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 .