INTRODUCTION TO ANTENNA

Jun 17 • Notes • 4297 Views • 5 Comments on INTRODUCTION TO ANTENNA

INTRODUCTION TO ANTENNA :

ANTENNA TERMINOLOGY :

Antenna can be defined as a strong metal structure and its form of wire is called aerial . This word antenna was developed by an Italian , Guglielmo  Marconi . He used long aerial wires and a wire attached with 2.5 meter vertical pole in his experiment which acted as a radiating and receiving element . It is mostly used in complete assembly also including support structure or enclosure etc .

RADIATION PATTERN AND RADIATION FIELD FROM CURRENT ELEMENT:

The radiation  waves emitted at different angles by the antenna along with its relative strength of field is called its radiation pattern . A three dimensional  graph is  used  to represent it . The sphere looking like antennas radiates easily in all directions and the non directional antennas only moves in horizontal direction . The maxima or lobes at various angles are separated by angles of zero radiation .  This is due to the interference of various radio waves that is emitted from various parts of the antenna . In directional antenna the radio waves are always directed in a particular direction and it represents main lobe . The unwanted radiations from other lobes are known as side lobes .

SELF OR MUTUAL IMPENDANCE OF ANTENNA :

A difference in impendence occurs when the electromagnetic wave passes through the various parts of the antenna system .Depending on impendence at each point or interface , some energy reflects back to the source which helps in creating a standing wave . When the difference in impendence becomes minimum the power transfer by each part of the system increases to maximum . The impendence in most of the time is corrected by adjusting the feedline impendence .

 EARTH’S EFFECT  ON VERTICAL PATTERN AND IMAGE ANTENNA  AND SQUARED AMPLITUDE PROPAGATION :

The earth behaves as a dielectric medium because the conduction capacity of earth is zero . The earths surface receives two electromagnetic  waves  , where one gets  reflected back and the other is captured by the dielectric .  This reflected wave has almost  similar amplitude  and the other wave is almost negligible . The waves sent by the antenna under the earth are moderate and hence are easily absorbrd by the earth . The radio waves reflected from the ground travels longer distance if placed at same height above the earth . these can be calculated by the squared amplitude propagation method where the amplitude is taken as a square calculation .

 ANTENNA ARRAY SYNTHESIS  : 

A group of isotropic radiators for example like running of current by them in different amplitudes and phrases . we use interferometric array of radio telemetres here . it consists of non uniform amplitude distribution . It uses certain mathematical formulas like fourier transform , laplace transform , Woodword Lawson method and Schelknoff polynomial method to calculate the synthesis of antenna array .

TYPES OF ANTENNAS : 

There are various types of antennas like in case of wire antennas we have :

1. short dipole antenna

2. dipole antenna

3.Broadband dipole antenna

4. monopole antenna

Similarly in case  of reflector antennas are of two types

1. rectangular microstrip antenna and

2. planner inverted F antenna . There are other antennas like loop antenna , helical antenna and biconical antenna and periodic antenna .

Periodic antennas range from 13MHz to  18GHz . It is a multi element antenna that has both impedance and radiation .The length of this increases logarithmically  starting from one end to the other end. The helical antenna has a wire that is conducting and it is always wound in thw form of a helix . These type of antennas are placed on a plane arranged in groups . The biconical antenna is also a type of broadband .It has objects that are conically conductive and they touch each point of one another . The loop antenna is a type of radio antenna and it consists of various loops .It has a transmission line which is balanced . These are mostly used in higher frequencies like VHF and UHF .

 RESISTANCE OF RADIATION  OF SHORT DIPOLE AND HALF WAVE DIPOLE ANTENNA:

The device was mainly designed to lessen the device reactance and it is also known as resistive load. When the reactance is eliminated , the sum of two parts represents pure resistance or radiation resistance . This power absorbed is then converted into waste heat and the absorbed power becomes radiated electromagnetic energy . The antenna becomes more efficient if the resistance of the radiation is greater .

QUESTIONS AND ANSWERS :

QUE > WRITE SHORT NOTE ON RADIATION RESISTANCE OF SHORT DIPOLE ?

ANS > It was mainly designed to lessen the device reactance and it is also called as resistive load. When the reactance is deleted , the addition of two parts presents pure resistance or radiation resistance .The power absorbed is then converted into waste heat and the absorbed power becomes radiated electromagnetic energy . The antenna becomes more efficient if the resistance of the radiation is greater .

QUE > WRITE SHORT NOTE ON SELF OR MUTUAL IMPEDANCE OF ANTENNA ?

ANS> A difference in impedence exists  when the electromagnetic wave moves through the various parts of the antenna  . Based on impedence at each point  , some energy reflects back to the source which helps in creating a standing wave . When the difference in impedence becomes less the power transfer by each part of the system increases to maximum . The impedence in most of the time is corrected by adjusting the feedline impedence .

GATE Syllabus-

1. Gate Syllabus for Electronics and Communication 2014

2. Gate Syllabus for Engineering Science 2014

IES Syllabus-

1. IES Syllabus for General Ability

2. IES Syllabus for Electronics and Telecomm

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