Antennas - Electrical Engineering - Lecture Slides, Slides of Electrical Engineering

Download Antennas - Electrical Engineering - Lecture Slides and more Slides Electrical Engineering in PDF only on Docsity!

ANTENNAS

HISTORY

The first antennas were built in 1888 by Germanphysicist

Heinrich

Hertz

in

his

pioneering

experiments to prove the existence of electromagneticwaves

predicted

by

the

theory

of

James

Clerk

Maxwell.

Hertz placed dipole antennas at the focal point of parabolic

reflectors

for

both

transmitting

and

receiving. He published his work in Annalen derPhysik und Chemie (vol. 36, 1889).

Transmitting

Antenna:

Any

structure

designed

to

efficiently

radiate

electromagnetic

radiation

in

a

preferred direction is called a

transmitting antenna

In reception, an antenna intercepts some of the powerof an electromagnetic wave in order to produce a tinyvoltage at its terminals, that is applied to a receiver tobe

amplified.

An

antenna

can

be

used

for

both

transmitting and receiving.

Receiving

Antenna:

Any

structure

designed

to

efficiently receive electromagnetic radiation is calleda receiving antenna

BASIC STRUCTURE

It

is

a

metallic

conductor

system

capable

of

radiating and receiving em waves.

Typically an antenna consists of an arrangement ofmetallic

conductors

(“elements"),

electrically

connected (often through a transmission line) to thereceiver or transmitter.

An oscillating current of electrons forced throughthe

antenna

by

a

transmitter

will

create

an

oscillating

magnetic

field

around

the

antenna

elements, while the charge of the electrons alsocreates

an

oscillating

electric

field

along

the

elements.

Antennas may also contain reflective or directiveelements

or

surfaces

not

connected

to

the

transmitter or receiver, such as parasitic elements,parabolic reflectors or horns, which serve to directthe radio waves into a beam or other desiredradiation pattern.

Antennas can be designed to transmit or receive radio

waves

in

all

directions

equally

(omnidirectional antennas), or transmit them in abeam in a particular direction, and receive fromthat one direction only ( directional or high gainantennas).

WHY ANTENNAS?

Need of antenna arisen when two person wanted tocommunicate

between

them

when

separated

by

some distance and wired communication is notpossible.

Antennas are required by any radio receiver ortransmitter to couple its electrical connection to theelectromagnetic field.

Radio

waves

are

electromagnetic

waves

which

carry signals through the air (or through space) atthe speed of light with almost no transmission loss.

WHERE USED?

Antennas are used in systems such as radio andtelevision

broadcasting,

point

to

point

radio

communication,

wireless

LAN,

radar

and

space

exploration

Antennas are most utilized in air or outer space

But can also be operated under water or even throughsoil and rock at certain frequencies for short distances

RADIATION MECHANISM

Ideally all incident energy must be reflected backwhen open circuit. But practically a small portion ofelectromagnetic energy escapes from the systemthat is it gets radiated.

This occurs because the line of force don’t undergocomplete phase reversal and some of them escapes.

G

To increase amount of radiated power open circuitmust be enlarged , by spreading the two wires.

Due

to

this

arrangement,

coupling

between

transmission line and free space is improved.

Also amount of cancellation has reduced.

The radiation efficiency will increase further if twoconductors of transmission line are bent so as to bringthem in same line.

TYPES OF ANTENNAS

According to their applications and technology available,antennas generally fall in one of two categories:

1.Omnidirectional or only weakly directional antennas which receive or radiate more or less in all directions.These are employed when the relative position of theother station is unknown or arbitrary. They are also usedat lower frequencies where a directional antenna wouldbe too large, or simply to cut costs in applications wherea directional antenna isn't required.

2. Directional or

beam

antennas which are intended to

preferentially radiate or receive in a particular directionor directional pattern.

RADIATION PATTERN

The radiation pattern of an antenna is a plot of therelative field strength of the radio waves emitted bythe antenna at different angles.

It is typically represented by a three dimensionalgraph, or polar plots of the horizontal and verticalcross sections. It is a plot of field strength in V/mversus the angle in degrees.

The pattern of an ideal isotropic antenna , whichradiates equally in all directions, would look like asphere.

Many non-directional antennas, such as dipoles, emitequal power in all horizontal directions, with thepower dropping off at higher and lower angles; this iscalled an omni directional pattern and when plottedlooks like a donut.

ANTENNA GAIN

Gain is a parameter which measures the degree ofdirectivity of the antenna's radiation pattern. A high-gain antenna will preferentially radiate in a particulardirection.

Specifically, the

antenna gain

, or

power gain

of an

antenna is defined as the ratio of the intensity (powerper

unit

surface)

radiated

by

the

antenna

in

the

direction

of

its

maximum

output,

at

an

arbitrary

distance, divided by the intensity radiated at the samedistance by a hypothetical isotropic antenna.

The gain of an antenna is a passive phenomenon -power

is

not

added

by

the

antenna,

but

simply

redistributed to provide more radiated power in acertain direction than would be transmitted by anisotropic antenna.

High-gain

antennas

have

the

advantage

of

longer

range and better signal quality, but must be aimedcarefully in a particular direction.

Low-gain

antennas

have

shorter

range,

but

the

orientation

of

the

antenna

is

relatively

inconsequential.