WIRELESS

COMMUNICATIONS

• Antenna – acts as the interface between a

transmitter or receiver and free space.

Antennas are a very important component of

communication systems.

- it is defined as a device used to transform an

RF signal, traveling on a conductor, into an

electromagnetic wave in free space.

- it can be a length of wire, a metal rod, or a

piece of tubing.

Antenna

the length of the conductor is dependent on

the frequency of operation.

- radiates most effectively when their length

Is directly related to the wavelength of the

transmitted signal.

- it demonstrate a property known as

reciprocity, which means that an antenna

will maintain the same characteristics

regardless if it is transmitting or receiving.

Most antennas are resonant devices, which

operate efficiently over a relatively narrow

frequency band.

Two types of antennas:

1. Transmitting antenna

- radiates the electromagnetic waves.

- converts the electrical energy traveling

along the transmission line into electromagnetic

waves in space.

-

2. Receiving antenna

- intercepts the electromagnetic wave,

- converts the electromagnetic wave into electrical signal down thru a transmission line.

It is analogous to a transmission line designed to radiate or receive energy.

There are parameters to consider:

a. Directivity

b. Gain

c. Input Impedance

d. Polarization

e. Bandwidth

f. Beamwidth

Antennas:

- are called passive devices, since the

power radiated by a transmitting antenna

cannot be greater than the power entering

from the transmitter because of losses due

to cable or transmission loss.

- but it can also be active devices.

in a high communication receiver or an

FM or television broadcast receiver where

the combination of a receiving antenna

with a low noise amplifier.

• Consider a transmission line connected to

a generator or a source of radio-frequency

signals. The generator will send these

radio-frequency signals down the

transmission line toward the load. The

radio or television transmitter can be

considered the generator and the

transmitting antenna as the load.

• Transmission line is open:

- the electric field and magnetic field

escape for the end of the line and it will

radiate into space, but, it is inefficient and

unsuitable for reliable xmission or

reception.

- Bending of the transmission line

conductors improved the efficiency of the

radiation and reception.

- Folding the conductors so that they are right

angle to the transmission line will give an

efficient radiation and reception.

- Because magnetic field will no longer cancel but

it will aid one another and the electric filed

spreads out from the conductor, then, it

becomes an ANTENNA.

However, optimum radiation occurs if the

segment of transmission wire is converted

to an antenna is one-quarter wavelength

long at the operating frequency.

Then, this makes an antenna that is one-

half wavelength long.

- Antenna radiates most effectively when

their length is directly related to the

wavelength of the transmitted signal.

• The electric field and the magnetic field is

perpendicular with each other and these

two field support each other.

• The ratio of the electric field strength to the

magnetic field strength is a constant and it

is called the Impedance of space or the

wave impedance and it is equal to 377

ohms.

• The resulting fields are radiated into space

at the speed of light.

• Radiation Resistance:

Antenna that is radiatingelectromagnetic energy appears to thegenerator as an ideally resistive electricalload so that the applied power isconsumed as radiated energy.

But antenna can also be reactive.

- The resistive component is called as theradiation resistance. This resistance doesnot dissipate power in the form of heat asin electronics circuit but instead the powerdissipated as radiated electromagneticenergy.

- at the frequency , the antenna appears to

be a pure resistance.

- for maximum power transfer it is important

that the impedance of the transmission

line match with the load.

- when the radiation resistance of the

antenna matches with the characteristic

impedance of the transmission line, then

SWR is minimum and therefore maximum

power reaches the antenna.

SWR – standing wave ratio

- Standing wave occurs when there is

mismatch between the transmission line

an the load.

it is the ratio of the maximum current to the

minimum current, or the maximum voltage

to the minimum voltage, along the line.

SWR = I max / I min, or

= V max / V min

• Since measuring the maximun nd

minimum voltage and current on a line is

not practical in the real world, then SWR is

the ratio of the load impedance to the

characteristic impedance:

SWR = Zl / Zo

Zl = Load impedance

Zo = characteristic impedance

• Antenna produce two sets of fields the

near field and the far field.

Near field – describes as the region

around the antenna where the electric and

magnetic fields are distinct.

- this field weaken with the distance from

the antenna, approximately by quadruple

power of the distance.

