15 Feb 2001Property of R. Struzak1 Antenna Fundamentals (3) R. Struzak ryszard.struzak@ties.itu.int...

15 Feb 2001 Property of R. Struzak 1

Antenna Fundamentals (3)

R. Struzakryszard.struzak@ties.itu.int

School on Digital and Multimedia Communications Using Terrestrial and Satellite Radio LinksThe Abdus Salam International Centre for Theoretical Physics ICTP Trieste (Italy) 12 February – 2 March 2001

• Note: These materials may be used for study, research, and education in not-for-profit applications. If you link to or cite these materials, please credit the author, Ryszard Struzak. These materials may not be published, copied to or issued from another Web server without the author's express permission. Copyright © 2001 Ryszard Struzak. All commercial rights are reserved. If you have comments or suggestions, please contact the author at ryszard.struzak@ties.itu.int.

Summary Slide

• Transmission vs. Reception

• Polarization

• More Complex Antennas

• Antenna Arrays, Adaptive Antennas

Polarization

Polarization ellipse

• The two linear far-field components radiated by the horizontal and the vertical antenna sum up to a resultant elliptically polarized wave

• The polarization ellipse is defined by its axial ratio N/M (ellipticity), tilt angle and sense of rotation

Polarization states

450 LINEAR

UPPER HEMISPHERE:ELLIPTIC POLARIZATIONLEFT_HANDED SENSE

LOWER HEMISPHERE:ELLIPTIC POLARIZATION RIGHT_HANDED SENSE

EQUATOR:LINEAR POLARIZATION

LATTITUDE:REPRESENTSAXIAL RATIO

LONGITUDE:REPRESENTSTILT ANGLE

POLES REPRESENTCIRCULAR POLARIZATIONS

(Poincaré sphere)

Antenna Polarization

• The polarization of an antenna in a specific direction is defined to be the polarization of the wave produced by the antenna at a great distance

Polarization Efficiency (1)

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

– the same axial ratio

– the same sense of polarization

– the same spatial orientation

as the polarization of the antenna in that direction.

• When the polarization of the incident wave is different from the polarization of the receiving antenna, then a loss due to polarization mismatch occurs

Polarization efficiency =

= (power actually received) / (power that would be received if the polarization of the incident wave were matched to the receiving polarization of the antenna)

450 LINEAR

2 Polarization efficiency = cos2

A: POLARIZATION OF RECEIVING ANTENNA W: POLARIZATION OF INCIDENT WAVE

Circularly-Polarized Antenna

• Radio wave of any polarization can be obtained by superposition of 2 linearly-polarized waves produced by 2 crossed dipoles and by controlling the amplitude- ratio and phase-difference of their excitations.

Ixcos(t+x)

Iycos(t+y)

More Complex Antennas

Antenna Over Ground: Image Theory

• Perfect ground = perfectly conducting plane surface

• Tangential electrical field component = 0– vertical components: the

same direction– horizontal components:

opposite directions

• The field (above the ground) is the same if the ground is replaced by the antenna image

2 Antennas

• 2 identical antennas– Excitation: I1 = I, I2 =Iej

• Ant#1 field-strength: E’= C*D(, )

• Ant#2 field-strength:E” = C*D(, )*ej(r+)

• E = E’ + E”

r = d*cos

Antenna Array Factor (AAF)

• Resultant field-strength E = E’ + E”

• E = C*D(, )*[1+ej(r+)] = C*D(, )*AAF(, ) Pattern multiplication

• |AAF(, )|2 = Antenna array factor = Gain of array of isotropic

antennas

2 Antenna Array Factor (1)

• AAF() = 1+ej(r+) ; (r+) = x• AAF() = 1+ejx = 2[(1/2)(e-jx/2 +ejx/2)]ejx/2

= 2cos(x/2)ejx/2

• |AAF()| = 2cos(x/2) = 2cos[(d/2)cos + /2) = 2cos[(d/)cos + /2]

