Television antenna, or TV aerial, is an antenna specifically designed for the reception of over-the-air broadcast television signals, which are transmitted at frequencies from about 41 to 250 MHz in the VHF band, and 470 to 960 MHz in the UHF band in different countries.
Television signals are strongest when the station transmitting tower and the home receiving antenna are in line-of-sight. If the line-of-sight is blocked or weakened by mountains, buildings or trees, the signal, likewise will be weakened or lost. The signal will also grow weaker as it travels farther. All indoor antennas can be adversely affected by the walls of a house, inadequate height, and by movement of people in the room.
Poor antenna system "Snow" is visual noise caused by a weak signal. poor antenna system, the set itself, or a combination of these. A good antenna system often solves these reception problems. An outdoor antenna is better than an indoor antenna. An indoor antenna is better than no antenna at all. If an outdoor antenna is needed for VHF, you'll almost certainly need an outdoor antenna for UHF. But if good VHF reception can be obtained with and indoor antenna, a loop or other UHF antenna on the set may also work.
This work focuses on designing a receiving antenna for VHF and UHF signal used for local television reception.
CHAPTER ONE
1.1 INTRODUCTION
An antenna is a specialized transducer that converts radio-frequency (RF) fields into alternating current (AC) or vice-versa. There are two basic types: the receiving antenna, which intercepts RF energy and delivers AC to electronic equipment, and the transmitting antenna, which is fed with AC from electronic equipment and generates an RF field.
At frequencies below 3 GHz, many different types of antennas are used. The simplest is a length of wire, connected at one end to a transmitter or receiver. More often, the radiating/receiving element is placed at a distance from the transmitter or receiver, and AC is delivered to or from the antenna by means of an RF transmission line, also called a feed line or feeder.
Our local television reception is occasionally affected by bad signal which is caused by buildings, trees and mountains. A good antenna system often solves these reception problems. An outdoor antenna is better than an indoor antenna. An indoor antenna is better than no antenna at all. If an outdoor antenna is needed for VHF, you'll almost certainly need an outdoor antenna for UHF. But if good VHF reception can be obtained with and indoor antenna, a loop or other UHF antenna on the set may also work.
All indoor antennas can be adversely affected by the walls of a house, inadequate height, and by movement of people in the room-as may be noticed with VHF "rabbit ears." Loop or single bow tie UHF antennas are usually not satisfactory. They are difficult to adjust for maximum signal pick up or for elimination of ghosts. Be sure their lugs are secure to the UHF antenna terminals on the back of the set. The indoor combination VHF/UHF antenna has a multi-position switch to get the best picture. As a rule, this type of antenna is not good for UHF. The rod antenna, or "monopole," is a VHF/UHF antenna found on some portable sets, Normally UHF reception is poor, but can be enhanced when the rod is telescoped to its smaller size.
For outdoor antenna, there are many different kinds of outdoor antennas available in a wide price range. The kind selected is determined primarily by the geographic location of the receiver.
Television signals are strongest when the station transmitting tower and the home receiving antenna are in line-of-sight. If the line-of-sight is blocked or weakened by mountains, buildings or trees, the signal, likewise will be weakened or lost. The signal will also grow weaker as it travels farther.
"Gain" is the measure of an antenna's sensitivity - and its ability to pick up signals. It is measured in decibels (dB). The farther away from the station tower, the more gain the antenna should have. Gain can also vary from channel to channel. For example, an antenna's advertised gain rating may be at Channel 20, but the gain may be much less at Channel 69.
Where buildings or other obstructions cause "ghosts," an antenna with good directivity is recommended. Directivity is the ability to receive only those signals at which an antenna is pointed. Highly directive antennas have narrow receiving angles (measured to degrees) and high "front to back ratios." To insure the best reception aim the antenna carefully.
If a good VHF antenna installation is already on hand, it will probably be less expensive to add a good quality UHF antenna on the same mounting mast, The separate UHF antenna also will permit pointing to VHF and UHF antennas independently.
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 receiving antenna for VHF and UHF signal used for local television reception.
1.3 SIGNIFICANCE OF THE PROJECT
The receiving antenna is very widely used as a high-gain antenna on the VHF and UHF bands. It has moderate gain which depends on the number of elements used, typically limited to about 40 dB, linear polarization, unidirectional (end-fire) beam pattern with high front-to-back ratio of up to 40 db. and is lightweight, inexpensive and simple to construct. The bandwidth of a receiving 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.
This receiving antenna is one of the most successful RF antenna designs for directive antenna applications.
The receiving antenna is used in a wide variety of applications where an RF antenna design with gain and directivity is required.
1.4 LIMITATION OF THE PROJECT
The receiving 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 <40dB
1.5 PURPOSE OF THE PROJECT
The purpose of constructing this receiving antenna is to provide to receive improving VHF/UHF television reception in the home.
1.6 APPLICATION OF THE PROJECT
Receiving antennas are essential components of all equipment that uses radio. They are used in systems such as radio receiver, television receiver, two-way radio receiver, communications receivers, radar, cell phones, and satellite communications, as well as other devices such as garage door openers, wireless microphones, Bluetooth-enabled devices, wireless computer networks, baby monitors, and RFID tags on merchandise.
1.7 TYPES OF ANTENNA
The types of antenna include whip antennas, dipole antennas, Yagi-Uda arrays, parabola antennas, loop antennas and so on.
Whip (rod) antennas |
Antennas used for mobile phones and the like. Non-directional antenna with equal sensitivity in any direction. |
Dipole antenna |
Used for amateur radio and so on. |
Yagi-Uda array |
Used as television antennas and so on. They have strong directivity, and must be aligned in the direction of the transmitting station. This is a dipole antenna with director and reflector elements attached to direct and reflect the radio wave. |
Parabola antenna |
Used for receiving satellite broadcasts. These antennas have very strong directivity and require fine directional adjustment, but they can use the power of the radio waves efficiently. |
Loop antenna |
Loop antennas capture the changes in the magnetic field of the radio waves. The radio waves propagate in the direction at right angles to the circle of the loop. |
Dielectric antenna |
Antennas using high frequency dielectric ceramics can be compact and achieve high performance. |
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