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  Types of Antenna

Introduction to Types of Antenna

DIPOLE ANTENNA:

This kind of outdoor antenna is made up of two pieces of aluminium poles. Therefore this kind of antenna is termed Dipole antenna. Suppose the length of this type of dipole antenna is 7.4feet. This antenna is able of attracting the signals related to it. However in the middle portion, current and voltage are high. Therefore, the transmission cable which is connected in this portion and taken to the TV receiver. The gain achieved by this antenna is very minimum, therefore it is modified.

FOLDER DI-POLE ANTENNA:

In this, a pole that is double the length of the di-pole is occupied and folded making equal to the length of di-pole. Because this structure is double such as the di-pole antenna, it is able of absorbing more signals. The gain achieved by this antenna is double like of the di-pole antenna. Therefore this kind of antenna is employed through the people bit far away from the transmitter.

REFLECTOR DI-POLE ANTENNA:

This category of antenna is employed at the distance of more than 50km from the transmitter. In this, together with the folded di-pole, an yet other pole that has bit more length than the di-pole is placed. This is termed as Reflector, because it imitates the waves to the di-pole and forms magnetic flux in it. The distance among the di-pole and the reflector is ë/4. Because, the phase of this wave and the wave attracted through the di-pole are identical. Therefore this kind of antenna is able of providing more gain.

YAGI ANTENNA:

The diagram depicts the structure of yagi antenna. As similar to reflector and yet other piece of pole is added before the di-pole. The length of the new piece have be equivalent to the di-pole. This is known as director.

In the diagram 'D' stands for director. Suppose that waves are coming from the arrow's direction, the director is the first (primary) element to receive the waves. At present this antenna could capable to produce three times higher gain (reflector, director, folded di-pole) than the strength of another antenna. Therefore the gain provided through this antenna is extremely high and it is extensively used even far away from the transmitter.

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Figure: yagi antenna

DISH ANTENNA:

The word dish antenna is frequently employed for a parabolic antenna. Parabolic antenna is a high-gain reflector antenna that employed for television and data communications, radio, and as well for radiolocation (radar), on the SHF and USF parts of the electromagnetic spectrum. At these frequencies the comparatively short wavelength of electromagnetic radiation permits reasonably sized reflectors to show the wanted highly directional response for both transmitting and receiving.

Heinrich Hertz built the first parabolic reflector antenna of world in year 1888. The antenna contain a focal distance of 0.12meters, an aperture 1.2 meters wide, and was employed at operating frequency of approximately 450MHz. the reflector was prepared from zinc sheet metal that supported through a wooden frame, and contain a spark-gap excited dipole along the focal line. Along with, two such types of antennas one employed for transmitting and another for receiving.

A general parabolic antenna contains a parabolic reflector along with a small feed antenna at its focus. To locate the focus, reflect the light of a flashlight off of the dish. The flashlight is at the focus, while the reflected beam is parallel.

The reflector is a metallic surface created into a parabolic of revolution and in a rounded rim which creates the diameter of the antenna. This paraboloid possesses a different focal point by virtue of containing the reflective property.

The feed antenna at the focus of reflector is generally a low gain like a half-wave dipole or a small waveguide horn. The feed antenna is linked to the related radio frequency (RF) transmitting or receiving equipment through way of a coaxial cable transmission line or hollow waveguide.

Referring the parabolic antenna like a circular aperture provides the subsequent approximation for the maximum gain: G ≈ (π2 D2) / λ2 or G() \

Where:

G stands for power gain,

D stands for reflector,

ë stands for wavelength

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