Antenna PARAMETERS

AJAL.A.J- AP ECE UNIVERSAL ENGG COLLEGE

RADIATION & PROPOGATIONRADIATION & PROPOGATION--Fundamental Parameters of AntennasFundamental Parameters of Antennas

AJAL.A.J Assistant Professor –Dept of ECE,

UNIVERSAL ENGINEERING COLLEGE

Mob: 8907305642 MAIL: [email protected]


An antenna is a way of converting the guided waves present in a waveguide, feeder cable or transmission line into radiating waves travelling in free space, or vice versa.

Full Null BeamwidthBetween1st NULLS

Radiation Pattern Lobes

Main lobe

Side lobes

Back lobes


Only accelerating charges produce radiation.

IdealizedPoint Radiator Vertical Dipole Radar Dish

Isotropic Omnidirectional Directional


Two fields regions:oNear field or Fresnel region: The region within the radius of the smallest sphere which completely encloses the antenna is called Fresnel region.In sitting an antenna ,it’s crucial to keep objects out of the near field region to avoid coupling the currents in the antenna with objects.

oFar Field or Fraunhofer region: The region beyond Fresnel region is called Fraunhofer region


Antenna parameters are: 1.Radiation Pattern

2.Directivity

3.Radiation Resistance and Efficiency

4.Power Gain

5.Bandwidth

6.Reciprocity

7.Effective Aperture

8.Beamwidth and Directivity

9.The Friis Formula: Antennas in Free Space

10.Polarisation Matching


The radiation pattern of an antenna is a plot of the far-field radiation from the antenna. More specifically, it is a plot of the power radiated from an antenna per unit solid angle, or its radiation intensity U [watts per unit solid angle]. This is arrived at by simply multiplying the power density at a given distance by the square of the distance r, where the power density S [watts per square metre] is given by the magnitude of the time-averaged Poynting vector: U=r^²S

Radiation IntensityRadiation Intensity

Aside on Solid Angles

lengtharcrad0.1

r

sr0.1

2rareasurface

radianscecircumfrantotal 2224 rrSareasurfacetotal o

srr

So2

ddrds )sin(2infinitesimal areaof surface of sphere

ddr

dsd )sin(

2



2.Directivity


4.Power Gain

5.Bandwidth

6.Reciprocity






The directivity D of an antenna, a function of directionis defined by the ratio of radiation intensity of antenna in direction to the mean radiation intensity in all directions.



2.Directivity


4.Power Gain

5.Bandwidth

6.Reciprocity






The resistive part of the antenna impedance is split into two parts, a radiation resistance Rr and a loss resistance Rl. The power dissipated in the radiation resistance is the power actually radiated by the antenna, and the loss resistance is power lost within the antenna itself. This may be due to losses in either the conducting or the dielectric parts of the antenna. Radiation efficiency e of the antenna as e is the ratio of power radiated to the power accepted by antenna

antenna with high radiation efficiency therefore has high associated radiation resistance compared with the losses. The antenna is said to be resonant if its input reactance Xa =0.



2.Directivity


4.Power Gain

5.Bandwidth

6.Reciprocity






The power gain G, or simply the gain, of an antenna is the ratio of its radiation intensity to thatof an isotropic antenna radiating the same total power as accepted by the real antenna. Whenantenna manufacturers specify simply the gain of an antenna they are usually referring to themaximum value of G.

Antenna GainAntenna Gain

inputP

UG

),(4),(

POWER DENSITY IN A CERTAIN DIRECTION

DIVIDED BY THE TOTAL POWER RADIATED

POWER DENSITY IN A CERTAIN DIRECTION

DIVIDED BY THE TOTAL INPUT POWER

TO THE ANTENNA TERMINALS (FEED POINTS)

IF ANTENNA HAS OHMIC LOSS…THEN, GAIN < DIRECTIVITY

DIRECTIVITY

GAIN



2.Directivity


4.Power Gain

5.Bandwidth

6.Reciprocity






The bandwidth of an antenna expresses its ability to operate over a wide frequency range. It is often defined as the range over which the power gain is maintained to within 3dB of its maximum value, or the range over which the VSWR is no greater than 2:1, whichever is smaller. The bandwidth is usually given as a percentage of the nominal operating frequency. The radiationpattern of an antenna may change dramatically outside its specified operating bandwidth.



2.Directivity


4.Power Gain

5.Bandwidth

6.Reciprocity






Reciprocity theorem:If a voltage is applied to the terminals of an antenna A and the current measured at the terminals of another antenna B then an equal current will be obtained at the terminals of antenna A if the same voltage is applied to the terminals of antenna B.



2.Directivity


4.Power Gain

5.Bandwidth

6.Reciprocity






Effective ApertureEffective Aperture

If an antenna is used to receive a wave with a power density S [W m2], it will produce apower in its terminating impedance (usually a receiver input impedance) of Pr watts. Theconstant of proportionality between Pr and S is Ae, the effective aperture of the antenna insquare metres:Pr = AeS For some antennas, such as horn or dish antennas, the aperture has an obvious physicalinterpretation, being almost the same as the physical area of the antenna, but the concept isjust as valid for all antennas. The effective aperture may often be very much larger than thephysical area, especially in the case of wire antennas. Note, however, that the effectiveaperture will reduce as the efficiency of an antenna decreases.The antenna gain G is related to the effective aperture as follows G=4pi/ (lamda)2Ae

Effective ApertureEffective Aperture

plane waveincident

AphysicalPload

incphysicalload WAP?