- is also referred as the Fresnel Zone.

- rarely used but some application as the

Radio Frequency Identification (RFID).

Far field – which approximately 10

wavelenghts from the antenna is the radio

wave with the composite electric and

magnetic fields.

- it strength also diminishes with the

distance but only at the square of the

distance.

- it is also called as Fraunhofer Zone.

- most wireless application use the far

field wave.

• Input Impedance

For an efficient transfer of energy, the impedance of the radio, of the

antenna and of the transmission cable connecting them must be the same.

- Transceivers and their transmission lines are typically designed for 50Ω

impedance. If the antenna has an impedance different from 50Ω, then

there is a mismatch and an impedance matching circuit is required.

The input impedance of an antenna is defined by

as “the impedance presented by an antenna at

its terminals or the ratio of the voltage to the

current at the pair of terminals or the

ratio of the appropriate components of the

electric to magnetic fields at a point”.

Hence the impedance of the antenna can be

written as:

Zin = Rin + jXin

where :

Zin is the antenna impedance at the terminals

Rin is the antenna resistance at the terminals

Xin is the antenna reactance at the terminals

• The imaginary part, X-in of the input impedance

represents the power stored in the near field of

the antenna.

The resistive part, R-in of the input impedance

consists of two components, the radiation

resistance Rr and the loss resistance Rl .

The power associated with the radiation

resistance is the power actually radiated by the

antenna, while the power dissipated in the loss

resistance is lost as heat in the antenna itself

due to dielectric or conducting losses.

• - Return loss

The return loss is another way of expressing

mismatch. It is a logarithmic

ratio measured in dB that compares the

power reflected by the antenna

to the power that is fed into the antenna

from the transmission line.

The relationship between SWR and return

loss is the following:

Return Loss (in dB)

= 20 log 10 SWR/SWR-1

According to their applications and technology

available, antennas generally fall in one of two

categories:

Omnidirectional or only weakly directional

antennas which receive or radiate more or

less in all directions. These are employed

when the relative position of the other station

is unknown or arbitrary. They are also used at

lower frequencies where a directional antenna

would be too large, or simply to cut costs in

applications where a directional antenna isn't

required.

• They are also used at lower frequencies

where a directional antenna would be too

large, or simply to cut costs in applications

where a directional antenna isn't required.

• Directional or beam antennas which are

intended to preferentially radiate or receive

in a particular direction or directional

pattern.

Antenna Characteristics:

Radiation Pattern

-The radiation pattern of an antenna is a

plot of the far-field radiation properties of

an antenna as a function of the spatial co-

ordinates which are specified by the

elevation angle θ and the azimuth angle φ.

-More specifically it is a plot of the power

radiated from an antenna per unit solid

angle which is nothing but the radiation

intensity .

http://en.wikipedia.org/wiki/File:Sidelobes_en.svg

Main Lobe: This is the radiation lobe containing

the direction of maximum radiation.

Minor Lobe: All the lobes other then the main

lobe are called the minor lobes.

These lobes represent the radiation in undesired

directions. The level of minor lobes is usually

expressed as a ratio of the power density in the

lobe in question to that of the major lobe. This

ratio is called as the side lobe level (expressed

in decibels).

Back Lobe: This is the minor lobe diametrically opposite the main lobe.

Side Lobes:

- These are the minor lobes adjacent to the main lobe and are separated by various nulls.

- are generally the largest among the minor lobes.

-No antenna is able to radiate all the energy in one preferred direction.

- Some is inevitably radiated in other directions. The peaks are referred to as sidelobes, commonly specified in dB down from the main lobe.

- Nulls

In an antenna radiation pattern, a null is a

zone in which the effective radiated power

is at a minimum. A null often has a narrow

directivity angle

• Bandwidth

- The bandwidth of an antenna refers to the range of frequencies over which the antenna can operate correctly. The antenna's bandwidth is the number of Hz for which the antenna will exhibit an SWR less than 2:1.

- The bandwidth can also be described in terms of percentage of the center frequency of the band.

BW = 100 × FH − FL/FC

where :

FH is the highest frequency in the band,

FL is the lowest frequency in the band

FC is the center frequency in the band.

In this way, bandwidth is constant relative to frequency.

