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Presentation On Antennas.
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Antennas
A good antenna worksA bad antenna is a waste of time & money
Antenna systems can be very inexpensive and simple
They can also be very, very expensive
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HEIGHT OF ANTENNAThe height of the antenna required for
transmission and reception of radio waves in
radio transmission is a function of wavelength
of the frequency used. The minimum height of
the antenna is given as /4. This wavelength is
given as c/f.
Where c is the velocity of light and f is the
frequency.
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Height of antenna
At low frequencies wavelength is very high and
hence the antenna height. For example, consider
the baseband signal with f= 15 kHz.
Height of antenna = 5000 metersThis 5000 meters height of a vertical antenna is
unthinkable and unpracticable. On the other hand,
if we consider a modulated signal with 1 MHzfrequency in the broadcast band the height of
antenna is given as
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Height of antenna = c / f*4
= 75 meters
This height of antenna is practical and such
antenna can be installed
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Antenna Considerations
The space available for an antenna
The proximity to neighbours
The operating frequencies we will use
The output power
Money
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Antenna Types
High Frequency
1.6 - 30 Mhz + 50 Mhz
160 - 6 metres
An antennas size/length depends on the frequency
Its functionality largely depends on the height above
ground, as well as the polarity and its configuration
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Types of Antennas
Simple wire
Dipole
Folded dipole
Trap dipole
Offset or Windom antenna
Phased dipoles
Vertical or horizontal (both)
Beverage wave antenna
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Types of Antennas
MetalVertical
Yagi
Trap Yagi
Phased arraysLoops
Vertical or Horizontal
Horns for super ultra high frequencies
Mobile antennas
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Antenna PolarizationVertical or horizontal
Electrical vs Magnetic radiation
(Diagram)
Vertical waves travel @ 90 to the earths surface
Horizontal waves travel parallel to the earths surface
Usually wire antennas are horizontal but an inverted V dipole has a verticalcomponent
Yagi type antennas can be either vertical or horizontal
Circular antennas can be both
Usually, horizontally polarized antennas hear less noise
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Isotropic AntennaThe isotropic antenna is a hypothetical point source.
It does not exist in reality but is considered as an important
starting point considering different
antennas from the theoretical to the practical
The pattern is a Cardioid - a donut shape or a sphere
Dipole Radiation
Pattern
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Polarization - Practical
Antennas radiating a vertical polarization are best receivedby an antenna of like polarization
Cross polarization reduces reception by as much as 30 db
Bouncing DX signals probably have both polarizations
Designing antenna polarization usually depends on the
frequency being used - at 70 cm in th eUHF band the
elements are very short so either polarization is possible.
Usually vertical is used as repeaters are vertically polarized.
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Resonance
Antenna length is dependant on frequency
The lower the frequency the longer the antenna elements
Examples
80 metres 3.750 Mhz 124 ft
40 7.055 66
10 28.5 16.4
6 52 92 145 3.2
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Wavelength vs Physical Length
The speed of a radio wave: is the same as the speed of light
The velocity of propagation of radio frequency energy in free spaceis: 300 000 kilometres per second
If an antenna is made longer, what happens to its resonantfrequency? It decreases
If an antenna is made shorter, what happens to its resonantfrequency? It increases
The resonant frequency of an antenna may be increased by:shortening the radiating element
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Wavelength vs Physical Length
To lower the resonant frequency of an antenna, the
operator should: lengthen it
Adding a series inductance to an antenna would:
decrease the resonant frequency
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Wavelength vs Physical Length
The wavelength for a frequency of 25 MHz is:
12 metres (39.4 ft)
The wavelength corresponding to a frequency of 2 MHz is:150 m (492 ft)
At the end of suspended antenna wire, insulators are used.These act to: limit the electrical length of the antenna
One solution to multi-band operation with a shortenedradiator is the "trap dipole" or trap vertical. These"traps" are actually: a coil and capacitor in parallel
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Presented by
Prerna Jha
090103163ECE C