Upcoming SlideShare
×

# Rf propagation in a nutshell

1,102 views

Published on

Fundamental Communication Engineering

0 Likes
Statistics
Notes
• Full Name
Comment goes here.

Are you sure you want to Yes No
• Be the first to comment

• Be the first to like this

Views
Total views
1,102
On SlideShare
0
From Embeds
0
Number of Embeds
3
Actions
Shares
0
0
0
Likes
0
Embeds 0
No embeds

No notes for slide

### Rf propagation in a nutshell

1. 1. Ron Milione Ph.D.Ron Milione Ph.D.W2TAPW2TAP
2. 2. Information Modulator AmplifierAntFeedlineTransmitterInformation Demodulator Pre-AmplifierAntFeedlineReceiverFilterFilterRF PropagationThis presentation concentrateson the propagation portion
3. 3.  As the wave propagates, thesurface area increases The power flux densitydecreases proportional to1/d2• At great enough distancesfrom the source, a portion ofthe surface appears as aplane• The wave may be modeledas a plane wave• The classic picture of an EMwave is a single ray out ofthe spherical wave
4. 4.  Most real antennas do notradiate spherically The wavefront will beonly a portion of a sphere• The surface area of the waveis reduced• Power density is increased!• The increase in powerdensity is expressed asAntenna Gain• dB increase in power along“best” axis• dBi = gain over isotropicantenna• dBd = gain over dipoleantennaGain inthis area
5. 5.  Radiated power often referenced to power radiatedby an ideal antennattGPEIRP =Pt= power of transmitterGt= gain of transmitting antenna system• The isotropic radiator radiates power uniformly in alldirections• Effective Isotropic Radiated Power calculated by:Gt = 0dB = 1 for isotropic antennaThis formula assumes power and gain is expressed linearly. Alternatively,you can express power and gain in decibels and add them: EIRP = P(dB) + G(dB)The exact same formulas andprinciples apply on thereceiving side too!
6. 6. λ22Dd f =• Large-scale (Far Field) propagation model• Gives power where random environmental effectshave been averaged together• Waves appear to be plane waves• Far field applies at distances greater than theFraunhofer distance:D = largest physical dimension of antennaλ = wavelength• Small-scale (Near Field) model applies for shorterdistances• Power changes rapidly from one area/time to the next
7. 7. 2222)4()4()(cfddPPlinlossFreert πλπ===For Free Space (no buildings, trees, etc.)dBdfcfddBlossFree 56.147log20log204log10)( 1010210 −+==πf = frequencyd = distance (m)λ= wavelength (m)c = speed of lighthb= base station antenna height (m)hm= mobile antenna height (m)a(hm) is an adjustment factor for the type of environment and theheight of the mobile.a(hm) = 0 for urban environments with a mobile height of 1.5m.Note: Hata valid only with d in range 1000-20000, hb in range 30-200m)3)(loglog55.60.44()(log82.13)6(log16.2655.69)(10101010−−+−−−+=dhhahfdBlossHatabmbFor Urban environments, use the Hata model
8. 8. A transmission system transmits a signal at 960MHz with a power of 100mW usinga 16cm dipole antenna system with a gain of 2.15dB over an isotropic antenna.At what distance can far-field metrics be used?λ = 3.0*108m/s / 960MHz = 0.3125 metersFraunhofer distance = 2 D2/ λ = 2(0.16m)2/0.3125 = 0.16mWhat is the EIRP?Method 1: Convert power to dBm and add gainPower(dBm) = 10 log10 (100mW / 1mW) = 20dBmEIRP = 20dBm + 2.15dB = 22.15dBmMethod 2: Convert gain to linear scale and multiplyGain(linear) = 102.15dB/10= 1.64EIRP = 100mW x 1.64 = 164mWChecking work: 10 log10 (164mW/1mW) = 22.15dBm