If bandwidth was expressed in absolute units of frequency, it would be different depending upon the center frequency.

Different types of antennas have

different bandwidth limitations.

- Directivity

is the ability of an antenna to focus energy in a particular direction when transmitting, or to receive energy better from a particular direction when receiving.

- In a static situation, it is possible to use the

antenna directivity to concentrate the radiation beam in the wanted direction. However in a dynamic system where the transceiver is not

fixed, the antenna should radiate equally in all directions, and this is known as an omni-directional antenna.

The relationship of the gain with respect to

Directivity:

Gain is the directivity multiplied by the

efficiency of the antenna:

G = Dn

where:

G = Gain, as ratio (not in dB)

D = Directivity

n = efficiency

where :

n = Pr / Pt

n= antenna efficiency

Pr= radiated power

Pt= power supplied to the antenna

Gain

is a parameter which measures the degree

of directivity of the antenna's radiation

pattern. A high-gain antenna will

preferentially radiate in a particular

direction.

it is defined as the output over the input

dB = 10 log P out / P in

• Specifically, the antenna gain, or power

gain of an antenna is defined as the ratio

of the intensity (power per unit surface)

radiated by the antenna in the direction of

its maximum output, at an arbitrary

distance, divided by the intensity radiated

at the same distance by a hypothetical

isotropic antenna.

• High-gain antennas have the advantage of

longer range and better signal quality, but

must be aimed carefully in a particular

direction.

• Low-gain antennas have shorter range,

but the orientation of the antenna is

relatively inconsequential.

- Beamwidth

The angular distance between the half power

points is defined as the beamwidth.

An antenna's beamwidth is usually

understood to mean the half-power

beamwidth.

The peak radiation intensity is found and then

the points on either side of the peak which

represent half the power of the peak intensity

are located.

- Half the power expressed in decibels is 3dB,

so the half power beamwidth is sometimes

referred to as the 3dB beamwidth.

Both horizontal and vertical beamwidths

are usually considered.

Assuming that most of the radiated power is

not divided into sidelobes, then the

directive gain is inversely proportional to

the beamwidth: as the beamwidth

decreases, the directive gain increases.

- Beamwidth – measure’s the antenna

directivity.

therefore, the narrower the beamwidth,

the better the directivity and more highly

focused the signal becomes.

• Let us consider the case of an isotropic

antenna.

An isotropic antenna is one which radiates

equally in all directions.

If the total power radiated by the isotropic

antenna is P , then the power is spread

over a sphere of radius r ,so that the

power density S at this distance in any

direction is given as:

HPBW: The half power beamwidth (HPBW) can

be defined as the angle subtended by the half

power points of the main lobe.

• Front to Back Ratio

- the direction of maximum radiation in the

horizontal plane is considered to be the front

of the antenna, and the back is the direction

180 deg. from the front.

- for a dipole, front and back ratio have the

same direction 180 deg. from the front.

• POLARIZATION

- The electric field determines the direction of polarization of the wave.

The most common types of polarization

include:

a. linear (horizontal or vertical) and

b. circular (right hand polarization or the left hand polarization).

In a vertically polarized wave, the electric lines of force lie in a vertical direction.

-In a horizontally polarized wave, the electric lines of force lie in a horizontal direction.

-

Circular polarization has the electric lines of

force rotating through 360 degrees with

every cycle of rf energy.

In a circularly polarized wave, the electric

field vector remains constant in length but

rotates around in a circular path. A left

hand circular polarized wave is one in

which the wave rotates counterclockwise

whereas right hand circular polarized wave

exhibits clockwise motion as shown in

Figure.

- In a vertically polarized wave, the electric

lines of force lie in a vertical direction.

-In a horizontally polarized wave, the

electric lines of force lie in a horizontal

direction.

• The electric field was chosen as the reference

field because the intensity of the wave is usually

measured in terms of the electric field intensity

(volts, millivolts, or microvolts per meter).

• When a single-wire antenna is used to extract

energy from a passing radio wave, maximum

pickup will result when the antenna is oriented in

the same direction as the electric field.

• Thus a vertical antenna is used for the efficient

reception of vertically polarized waves, and a

horizontal antenna is used for the reception of

horizontally polarized waves.

