radio propagation: the key to communication
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Radio Propagation: the key to communication. … and its role in clandestine operations during WWII. Some basic theory. Free space propagation The Friis equation - the effects of the TX and RX antennas - the effect of frequency - the effect of distance. - PowerPoint PPT PresentationTRANSCRIPT
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Radio Propagation: the key to Radio Propagation: the key to communicationcommunication
… … and its role in clandestine and its role in clandestine operations during WWII.operations during WWII.
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Some basic theorySome basic theory
Free space propagationFree space propagation
The Friis equationThe Friis equation - - the effects of the TX and RX antennasthe effects of the TX and RX antennas
- the effect of frequency- the effect of frequency
- the effect of distance.- the effect of distance.
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The effect of NOISEThe effect of NOISE
The ultimate criterion of communication The ultimate criterion of communication performance is the Signal-to-Noise Ratio performance is the Signal-to-Noise Ratio or SNR …or SNR …
SNR = (S + N)/NSNR = (S + N)/N
So we must always consider NOISE too.So we must always consider NOISE too.
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Noise characteristicsNoise characteristics
Atmospheric noiseAtmospheric noiseMan-made noiseMan-made noiseGalactic noiseGalactic noiseFrequency dependenceFrequency dependence
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Clandestine comms between Clandestine comms between occupied Europe and Englandoccupied Europe and England
DistanceDistance
TimeTime
Mode(s) of propagationMode(s) of propagation
EquipmentEquipment
Operator skillOperator skill
Other?Other?
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TheThe options for clandestine radio options for clandestine radiobetween occupied France & England between occupied France & England
Line of sightLine of sight
Ground wavesGround waves
Troposcatter Troposcatter
Ionospheric modesIonospheric modes
Other?Other?
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Line of Sight (“LOS”)Line of Sight (“LOS”)
Means what it says – more or lessMeans what it says – more or less
Problems with earth curvature & obstaclesProblems with earth curvature & obstacles
Antenna height is crucialAntenna height is crucial
Free space?Free space?
… … Remember the Friis equationRemember the Friis equation
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Ground WavesGround Waves
Follow the earth (“surface waves”)Follow the earth (“surface waves”)
The lower the frequency the betterThe lower the frequency the better
Must be vertically polarisedMust be vertically polarised
Ground conductivity is importantGround conductivity is important
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Medium Wave AntennaMedium Wave Antenna
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TroposcatterTroposcatter
Troposphere – “weather zone”Troposphere – “weather zone”
Scatter means reflection or refractionScatter means reflection or refraction
Requires focusing: … high gain antennasRequires focusing: … high gain antennas
Therefore UHF and aboveTherefore UHF and above
Predominantly large fixed installationsPredominantly large fixed installations
Requires high transmitter powerRequires high transmitter power
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The IonosphereThe Ionosphere
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IonosondeIonosonde
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Ionogram (Day)Ionogram (Day)
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Ionogram (night)Ionogram (night)
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AbsorptionAbsorption
The D layerThe D layer
- frequency dependent as 1/f squared,- frequency dependent as 1/f squared,
so low frequencies are most affected;so low frequencies are most affected;
- time dependent because D layer - time dependent because D layer disappears soon after sunset.disappears soon after sunset.
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The equipment requirementsThe equipment requirements
Small, lightweight and easily hiddenSmall, lightweight and easily hidden
Simple to operateSimple to operate
Low power consumptionLow power consumption
CW onlyCW only
Xtal controlXtal control
Selective receiverSelective receiver
Other …Other …
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THE NUMBERS …
LINK: Calais - Whaddon Marseilles - Whaddon
Frequency 4.7 MHz 10.3 MHz
Wavelength 64 m 29 m
TX antenna gain (max) - 0.46 dBi - 0.1 dBi
RX antenna gain (max) 10 dBi 16 dBi
TX output power 5 W 20 W
Path length vs F2 layer 707 km 1208 km
Absorption loss (January) 8.1 dB 4.6 dB
Absorption loss (June) 12.1 dB 5.5 dB
Polarisation loss 3 dB 3 dB
Atmospheric noise 32 dB rel to ambient 27 dB rel to ambient
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… … and today?and today?