This project is on yagi-uda antenna. Yagi-Uda antenna is a directional antenna consisting of multiple parallel elements in a line, usually made of metal rods. Yagi-Uda antennas consist of a single driven element connected to the transmitter or receiver with a transmission line, and additional parasitic elements: a so-called reflector and one or more directors.
The reflector element is slightly longer than the driven dipole, whereas the directors are a little shorter. This design achieves a very substantial increase in the antenna's directionality and gain compared to a simple dipole.
Yagi is very widely used as a high-gain antenna on the HF, VHF and UHF bands. It has moderate gain which depends on the number of elements used, typically limited to about 17 dBi, linear polarization, unidirectional (end-fire) beam patternwith high front-to-back ratio of up to 20 db. and is lightweight, inexpensive and simple to construct.
INTRODUCTION
1.1 CHAPTER ONE
A Yagi-Uda antenna, commonly known as a Yagi antenna, is a directional antenna consisting of multiple parallel elements in a line, usually made of metal rods. Yagi-Uda antennas consist of a single driven element connected to the transmitter or receiver with a transmission line, and additional parasitic elements: a so-called reflector and one or more directors.
The reflector element is slightly longer than the driven dipole, whereas the directors are a little shorter. This design achieves a very substantial increase in the antenna's directionality and gain compared to a simple dipole.
Conveniently, the parasitic elements have a node (point of zero RF voltage) at their centre, so they can be attached to a conductive metal support at that point without need of insulation, without disturbing their electrical operation. They are usually bolted or welded to the antenna's central support boom. The driven element is fed at centre so its two halves must be insulated where the boom supports them.
The gain increases with the number of parasitic elements used. Only one reflector is used since the improvement of gain with additional reflectors is negligible, but Yagis have been built with up to 30-40 directors.
The bandwidth of the antenna is the frequency range between the frequencies at which the gain drops 3 dB (one-half the power) below its maximum. The Yagi-Uda array in its basic form has very narrow bandwidth, 2 - 3 percent of the centre frequency. There is a tradeoff between gain and bandwidth, with the bandwidth narrowing as more elements are used. For applications that require wider bandwidths, such as terrestrial television, Yagi-Uda antennas commonly feature trigonal reflectors, traps and larger diameter conductors, in order to cover the relevant portions of the VHF and UHF bands.
1.2 OBJECTIVE OF THE PROJECT
This antenna is a particularly useful form of RF antenna design. It is widely used in applications where an RF antenna design is required to provide gain and directivity. The objective of this work is to construct a yagi uda antenna for local television stations.
1.3 SIGNIFICANCE OF THE PROJECT
The Yagi is very widely used as a high-gain antenna on the HF, VHF and UHF bands. It has moderate gain which depends on the number of elements used, typically limited to about 17 dBi, linear polarization, unidirectional (end-fire) beam pattern with high front-to-back ratio of up to 20 db. and is lightweight, inexpensive and simple to construct. The bandwidth of a Yagi antenna, the frequency range over which it has high gain, is narrow, a few percent of the center frequency, and decreases with increasing gain, so it is often used in fixed-frequency applications. The largest and most well-known use is as rooftop terrestrial television antennas, but it is also used for point-to-point fixed communication links, in radar antennas, and for long distance shortwave communication by shortwave broadcasting stations and radio amateurs.
The Yagi antenna or Yagi-Uda antenna / aerial is one of the most successful RF antenna designs for directive antenna applications.
The Yagi or Yagi-Uda antenna is used in a wide variety of applications where an RF antenna design with gain and directivity is required.
The Yagi has become particularly popular for television reception, but it is also used in very many other domestic and commercial applications where an RF antenna is needed that has gain and directivity.
Not only is the gain of the Yagi antenna important as it enables better levels of signal to noise ratio to be achieved, but also the directivity can be used to reduce interference levels by focussing the transmitted power on areas where it is needed, or receiving signals best from where the emanate.
1.4 SCOPE OF THE PROJECT
The Yagi-Uda antenna consists of a number of parallel thin rod elements in a line, usually half-wave long, typically supported on a perpendicular crossbar or "boom" along their centers. There is a single driven element driven in the center (consisting of two rods each connected to one side of the transmission line), and a variable number of parasitic elements, a single reflector on one side and optionally one or more directors on the other side. The parasitic elements are not electrically connected to the transmitter or receiver, and serve as passive radiators, reradiating the radio waves to modify the radiation pattern. Typical spacings between elements vary from about 1/10 to 1/4 of a wavelength, depending on the specific design. The lengths of the directors are slightly shorter than that of the driven element, while the reflector(s) are slightly longer. The radiation pattern is unidirectional, with the main lobe along the axis perpendicular to the elements in the plane of the elements, off the end with the directors.
1.5 LIMITATION OF THE PROJECT
The Yagi antenna also has a number of limitations that need to be considered.
- For high gain levels the antenna becomes very long
- Gain limited to around 20dB or so for a single antenna
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