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

2. 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.

4. 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.

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

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

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

9. 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

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

13. Medium Wave Antenna

14. 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

17. The Ionosphere

20. Ionosonde

21. Ionogram (Day)

22. Ionogram (night)

23. 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.

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

34. THE NUMBERS … LINK: Calais - WhaddonMarseilles - Whaddon Frequency4.7 MHz10.3 MHz Wavelength64 m29 m TX antenna gain (max)- 0.46 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

36. … and today?