AntennaFundamentals&wireless.ictp.it/school_2014/Lectures/Day2/Antenna_Fundamentals.pdf · (CC)&RStruzak&& 14 Transming&antennaequivalentcircuit& TransmiLer& Transm.line& Antenna

Antenna Fundamentals

Prof. Ryszard Struzak

ICTP School on Applica0ons of Open Spectrum and White Spaces Technologies ICTP, Trieste-‐Miramare, 3 -‐ 14 March 2014

(CC) R Struzak <r.struzakATieee.org> 2

• Beware of misprints! These materials are preliminary notes intended for my lectures only and may contain misprints.

• Feedback is welcome: if you noIce faults, or you have improvement suggesIons, please let me know.

• This work is licensed under the CreaIve Commons ALribuIon License (hLp://creaIvecommons.org/ licenbses/by/1.0) and may be used freely for individual study, research, and educaIon in not-‐for-‐profit applicaIons. Any other use requires the wriLen author’s permission.

• These materials and any part of them may not be published, copied to or issued from another Web server without the author's wriLen permission.

• If you cite these materials, please credit the author and ICTP. • Copyright © 2012 Ryszard Struzak.

•  ObjecIve: to refresh basic concepts related to the antenna physics –  needed to understand beLer the operaIon of wireless (radio) links/ networks

•  Topics for discussion: 1.  Antenna funcIons 2.  Antenna matching 3.  Antenna polarizaIon 4.  Antenna direcIvity 5.  Antenna arrays

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Antennas for laptop applicaIons

Source: D. Liu et al.: Developing integrated antenna subsystems for laptop computers; IBM J. RES. & DEV. VOL. 47 NO. 2/3 MARCH/MAY 2003 p. 355-‐367

Linksys

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Antenna funcIons

•  TransformaIon of a guided EM wave into an unguided wave freely propagaIng in space (or the opposite) –  a Ime-‐funcIon in 1-‐D space à a Ime-‐funcIon in 3-‐D space

–  The specific form of the radiated wave is defined by the antenna structure and the environment

Space wave

Guided wave

TEM -‐ simplest EM wave

Linearly-‐polarized plane wave traveling in vacuum with the speed of light: (x, t) = A sin[ω(t -‐ x/c) + ϕ]; ω = 2πF; c ~3.108m At large distances spherical wave-‐front ~ plane

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Java applet plane wave: hLp://www.amanogawa.com/archive/wavesA.html

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Power Flow

•  In free space, the radiated energy streams from the antenna in radial lines, i.e. the PoynIng vector has only the radial component

•  A source that radiates uniformly in all direcIons is an isotropic source (radiator, antenna). For such a source the radial component of the PoynIng vector is independent of θ and ϕ.

Topics for discussion

1.  Antenna funcIons 2.  Antenna matching 3.  Antenna polarizaIon 4.  Antenna direcIvity 5.  Antenna arrays

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13

Basic transmiLer/ receiver parts

•  The transmission line •  The juncIon •  The antenna radiator (size comparable with λ/2) •  The EM wave

•  Notes: –  Possible power reflecIons (impedance matching) –  Possible resonaces (for broadband applicaIons must be aLenuated)

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Transmisng antenna equivalent circuit

TransmiLer Transm. line

Antenna

Gen

erat

or

RG

jXG

VG

jXA

Rr

Rl

The transmiLer with the transmission line is represented by an (Thevenin) equivalent generator

The antenna is represented by its input

impedance (which is frequency-‐dependent and is influenced by objects nearby) as seem from the generator

jXA represents energy stored in electric (Ee) and magneIc (Em) near-‐field components; if |

Ee| = |Em| then XA = 0 (antenna resonance) Can be approximated by the impedance of a transmission line

Rr represents energy radiated into space (far-‐field components) Rl represents energy lost, i.e. transformed into

heat in the antenna structure

Radio wave

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Power transfer

•  The maximum power is delivered to (or from) the antenna when the antenna impedance and the impedance of the equivalent generator (or load) are matched

0

0.5

1

0.1 1 10

PA /

PAm

ax

RA / RG; (XA+XG = 0)

