INTRODUCTION. TO. What is an antenna?. A metallic apparatus for sending and receiving electromagnetic waves . A usually metallic device (as a rod or wire) for radiating or receiving radio waves . A transitional structure between free-space and a guiding structure.
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The antenna can be defined as,
“A truncated transmission line with output impedance equal to the impedance of the free space i.e. 377 Ω”
“The ratio of voltage to current at pair of terminals”
What does the reactance part indicate?
The reactance part of this impedance represents power temporarily stored by the antenna.
What is the radiation resistance?
The antennas appear to the transmission line as a resistance Rr called as radiation resistance.
Antenna resistance consists of two components:
Rr = Radiation Resistance
RL = Loss resistance i.e. ohmic resistance
“The property of an electromagnetic wave describing the time varying direction and relative magnitude of the electric field vector”.
Types of polarization :
Linear (horizontal or vertical)
Circular (right hand polarization or the left hand polarization)
A left hand circular polarized wave is one in which the wave rotates counter-clockwise whereas right hand circular polarized wave exhibits clockwise motion as shown in Figure (d).
If the path of the electric field vector is back and forth along a line, it is said to be linearly polarized.
In a circularly polarized wave, the electric field vector remains constant in length but rotates around in a circular path.
Antenna efficiency :
“ The property of radiating more strongly in some directions than in others is called the directivity of the antenna”
It can also be called as the ratio of the maximum power density to its average value over a sphere as observed in the far field of an antenna.
“ The gain of an antenna is an actual or realized quantity which is less than the directivity”
Gain can be measured by comparing the maximum power density of the antenna of known gain.
The Yagi-Uda Antenna is a “Directional antenna”
1) Dr.Hidetsugu Yagi [Tohoku Imperial University ]
2)Dr. Shintaro Uda.
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Parabolic - Dish Antennas
Methods of feeding parabolic reflectors
Front – fed reflectors
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Advantages : (Axial mode)