Atmospheric Layers
The Earths atmosphere consists of a number of different layers that can effect long distance radio communications.
The Ionosphere has the biggest impact on HF signals:
- The height of the ionospheric layers is approximate
- The sun ionises or charges the air particles
- When radio waves hit the ionosphere they can be bent back to earth
- The bending is dependent on the layer, its density and the frequency of operation
Ionospheric refraction
- A signal travelling from the transmitter to the receiver via two paths can cause multipath fading
- Selective fading occurs when the frequency components that make up the signal are refracted by different amounts
Effect of obstacles and structures on VHF and UHF signals
- VHF, UHF and frequencies above UHF are dependent on an almost clear line of sight path from transmitter to receiver.
- VHF and UHF are generally obstructed by hills and other large structures.
- VHF and UHF signals can be bent or diffracted over some obstacles.
- Temperature changes in the troposphere results in ducts that can cause VHF and UHF signals to be propagated over long distances.
VHF / UHF Radio signals can be impacted by knife edge diffraction as shown below
Long distance communications on VHF / UHF can be achieved by tropospheric ducting. This is where a warm layer of air is trapped between two (2) cold layers as shown below
Factors effecting HF propagation
The ionosphere refracts the radio wave and the amount of refraction is dependent on:
- Sun spot activity
- Time of day
- The season
The ionosphere is reliant on ultraviolet radiation from the sun and Sunspots occur in an 11 year cycle.
