, polarization, side lobe level, gain, beam direction) conforms to a specified criterion.

Polarization : Polarization of a radiated wave is defined as the time-varying direction and relative magnitude of the electric field.

Polarization Types:- Linear Polarization Circular Polarization

Elliptical Polarization

Fundamental Parameters(contd.)

Input Impedance:The input impedance of an antenna is defined as the impedance measured at its input port, and can be expressed as Za = Ra + jXa. The real part can be decomposed into Ra = Rr + Rc where Rr is the radiation resistance and Rc is the loss resistance. The internal impedance of the signal generator can be expressed as Zg = Rg + jXg.

The input impedance is a function of frequency, and the antenna can be matched to the transmission line reasonably well only over certain bandwidth.

Reciprocity: Reciprocity can be understood by considering two antennas in a linear, passive and isotropic medium, where one antenna is located in the far-field region of the other.

Methods of Analysis

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.

Methods of Analysis(contd.) Cavity Model: 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 for this assumption is the following observations for thin substrates (h<<).

Design Specifications of Microstrip Patch Antenna

Frequency of operation (fo): The resonant frequency of the antenna must be selected appropriately. The Personal Communication System (PCS) uses the frequency range from 1850-1990 MHz. Hence the antenna designed must be able to operate in this frequency range.

Dielectric constant of the substrate (εr): The dielectric material selected for my design is PVC .A substrate with a high dielectric constant must be selected since it reduces the dimensions of the antenna.

Height of dielectric substrate (h): For the microstrip patch antenna to be used in cellular phones, it is essential that the antenna is not bulky. Hence, the height of the dielectric substrate must be very small.

Feed Techniques

Microstrip Line Feed:-In this type of feed technique, a conducting strip is connected directly to the edge of the microstrip patch.

Coaxial Feed:- the inner conductor of the coaxial connector extends through the dielectric and is soldered to the radiating patch, while the outer conductor is connected to the ground plane.

Aperture Coupled Feed:- In this type of feed technique, the radiating patch and the microstrip feed line are separated by the ground plane.

Coupling between the patch and the feed line is made through a slot or an aperture in the ground plane.

Proximity Coupled Feed:- This type of feed technique is also called as the electromagnetic coupling scheme. 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.

Comparing the different feed techniques

Advantages

Low cost fabrication. Can easily conform to a curved surface of a vehicle or product. Resistant to shock & vibrations. Many designs readily produce linear or circular polarization. Considerable range of gain & pattern options (2.5 to 10.0dB). Other microwave devices realizable in microstrip may be

integrated with a microstrip antenna with no extra fabrication steps.

Antenna thickness is small. Can be easily integrated with microwave integrated circuits

(MICs). Capable of dual and triple frequency operations. Mechanically robust when mounted on rigid surfaces

Disadvantages

Narrow bandwidth. Dielectric & conductor losses can be large for

thin patches resulting in poor antenna efficiency. Sensitivity to environmental factors such as

temperature & humidity. Low Gain. Extraneous radiation from feeds and junctions. Poor end fire radiator except tapered slot

antennas. Low power handling capacity. Surface wave excitation.

Applications of Microstrip Antenna

The telemetry communication antennas on missiles need to be thin and conformal and are often Microstrip patch antennas.

They are extremely compatible for embedded antennas in handheld wireless devices such as cellular phones.

Antennas are usually employed for radio and television broadcasting to the large parabolic reflectors used to receive satellite signals and the radio waves generated by distant astronomical objects.

Microstrip antenna being applicable to many military and commercial devices, such as use on aircraft or space antennas.

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