Prerna Jha Presentation Comm

8/3/2019 Prerna Jha Presentation Comm

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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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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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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

Prerna Jha Presentation Comm

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