• In some cases the orientation of the

electric field does not remain constant

• Instead, the field rotates as the wave

travels through space. Under these

conditions both horizontal and vertical

components of the field exist and the wave

is said to have an elliptical polarization.

• IMPEDANCE MATCHING:

- One of the most critical aspects of any

antenna system is to ensure maximum

power transfer from the transmitter to the

antenna.

- when the SWR is equal to 1:1, maximum

power transfer will take place.

- then, characteristic impedance of the

transmission line matches the output

impedance of the transmitter and the

impedance of the antenna itself.

To prevent mismatch between the antenna

and transmission line is through correct

design.

When mismatches occur, some corrections

are possible:

1. Tune the antenna by adjusting its length

to minimize SWR.

2. Insert impedance-matching circuit or

antenna tuner between the transmitter and

the transmission line.

Impedance matching circuits:

1. Balun or LC

2. L

3. T

4. pie

The ideal is an SWR of 1, but any SWR

value below 2 is usuallu acceptable.

Basic antenna models.

ISOTROPIC RADIATOR:

- is a purely theoretical antenna that radiates all the electrical power supplied to it and equally distribute in all directions.

- it is considered to be a point in space with no dimensions and no mass.

-This antenna cannot physically exist, but is useful as a theoretical model for comparison with all other antennas.

Most antennas' gains are measured with reference to an isotropic radiator, and are rated in dBi (decibels with respect to an isotropic radiator)

Dipole antenna:

is simply two wires pointed in opposite

directions arranged either horizontally or

vertically, with one end of each wire connected

to the radio and the other end hanging free in

space.

Dipole – means it has two parts

- it does not mean that dipole is always one-half

wavelength in length but we allow it for

impedance matching.

Since this is the simplest practical antenna, it

is also used as a reference model for other

antennas; gain with respect to a dipole is

labeled as dBd.

NOTE: 0 dBd = 2.15 dBi. It is vital in

expressing gain values that the reference point

be included. Failure to do so can lead to

confusion and error.

The directive gain of a half wave dipole is

known to be 1.64 & it can be made nearly

100% efficient. Since gain has been

measured with respect to this reference

antenna.

GdBd = 10 log G/1.64

G(dBd)= G(dBi) -2.15 dB

Where: G(dBd) =gain of antenna in decibels

with respect to a half-wave dipole.

G(dBi) = gain of the antenna in

decibels with respect to an isotropic

radiator.

Generally, the dipole is considered to be

omnidirectional in the plane perpendicular

to the axis of the antenna, but it has deep

nulls in the directions of the axis.

Variations of the dipole include the folded

dipole, the half wave antenna, the ground

plane antenna, the whip, and the J-pole.

Yagi-Uda antenna

is a directional variation of the dipole with

parasitic elements added which are

functionality similar to adding a reflector

and lenses (directors) to focus a filament

light bulb.

Random wire antenna

is simply a very long (at least one quarter

wavelength) wire with one end connected

to the radio and the other in free space,

arranged in any way most convenient for

the space available.

Folding will reduce effectiveness and

make theoretical analysis extremely

difficult. (The added length helps more

than the folding typically hurts.) Typically,

a random wire antenna will also require an

antenna tuner, as it might have a random

impedance that varies non-linearly with

frequency.

Horn antenna

is used where high gain is needed, the

wavelength is short (microwave) and space

is not an issue. Horns can be narrow band or

wide band, depending on their shape. A horn

can be built for any frequency, but horns for

lower frequencies are typically impractical.

Horns are also frequently used as reference

antennas.

Parabolic antenna

- consists of an active element at the focus of a

parabolic reflector to reflect the waves into a

plane wave. Like the horn it is used for high

gain, microwave applications, such as satellite

dishes.

Patch antenna

- consists mainly of a square conductor mounted

over a groundplane. Another example of a

planar antenna is the tapered slot antenna

(TSA), as the Vidaldi-antenna.

Yagi-Uda beam antenna

Rooftop TV antenna. It is actually three Yagi antennas. The longest elements are

for the low band, while the medium and short elements are for the high and UHF

band.

A terrestrial microwave radio antenna array.

Examples of US 136-174 MHz base station antennas.