What is Antenna?

                                      What is Antenna?


Antennas or aerials are necessary for the radiations or reception of radio wave which are electromagnetic in character. A system of elevated conductors that couple or match the transmitter or the receiver to space, can serves as antenna. A transmission line is employed to connect the transmitting antenna to the transmitter. In according with Maxwell’s theory, electromagnetic energy having the same frequency as the RF current flowing through the antenna, is radiated in space For a straight-wire antenna, the radiation COMPARISION AN ELECTRIC FIELD VECTOR IN A PLANE CONTAINING THE ANTENNA, AND A MAGNETIC FIELD VECTOR TRANSVERES TO THE ELECTRIC FIELD. 

The electric and the magnetic fields oscillate At the frequency of the RF currents in the antenna. The amplitudes of the fields vary directly as the amplitude of the RF current, and they fall off inversely as the distance from the antenna. The fields travel through the space with the velocity of light,. The orientation of the electric fields with respect to the earth is referred to as the polarization of the electromagnetic wave. The wave is horizontally or vertically polarized in accordance as the electric field vector is parallel to the earth or vertical. The orientation of the antenna determines the polarization of the wave. The antenna rods should be horizontal to produce a horizontally polarized  waves, whereas they should be vertical to obtain a vertically polarized wave.


Maximum radiation is found to occur in a direction at right angles, i,e., broadside to the antenna. Parallel to the antenna the radiation is a minimum. So, for a maximum radiation of energy in the horizontal direction, the antenna should be vertical, whereas for a maximum radiation in the vertical direction, the antenna should be horizontal.

The frequency of the radiated signal determines the length of the antenna for effective radiation. The higher the frequency, the smaller the antenna length since for efficient radiation the antenna length is of the same order as the wavelength to be radiated. As the wavelength of audio frequency lie in the range from about 107 to 104 m, inconveniently large antenna would be necessary for a direct radiation of audio frequency.

 On the other hand, the wave length of a 1MHz radio wave is 300 m and antenna of this order of size can be easily constructed. Therefore, the audio signals containing the intelligence are used to modulate an RF CARRIER WHICH IS THEN RADIATED FROM THE TRANSMITTER. For long distance communication, the choice of the carrier frequency is determined, apart from the antenna size, by the absorption characteristics of the propagating medium for the wave.


When programmes are broadcast simultaneously by different transmitters, a different carrier frequency must be chosen for each transmitter to avoid interference between programmes. Suppose that one transmitter A broadcasts a speech and another transmitter B Simultaneously broadcasts a music. Each programme contains audio frequencies from 0 to 500 kHz. If the
transmitter A uses a carrier frequency 700 kHz and the transmitter B a carrier frequency 750 kHz, the programme radiated by transmitter A is contained in the frequency range 695 to 705 kHz and that radiated by transmitter B is contained in the frequency range 745 to 755 kHz for amplitude modulation. The two programmes are thus shifted to different regions of the frequency scale. By tuning the receiver to 700 kHz or 750 kHz, either programme can be distinctly heard.
 A transmitting antenna can also be used as a receiving antenna,. The action of receiving antenna is opposite to that of a transmitting antenna. A small portion of the radio waves radiated by the transmitting antenna is intercepted by a receiving antenna. If the rods of the receiving antenna are parallel to the electric field vector of the radiated wave an electric currents is induced in the rods.  The frequency and the waveform of the induced current are the same as those of the radiated wave. The small voltage produced by the induced currents is amplified and demodulated by the receiver. The intelligence is thus extracted.
In principle, the design and the sharp of the receiving antenna are the same as those of the transmitting antenna. However in practical situations a convenient length or loop of wires often serves as a receiving antenna due to space restrictions. In the neighbourhood of the transmitter, these makeshift antenna give sufficient voltage pickup, but in regions away from the transmitter, the design of receiving antenna becomes elaborate. The shape and size of      the antennas depend on several factors including the frequency and the directivity of the radiation and the objective required    to be served by the antenna.

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