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Refraction, diffraction, reflection
Radio waves normally travel in straight lines but they can also be refracted, diffracted and reflected.
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Refraction is when a wave bends as it passes through one medium to another. This occurs because waves travel at different speeds through different materials.
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Diffraction is when the wave bends around obstacles or spreads outwards after passing through a narrow opening.
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Reflection is when the wave bounces right off a surface without penetrating or interacting with it (like a ball bouncing off a wall). The angle of incidence (entry) equals the angle of reflection (exit)
Ionosphere and troposphere
Both the ionosphere and troposphere are substrata of the atmosphere.
The troposphere is the lowest and most dense layer of the Earth's atmosphere. It extends from ground level to approximately 13km up.
It contains the 80% of the atmosphere's mass and 99% of its water vapour. It is where nearly all weather events occur.
The ionosphere is much higher than the troposphere however it is defined by its behaviour (ionisation) rather than by its altitude (in contrast to the other regions of the atmosphere). That said, it is detectable in the region between 70-400km above the surface of the Earth, cutting across the mesosphere, thermosphere, and exosphere.
It comprises layers of ionised gases - gases where atoms or molecules have lost of gained electrons giving them an electrical charge rather than overall balanced neutrality. As a result you end up with free-floating charged particles. The ionisation is caused by the ultraviolet rays of the sun, splitting off particles.
Relation to radio
The ionosphere is used to propagate HF-band frequencies. When the waves hit the ionosphere, the charged particles interact with the wave and can bend it back towards the Earth. This facilitates long-distance radio communication.
More specifically, the wave bends as it travels through the ionosphere's different density layers, eventually curving back towards Earth.
We say that a band is "open" when it supports "skywave" propagation via the ionosphere.
HF propagation via the ionosphere is affected by the following factors:
- the time of day and season (more sun means more ionisation)
- solar activity (sun spots and solar flares)
- the frequency of the transmission
- the latitude of the transmission
Line-of-sight propagation
In contrast to HF, VHF/UHF usually occurs at the level of the troposphere. As such it is reliant on line-of-sight. This bandwidth does not travel as far as HF because:
as the frequency of a wave increases, its range decreases
As a result, the range of transmission for VHF/UHF is dependent on:
- antenna height
- antenna gain
- a clear line-of-sight
- the power of the transmitter
Other factors affecting propagation are hill and signals becoming weaker as they penetrate buildings.
Snow, hailstones and heavy rain reduce the strength of UHF signals. Reduction of signal strength is called attenuation.