• |AAF()|2 Antenna Array Factor

2 Antenna Array Factor (2)

• |AAF()|2 = {2cos[(d/)cos + /2]}2

• Gain: Max{|AAF()|2} = 4 (6 dBi)when (d/)cos + /2 = 0, , …, k

• Nulls: when (d/)cos + /2 = /2, …, (k + 1)/2

• Relative gain = |AAF()|2 / Max{|AAF()|2}

Demonstration (Simulation)

Array2antThis program simulates radiation pattern

of 2 antenna-array factor. It produces 2D diagrams showing

how the radiation lobes maximums and minimums depends on the antennas

distance and excitation phases and magnitudes

Antenna Arrays

Yagi-Uda Arrays

• Only one antenna- element fed

• Other elements unexcited (parasitic)

• Non-identical elements• Non-identical distances

Directors

Reflector Driver

Linear Array of n Antennas

• equally spaced antennas in line

• currents of equal magnitude

• constant phase difference between adjacent antennas

• numbered from 0 to (n-1)

• F = 1+ejx+ej2x+ej3x+…+ej(N-1)x

= (1-ejNx) / (1-ejx)

• |F| = |(1-ejNx) / (1-ejx)| = [sin(Nx/2) / sin(x/2)] = F() array factor

• x/2 = (d/)cos + /2

Demonstration (Simulation)

Array_NanThis program simulates radiation pattern

of N - antenna-array factor. It produces 2D diagrams showing

how the radiation lobes maximums and minimums depends on the antenna

distance increment and on excitation phase and magnitude functions

Mutual Impedance

Array of antennas

V1 = I1Z11+I2Z12+…+InZ1n

V2 = I1Z12+I2Z22+…+InZ2n

.-……

Vn = I1Z1n+I2Z2n+…InZnn

Z1input = V1/I1= Z11+(I2/I1)Z12+…+(In/I1)Z1n

The input impedance depends on mutual impedance (coupling) with other antennas and on relative currents

Example: Impedance of Dipole

~73 ~300/2

Phased Arrays

• Array of N antennas in a linear or spatial configuration

• The amplitude and phase excitation of each individual antenna controlled electronically (“software-defined”)– Diode phase shifters – Ferrite phase shifters

• Inertia-less beam-forming and scanning (sec) with fixed physical structure

Antenna Arrays: Benefits• Possibilities to control

– Direction of maximum radiation

– Directions (positions) of nulls

– Beam-width

– Directivity

– Levels of sidelobes

using standard antennas (or antenna collections) independently of their radiation patterns

• Antenna elements can be distributed along straight lines, arcs, squares, circles, etc.

Beam Steering

• Beam-steering using phase shifters at each radiating element

Radiatingelements

Powerdistribution

Phaseshifters

Equi-phasewave front

= [(2/)d sin]

Beam direction

4-Bit Phase-Shifter (Example)

00 or 22.50 00 or 450 00 or 900 00 or 1800Input Output

Bit #4 Bit #3 Bit #2 Bit #1

Steering/ Beam-forming Circuitry

Switched-Line Phase Bit

2 delay lines and 4 diodes per bit

Input Output

Diode switch

Delay line

Switching Diode Circuit

a: RF short-circuited in forward biasb: RF short-circuited in reverse bias

PINdiode

Tuningelement

PINdiode

Tuningelement

Adaptive “Intelligent” Antennas

Adaptive (“Intelligent”)Antennas• Array of N antennas in a linear

or spatial configuration• Used for receiving signals from

desired sources and suppress incident signals from undesired sources

• The amplitude and phase excitation of each individual antenna controlled electronically (“software-defined”)

• The weight-determining algorithm uses a-priori and/ or measured information

• The weight and summing circuits can operate at the RF or at an intermediate frequency

Weight-determining algorithm

15 Feb 2001Property of R. Struzak1 Antenna Fundamentals (3) R. Struzak ryszard.struzak@ties.itu.int...

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