RADIO SYSTEMS – ETIN15 2 Lecture no: Propagation mechanisms Ove Edfors, Department of Electrical and Information Technology Ove.Edfors@eit.lth.se 2011-03-18 Ove Edfors - ETIN15 1
Contents • Short on dB calculations • Basics about antennas • Propagation mechanisms – Free space propagation – Reflection and transmission – Propagation over ground plane – Diffraction • Screens • Wedges • Multiple screens – Scattering by rough surfaces – Waveguiding 2012-03-14 Ove Edfors - ETIN15 2
DECIBEL 2012-03-14 Ove Edfors - ETIN15 3
dB in general When we convert a measure X into decibel scale, we always divide by a reference value X ref : Independent of the dimension of X X ∣ non − dB (and X ref ), this value is always X ref ∣ non − dB dimension-less. The corresponding dB value is calculated as: X ∣ dB = 10log X ref ∣ non − dB X ∣ non − dB 2012-03-14 Ove Edfors - ETIN15 4
Power We usually measure power in Watt [W] and milliWatt [mW] The corresponding dB notations are dB and dBm Non-dB dB P ∣ dB = 10 log W = 10log P ∣ W P ∣ W P ∣ Watt: W 1 ∣ P ∣ dBm = 10log 1 ∣ mW = 10 log P ∣ mW P ∣ mW P ∣ mW milliWatt: P ∣ dBm = 10log 0.001 ∣ W = 10 log P ∣ P ∣ W RELATION: W 30 ∣ dB = P ∣ dB 30 ∣ dB 2012-03-14 Ove Edfors - ETIN15 5
Example: Power Sensitivity level of GSM RX: 6.3x10 -14 W = -132 dB or -102 dBm Bluetooth TX: 10 mW = -20 dB or 10 dBm GSM mobile TX: 1 W = 0 dB or 30 dBm ERP – Effective GSM base station TX: 40 W = 16 dB or 46 dBm Radiated Power Vacuum cleaner: 1600 W = 32 dB or 62 dBm Car engine: 100 kW = 50 dB or 80 dBm ”Typical” TV transmitter: 1000 kW ERP = 60 dB or 90 dBm ERP ”Typical” Nuclear powerplant : 1200 MW = 91 dB or 121 dBm 2012-03-14 Ove Edfors - ETIN15 6
Amplification and attenuation (Power) Attenuation: (Power) Amplification: P in P out P in P out G 1 / L Note: It doesn’t matter if the power is in mW or W. Same result! P in P in P out P out = L ⇒ L = P out = GP in ⇒ G = P out P in The attenuation is already The amplification is already dimension-less and can be converted dimension-less and can be converted directly to dB: directly to dB: L ∣ dB = 10log 10 L G ∣ dB = 10log 10 G 2012-03-14 Ove Edfors - ETIN15 7
Example: Amplification and attenuation High frequency cable RG59 140 Attenuation [dB/100m] 120 30 m of RG59 feeder cable for an 1800 MHz application 100 has an attenuation: 80 L ∣ dB / 100 m G ∣ dB = 30 58 100 60 dB / 1m 40 = 30 58 100 = 17.4 20 1800 0 0 1000 2000 3000 4000 5000 Frequency [MHz] 2012-03-14 Ove Edfors - ETIN15 8
Example: Amplification and attenuation Ampl. Cable Ampl. Ampl. Detector A B 4 dB 30 dB 10 dB 10 dB The total amplification of the (simplified) receiver chain (between A and B) is G A, B ∣ dB = 30 − 4 10 10 = 46 2012-03-14 Ove Edfors - ETIN15 9
ANTENNA BASICS 2012-03-14 Ove Edfors - ETIN15 10
The isotropic antenna The isotropic antenna radiates Elevation pattern equally in all directions Radiation pattern is spherical Azimuth pattern This is a theoretical antenna that cannot be built. 2012-03-14 Ove Edfors - ETIN15 11
The dipole antenna Elevation pattern λ / 2 -dipole This antenna does not radiate straight up or down. Therefore, more energy is available in other directions. λ / 2 Feed THIS IS THE PRINCIPLE Azimuth pattern BEHIND WHAT IS CALLED ANTENNA GAIN . A dipole can be of any length, but the antenna patterns shown Antenna pattern of are only for the λ/2-dipole. isotropic antenna. 2012-03-14 Ove Edfors - ETIN15 12
Antenna gain (principle) Antenna gain is a relative measure. We will use the isotropic antenna as the reference. Radiation pattern Isotropic and dipole, The increase of input with equal input power to the isotropic power! antenna, to obtain the same maximum radiation is called the antenna gain ! Isotropic, with increased input power. Antenna gain of the λ/2 dipole is 2.15 dB . 2012-03-14 Ove Edfors - ETIN15 13
Antenna beamwidth (principle) Radiation pattern The isotropic antenna has ”no” beamwidth. It radiates equally in all directions. 3 dB The half-power beamwidth is measured between points were the pattern as decreased by 3 dB. 2012-03-14 Ove Edfors - ETIN15 14
