Antenna Fundamentals Prof. Ryszard Struzak Na8onal Ins8tute of Telecommunica8ons, Poland r.struzakATieee.org •  Beware of misprints!!! These materials are preliminary notes intended for my lectures only and may contain misprints. Feedback is welcome. If you no8ce serious faults, or you have an improvement sugges8on, please describe it exactly and I will try to modify the materials. •  This work is licensed under the Crea8ve Commons ANribu8on License (
hNp://crea8vecommons.org/ licenbses/by/1.0) and may be used freely for individual study, research, and educa8on in not-­‐for-­‐profit applica8ons. Any other use requires the wriNen author s permission. If you cite these materials, please credit the author, 8tle, and place. •  These materials and any part of them may not be published, copied to or issued from another Web server without the author's wriNen permission. •  Copyright © 2012 Ryszard Struzak. (CC) R Struzak <r.struzakATieee.org> 2 Objec8ve •  to refresh basic concepts related to the antenna physics –  needed to understand beNer the opera8on and design of microwave links and networks (CC) R Struzak 3 Antennas for laptop applica8ons 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 func8ons Space wave
Guided wave
•  Transforma8on of a guided EM wave into an unguided wave freely propaga8ng in space (or the opposite) –  from 8me-­‐func8on in one-­‐
dimensional space into 8me-­‐
func8on in 3-­‐D space –  The specific form of the radiated wave is defined by the antenna structure and the environment (CC) R Struzak 6 •  The junc8on –  power reflec8ons, use matching devices •  Radiator –  Must radiate efficiently – must be of a size comparable with the half-­‐wavelength •  Resonaces –  Unavoidable -­‐ for broadband applica8ons resonances must be aNenuated 7 Transmiong antenna equivalent circuit Antenna TransmiNer jXA
RG
VG
Generator
jXG
Rr
Rl
Transm. line Radio wave The transmiNer 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 magne8c (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 8 Receiving antenna equivalent circuit Antenna Radio wave Rr
Rl
Antenna
jXA
jXL
RL
VA
Transm.line 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 9 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 10 •  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 reradiated (scaNered) back into space •  Even when the antenna losses tend to zero, s8ll only half of the power captured is delivered to the load (in the case of conjugate matching), the other half being scaNered back into space (CC) R Struzak 11 •  When the antenna impedance is not matched to the transmiNer 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 oten have significant losses (CC) R Struzak 12 Radia8on efficiency •  The radia8on efficiency e indicates how efficiently the antenna uses the RF power •  It is the ra8o of the power radiated by the antenna and the total power delivered to the antenna terminals (in transmiong mode). In terms of equivalent circuit parameters: rrlReRR=+
(CC) R Struzak 13 EM Field of Current Element Er
z
Eθ
OP
ϕ
I, dz
r
Eϕ
!
!
" !
E = Er + E" + E!
! !
!
!
H = H r + H" + H!
y
θ
x
I: current (monochromatic) [A]; dz: antenna element (short) [m]
(CC) R Struzak 14 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 propor8onal to the current moment Idz Have 3 components that decrease with the distance-­‐to-­‐wavelength ra8o as –  (r/λ)-­‐2 & (r/λ)-­‐3: near-­‐field, or induc8on field. The energy oscillates from en8rely electric to en8rely magne8c and back, twice per cycle. Modeled as a resonant LC circuit or a transmission-­‐line resonator; –  (r/λ)-­‐1: far-­‐field or radia8on field –  These 3 component are all equal at (r/λ) = 1/(2π) (CC) R Struzak 15 β
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 16 Field 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 17 Far-­‐Field, Near-­‐Field •
Near-­‐field region: –  Angular distribu8on of energy depends on distance from the antenna; –  Reac8ve field components dominate (L, C) •
Far-­‐field region: –  Angular distribu8on of energy is independent on distance; –  Radia8ng field component dominates (R) –  The resultant EM field can locally be treated as uniform (TEM) (CC) R Struzak 18 Poyn8ng vector •  The 8me-­‐rate of EM energy flow per unit area in free space is the Poyn1ng vector (see hNp://www.amanogawa.com/archive/docs/EM8.pdf). •  It is the cross-­‐product (vector product, right-­‐hand screw direc8on) of the electric field vector (E) and the magne8c field vector (H): P = E x H. •  For the elementary dipole Eθ ⊥ Hθ and only EθxHθ carry energy into space with the speed of light. (CC) R Struzak 19 Power Flow •  In free space and at large distances, the radiated energy streams from the antenna in radial lines, i.e. the Poyn8ng vector has only the radial component in spherical coordinates. •  A source that radiates uniformly in all direc8ons is an isotropic source (radiator, antenna). For such a source the radial component of the Poyn8ng vector is independent of θ and ϕ. (CC) R Struzak 20 Linear Antennas •  Summa8on 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 –  Excita8on phase difference –  Path distance difference •  Method of