RADIO SYSTEMS – ETI 051 Contents Lecture no: 2 • Short on dB calculations • Basics about antennas • Propagation mechanisms – Free space propagation – Reflection and transmission Propagation – Propagation over ground plane – Diffraction mechanisms • Screens • Wedges • Multiple screens – Scattering by rough surfaces – Waveguiding Ove Edfors, Department of Electrical and Information Technology Ove.Edfors@eit.lth.se 2010-03-17 Ove Edfors - ETI 051 1 2010-03-17 Ove Edfors - ETI 051 2 dB in general When we convert a measure X into decibel scale, we always divide by a reference value X r f : e Independent of the dimension of X X ∣ non − dB (and X r f ), this e value is always X ref ∣ non dimension-less. − dB DECIBEL The corresponding dB value is calculated as: X ∣ dB = 10log X ref ∣ non − dB X ∣ non − dB 2010-03-17 Ove Edfors - ETI 051 3 2010-03-17 Ove Edfors - ETI 051 4
Power Example: Power We usually measure power in Watt [W] and milliWatt [mW] 4 W = -132 dB or -102 dBm The corresponding dB notations are dB and dBm Sensitivity level of GSM RX: 6.3x10 - 1 Non-dB dB Bluetooth TX: 10 mW = -20 dB or 10 dBm P ∣ dB = 10log W = 10log P ∣ W P ∣ P ∣ W GSM mobile TX: 1 W = 0 dB or 30 dBm Watt: W 1 ∣ ERP – Effective GSM base station TX: 40 W = 16 dB or 46 dBm P ∣ dBm = 10log 1 ∣ mW = 10log P ∣ mW P ∣ mW Radiated Power P ∣ mW milliWatt: Vacuum cleaner: 1600 W = 32 dB or 62 dBm Car engine: 100 kW = 50 dB or 80 dBm P ∣ dBm = 10log 0.001 ∣ W = 10 log P ∣ P ∣ W ”Typical” TV transmitter: 1000 kW ERP = 60 dB or 90 dBm ERP RELATION: W 30 ∣ dB = P ∣ dB 30 ∣ dB Nuclear powerplant (Barsebäck): 1200 MW = 91 dB or 121 dBm 2010-03-17 Ove Edfors - ETI 051 5 2010-03-17 Ove Edfors - ETI 051 6 Example: Amplification and Amplification and attenuation attenuation (Power) Amplification: (Power) Attenuation: High frequency cable RG59 140 P in P out P in P out Attenuation [dB/100m] G 1 / L 120 30 m of RG59 feeder cable Note: It doesn’t for an 1800 MHz application matter if the power 100 has an attenuation: is in mW or W. Same result! 80 L ∣ dB / 100 m P out P in P in G ∣ dB = 30 P out = L ⇒ L = 58 P out = GP in ⇒ G = 100 60 P out P in dB / 1m 40 = 30 58 The attenuation is already The amplification is already 100 = 17.4 dimension-less and can be converted dimension-less and can be converted 20 directly to dB: directly to dB: 1800 0 L ∣ dB = 10log 10 L G ∣ dB = 10log 10 G 0 1000 2000 3000 4000 5000 Frequency [MHz] 2010-03-17 Ove Edfors - ETI 051 7 2010-03-17 Ove Edfors - ETI 051 8
Example: Amplification and attenuation Ampl. Cable Ampl. Ampl. Detector A B 4 dB 30 dB 10 dB 10 dB ANTENNA BASICS The total amplification of the (simplified) receiver chain (between A and B) is G A, B ∣ dB = 30 − 4 10 10 = 46 2010-03-17 Ove Edfors - ETI 051 9 2010-03-17 Ove Edfors - ETI 051 10 The isotropic antenna The dipole antenna The isotropic antenna radiates Elevation pattern Elevation pattern equally in all directions λ / 2 -dipole Radiation pattern is This antenna does not spherical radiate straight up or down. Therefore, more energy is available in other directions. λ / 2 Feed THIS IS THE PRINCIPLE Azimuth pattern Azimuth pattern BEHIND WHAT IS CALLED ANTENNA GAIN . This is a theoretical A dipole can be of any length, antenna that cannot but the antenna patterns shown be built. are only for the λ/2-dipole. Antenna pattern of isotropic antenna. 2010-03-17 Ove Edfors - ETI 051 11 2010-03-17 Ove Edfors - ETI 051 12
