Antenna Design and Link Engineering

Pattern lobes

Pattern lobe is a portion of the radiation pattern with a local maximum

Lobes are classified as: major, minor, side lobes, back lobes.

Practical Antennas.Directional Antennas:• this type of antenna has a narrow beam width; with the

power being more directional, greater distances are usually achieved but area coverage is sacrificed

• - Yagi, Sector and Parabolic antennas

- Yagi better suited for shorter links

- lower dBi gain; usually between 7 and 15 dBi

Parabolic- used in medium to long links

- gains of 18 to 28 dBi

- most common

TEM Waves: (Transverse Electromagnetic ) Wave:

In TEM waves all the vectors E, H and direction of propagation are perpendicular to each other.

Polarization:

An electromagnetic wave propagates through the atmosphere in the form of TEM i.e. the E field ,H field and direction of propagation are perpendicular to each other. Polarization is defined on the bases of the direction of E field.

.

(1 )Polarization

Principal patternsPrincipal patterns are the 2-D patterns of linearly polarized antennas, measured in 2 planes

1. the E-plane: a plane parallel to the E vector and containing the direction of maximum radiation, and

2. the H-plane: a plane parallel to the H vector, orthogonal to the E-plane, and containing the direction of maximum radiation

(1 )Polarization Contd.• Types Of Polarization:

(i) Horizontal Polarization:If the direction of E field is horizontal to the direction of propagation the wave is called horizontally polarized wave. For the transmission and reception of such waves horizontal polarized antennas are used.

(ii) Vertical Polarization:When the direction of the electric field is vertical to the direction of propagation it is called a vertically polarized wave. For the transmission and reception of such waves vertical antennas are used. Mostly this vertical polarization is used.

The polarization must be the same in the receive and transmit antennas. The polarization is the direction of the electric filed.

Example

Source: NK Nikolova

Horizontal

Vertical

Types of Polarization Contd.(iii ) Circular Polarization:

This is a combination of vertical and horizontal polarization. In this polarization the electric field vector rotates around the direction of propagation clockwise or anti-clock wise during the transmission. For the transmission and reception of such of such waves circular dishes are used.

--With this circular polarization we can implement the freq. reuse technique to increase BW. In this case same freq. can be used for transmission and reception purposes with the use of orthogonal polarization.

Note: No matter what polarity you choose, all antennas in the same RF network must be polarized identically regardless of the antenna type

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Polarization Efficiency

• The power received by an antenna from a particular direction is maximal if the polarization of the incident wave and the polarization of the antenna in the wave arrival direction have:

• the same axial ratio• the same sense of polarization• the same spatial orientation

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Radiation pattern interrelated to dimension• The radiation pattern depends mainly on the length of the

antenna. The length of an antenna can be calculated using the following equation,

• where c is the speed of light

• L is the length in meters• f is frequency in Hertz• k is the velocity factor

kf

cL

Antenna resistance• For power to get to an antenna it must be connected to a

transmission line.• To prevent standing waves from occurring within the line and for

maximum power transfer, the resistance of the transmission line must be equal to the resistance of the antenna.

• The antenna resistance is termed radiation resistance. This is defined as a fictitious resistance which would dissipate as much power as an antenna in question is radiating if it were connected to the same transmission line.

• If an antenna is radiating 100 W when drawing a current of 2 A then its radiation resistance will be 25 ohm. (P=I2R).

Antenna resistance Cont. Not all energy absorbed by an antenna is radiated.

• Losses can occur within the antenna (imperfect dielectrics, eddy currents etc), as such antenna efficiency is defined

• Rr is the resistance of the antenna • Rl is resistance due to losses

lr

r

input

dtransmitte

RR

R

P

P

Maximum power transfer:Max Power Transfer theorem:

For the maximum transfer of power from a receiving antenna to a receiver , the impedance of the antenna should be matched to the input impedance of the receiver, in accordance with the maximum power transfer theorem.

As the antenna impedance is normally resistive , this means that the input impedance of the receiver should also be resistive.