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•  When the antenna impedance is not matched to the transmiLer output impedance (or to the receiver input impedance) or to the transmission line between them, impedance-‐matching devices must be used for maximum power transfer

•  Inexpensive impedance-‐matching devices are usually narrow-‐band

•  Transmission lines oven have significant losses

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•  When the impedances are matched –  Half of the source power is delivered to the load and half is dissipated within the (equivalent) generator (as heat)

–  In the case of receiving antenna, a part (Pl) of the power captured is lost as heat in the antenna elements, the other part being re-‐radiated (scaLered) back into space

•  Even when the antenna losses tend to zero, sIll only half of the power captured is delivered to the load (in the case of conjugate matching), the other half being scaLered back into space

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Receiving antenna equivalent circuit

The antenna with the transmission line is represented by an (Thevenin) equivalent generator The receiver is represented by its input impedance as seen from the antenna terminals (i.e. transformed by the transmission line) VA is the (induced by the incident wave) voltage at the antenna terminals determined when the antenna is open circuited Note: The antenna impedance is the same when the antenna is used to radiate and when it is used to receive energy

Radio wave Receiver Transm.line

Antenna

Ant

enna

Rr

jXA

VA

jXL

RL Rl

Thevenin equivalent

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RadiaIon efficiency

•  The radiaIon efficiency e indicates how efficiently the antenna uses the RF power

•  It is the raIo of the power radiated by the antenna into the space and the total power delivered to the antenna terminals (in transmisng mode). In terms of equivalent circuit parameters:

e = RrRr + Rl



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Antenna polarizaIon

•  The polarizaIon of an antenna in a specific direcIon is defined to be the polarizaIon of the wave produced by the antenna at this direcIon at a great distance

•  By convenIon the "polarizaIon" of an EM wave refers to the polarizaIon (direcIon) of oscillaIons of the electric field vector.

•  The oscillaIon may be in a single direcIon (linear polarizaIon), or the field may rotate (circular or ellipIcal polarizaIon).

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PolarizaIon filters/ reflectors

•  At the surface of ideal conductor the tangenIal electrical field component = 0

|E1|>0 |E2| = 0

Vector E ⊥ wires Vector E || wires

|E1|>0 |E2| ~ |E2|

Wall of thin parallel wires (conductors)

Wire distance ~ 0.1λ

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PolarizaIon states

450 LINEAR

UPPER HEMISPHERE: ELLIPTIC POLARIZATION LEFT_HANDED SENSE

LOWER HEMISPHERE: ELLIPTIC POLARIZATION RIGHT_HANDED SENSE

EQUATOR: LINEAR POLARIZATION

LATTITUDE: REPRESENTS AXIAL RATIO

LONGITUDE: REPRESENTS TILT ANGLE

POLES REPRESENT CIRCULAR POLARIZATIONS

LHC

RHC

(Poincaré sphere)



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Point Source

•  For many purposes, it is sufficient to know the direcIon (angle) variaIon of the power radiated by antenna at large distances.

•  For that purpose, any pracIcal antenna, regardless of its size and complexity, can be represented as a (distant) point-‐source.

•  The actual field near the antenna is then disregarded.

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•  The EM field at large distances from an antenna can be treated as originated at a point source -‐ ficIIous volume-‐less emiLer.

•  The EM field in a homogenous unlimited medium at large distances from an antenna can be approximated by an uniform plane TEM wave

15 Feb 2001 Property of R. Struzak 27

PFD: Isotropic Radiator

•  Loss-‐less propagaIon medium assumed

•  Isotropic radiator cannot be physically realized

•  PFD does not depend on frequency/ wavelength

24 rPPFD T

π=

r

Power Flux Density (PFD)


PFD: Example 1

•  What is the PFD from TV broadcast GEO satellite at ICTP?

•  EIRP = 180 kW (52.5 dB(W))

•  Distance: ~38'000 km •  Free space

PFD =1.8•102 •103

4•! •(38•106 )2

!1.8•105

1.8•1016 ![Wm-2 ]

=1•10"11 [Wm-2 ]= "100 [dB(Wm"2 )]


PFD: Example 2

•  What is the PFD from a hand-held phone at the head?

