Chelmsford Amateur Radio Society Intermediate Course (6) Propagation

1. Chelmsford Amateur Radio SocietyIntermediate Course(6)Propagation

2. v f l Velocity of Propagation • Candidates should know the formula v = f x l. • Try to remember another triangle formula. v is the speed of radio waves / light :- v = 300,000,000 m/s. f is the frequency in Hz. is the wavelength in metres. • Example: What's l f = 3.7MHz  = 300,000,000 3,700,000 Or 300 3.7 Thus  = 81.08 metres

3. 11 Year Sunspot Cycle • 11 year sunspot cycle; more sunspots leads to higher ionisation. • More sunspots gives better HF propagation as a result of increased ionisation. • The higher the ionisation, the better the reflection, and the higher the HF frequencies can be reflected.

4. Ionosphere D,E and F1/F2 • The basic structure of the ionosphere: D, E and F1 & F2 layers • Ionisation is caused mainly by ultraviolet rays from the sun • Ionisation makes the gasses partially conduct which causes the refraction (bending back) of HF radio waves.

5. The F layer is the main layer reflecting HF signals. The level of ionisation changes with the time of day and the time of the year. Ionisation greatest during the day/early afternoon. Because its colder the F layer remains more ionised in the winter. F1/F2 merge layers and remain ionised overnight, but is much weaker. Ionosphere Day and Night Night-time Daytime

6. F1/F2, E, D layers are ionised during the sunlight hours. HF propagation over great distance is possible. On LF bands range is relatively similar. F1 /F2 (no E, D) layers merge and remain ionised overnight but are much weaker. HF propagation over great distances reduces. On LF bands range is better. Propagation Day and Night < Daytime Night-time >

7. Ground wave hugs earth but quickly gets weaker, range only a few km at best. Skip distance is measured from transmitter to first point of skywave return. Skip zone is annulus between ground wave coverage and return of sky wave. No signal is received in the skip zone. Ground Wave, Skip Distance & Skip Zone

8. VHF signals can occasionally be increased by reflection of highly ionised areas in the E layer (Sporadic E). Small areas of the E layer can become very highly ionised so that VHF signals can be reflected. This allows the signals to be received at distances up to 2000km. Sporadic E is the most common mode of VHF dx communication. Propagation at VHF

9. VHF & UHF Signals • VHF and UHF normally go through ionosphere and can reach satellites or the moon. • Signals from satellites (satellite TV) travel down through the ionosphere. • Sporadic E is as its name suggests - ‘sporadic’ and unusual. • At these frequencies propagation is within the troposphere, which is situated below the ionosphere.

10. High atmospheric pressure can cause tropospheric ducting to extend the range of VHF and UHF signals to beyond the normal line of sight. The basis of ducting is that the wave is trapped vertically either between two atmospheric layers or between the ground and a layer. This occurs at relatively low heights. Tropospheric ducting is the most common mode of dx communication for UHF signals. Tropospheric Ducting

11. Attenuated UHF and Higher Frequency Signals • Snow and ice and heavy rain can attenuate UHF and higher frequency signals. • Even professional microwave links can drop out if the rain is very heavy. • Heavy rain can affect satellite TV downlinks.

12. Aurora On SSB, no tonal content to voice, whisper-like sound Other Mode of PropagationNot for the Exam