Courses for Telecommunications Antennae
These are elements converting an electric signal into an electromagnetic wave (shows) or otherwise (reception). It is important to know their impedance, their gain, their directivity.Introduction
An electrical conductor, supplies energy in the form of heat (heating), light (lamp). Also it provides a magnetic field (transformer principle) and electrical (principle of the capacitor, the cathode ray tube). The principle of the transmission antenna is to ensure that the electromagnetic field is important, without heat or light up. This field is propagated, then it is an electromagnetic wave. A hot spring can be transformed into electrical energy (temperature sensor), as a light source (solar panel), an electromagnetic source is transformed into electrical energy through a electrical conductor. The principle of the receiving antenna is to ensure that the electromagnetic fields from the air is converted into usable energy. Of active energy Why and how are these energy exchanges are physical problems that m 'escape completely.
Operation
Study of the receiving antenna (the transmitting antenna is same). Any transmission line transforms electromagnetic fields of electrical energy. The only usable electrical energy is active energy. This requires that the line is producing the active energy, so that its impedance or resistance. The most suitable length line is 1/4, 1/2 etc. of the wavelength (see course of transmission lines). To this line to be subjected to more possible fields, he must be large as possible. The power line is thus unfolded. It is a doublet (dipole wire antenna). It is possible to eliminate one of the two strands of the line, then it is the soil that serves as second strand. It is a wire antenna (FM). If we can not have a long enough line (capacitive line), just add an inductor at the base. (this is the case in BC, for laptops).
It is possible to have directional antennas that are actually doublets with mechanical systems (This is even physical) to promote a direction (UHF antenna VHF). We only studied the lines ending with an open circuit (see transmission lines part), but we can also have lines terminated by a short circuit. These lines also by length a variable impedance, we can turn them into antenna (circular indoor antenna).
Feeder and impedance antenna
The line that carries power to the antenna is called feeder. It is necessary that this energy does not come back, but remains in the antenna. This requires that the antenna has the same impedance as the characteristic impedance of the feeder (TOS or ROS = 1 = 1 for amateurs). If the feeder is absent, it is necessary that the floor of the output impedance end is identical to the impedance of the antenna.
Radiation pattern
The shape of each antenna determines its strength, receiving emission direction.These directions are represented by a diagram. The pattern can be in the horizontal or vertical plane. The lobes represent the space where the field is present.Gain
The antenna gain is the amount of energy received or transmitted in one direction relative to the amount of received energy emitted from a reference antenna.
The reference antenna can be a perfect antenna has a diagram of circular radiation, it is called isotropic gain.
The reference antenna can be the 1/2 wave dipole (see below).
Different types of antenna
The half-wave antenna
The antenna of the best known and 1/2 wave dipole type. this is a transmission line terminated by an open circuit (CO) whose length is equal to a quarter of the wavelength of the carrier. When this line is unfolded, it then measures a half wavelength. Its impedance is a pure resistance of 73 ohms. The radiation pattern in the horizontal plane is visible below cons. Example: an FM receiver is tuned to 100 MHz . The wavelength is 3 m. The length of the antenna will be equal to twice 75 cm.
The ground plane antenna.
Is the previous antenna with one of the two strands is the ground. This is a vertical antenna. Its theoretical impedance is a pure resistance of 36 ohms. The radiation pattern in the horizontal plane is visible below cons. With the above example the length of the antenna would be 75 cm.
The Yaggi antenna (the most common antenna UHF)
It consists of a half wave dipole, one or more managers, a reflector. The whole is held by an insulator. Its directivity depends on the number of directors. Its impedance is a pure resistance of 52 ohms. The radiation pattern in the horizontal plane is visible below cons. Its gain can reach 15 DB relative to the quarter-wave dipole.Litters antennas
According to the desired transmission power and reception power, one can calculate a theoretical range in direct wave.
Unfortunately, the wave is reflected from the floor, the walls. This reflection can cause diffraction etc. For all this, the theory speaks only about 60%. It is therefore necessary to have some orders of magnitude on the bearing surfaces. These orders of magnitude are given in the section units, sizes, laid.