ICTP School on Applica0ons of Open Spectrum and White Spaces Technologies ICTP, Trieste-­‐Miramare, 3 -­‐ 14 March 2014 Antenna Fundamentals Prof. Ryszard Struzak • 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 • These materials and any part of them may not be published, copied to or issued from another Web server without the author's wriLen permission.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. (CC) R Struzak <r.struzakATieee.org> 2 •  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 (CC) R Struzak 3 Antennas for laptop applicaIons Linksys 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 (CC) R Struzak 4 Antenna funcIons Space wave
•  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 Guided wave
(CC) R Struzak 9 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 Java applet plane wave: hLp://www.amanogawa.com/archive/wavesA.html R. Struzak 10
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. (CC) R Struzak 11 Topics for discussion 1.
2.
3.
4.
5.
Antenna funcIons Antenna matching Antenna polarizaIon Antenna direcIvity Antenna arrays (CC) R Struzak 12 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) 13 Transmisng antenna equivalent circuit Antenna TransmiLer Transm. line jXA
RG
VG
Generator
jXG
Rr
Rl
Radio wave 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 (CC) R Struzak 14 Power transfer PA / PAmax
1
0.5
0
0.1
1
RA / RG; (XA+XG = 0)
10
•  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 (CC) R Struzak 15 •  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 (CC) R Struzak 16 •  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 (CC) R Struzak 17 Receiving antenna equivalent circuit Antenna Transm.line Radio wave Rr
Rl
Antenna
jXA
jXL
RL
VA
Receiver 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 Thevenin equivalent (CC) R Struzak 18 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: Rr
e=
Rr + Rl
(CC) R Struzak 19 Topics for discussion 1.
2.
3.
4.
5.
Antenna funcIons Antenna matching Antenna polarizaIon Antenna direcIvity Antenna arrays (CC) R Struzak 20 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). (CC) R Struzak 21 PolarizaIon filters/ reflectors Wall of thin parallel wires (conductors)
|E1|>0
|E1|>0
|E2| = 0
|E2| ~ |E2|
Vector E ⊥ wires
Vector E || wires
Wire distance ~ 0.1λ
•  At the surface of ideal conductor the tangenIal electrical field component = 0 (CC) R Struzak 22 PolarizaIon states LHC UPPER HEMISPHERE: ELLIPTIC POLARIZATION LEFT_HANDED SENSE LATTITUDE: REPRESENTS AXIAL RATIO EQUATOR: LINEAR POLARIZATION LOWER HEMISPHERE: ELLIPTIC POLARIZATION RIGHT_HANDED SENSE POLES REPRESENT CIRCULAR POLARIZATIONS (Poincaré sphere)
450 LINEAR RHC (CC) R Struzak LONGITUDE: REPRESENTS TILT ANGLE 23 Topics for discussion 1.
2.
3.
4.
5.
Antenna funcIons Antenna matching Antenna polarizaIon Antenna direcIvity Antenna arrays (CC) R Struzak 24 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. (CC) R Struzak 25 •  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 (CC) R Struzak 26 PFD: Isotropic Radiator Power Flux Density (PFD) PT
PFD =
2
4πr
r •  Loss-­‐less propagaIon medium assumed •  Isotropic radiator cannot be physically realized •  PFD does not depend on frequency/ wavelength 15 Feb 2001 Property of R. Struzak 27 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 15 Feb 2001 1.8•10 2 •10 3
PFD =
4 • ! •(38•10 6 )2
Property of R. Struzak 1.8•10 5
-2
!
![Wm
]
16
1.8•10
= 1•10 "11 [Wm -2 ]
= "100 [dB(Wm "2 )]
28 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
15 Feb 2001 1.8
4 • ! •(3.8•10 !2 )2
1.8
-2
"
![Wm
]
!2
1.8•10
100 •10 3
-2
= 100 [Wm ] =
[mWcm -2 ]
4
10
= 10![mWcm -2 ]
PFD =
Property of R. Struzak 29 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) (CC) R Struzak 30 Field components 1000
C
Relative fieldstrength
100
Q
C, Q: Induction fields
10
FF
1
FF: Radiation field
0.1
FF
Q
0.01
C
0.001
0.1
1
10
Relative distance, Br
(CC) R Struzak 31 EM field intrinsic impedance 100
Short dipole
Z / 377
10
1
0.1
Small loop
0.01
0.01
0.1
1
10
Distance / (lambda/ 2Pi)
(CC) R Struzak 100
Field impedance Z = E/H depends on the antenna type and on distance 32 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; (CC) R Struzak 33 •  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 A Solar Power Satellite beams RF energy from a geostaIonary satellite down to a receiving site on the ground of 10 km diameter. •  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) Short antenna radiaIon paLern (CC) R Struzak 36 Linear Antennas •  SummaIon of all vector components E (or H) produced by each antenna !
" ! !
element E = E1 + E2 + E3 + ...
! !
!
!
H = H1 + H 2 + H 3 + ...
O
•  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 (CC) R Struzak 37 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) 15 Feb 2001 Property of R. Struzak 38 Satellite antenna mask (example) 0dB 0
RR/1998 APS30 Fig.9
Phi0/2 Relative gain (dB)
-10
COPOLAR
-­‐3dB -20
Phi -30
-40
CROSSPOLAR
-50
0.1
1
10
100
Phi/Phi0
Reference paLern for co-­‐polar and cross-­‐polar components for satellite transmisng antennas in Regions 1 and 3 (BroadcasIng ~12 GHz) 15 Feb 2001 Property of R. Struzak 39 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
(CC) R Struzak 40 Topics for discussion 1.
2.
3.
4.
5.
Antenna funcIons Antenna matching Antenna polarizaIon Antenna gain Antenna arrays (CC) R Struzak 41 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 (CC) R Struzak 42 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. (CC) R Struzak 43 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 (CC) R Struzak 44 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 (CC) R Struzak 45 2 GHz adapIve antenna 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 (CC) R Struzak 46 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 15 Feb 2001 1 w1 Σ wN N Property of R. Struzak Weight-­‐determining algorithm 47 2 omnidirecIonal antennas 1
1
1
0.5
0.5
0.5
-0.5
0
0
0
-1
0
0.5
1
-1
-0.5
0
0.5
1
-1
-0.5
0
0.5
1
-0.5
-0.5
-0.5
-1
-1
-1
D = 0.5λ, θ= 900
D = 0.5λ, θ= 1800
D = 0.5λ, θ= 00
Run simulaIon program: Array2ant_Demo1.xlsm (CC) R Struzak 48 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 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 (CC) R Struzak 50 Thank you for your aLenIon (CC) R Struzak 51