9. 9. A transmission system transmits a signal at 960MHz with a power of 100mWusing a 16cm dipole antenna system with a gain of 2.15dB over an isotropicantenna.What is the power received at a distance of 2km (assuming free-spacetransmission and an isotropic antenna at the receiver)?Loss(dB) = 20 log10(960MHz) + 20 log10(2000m) – 147.56dB= 179.6dB + 66.0dB – 147.56dB = 98.0dBReceived power(dBm) = EIRP(dB) – loss= 22.15dBm – 98.0dB = -75.85dBmReceived power(W) = EIRP(W)/loss(linear)= 164mW / 1098.0dB/10= 2.6 x 10-8mW = 2.6 x 10-11WChecking work: 10 -75.85dBm/10= 2.6x 10-8mWWhat is the power received at a distance of 2km (use Hata model with baseheight 30 m, mobile height 1.5 m, urban env.)?Loss(dB) = 69.55+26.16(log(f)-6) – 13.82(log(hb)) – a(hm)+ 44.9-6.55(log(hb))(log(d)-3)=69.55 + 78.01 – 27.79 – 0 + (35.22)(0.30)= 130.34 dB  Received power = 22.15dBm – 130.34dB = -108.19dBm
10. 10.  A Link Budget analysis determines if there isenough power at the receiver to recover theinformationInformation Modulator AmplifierAntFeedlineTransmitterInformation Demodulator Pre-AmplifierAntFeedlineReceiverFilterFilterRF PropagationGainGainLoss
11. 11.  Begin with the power output of the transmit amplifier Subtract (in dB) losses due to passive components in the transmitchain after the amplifier Filter loss Feedline loss Jumpers loss Etc. Add antenna gain dBi Result is EIRPInformation Modulator AmplifierAntFeedlineTransmitterFilterRF Propagation
12. 12. dBi12Antenna gaindB(1.5)150 ft. at 1dB/100 footFeedline lossdB(1)Jumper lossdB(0.3)Filter lossdBm4425 WattsPower AmplifierScaleValueComponentdBm53TotalAll values are example values
13. 13.  The Receiver has several gains/losses Specific losses due to known environment around the receiver Vehicle/building penetration loss Receiver antenna gain Feedline loss Filter loss These gains/losses are added to the received signal strength The result must be greater than the receiver’s sensitivityInformationDemodulatorPre-AmplifierAntFeedlineReceiverFilter
14. 14.  Sensitivity describes the weakest signal power levelthat the receiver is able to detect and decode Sensitivity is dependent on the lowest signal-to-noise ratioat which the signal can be recovered Different modulation and coding schemes have differentminimum SNRs Range: <0 dB to 60 dB Sensitivity is determined by adding the requiredSNR to the noise present at the receiver Noise Sources Thermal noise Noise introduced by the receiver’s pre-amplifier
15. 15.  Thermal noise N = kTB (Watts) k=1.3803 x 10-23J/K T = temperature in Kelvin B=receiver bandwidth Thermal noise is usually very small for reasonablebandwidths Noise introduced by the receiver pre-amplifier Noise Factor = SNRin/SNRout (positive becauseamplifiers always generate noise) May be expressed linearly or in dB
16. 16.  The smaller the sensitivity, the better the receiver Sensitivity (W) =kTB * NF(linear) * minimum SNR required (linear) Sensitivity (dBm) =10log10(kTB*1000) + NF(dB) + minimum SNRrequired (dB)
17. 17.  Example parameters Signal with 200KHz bandwidth at 290K NF for amplifier is 1.2dB or 1.318 (linear) Modulation scheme requires SNR of 15dB or 31.62 (linear) Sensitivity = Thermal Noise + NF + Required SNR Thermal Noise = kTB =(1.3803 x 10-23J/K) (290K)(200KHz)= 8.006 x 10-16W = -151dBW or -121dBm Sensitivity (W) = (8.006 x 10-16W )(1.318)(31.62) = 3.33 x 10-14W Sensitivity (dBm) = -121dBm + 1.2dB + 15dB = -104.8dBm Sensitivity decreases when: Bandwidth increases Temperature increases Amplifier introduces more noise
18. 18.  Transmit/propagate chain produces a receivedsignal has some RSS (Received Signal Strength) EIRP minus path loss For example 50dBm EIRP – 130 dBm = -80dBm Receiver chain adds/subtracts to this For example, +5dBi antenna gain, 3dB feedline/filterloss  -78dBm signal into receiver’s amplifier This must be greater than the sensitivity of thereceiver If the receiver has sensitivity of -78dBm or lower, thesignal is successfully received.