Radio Propagation: the key to communication

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 effects of the TX and RX antennas - the effect of frequency - 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 So we must always consider NOISE too.

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

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

8. The options for clandestine radiobetween 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

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, so low frequencies are most affected; - 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 …

36. … and today?