Mathematically Pr (max)= v²/4Ri Watts

Where V = Induced Voltage Ri= Input Resistance

Effective Aperture OR Effective Area

•It is the collecting area of a receiving antenna.•Logical Concept

Mathematical representation of effective area

If Pd is the power density at the receiving antenna and Pr is the received power then

PdAe

Pr

Power gain i.e. GAs we know that radiation pattern is a graphical plot of the

power strength in different directions. Due to this pattern the power may be more concentrated in

a particular direction. This property is usually expressed in terms of power gain

G which is normally defined in the direction of maximum radiation per unit area

Power output, in a particular direction, compared to that produced in any direction by a perfect reference antenna.

If an antenna is said to have a gain of 10dB, it means it improves upon the reference antenna in that direction by 10dB.

Gain expressed as ratio•Mathematical Representation Of Power Gain:

It is a power ratio of outputs taken from an actual antenna and a reference antenna by applying same input power

antennareferenceabyradiatedPower

antennaactualanbyradiatedPowerG

.....

.....

Power gain Contd.

antennaisotropicanbyradiatedPower

antennaactualanbyradiatedPowerdBiG

.....

.....)(

antennadipoleabyradiatedPower

antennaactualanbyradiatedPowerdBdG

.....

.....)(

1

2log10)(P

PdBG

Mathematical Representation Of Power Gain:

(11)The received signal strength & Received Signal Power

Received Signal Strength: is described in terms of electric field strength. If a signal induces a 15 mV in a receiving antenna 3m long, then the field strength is 15 mV/3m or 5 mV/m.

The power density P(d) at a distance (d) from the isotropic source is related to the transmitted power Pt. P(d) = Pt/4*pi*d2

Where 4*pi*d2 is an area of sphere around the isotropic antenna.

Received Signal Power:P(r) = P(d) * Aer = Pt*Aer/4*pi*d2

The actual amount of power received by an antenna through free space can be predicted by use of the following equation

Antenna arrays• Consist of multiple (usually identical) antennas (elements)

‘collaborating’ to synthesize radiation characteristics not available with a single antenna. They are able• to match the radiation pattern to the desired coverage area • to adapt to changing signal conditions• to increase transmission capacity by better use of the radio

resources and reducing interference • Complex & costly

• Intensive research related to military, space, etc. activities• Smart antennas, signal-processing antennas, phased arrays, etc.

Famous BS Antenna Types• Following are some famous types of BS antennas.

1) Dual Frequency Antenna

2) Dual Beam Antenna

3) Space Diversity Antenna

4) Polarization Diversity Antenna

(1 )Dual Frequency Antenna•Mostly used in three sector cell configuration.

•Array elements are composed to have dual freq. operation.•26 elements are selected to achieve the required antenna gain.

•Array element spacing for 1500 MHz

0.6 for 900 MHz6 Sub arrays with number of elements (5,4,4,4,4,5 =26)

(1 )Dual Frequency Antenna• Mostly used in three sector cell configuration.• Array elements are composed to have dual freq. operation.• 26 elements are selected to achieve the required antenna gain.• Array element spacing

for 1500 MHz

0.6 for 900 MHz6 Sub arrays with number of elements (5,4,4,4,4,5 =26)

Exterior View of dual freq. antenna

Configuration of dual freq. antenna.

(2 )Dual Beam Antenna• Used in six-sector cell configuration.• This dual beam antenna radiates two beams of transmission at

same freq. to give proper coverage of the sector. • With out dual beam antennas for diversity reception we would

require 12 antennas on the base station.• Two radiators are combined by 90 deg hybrid circuit.• By excitation of the A and B ports respectively beams of A and

B are generated.

Fig .

Dual Beam base station antenna configuration

Dual Beam Antenna Contd.• In the previous figure two vertical array antennas are

arranged side by side. • By connecting each port of the hybrid circuits to a beam

tilt panel , electrical beam tilt is achieved.

Four beam antenna configuration of dual beam antenna

Dual beam antenna layout for six-sector cell configuration

beamwidth

antenna

• A

Power 3dB down from maximum point A

Max power

2 dipole

Directional Antenna

Radiated energy is focused in a specific direction