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Antenna Fundamentals 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 Creave Commons ALribuIon License (hLp://creavecommons.org/ licenbses/by/1.0) and may be used freely for individual study, research, and educaon in not- for-profit applicaons. 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 • These materials and any part of them may not be published, copied to or issued from another Web server without the author's wriLen permission.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 • Objecve: • Topics for discussion: to refresh basic 1. Antenna funcIons concepts related to the 2. Antenna matching antenna physics 3. Antenna polarizaon – needed to understand beLer the operaon of 4. Antenna direcIvity wireless (radio) links/ 5. Antenna arrays networks (CC) R Struzak 3 Antennas for laptop applicaons 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 funcons • Transformaon of a guided EM wave into an unguided wave Space wave freely propagang 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. Antenna funcIons 2. Antenna matching 3. Antenna polarizaon 4. Antenna direcIvity 5. Antenna arrays (CC) R Struzak 12 Basic transmiLer/ receiver parts • The transmission line • The juncon • The antenna radiator (size comparable with λ/2) • The EM wave • Notes: – Possible power reflecIons (impedance matching) – Possible resonaces (for broadband applicaons must be aenuated) 13 Transming antenna equivalent circuit Antenna TransmiLer Transm. line Radio wave The transmiLer with the transmission line is represented by an (Thevenin) equivalent generator jX jXA G The antenna is represented by its input impedance (which is frequency-dependent and is influenced by objects nearby) as seem from RG the generator R r jXA represents energy stored in electric (Ee) and magneIc (Em) near-field components; if | Ee| = |Em| then XA = 0 (antenna resonance) Generator Generator Can be approximated by the impedance of a transmission line R Rr represents energy radiated into space (far- VG l field components) Rl represents energy lost, i.e. transformed into heat in the antenna structure (CC) R Struzak 14 Power transfer 1 • The maximum power is delivered to (or from) the 0.5 antenna when the PA / PAmax antenna impedance and the impedance 0 of the equivalent 0.1 1 10 RA / RG; (XA+XG = 0) 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 (scaered) 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 scaered back into space (CC) R Struzak 17 Receiving antenna equivalent circuit Antenna Radio wave Transm.line Receiver The antenna with the transmission line is represented by an (Thevenin) equivalent generator jXA The receiver is represented by its input impedance as seen from the antenna jXL terminals (i.e. transformed by the Rr transmission line) V is the (induced by the incident wave) voltage at the antenna terminals A R Antenna R l L determined when the antenna is open circuited V Note: The antenna impedance is the same when the A antenna is used to radiate and when it is used to receive energy Thevenin equivalent (CC) R Struzak 18 Radiaon efficiency • The radiaon efficiency e indicates how efficiently the antenna uses the RF power • It is the rao of the power radiated by the antenna into the space and the total power delivered to the antenna terminals (in transming mode). In terms of equivalent circuit parameters: R e = r Rr + Rl (CC) R Struzak 19 Topics for discussion 1. Antenna funcIons 2. Antenna matching 3. Antenna polarizaon 4. Antenna direcIvity 5. Antenna arrays (CC) R Struzak 20 Antenna polarizaon • The polarizaon of an antenna in a specific direcIon is defined to be the polarizaon of the wave produced by the antenna at this direcIon at a great distance • By convenIon the "polarizaon" of an EM wave refers to the polarizaon (direcIon) of oscillaons of the electric field vector. • The oscillaon may be in a single direcIon (linear polarizaon), or the field may rotate (circular or ellipIcal polarizaon). (CC) R Struzak 21 Polarizaon filters/ reflectors Wall of thin parallel wires (conductors) |E |>0 1 |E | = 0 |E1|>0 2 |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 Polarizaon states LHC (Poincaré sphere) UPPER HEMISPHERE: ELLIPTIC POLARIZATION LEFT_HANDED SENSE LATTITUDE: REPRESENTS AXIAL RATIO EQUATOR: LINEAR POLARIZATION LOWER HEMISPHERE: 450 LINEAR ELLIPTIC POLARIZATION RIGHT_HANDED SENSE LONGITUDE: REPRESENTS TILT ANGLE RHC POLES REPRESENT CIRCULAR POLARIZATIONS (CC) R Struzak 23 Topics for discussion 1. Antenna funcIons 2. Antenna matching 3. Antenna polarizaon 4. Antenna direcIvity 5. Antenna arrays (CC) R Struzak 24 Point Source • For many purposes, it is sufficient to know the direcIon (angle) variaon 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) P PFD = T r 4πr 2 • Loss-less propagaon 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 1.8•102 •103 PFD = TV broadcast GEO 4•! •(38•106 )2 satellite at ICTP? 5 1.8•10 -2 • EIRP = 180 kW ! 16 ![Wm ] 1.8•10 (52.5 dB(W)) =1•10"11 [Wm-2 ] • Distance: ~38'000 km = "100 [dB(Wm"2 )] • Free space 15 Feb 2001 Property of R. Struzak 28 PFD: Example 2 • What is the PFD 1.8 from a hand-held PFD = 4•! •(3.8•10!2 )2 phone at the head? 1.8 " ![Wm-2 ] • EIRP = 1.8 W 1.8•10!2 3 -2 100•10 -2 =100 [Wm ] = [mWcm ] • Distance = ~3.8 cm 104 -2 • Free space =10![mWcm ] 15 Feb 2001 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 rao 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 radiaon field – These 3 component are all equal at (r/λ) = 1/(2π) (CC) R Struzak 30 Field components 1000 C C, Q: Induction fields 100 Q 10 FF 1 FF 0.1 FF: Radiation field Q Relative fieldstrength 0.01 C 0.001 0.1 1 10 Relative distance, Br (CC) R Struzak 31 EM field intrinsic impedance 100 Field Short dipole impedance 10 z = E/H depends 1 Z / 377 on the 0.1 antenna Small loop type and 0.01 0.01 0.1 1 10 100 on Distance / (lambda/ 2Pi) distance (CC) R Struzak 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: – Radiang 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 operang at a frequency of 12GHz, (a wavelength of 25mm), R1 = 0.85m and R2 = 29m.
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