Radio Propagation: the key to communication … and its role in clandestine operations during WWII.

Some basic theory Free space propagation The Friis equation The Friis equation - the effects of the TX and RX antennas - the 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 NOISE The ultimate criterion of communication performance is the Signal-to-Noise Ratio or SNR … SNR = (S + N)/N SNR = (S + N)/N So we must always consider NOISE too.

Noise characteristics Noise characteristics Atmospheric noise Man-made noise Galactic noise Frequency dependence

Clandestine comms between occupied Europe and England DistanceTime Mode(s) of propagation Equipment Operator skill Other?

The options for clandestine radio between occupied France & England Line of sight Ground waves Troposcatter Ionospheric modes Other?

Line of Sight (“LOS”) Means what it says – more or less Problems with earth curvature & obstacles Antenna height is crucial Free space? … Remember the Friis equation … Remember the Friis equation

Ground Waves Follow the earth (“surface waves”) The lower the frequency the better Must be vertically polarised Ground conductivity is important

Medium Wave Antenna

Troposcatter Troposphere – “weather zone” Scatter means reflection or refraction Requires focusing: … high gain antennas Therefore UHF and above Predominantly large fixed installations Requires high transmitter power

The Ionosphere

Ionosonde

Ionogram (Day)

Ionogram (night)

Absorption The 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 disappears soon after sunset. - time dependent because D layer disappears soon after sunset.

The equipment requirements Small, lightweight and easily hidden Simple to operate Low power consumption CW only Xtal control Selective receiver Other …

THE NUMBERS … LINK: Calais - WhaddonMarseilles - Whaddon Frequency4.7 MHz10.3 MHz Wavelength64 m29 m TX antenna gain (max) dBi- 0.1 dBi RX antenna gain (max)10 dBi16 dBi TX output power5 W20 W Path length vs F2 layer707 km1208 km Absorption loss (January)8.1 dB4.6 dB Absorption loss (June)12.1 dB5.5 dB Polarisation loss3 dB Atmospheric noise32 dB rel to ambient27 dB rel to ambient

… and today?