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.

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.

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.

Noise characteristicsNoise characteristics

Atmospheric noiseAtmospheric noiseMan-made noiseMan-made noiseGalactic noiseGalactic noiseFrequency dependenceFrequency dependence

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?

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?

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

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

Medium Wave AntennaMedium Wave Antenna

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

The IonosphereThe Ionosphere

IonosondeIonosonde

Ionogram (Day)Ionogram (Day)

Ionogram (night)Ionogram (night)

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.

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 …

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

… … and today?and today?