Presentation 3. Antenna Parameters

8/2/2019 Presentation 3. Antenna Parameters

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Lecture 3. ANTENNA PARAMETERS

Dr Hasanovic: ADVANCED ANTENNA THEORY (Spring 2012

Properties of antennas are described through antenna parameters Antenna parameters are also used for comparing various antennaconfigurations

Antenna parameters define the performance of an antenna, from variousperspectives

Certain antenna parameters are mutually related to each other

ANTENNA PARAMETERS:[1] radiation pattern

[2] directivity[3] gain[4] efficiency[5] axial ratio[6] input impedance[7] frequency band, etc.


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RADIATION PATTERN[1] this is the most important and the mostcomprehensive antenna parameter

[2] graphical representation ofelectromagnetic radiation thatcharacterizes an antenna, as a functionof three dimensional space coordinates

Field pattern:

Power pattern:


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RADIATION PATTERN (cont.)

DIJAGRAM POLJA(linearna skala)

DIJAGRAM SNAGE(linearna skala)

DIJAGRAM SNAGE(logaritamska skala)

DIJAGRAM POLJA(linearna skala)

DIJAGRAM SNAGE(linearna skala)

DIJAGRAM SNAGE(logaritamska skala)

POWER RADIATION PATTERN(linear scale)

FIELD RADIATION PATTERN(linear scale)

POWER RADIATION PATTERN(logarithmic scale)

GLAVNA LEPEZA

SPOREDNE LEPEZE

POZADINSKA LEPEZA

BONA LEPEZASPOREDNE

LEPEZE

IRINA SNOPAIZMEU PRVIH NULA

(FNBW)

IRINA SNOPASA 50% SNAGE(HPBW)

GLAVNA LEPEZA

SPOREDNE LEPEZE

POZADINSKA LEPEZA

BONA LEPEZASPOREDNE

LEPEZE


(FNBW)

IRINA SNOPASA 50% SNAGE(HPBW)

FIRST NULL BEAM WIDTH(FNBW)

HALF POWER BEAM WIDTH(HPBW)

MAIN LOBE (BEAM)

SIDE LOBE

BACK LOBE

MINOR LOBES

MINORLOBES

LOBE (BEAM)MAJOR LOBEMINOR LOBE

side lobeback lobe


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RADIATION PATTERN (cont.)

GLAVNA LEPEZA

SPOREDNE LEPEZE

POZADINSKA LEPEZA

BONA LEPEZASPOREDNE

LEPEZE


(FNBW)

IRINA SNOPASA 50% SNAGE

(HPBW)

GLAVNA LEPEZA

SPOREDNE LEPEZE

POZADINSKA LEPEZA

BONA LEPEZASPOREDNE

LEPEZE


(FNBW)

IRINA SNOPASA 50% SNAGE

(HPBW)

FIRST NULL BEAM WIDTH(FNBW)

HALF POWER BEAM WIDTH(HPBW)

MAIN LOBE (BEAM)

SIDE LOBE

BACK LOBE

MINOR LOBES

MINORLOBES

The First Null Beam Width (FNBW)represents an angle between the twostraight lines with an origin in thecenter of the radiation pattern that passthrough the two points on the oppositesides of the beam characterized by thezero radiation.

The Half Power Beam Width (HPBW)is an angle between the two straightlines with the origin at the center of theradiation pattern that pass through thetwo points on the opposite sides of thebeam characterized by a radiationintensity equal to the half of theintensity present on the main axis ofthe beam (maximum intensity).


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RADIATION INTENSITY

S

2

SSS

dds inr)(*)(21

d)(*)(21

d)(*)(21

d)(P

rrHrE

srHrEsrHrEsrS

The total power that an antenna radiates through a specified surface S that is apart of a sphere of radius r :

The total power that an antenna radiates through a unitspace angle d=sindd or through any other spaceangle obtained through integration of unit angles, isconstant


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RADIATION INTENSITY (cont.)To avoid dependence on r, we introduce a new quantity called RADIATIONINTENSITY :

Radiation intensity is proportional to the antenna radiation pattern because it can

be shown that :

Average radiation intensity:

Coefficient 4 represents the value of thetotal space angle (units steradians)


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BEAM SOLID ANGLE

The beam solid angle A physically represents a solid angle through which anantenna would radiate if its radiation intensity would have been constant and equalto the maximum value U max for all directions within the angle A.


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DIRECTIVITYAntenna directivity D is defined as the ratio of the radiation intensity in acertain direction to the average radiation intensity in all directions.

If the direction for which the directivity is considered is not specified, weassume the direction in which the antenna has maximum radiation, i.e. weare considering the maximum value of the antenna directivity

Antenna radiation directivity D and the beamsolid angle A are interrelated through thefollowing expression


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OMNIDIRECTIONAL PATTERN

Figure 3.5. Omnidirectional radiation

pattern

Antenna hasomnidirectional radiation pattern if itsradiation pattern ischaracterized by theuniform radiationintensity in the horizontalplane, i.e. for a constant

value of angle .


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ANTENNA EFFICIENCYAntenna represents a device that transformsenergy brought on its input terminals into theenergy of an electromagnetic wave radiatedinto the surrounding environment, or opposite.

This transformation of energyis characterized by certainenergy losses.A measure of these losses isgiven through an antennaparameter called antennaefficiency (e).Types of losses:

[1] conductor losses,[2] dielectric losses,[3] mismatch losses


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ANTENNA EFFICIENCY (cont.)

ANTENNA RADIATION EFFICIENCY(depends on the antenna only and NOT on the feeding line):

Lc coefficient of conductor losses,Ld coefficient of dielectric losses,L coefficient of mismatch losses, voltage reflection coefficient on the antennainput terminals [VSWR= (1+| |/(1 -||)]


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ANTENNA GAIN Antenna gain (G) is anantenna parameter thattakes into account both itsefficiency and its directionalproperties.