moments -­‐ NEC (CC) R Struzak 21 Point Source •  For many purposes, it is sufficient to know the direc8on (angle) varia8on of the power radiated by antenna at large distances. •  For that purpose, any prac8cal antenna, regardless of its size and complexity, can be represented as a point-­‐source. •  The actual field near the antenna is then disregarded. (CC) R Struzak 22 •  The EM field at large distances from an antenna can be treated as originated at a point source -­‐ fic88ous volume-­‐less emiNer. •  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 23 Antenna radia8on paNern is 3-­‐dimensional The 3-­‐D plot assumes both angles θ and ϕ varying Difficult to produce and to interpret (CC) R Struzak 24 2-­‐D paNern •  Usually the antenna paNern is presented as a 2-­‐D plot, with only one of the direc8on angles, θ or ϕ varies •  It is an intersec8on of the 3-­‐D one with a given plane –  usually it is a θ = const plane or a ϕ= const plane that contains the paNern s maximum Two 2-­‐D paNerns Source: NK Nikolova
(CC) R Struzak 25 Isotropic antenna •  Isotropic antenna or isotropic radiator is a hypothe8cal (not physically realizable) concept, used as a useful reference to describe real antennas. •  Isotropic antenna radiates equally in all direc8ons. –  Its radia8on paNern is represented by a sphere whose center coincides with the loca8on of the isotropic radiator. Source: NK Nikolova
(CC) R Struzak 26 Anisotropic sources: gain Hypothetic •  Every real antenna radiates more energy in some direc8ons than in others (i.e. has directional
direc8onal proper8es) antenna
Hypothetic
isotropic
antenna
•  Idealized example of direc8onal antenna: the radiated energy is concentrated in the yellow region (cone). •  Direc8ve antenna gain: the power flux density is increased by (roughly) the inverse ra8o of the yellow area and the total surface of the isotropic sphere –  Gain in the field intensity may also be considered -­‐ it is equal to the square root of the power gain. (CC) R Struzak 27 Example Source: NK Nikolova
(CC) R Struzak 28 Volts Equivalent half-­‐power beamwidth representa8ons of an antenna s radia8on paNern. (CC) R Struzak 29 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 30 Antenna Polariza8on •  The polariza8on of an antenna in a specific direc8on is defined to be the polariza8on of the wave produced by the antenna at a great distance at this direc8on (CC) R Struzak 31 Polariza8on 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 32 Polariza8on 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 tangen8al electrical field component = 0 (CC) R Struzak 33 Antenna arrays •  Consist of mul8ple (usually iden8cal) antennas (elements) collabora8ng to synthesize radia8on characteris8cs not available with a single antenna. They are able –  to match the radia8on paNern to the desired coverage area –  to change the radia8on paNern electronically (electronic scanning) through the control of the phase and the amplitude of the signal fed to each element –  to adapt to changing signal condi8ons –  to increase transmission capacity by beNer use of the radio resources and reducing interference •  Complex & costly –  Intensive research related to military, space, etc. ac8vi8es »  Smart antennas, signal-­‐processing antennas, tracking antennas, phased arrays, etc. Source: adapted from N Gregorieva (CC) R Struzak 34 2 GHz adap8ve antenna •  A set of 48 2GHz antennas –  Source: Arraycomm (CC) R Struzak 35 Phased Arrays •  Array of N antennas in a linear or two-­‐dimensional configura8on + beam-­‐forming & control device •  The amplitude and phase excita8on of each individual antenna controlled electronically ( sotware-­‐defined ) –  Diode phase shiters –  Ferrite phase shiters •  Iner8a-­‐less beam-­‐forming and scanning (µsec) with fixed physical structure (CC) R Struzak 36 •  Switched beam antennas –  Based on switching func8on between separate direc8ve antennas or predefined beams of an array •  Space Division Mul1ple Access (SDMA) = alloca8ng an angle direc8on sector to each user –  In a TDMA system, two users will be allocated to the same 8me slot and the same carrier frequency –  They will be differen8ated by different direc8on angles (CC) R Struzak 37 2 omnidirec8onal 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
(CC) R Struzak 38 Antenna Arrays: Benefits •  Possibili8es to control electronically –
–
–
–
–
Direc8on of maximum radia8on Direc8ons (posi8ons) of nulls Beam-­‐width Direc8vity Levels of sidelobes using standard antennas (or antenna collec8ons) independently of their radia8on paNerns •  Antenna elements can be distributed along straight lines, arcs, squares, circles, etc. (CC) R Struzak 39 Antenna simulators •  Polariza8on: –  hNp://www.amanogawa.com/archive/wavesA.html •  Linear dipole antennas: –  hNp://www.amanogawa.com/archive/DipoleAnt/DipoleAnt-­‐2.html –  hNp://www.amanogawa.com/archive/Antenna1/Antenna1-­‐2.html •  2 antennas: –  hNp://www.amanogawa.com/archive/TwoDipole/Antenna2-­‐2.html •  Antenna design using MiniNEC –  hNp://www.sotpedia.com/get/Science-­‐CAD/Expert-­‐MININEC-­‐
Classic.shtml (CC) R Struzak 40 Thank you for your aNen8on (CC) R Struzak 41