Antenna gain (principle) Antenna beamwidth (principle) Antenna gain is a relative measure. Radiation pattern We will use the isotropic antenna as the reference. The isotropic antenna has ”no” beamwidth. It radiates equally Radiation pattern in all directions. Isotropic and dipole, 3 dB The increase of input with equal input The half-power beamwidth power to the isotropic power! is measured between points antenna, to obtain the were the pattern as decreased same maximum by 3 dB. radiation is called the antenna gain ! Isotropic, with increased input power. Antenna gain of the λ/2 dipole is 2.15 dB . 2010-03-17 Ove Edfors - ETI 051 13 2010-03-17 Ove Edfors - ETI 051 14 Receiving antennas A note on antenna gain In terms of gain and beamwidth, an antenna has the same properties when Sometimes the notation dBi is used for antenna gain (instead of dB). used as transmitting or receiving antenna. The ” i ” indicates that it is the gain relative to the A useful property of a receiving antenna isotropic antenna (which we will use in this course). It can be shown that the is its ” effective area ”, i.e. the area from effectiva are of the isotropic antenna is: which the antenna can ”absorb” the power 2 from an incoming electromagnetic wave. A ISO = Another measure of antenna gain frequently encountered is dBd , which is relative to the λ/2 dipole. 4 Effective area A R X of an antenna is Be careful ! Sometimes connected to its gain: it is not clear if the G ∣ dBi = G ∣ dBd 2.15 antenna gain is given A RX = 4 Note that A IS O becomes G RX = 2 A RX in dBi or dBd. smaller with increasing A ISO frequency, i.e. with smaller wavelength. 2010-03-17 Ove Edfors - ETI 051 15 2010-03-17 Ove Edfors - ETI 051 16
EIRP EIRP and the link budget Effective Isotropic Radiated Power ”POWER” [dB] EIRP = Transmit power (fed to the antenna) + antenna gain EIRP G TX ∣ dB EIRP ∣ dB = P TX ∣ dB G TX ∣ dB P TX ∣ dB Gain Answers the questions: Loss 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 EIRP ∣ dB = P TX ∣ dB G TX ∣ dB limit on EIRP is a limit on the radiation in the maximal direction. 2010-03-17 Ove Edfors - ETI 051 17 2010-03-17 Ove Edfors - ETI 051 18 Propagation mechanisms • We are going to study the fundamental propagation mechanisms • This has two purposes: PROPAGATION MECHANISMS – 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 2010-03-17 Ove Edfors - ETI 051 19 2010-03-17 Ove Edfors - ETI 051 20
Free-space loss Derivation Assumptions: Isotropic TX antenna TX power P T X Distance d RX antenna with effective P T area A R X X d Relations: FREE SPACE PROPAGATION A tot = 4 d Area of sphere: 2 A R X Received power: P RX = A RX P TX A tot Attenuation between two = A RX isotropic antennas in free 2 P TX 4 d space is (free-space loss): If we assume RX antenna to be isotropic: L free d = 2 4 d 2 λ π λ 2 / 4 = = P P P RX TX TX π 2 π 4 d 4 d 2010-03-17 Ove Edfors - ETI 051 21 2010-03-17 Ove Edfors - ETI 051 22 Free-space loss Free-space loss Non-isotropic antennas Non-isotropic antennas (cont.) Let’s put Friis’ law into the link budget Received power, with isotropic antennas ( G T X = G R X =1): Received power ”POWER” [dB] decreases as 1/ d 2 , P TX which means a P RX d = G TX ∣ dB L free d propagation exponent P TX ∣ dB L free ∣ dB d = 20log 10 λ of n = 2. Gain 4πd Received power, with antenna gains G T X and G R X : How come that Loss the received P RX d = G RX G TX P RX ∣ dB P RX ∣ dB d = P TX ∣ dB G TX ∣ dB − L free ∣ dB d G RX ∣ dB power decreases P TX G RX ∣ dB L free d = P TX ∣ dB G TX ∣ dB − 20log 10 G RX ∣ dB with increasing 4 d frequency (decre- asing λ)? = G RX G TX 2 P TX This relation is called Friis’ law Does it? 4 d P RX ∣ dB d = P TX ∣ dB G TX ∣ dB − L free ∣ dB d G RX ∣ dB 2010-03-17 Ove Edfors - ETI 051 23 2010-03-17 Ove Edfors - ETI 051 24
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