1. Chelmsford Amateur Radio Society Intermediate Course (6) Propagation

2. 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 v l f
Example: What's l
f = 3.7MHz
 = 300,000,000
3,700,000 Or
300
3.7
Thus  = metres

3. The Sun & Sunspot Cycle
Solar ultraviolet radiation and extra emissions from sunspots leads to ionisation in the earth’s ionosphere
Local ultraviolet ionisation varies with time of day and time of year
Sunspots vary over an 11 year cycle
Higher sunspots numbers at the peak of the cycle add to ionisation levels and enhance HF propagation
Higher ionisation influences the level and maximum usable frequencies at HF – but has minimal affect at VHF/UHF which passed through the ionosphere

4. Ionosphere D,E and F1/F2
The basic structure of the ionosphere has 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. Ionosphere Day and Night
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
Night-time
Daytime
Ionisation is greatest during the day/early afternoon
Because it is colder, the F layer remains more ionised in the winter.
At night the F1/F2 layers merge and remain ionised, but the F-region is overall rather weaker

6. Propagation Day and Night
< Daytime
Night-time >
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

7. Ground Wave, Skip Distance & Skip Zone
Ground wave hugs earth but quickly gets weaker, range only a few km at best. Long distance HF communications relies on the sky wave
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.

8. Propagation at VHF
VHF signals can occasionally be increased by reflection from 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
However as its name suggests - it is ‘sporadic’ and unusual

9. VHF & UHF Signals
VHF and UHF normally go through ionosphere and can reach satellites or the moon
Signals from satellites travel down through the ionosphere. For example reception of Satellite TV is at microwave frequencies
Most VHF-UHF frequencies propagate within the troposphere, which is situated well below the ionosphere

10. Tropospheric Ducting
High atmospheric pressure can cause tropospheric ducting to extend the range of VHF and UHF signals to beyond the normal line of sight
Ducting occurs when the wave is trapped vertically either between two atmospheric layers or between the ground and a layer
The duct guides the wave (similar to a mirage) - and occurs at relatively low heights
Tropospheric ducting is the most common mode of DX communication for UHF signals

11. Attenuated UHF and Higher Frequency Signals
Snow, 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. Other Modes of Propagation Not for the Exam
Aurora
On SSB, no tonal content to voice, whisper-like sound