Define Antenna

Explain with an ilustration types of

microwave antenna

First antenna used to transmit signal wirelessly is demonstrated by

Guglielmo Marconi in 1901. It was a simple quarter wavelength monopole

antenna and he managed to send message using radio telegraphed

across the Atlantic Ocean.

Antenna is a means of

converting the electrical energy in

the transmission line into

electromagnetic waves in the free

space.

Defined as a transducer between

guided wave propagating in a

transmission line and an electromagnetic wave propagating in unbounded media (free space) and

vice versa.

Antenna are made in various shape, sizes and are used in radio and television

broadcasting and reception, radio wave communication systems, cellular

telephones, radar systems, anti-collision automobile sensor and many more.

Any conducting material can becomes an antenna, however an antenna is

design to radiate or receive electromagnetic energy with directional

and polarization suitable for intended application.

At the receiving end, antenna will

convert EM waves in the space into

electrical energy on a transmission line.

At the transmitting end of a radio

communication system, antenna will

convert electrical energy travelling

along the transmission line into an EM

waves that are emitted into space.

to minimize losses at the input of

the antenna, it is important to

know the impedance of the

antenna and to match it to the

transmission line.

To ensure the impedance of the

transmission lines is equal to the

impedance of the free space.

To transmit / radiate the energy

with high efficiency.

To receive the lowest energy from

the space ( in mW )

To radiate the energy in the

direction in favour and to distract

the energy from the unwanted

direction.

Antenna reciprocity

An antenna is a reciprocal device which

means that the transmitted and received

characteristic and performances are

identical : gain, directivity, frequency

of operation, BW, radiation resistance,

etc).

A basic antenna is a passive reciprocal

device. Passive in a sense that it cannot

actually amplify a signal.

Active antenna is a combination of a

passive antenna and low noise amplifier

(LNA). The real active antenna does not

exist in practice. Active antenna is not

reciprocal and it can only transmit or

receive signal but not both.

Radiation pattern › It is a polar diagram or graph

representing field strength of power

densities at various angular position relative to antenna.

Radiation pattern

Back lobe: lobe in direction exactly opposite the front lobe.

Side lobe : Secondary Beam :

› Minor beam.

› Normally represent undesired radiation or reception.

› Adjacent to front load (in 180 directions).

Major lobe: Primary Beam :

› Can be more than 1 major lobe.

› Propagates and receives the most energy.

› Also called front lobe (front of the antenna).

Line of shoot: the line bisecting the major lobe or pointing from the center of antenna in direction of maximum radiation.

From radiation pattern, there are two qualities that can be calculated:

› Front-to-back ratio : Ratio of the power in the direction of

propagation (front lobe) to the power opposite the direction of propagation (back lobe).

Front-to-back ratio = front lobe power / back lobe power

Front-to-side ratio:

Ratio of the power in the direction of propagation (front lobe) to the power in the side direction (side lobe).

Front-to-side ratio

= front lobe power / side lobe power

Antenna Beamwidth, θ It is the angular separation between the

two half power (-3dB) points or 0.707

from the maximum value on the major

lobe of an antenna’s plane radiation pattern. Also called the Half Power Beam

Width (HPBW).

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Antenna Beamwidth, θ

Beamwidth is inversely proportional to

antenna gain, i.e. the higher the gain,

the narrower the beamwidth.

For omni directional antenna: Gp=1 ,

θ = 360 ˚

Typical antenna has beamwidth in the

range of 30˚ < θ < 60˚

Beamwidth is sometime given for the

null-to-null angle

Antenna bandwidth

Defined as “the range of frequencies within which the performance of the antenna, with respect to some characteristic, conforms to a specified standard”.

Can be considered to be the range of frequencies, on either side of a center frequency (usually the resonance frequency for dipole), where the antenna characteristics (such as input impedance, pattern, beam-width, polarization, side-lobe level, gain, beam direction, radiation efficiency) are within an acceptable value of those at the center frequency.

Antenna Polarization

It is the direction in space of the

electric vector of the EM waves

radiated from an antenna and is

parallel to the antenna itself.

An EM waves are said to be

polarized if all its electric field vector

has the same alignment in space.

1. Vertically polarized (linear polarization) :

If an antenna radiates a vertically polarized EM wave.

Example a dipole in vertical position.

2. Horizontally polarized (linear polarization) :

If an antenna radiates a horizontally polarized EM wave.

Example a dipole in horizontal position.

3. Elliptically Polarized (elliptical polarization) :

If the radiated electric field rotates in an elliptical pattern.

Example a helix antenna.

4. Circularly polarized (circular polarization) :

If the radiated electric field rotates in a circular pattern.

Example a helix antenna.

Fig 3.7: Antenna polarization : (a)

linear, (b) elliptical polarization.

Antenna Gain

When calculating power density, we have

discovered that an antenna will introduce

gain to the power transmitted.

We also assume that the antenna that we

used (isotropic and other types of antenna)

have gain of G and unity efficiency.

We shall now introduce to the concept of

gain in antenna.

Antenna Gain Is the comparison of transmitted /

received Power of an antenna in the maximum direction of radiation between the transmitted/ received power of a reference antenna ( omni –directional antenna)

There are two types of gain in antenna: Directive gain Gd.

Power gain Gp.

Antenna Gain› Directive Gain, Gd It is the ratio of the power density radiated in

particular direction to the power density radiated to the same point by a reference antenna, assuming both antennas are radiating the same amount of power.

› Power gain, Gp It is the same as the directive gains except that

the total power that is fed to the antenna is

used, that is antenna efficiency is considered.

