An aerials or antenna is a device used to trasmit or receive radio waves. most antennae for radio or television consist of metal wires or rods connected to trasmitter or receiver .
When the antennae is used for trasmission,electric currents are made to oscillate along the wires by the trasmitter .
Energy from this oscillating charge is emitted into the space
When the antenna is used for reception these waves induce a weak electric current in the antenna wire. this current is then employed by the radio receiver
the radio wavelength determines the dimension of the antenna
For the antenna to resonate electrically at the desired wavelength, its should be one half or one quarter of the wavelength of the radio waves
-The simplest aerial called a monopole consist of a single wire suspended horizontally, or of a straight metallic rod.
A single-wire aerial is used in many short-wave receivers,
the extendible rod aerials are used in portable devices, such as Fm radios or television .
-The most widely used TV aeriales dipole, which consist of a straight metal rod is split in the centre .
-An other type is multiple element antenna,it consist more than one dipole that gives a better performance
-Dish aerials are bowl-shaped. the antenna is section of a parabola and a small dipole is placed in its focus ,a dish aerial is highly directional and can trasmit or receive radio waves in narrow beam these aerials are used to trasmit signal to satellites and signal overland.
The location of trasmitting aerials is also imprtant. Fm and television aerials must be placed as high as possible to increase their range.