•  EIRP = 1.8 W •  Distance = ~3.8 cm •  Free space

PFD =1.8

4•! •(3.8•10!2 )2

"1.8

1.8•10!2 ![Wm-2 ]

=100 [Wm-2 ]= 100•103

104 [mWcm-2 ]

=10![mWcm-2 ]

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Short dipole antenna: summary •  Eθ & Hθ are maximal in the equatorial plane, zero along the antenna axis

•  Er is maximal along the antenna axis dz, zero in the equatorial plane

•  All show axial symmetry •  All are proporIonal to the current moment Idz •  Have 3 components that decrease with the distance-‐to-‐wavelength raIo as –  (r/λ)-‐2 & (r/λ)-‐3: near-‐field, or inducIon field. The energy oscillates from enIrely electric to enIrely magneIc and back, twice per cycle. Modeled as a resonant LC circuit or a transmission-‐line resonator;

–  (r/λ)-‐1: far-‐field or radiaIon field –  These 3 component are all equal at (r/λ) = 1/(2π)

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Field components

0.001

0.01

0.1

1

10

100

1000

0.1 1 10

Relative distance, Br

Rela

tive

field

stre

ngth

FF

FF

Q

Q

C

C

FF: Radiation field

C, Q: Induction fields

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EM field intrinsic impedance

Field impedance Z = E/H depends on the antenna type and on distance

0.01

0.1

1

10

100

0.01 0.1 1 10 100

Distance / (lambda/ 2Pi)

Z / 3

77

Short dipole

Small loop

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Far-‐Field, Near-‐Field

•  Near-‐field region: –  ReacIve field components dominate (L, C) –  The resultant EM field highly non-‐uniform –  Angular distribuIon of energy depends on

distance from the antenna; •  Far-‐field region:

–  RadiaIng field component dominates (R) –  The resultant EM field can locally be treated as

uniform (TEM) –  Angular distribuIon of energy is independent on

distance;

•  For a 60 cm diameter satellite TV antenna operaIng at a frequency of 12GHz, (a wavelength of 25mm), R1 = 0.85m and R2 = 29m.

Example

•  The power density in the beam exceeds that from isotropic antenna by the raIo of the area of a sphere of radius equal to the antenna to spot distance to the area of the spot. This is the ‘Gain’ of the antenna system: = 4(r/ra)2.

•  With the satellite alItude r = 35786 km, earth radius R = 6371 km and the rectenna radius ra = 5 km, the gain is ~2.84 million or 84dB.

•  This would require antenna about 5000 wavelengths across (600m at 2.4GHz), or in wavelength terms about the same as the human eyeball! Source: A. Marvin: IntroducIon to ElectromagneIc Fields and Waves (slides)

A Solar Power Satellite beams RF energy from a geostaIonary satellite down to a receiving site on the ground of 10 km diameter.

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Short antenna radiaIon paLern

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Linear Antennas •  SummaIon of all vector

components E (or H) produced by each antenna element

•  In the far-‐field region, the vector components are parallel to each other

•  Phase difference due to –  ExcitaIon phase difference –  Path distance difference

•  Method of moments -‐ NEC

...

...

321

321

+++=

+++=

HHHH

EEEE!!!!

!!!"

O


Reference Antennas

•  Isotropic radiator – isolated in space (Gi, absolute gain, or isotropic gain)

•  Half-‐wave dipole – isolated in space, (Gd, gain relaIve to λ/2 dipole)


Satellite antenna mask (example)

-50

-40

-30

-20

-10

0

0.1 1 10 100

Phi/Phi0

Rela

tive

gain

(dB)

RR/1998 APS30 Fig.9

COPOLAR

CROSSPOLAR

Reference paLern for co-‐polar and cross-‐polar components for satellite transmisng antennas in Regions 1 and 3 (BroadcasIng ~12 GHz)

0dB

-‐3dB

Phi0/2

Phi

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Typical Gain and Beamwidth

Type of antenna Gi [dB] BeamW.