Allowing for the antenna loss in the near field

and the radiating structure.

Antenna Gain

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› Antenna Efficiency, ƞr

It is a measure of how efficient is an antenna in converting all of its input electrical energy into electromagnetic waves in free space.

It is the ratio of the power radiated by antenna to the total input power.

ƞr = PR / ( PR + PLOSS )

where PR = radiated power = I2 Rradiation

PLOSS = power loss = I2 Rloss

Rloss = ohmic loss Rradiation = radiation resistance

The efficiency of the antena can be found

without knowing the current feed in the

antena , hence the effieciency can be

reduced to

Efficiency , ƞr = RR / ( RR + RLOSS )

The effectiveness of an entire transmitting/ receiving system depends largely on impedance matching between the elements of the system. Impedance matching is particularly critical at the antenna connection.

If a good impedance match is maintained between the system and the antenna throughout the operating frequency band, power transfer to and from the antenna is always maximum.

The transmission line or waveguide used to transport energy to and from the antenna should have a characteristic impedance equal to that of the antenna.

Characteristic impedance of free space,

Z = E / H = electric field( V/m)

magnetic field (A/m)

Effective Isotropic Radiated Power

(EIRP)

It is the equivalent power that an isotropic

antenna would have to radiate to achieve

the same power density in chosen

direction at a given point as another antenna.

For instance: if a given transmit antenna

has a power gain of 10, the given antenna

effective 10 times as much power as an

isotropic antenna with the same input

power and efficiency.

We can measure the electric field and

magnetic field strengths or intensities of

a radio wave. An isotropic source is used

as the basis for calculation :

E = √30 Pt / d ( V/m )

H = √ Pt / 68.8d ( A/m)

where Pt = isotropic radiated power

d = distance from the point source

The electric field and magnetic field combine form power field density. Represented by the area called wavefront.

As the wavefront moves away from the center of the point source, the power density is spread over a rapidly increasing area.

Can be calculated as

P = E x H = [√30 Pt / d ]x [√ Pt / 68.8d]

= Pt / 12.56d2 = Pt/4π d 2

DIRECTIVITY

refers to the direction in which an

antenna radiates and the narrowness of

the radiated beam in DIRECTIONAL

ANTENNAS.

OMNIDIRECTIONAL ANTENNAS:

radiate and receive in all directions at

once.

HORN ANTENNA

Conical, Sectorial And Piramidal

Radiation pattern of horn antenna

PARABOLIC DISH ANTENNAS

PARABOLIC REFLECTIVE ANTENNA

REFLECTOR ANTENNAS are antennas that use a reflector to focus electromagnetic energy into a beam that is directional in either the vertical plane, the horizontal plane, or both planes at once.

The PARABOLIC REFLECTOR is most often used for high directivity.

Microwaves travel in straight lines as do light rays. They can also be focused and reflected just as light rays can, as illustrated by the antenna shown in figure 3-2.

Figure 3-2.—

Parabolic

reflector

radiation.

A microwave source is placed at focal point F. The field leaves this antenna as a spherical wavefront. As each part of the wavefront reaches the reflecting surface, it is phase-shifted 180 degrees. Each part is then sent outward at an angle that results in all parts of the field traveling in parallel paths.

Because of the special shape of a parabolic surface, all paths from F to the reflector and back to line XY are the same length. Therefore, when the parts of the field are reflected from the parabolic surface, they travel to line XY in the same amount of time.

Collimation characteristic: able to change wavefront from sphere to flat wavefront and vice verse.

Reciprocal (Ciri kesalingan): receiving antenna and transmitting antena have the same path (laluan sinaran yang sama).

Feeder : used together with antenna

Feeder is must be mounted at the focal point of the antena primer to achieve correctly the receive and transmit rays.

The source of transmitting at focal points is classified into two :

› Front feed:

dipole feed and horn feed

› Rear feed:

Feed mechanism :

Its Function: to radiate toward the reflector.

An ideal feed mechanism should direct all the energy toward the parabolic reflector.

It is usually a dipole or a dipole array or a horn.

Primary types of feed mechanism for parabolic antenna are center feed, horn feed and Cassegrain feed.

(a) Parabolic Antenna with a

Horn Feed,

(b) Cassegrain Feed.

Dipole Antenna

It is the basic antenna.

It is an electrically short

dipole, elementary dipole.

Generally, any dipole that is

less than is considered

electrically short.

Antenna dipole &

radiation pattern

Slot in the wall of a waveguide acts as an

antenna

Slot should have length λg/2

Slots and other basic antennas can be

combined into phased arrays with many

elements that can be electrically steered

Basic antenna:

folded dipole antenna, turnstile antenna, loop antenna, long wire antenna

Frequency independent antenna:

Log Periodic antenna

Broadband antenna: helical antenna

Antenna array:

broadside antenna, end fire antenna, rhombic antenna, Yagi –Uda antenna

Microwave Antennas

Conventional antennas can be adapted

to microwave use

The small wavelength of microwaves

allows for additional antenna types

The parabolic dish already studied is a

reflector not an antenna but we saw that

it is most practical for microwaves

Horn Antennas

• Not practical at low frequencies

because of size

• Can be E-plane, H-plane, pyramidal or

conical

• Moderate gain, about 20 dBi

• Common as feed antennas for dishes

Fresnel Lens

• Lenses can be used for radio waves

just as for light

• Effective lenses become small enough

to be practical in the microwave region

• Fresnel lens reduces size by using a

stepped configuration