ANTENNA PRESENTATION

GROUP MEMBERS

ZIAFAT ALI Impedance and gain of antenna. SAAD ASGHAR Beamwidth,Bandwidth,Polarization. SAMEER SHABEER

Antenna apparture,effective area,efficiency

What is an antenna ?

“An antenna is a metallic conductor system capable of radiating and receiving electromagnetic waves.”

Reciprocity Theorem

Antenna Parameters

Antenna Impedance , Za “It is defined as the ratio of input

voltage to input current Za = V / I Units= Ω

Za is the complex quantity and it is written as

Za = Ra + jXa

Xa = reactive part results from fields surrounding the antenna

Ra= resistive part

Two kinds of resistance

Ohmic resistance ( Rl ) Radiation Resistance ( Rr )

Ra =Rl +Rr

Rl =Losses in the antenna or ohmic resistance

Rr=is called radiation resistance

Radiation Resistance, Rr

is defined as a hypothetical resistance that would dissipate an amount of power equal to the radiated power

Rr = power radiated / I² rms

Ohmic Resistance , Rl

Heating loss present in a conductor is called ohmic resistance

Pomic = I²R

As our resistance is given as

Ra = Rl + Rr so our power is given as

Total power = Pl + Pr

P = I² R

GAIN

Saving of energy is called the gain of an antenna

Gain depends upon Beam Width G =4 π / Θφ

Beam width = Θ = ּג / l

Maximum amount of energy which our antenna can send in the desired direction is called Gain or Directive gain

DIRECTIVE GAIN

Directive Gain (Dirgain) is referred to the ratio of the power density

radiated in a particular direction to the power density radiated to the same point by a reference antenna(isotropic antenna) ,assuming both antenna radiate the same power level.

Dirgain = P/Pref

P = power density at some point with a given antenna (W/m2)

Pref = power density at the same point with a reference antenna(W/m2)

BEAMWIDTH

Beam width is the angular separation of the half-power points of the radiated pattern.

When there is 50% energy present this is called the

3-db beamwidth.

We consider the beamwidth in 3-Dimensions.

The 1st dimension is for horizontal beamwidth,2nd is for vertical beamwidth and 3rd is for the length of beamwidth.

BANDWIDTH OF ANTENNA

The bandwidth of antenna is defined as the range of frequencies in which the performance of antenna with respect to some characteristics is good.

We consider the bandwidth of antenna with respect to 3-db beamwidth as whole of beamwidth is useless and there is no 50% energy outside the 3-db beamwidth.

BANDWIDTH

There are different types of frequencies bands for different types of antennas like for yagi are VHF(30MHz-300MHz),UHF(300MHz-3GHz).

The frequency band for microwave antennas are from 3GHz-300GHz.

POLARIZATION

An antenna polarization is relative to the E-field of antenna.

E-field is responsible for carrying the energy and magnetic field is not responsible for this.

The receiver and transmitter antennas need to possess the same polarization.

TYPES

LINEAR POLARIZATION Horizontal Vertical Theta

CIRCULAR POLARIZATION.

ELLIPTICAL POLARIZATION

LINEAR POLARIZATION

VERTICAL POLARIZATION

An antenna is vertically linear polarized when its electric field is perpendicular to the Earth’s surface.

An example of a vertical antenna is a broadcast tower for AM radio or the whip antenna on an automobile.

LINEAR POLARIZATION

HORIZONTAL POLARIZATION Horizontally linear polarized

antennas have their electric field parallel to the Earth's surface.

For example, television transmissions use horizontal polarization. Thus, TV antennas are horizontally-oriented.

THETA POLARIZATION It is a type of polarization in

which electric field is neither parallel, nor perpendicular but making some angle with the earth surface.

CIRCULAR POLARIZATION

A circularly polarized antenna radiates energy in the horizontal plane, vertical planes as well as every plane in between and making one complete revolution during each interval.

If the rotation is clockwise looking in the direction of propagation than this is called right hand polarization.

If the rotation is counterclockwise looking in the direction of propagation than this is called left hand rotation.

ADVANTAGES

REFLECTIVITY; Radio signals are reflected or absorbed

by materials but circular polarized antennas send in all planes and the signal strength is not lost.

LINE-OF-SIGHT When line-of-sight is not clear and

there are obstacles like buildings which effect the transmission than we use circular polarization.

ELLIPTICAL POLARIZATION

A elliptical polarized wave radiates energy in the form of ellipse, in the horizontal, vertical planes as well as every plane in between.

If the rotation is clockwise looking in the direction of propagation than this is called right hand polarization.

If the rotation is counterclockwise looking in the direction of propagation than this is called left hand rotation.

ANTENNA APERTURE (Aa)

It is defined as the area of an antenna which is responsible for transmitting and receiving energy.

It is the primary parameter which tells about the performance of antenna.

It is always lesser than that of the actual area of an antenna.

EFFECTIVE AREA (Ae)

It is defined as the product of antenna aperture and the total area of an antenna, that is,

Ae = Aa * A

Antenna aperture and effective area are related as by the above relation.

The gain of antenna is related to the effective area of antenna, that is,

G = 4 π * Ae /λ²

ANTENNA EFFICIENCY(η)

It is defined as the ratio of radiated power to the input power, that is,

η= Pr / Pt