Isotropic 0 3600x3600

Half-wave Dipole 2 3600x1200

Helix (10 turn) 14 350x350

Small dish 16 300x300

Large dish 45 10x10


1.  Antenna funcIons 2.  Antenna matching 3.  Antenna polarizaIon 4.  Antenna gain 5.  Antenna arrays

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Antenna arrays •  MulIple antennas collaboraIng’ to synthesize radiaIon characterisIcs not available with a single antenna, able –  to match the radiaIon paLern to the desired coverage area –  to change the radiaIon paLern electronically (scanning) through the

control of the phase & amplitude of the signals in each element –  to dynamically adapt to changing signal condiIons –  to increase transmission capacity by beLer use of the radio resources

and reducing interference

•  Complex & costly –  Intensive research related to military, space, etc. acIviIes

»  Smart antennas, signal-‐processing antennas, tracking antennas, phased arrays, MIMO anternnas, etc.

•  Passive & unintenIonal antennas Source: adapted from N Gregorieva

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Antenna Arrays: posibiliIes

•  PossibiliIes to control electronically –  DirecIon of maximum radiaIon –  DirecIons (posiIons) of nulls –  Beam-‐width –  DirecIvity –  Levels of sidelobes

using standard antennas (or antenna collecIons) independently of their radiaIon paLerns

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

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Switched arrays •  Switched beam antennas

–  Based on switching funcIon between separate direcIve antennas or predefined beams of an array

•  Space Division Mul0ple Access (SDMA) = allocaIng an angle direcIon sector to each user –  In a TDMA system, two users will be

allocated to the same Ime slot and the same carrier frequency

–  They will be differenIated by different direcIon angles

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Phased Arrays

•  Array of N antennas in a linear or two-‐dimensional configuraIon + beam-‐forming & control device

•  The amplitude and phase excitaIon of each individual antenna controlled electronically (“sovware-‐defined”) –  Diode phase shivers –  Ferrite phase shivers

•  InerIa-‐less beam-‐forming and scanning (µsec) with fixed physical structure

2 GHz adapIve antenna

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An array of 48 2.4 GHz antennas Source: Arraycomm

The Square Kilometre Array (SKA). A radio telescope in development in Australia/New Zealand/Africa. Radius: 3000 km; Budget: €1.5 billion


AdapIve (“Intelligent”)Antennas

•  Array of N antennas in a linear or spaIal configuraIon

•  Used for receiving signals from desired sources and suppress incident signals from undesired sources

•  The amplitude and phase excitaIon of each individual antenna controlled electronically (“sovware-‐defined”)

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

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

w1

wN

Σ

Weight-‐determining algorithm

1

N

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2 omnidirecIonal antennas

Run simulaIon program: Array2ant_Demo1.xlsm

-1

-0.5

0

0.5

1

-1 -0.5 0 0.5 1

D = 0.5λ, θ= 900

-1

-0.5

0

0.5

1

-1 -0.5 0 0.5 1

-1

-0.5

0

0.5

1

-1 -0.5 0 0.5 1

D = 0.5λ, θ= 00 D = 0.5λ, θ= 1800

What we have learned

•  Symmetrical role of transmisng and receiving antennas

•  CriIcal elements of transmission chain – Power matching between transmission-‐line and antenna

– PolarizaIon matching between antennas – DirecIon matching of transmisng and receiving antennas

– UnintenIonal antennas

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Antenna simulators •  PolarizaIon:

–  hLp://www.amanogawa.com/archive/wavesA.html •  Linear dipole antennas:

–  hLp://www.amanogawa.com/archive/DipoleAnt/DipoleAnt-‐2.html –  hLp://www.amanogawa.com/archive/Antenna1/Antenna1-‐2.html

•  2 antennas: –  hLp://www.amanogawa.com/archive/TwoDipole/Antenna2-‐2.html

•  Antenna design using MiniNEC –  hLp://www.sovpedia.com/get/Science-‐CAD/Expert-‐MININEC-‐

Classic.shtml

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Thank you for your aLenIon

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AntennaFundamentals&wireless.ictp.it/school_2014/Lectures/Day2/Antenna_Fundamentals.pdf · (CC)&RStruzak&& 14 Transming&antennaequivalentcircuit& TransmiLer& Transm.line& Antenna