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POLARIZATION Consider the vector of electricfield E(r,t) and thecorresponding phasor formE(r) where E(r,t)=Re{ E(r)e - jt }then we may notice that theamplitude of this vector forms

a spiral path in time as shownin Figure on the right.

If we fix the point at which we observe thevector of electric field, i.e. E(r=const,t), then this

vector is exclusively a function of the timevariable t.

Antenna polarization is defined as a curve made by the end point of thevector E(r=const,t) in a plane perpendicular to the direction ofelectromagnetic wave propagation.


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POLARIZATION (demo)


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POLARIZATION (cont.)

Axial Ratio:

GLAVNA OSA SPOREDNA OSA

Ex(r ,t)

Ey(r ,t)

GLAVNA OSA SPOREDNA OSA

Ex(r ,t)

Ey(r ,t)

MAJOR AXIS MINOR AXIS

Antenna polarization is defined as a curve made bythe end point of the vector E(r=const,t) in a plane

perpendicular to the direction of electromagneticwave propagation.

Three types of polarization:[1] linear (AR=0 or AR=infinity),[2] circular (AR=1)[3] elliptical


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POLARIZATION (cont.)

E(r =const,t)

E(r =const,t)

E(r =const,t)

E(r =const,t)

E (r =const,t)

E(r =const,t)

VERTIKALNA LINEARNA POLARIZACIJA HORIZONTALNA LINEARNA POLARIZACIJA

DESNA KRU NA POLARIZACIJA LIJEVA KRU NA POLARIZACIJA

DESNA ELIPTI KA POLARIZACIJA LIJEVA ELIPTI KA POLARIZACIJA

E(r =const,t)

E(r =const,t)

E(r =const,t)

E(r =const,t)

E (r =const,t)

E(r =const,t)


DESNA KRU NA POLARIZACIJA LIJEVA KRU NA POLARIZACIJA

DESNA ELIPTI KA POLARIZACIJA LIJEVA ELIPTI KA POLARIZACIJA

VERTICAL LINEAR POLARIZATION

RIGHT CIRCULAR POLARIZATION

RIGHT ELLIPTICAL POLARIZATION

HORIZONTAL LINEAR POLARIZATION

LEFT CIRCULAR POLARIZATION

LEFT ELLIPTICAL POLARIZATION


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LINEAR POLARIZATION

In practice, it is impossible to achieve ideal linear polarization for which OA or OB isequal to zero.

Let us assume that we have polarization for which OA0.

In this case, the component of electric field in the direction of desired polarization(OB) is called the copolarized component while undesired field component (in thedirection of OA) is called the crosspolarized component. Very often in practice,the goal is to reduce the undesired component (crosspolarized) component. Byreducing the crosspolarized component, the quality of linear polarization isimproved.

E(r =const,t)

E(r =const,t)


E(r =const,t)

E(r =const,t)


VERTICAL LINEAR POLARIZATION HORIZONTAL LINEAR POLARIZATION


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LINEAR POLARIZATION (demo):


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CIRCULAR POLARIZATION (demo)


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INPUT IMPEDANCE OF ANTENNA ANTENAGENERATOR

(Zg)

Xg

Rg

XA

RA

GENERATOR ANTENA

ANTENA

GENERATOR(Zg)

Xg

Rg

XA

RA

GENERATOR ANTENA

Input impedance of an antenna is equal to the ratioof voltage and current on the antenna inputterminals when the antenna is fed from a generator

Real part of antenna input impedance is calledANTENNA INPUT RESISTANCE :

Antenna input resistance represents the measure of power dissipation on theantenna that occurs through two different processes:[1] the portion of the power is being radiated into the surroundingenvironment as an electromagnetic wave[2] the portion of the power is irreversibly lost as a heat in the antennastructure .


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Generator Impedance:

Impedance Matching:

ggg jXRZ

AgAg XX , RR

2Al

2Arlrul IR2

1IR

21

PPP

ANTENA

GENERATOR(Zg)

Xg

Rg

XA

RA

GENERATOR ANTENA

ANTENA

GENERATOR(Zg)

Xg

Rg

XA

RA

GENERATOR ANTENA

lr

r

RRR

Re

lr

rr RR

Re

INPUT IMPEDANCE OF ANTENNA (cont.) Antenna Input Impedance:

If above conditions are met, then the maximum powerfrom the generator will be delivered to the antenna:


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ANTENNA FREQUENCY BAND

1. Antenna frequency band is defined as a frequency rangein which antenna meets certain standards relative to aspecified antenna parameter.

2. In other words, for a specified antenna parameter (inputimpedance, beam width, axial ratio, efficiency, etc), acorresponding criterion is defined and then the frequencyband would be the range of frequencies for which that

criterion is satisfied.3. In practice, two frequency band definitions are used onefor the radiation pattern and another for the antennainput impedance .


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D H i ADVANCED ANTENNA THEORY (S i 2012

ANTENNA FREQUENCY BAND (cont.)

1. There exist different definitions of antenna frequency band.2. For broadband antennas , frequency band is usually

defined as the ratio of the highest to the lowest frequency

in the band (for example, 10:1 frequency band means thatthe highest frequency in the band is ten times higher thanthe lowest frequency in the band).

3. For narrowband antennas , frequency band is defined asthe difference between the highest and the lowestfrequency given as a percentage of the center frequency.For example, a frequency band of 5% means that thedifference between the highest and the lowest frequency isequal to 5% of the center frequency.

Presentation 3. Antenna Parameters

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