Receiving antennas In terms of gain and beamwidth, an antenna has the same properties when used as transmitting or receiving antenna. A useful property of a receiving antenna It can be shown that the effectiva are of the isotropic is its ” effective area ”, i.e. the area from antenna is: which the antenna can ”absorb” the power 2 A ISO = from an incoming electromagnetic wave. 4 Effective area A RX of an antenna is connected to its gain: A RX = 4 Note that A ISO becomes G RX = 2 A RX smaller with increasing A ISO frequency, i.e. with smaller wavelength. 2012-03-14 Ove Edfors - ETIN15 15
A note on antenna gain Sometimes the notation dBi is used for antenna gain (instead of dB). The ” i ” indicates that it is the gain relative to the isotropic antenna (which we will use in this course). Another measure of antenna gain frequently encountered is dBd , which is relative to the λ/2 dipole. Be careful ! Sometimes it is not clear if the G ∣ dBi = G ∣ dBd 2.15 antenna gain is given in dBi or dBd. 2012-03-14 Ove Edfors - ETIN15 16
EIRP Effective Isotropic Radiated Power EIRP = Transmit power (fed to the antenna) + antenna gain EIRP ∣ dB = P TX ∣ dB G TX ∣ dB Answers the questions: How much transmit power would we need to feed an isotropic antenna to obtain the same maximum on the radiated power? How ”strong” is our radiation in the maximal direction of the antenna? This is the more important one, since a limit on EIRP is a limit on the radiation in the maximal direction. 2012-03-14 Ove Edfors - ETIN15 17
EIRP and the link budget ”POWER” [dB] EIRP G TX ∣ dB P TX ∣ dB Gain Loss EIRP ∣ dB = P TX ∣ dB G TX ∣ dB 2012-03-14 Ove Edfors - ETIN15 18
PROPAGATION MECHANISMS 2012-03-14 Ove Edfors - ETIN15 19
Propagation mechanisms • We are going to study the fundamental propagation mechanisms • This has two purposes: – Gain an understanding of the basic mechanisms – Derive propagation losses that we can use in calculations • For many of the mechanisms, we just give a brief overview 2012-03-14 Ove Edfors - ETIN15 20
FREE SPACE PROPAGATION 2012-03-14 Ove Edfors - ETIN15 21
Free-space loss Derivation Assumptions: Isotropic TX antenna TX power P TX Distance d RX antenna with effective P TX area A RX d Relations: 2 Area of sphere: A tot = 4 π d A RX Received power: P RX = A RX P TX A tot Attenuation between two = A RX isotropic antennas in free 2 P TX space is (free-space loss): 4 π d If we assume RX antenna to be isotropic: L free ( d )= ( 4 π d λ ) 2 2 / 4 π P RX =λ 2 2 P TX = ( 4 π d ) λ P TX 4 π d 2012-03-14 Ove Edfors - ETIN15 22
Free-space loss Non-isotropic antennas Received power, with isotropic antennas ( G TX = G RX =1): P TX P RX ( d )= L free ( d ) Received power, with antenna gains G TX and G RX : P RX d = G RX G TX P RX ∣ dB d = P TX ∣ dB G TX ∣ dB − L free ∣ dB d G RX ∣ dB P TX L free d = P TX ∣ dB G TX ∣ dB − 20 log 10 G RX ∣ dB 4 d = G RX G TX 2 P TX This relation is called Friis’ law 4 d 2012-03-14 Ove Edfors - ETIN15 23
Free-space loss Non-isotropic antennas (cont.) Let’s put Friis’ law into the link budget Received power ”POWER” [dB] decreases as 1/ d 2 , which means a G TX ∣ dB propagation exponent P TX ∣ dB L free ∣ dB d = 20log 10 λ Gain of n = 2. 4πd How come that Loss the received P RX ∣ dB G RX ∣ dB power decreases with increasing frequency (decre- asing λ)? Does it? P RX ∣ dB d = P TX ∣ dB G TX ∣ dB − L free ∣ dB d G RX ∣ dB 2012-03-14 Ove Edfors - ETIN15 24
Free-space loss Example: Antenna gains Assume following three free-space scenarios with λ/2 dipoles and parabolic antennas with fixed effective area A par : Antenna gains G dip ∣ dB = 2.15 D-D: G par ∣ dB = 10log 10 A iso A par D-P: = 10log 10 2 / 4π A par 2 = 10log 10 4 A par P-P: 2012-03-14 Ove Edfors - ETIN15 25
Free-space loss Example: Antenna gains (cont.) Evaluation of Friis’ law for the three scenarios: D-D: P RX ∣ dB d = P TX ∣ dB 2.15 − 20 log 10 2.15 4 d = P TX ∣ dB 4.3 − 20 log 10 4 d 20log 10 Received power decreases with decreasing wavelength λ, i.e. with increasing frequency . 2 10log 10 D-P: P RX ∣ dB d = P TX ∣ dB 2.15 − 20 log 10 4 A par 4 d = P TX ∣ dB 2.15 − 20log 10 4 d 10log 10 4 A par Received power independent of wavelength, i.e. of frequency . 2 − 20log 10 2 P RX ∣ dB d = P TX ∣ dB 10 log 10 10 log 10 P-P: 4 A par 4 A par 4 d = P TX ∣ dB 20log 10 4 A par − 20 log 10 4 d − 20log 10 Received power increases with decreasing wavelength λ, i.e. with increasing frequency . 2012-03-14 Ove Edfors - ETIN15 26
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