Question:

Answer: Usually NOTinc

loadeffinceffload W

PAWAP



2.Directivity


4.Power Gain

5.Bandwidth

6.Reciprocity






The directivity of an antenna increases as its beamwidth is made smaller, as the energyradiated is concentrated into a smaller solid angle



2.Directivity


4.Power Gain

5.Bandwidth

6.Reciprocity






rotot

r DDRP

P2

4

Directivity and Maximum Effective Aperture Directivity and Maximum Effective Aperture (no losses)(no losses)

Antenna #2

transmit receiver

R

Direction of wave propagation

Antenna #1

Atm, DtArm, Dr

oem DA4

2

Directivity and Maximum Effective Aperture Directivity and Maximum Effective Aperture (include losses)(include losses)

Antenna #2

transmit receiver

R

Direction of wave propagation

Antenna #1

Atm, DtArm, Dr

2*2

2 ˆˆ4

)1( awocdem DeA

conductor and dielectric losses reflection losses

(impedance mismatch)polarization mismatch

Friis Transmission Equation (no loss)Friis Transmission Equation (no loss)

Antenna #2

Antenna #1

R

transmit

Atm , D

treceiver

Arm, D

r

The transmitted power density supplied by Antenna #1at a distance R and direction rr)is given by:

24

),(

R

DPW ttgttt

tt)

rr)

The power collected (received) by Antenna #2 is given by:

),(),(4

4

),(

4

),(

4

),(

2

2

22

rrgrttgtt

r

rrgrttgttr

ttgttrtr

DDRP

P

D

R

DPA

R

DPAWP

Friis Transmission Equation (no loss)Friis Transmission Equation (no loss)

Antenna #2

Antenna #1

R

transmit

Atm , D

treceiver

Arm, D

r

tt)

rr)

),(),(4

2

rrgrttgtt

r DDRP

P

If both antennas are pointing in the direction of their maximum radiation pattern:

rotot

r DDRP

P2

4



2.Directivity


4.Power Gain

5.Bandwidth

6.Reciprocity






The polarisation mismatch loss is the ratio between the power received by the antenna and the power which would be received by an antenna perfectly matched to the incident wave

AppendicesAppendices

Friis Transmission Equation: Example #1Friis Transmission Equation: Example #1

A typical analog cell phone antenna has a directivity of 3 dBi at its operating frequency of 800.0 MHz. The cell tower is 1 mile away and has an antenna with a directivity of 6 dBi. Assuming that the power at the input terminals of the transmitting antenna is 0.6 W, and the antennas are aligned for maximum radiation between them and the polarizations are matched, find the power delivered to the receiver. Assume the two antennas are well matched with a negligible amount of loss.

nWwattsPr 65.142 609.344 14

375.06.0

2

2*maxmax

222 ˆˆ

4)1)(1( awrttrcdrcdt

t

r DDR

eeP

P

= 0 = 0= 1= 1 = 1

0.410

0.210

375.06800

83

10/6max

10/3max

r

t

D

D

me

e

f

c


A half wavelength dipole antenna (max gain = 2.14 dBi) is used to communicate from an old satellite phone to a low orbiting Iridium communication satellite in the L band (~ 1.6 GHz). Assume the communication satellite has antenna that has a maximum directivity of 24 dBi and is orbiting at a distance of 781 km above the earth. Assuming that the power at the input terminals of the transmitting antenna is 1.0 W, and the antennas are aligned for maximum radiation between them and the polarizations are matched, find the power delivered to the receiver. Assume the two antennas are well matched with a negligible amount of loss.

pWwattsPr 15.025164.1 781,0004

1875.00.1

2

2*maxmax

222 ˆˆ


t

r DDR

eeP

P

= 0 = 0= 1= 1 = 1

0.25110

64.110

1875.06800

83

10/24max

10/14.2max

r

t

D

D

me

e

f

c


A roof-top dish antenna (max gain = 40.0 dBi) is used to communicate from an old satellite phone to a low orbiting Iridium communication satellite in the Ku band (~ 12 GHz). Assume the communication satellite has antenna that has a maximum directivity of 30 dBi and is orbiting at a distance of 36,000 km above the earth. How much transmitter power is required to receive 100 pW of power at your home. Assume the antennas are aligned for maximum radiation between them and the polarizations are matched, find the power delivered to the receiver. Assume the two antennas are well matched with a negligible amount of loss.

Wwatts

Pt 82

1000000,10 36,000,0004

025.0

101002

12

2*maxmax

222 ˆˆ


t

r DDR

eeP

P

= 0 = 0= 1= 1 = 1

0.100010

000,1010

025.06800

83

10/30max

10/40max

t

r

D

D

me

e

f

c

Antenna PARAMETERS

Education

antenna antenna

antenna parameters

isotropic antenna

antenna impedance

antenna gainu

real antenna

antenna manufacturers

radiation intensityaside