Friday, 26 June 2015

Microwave Antenna System

Antenna : It's define as a transducer that converts electric signal to electromagnetic signal and vice versa .

Antenna Parameters  :

1.1 – Radiation Pattern :
Radiation Pattern : It's defined as  a graphical representation of the radiation properties of the antenna as a function of space coordinates .
Power Pattern : Is trace of the received power at constant radius .
Field Pattern : Is a graph of the variation of E̅ or ( H̅ ) at a constant radius .

Types of Radiation Pattern :

1.A – Isotropic Radiation
An Isotropic radiator is defined as " hypothetical antenna having equal radiation in all directions ".
Point source would be an example of such a radiator , although it is ideal and not physically realizable , it's often taking as a reference .

1.B – Directional Antenna :
Is one " having the property of radiating or receiving electromagnetic wave more effectively in some directions than in others .

1.C – An Omni Directional Antenna :
Is one " having an essentially non directional (  isotropic ) pattern in horizontal plane    ( azimuth ) and a directional pattern in vertical plane ( elevation ).
    
1.2 Principle plane

Antenna performance is often described in terms
 of it's principle E and H-Plane .
         
The E-Plane pattern is defined as " The plane containing the electric field vector E̅ and the direction of propagation .
The H-Plane pattern is defined as " The plane containing the magnetic field vector H̅ and the direction of propagation .

1.3 Radiation pattern lobes

A radiation lobe is " a portion of the radiation pattern bounded by regions of relatively weakly radiation intensity " 

The lobes may be sub classified into :
A – Major ( Main ) Lobe  : Is defined as  " The radiation lobe containing the direction of maximum power ( radiation ) " .
In some antennas ,their may exist more than one major lobe ( Greating Lobes ).
B – Side Lobe  : Is " A radiation lobe in any direction other than main lobe  " .
C – Back Lobe  : Usually  refers to a minor lobe that exist in opposite direction of the major lobe .
D – Side Lobe Level : Is expressed as a ratio of the maximum power of the side lobe to the maximum power of the major lobe .
Side lobe levels of ( -20 dB or smaller , -30 dB , -40 dB ,..) are usually not very harmful in most application .

1.4 – Filed Regions :
A – Reactive Near Filed Region : is defined as  " that region of the filed immediately surrounding the antenna wherein the reactive field  ( Imaginary Part ) predominate .
Where D is the largest dimension of the antenna  .

    

B – Radiating near field  Region :
is defined as " that region of the field wherein the angular field distribution as function of ( θ , ɸ ) is dependent upon a distance

C – Far Field Region : is defined as  " That region of the field of an antenna wherein the angular is independent of the distance from the antenna ".
And their only two components of the field
Eθ (θ,ɸ) , Eɸ (θ,ɸ) where Er = 0


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