radio propagation

zk/Radio_wave_propagation.md

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+---
+tags: [radio, ham-study]
+---
+
+## Refraction, diffraction, reflection
+
+Radio waves normally travel in straight lines but they can also be
+**refracted**, **diffracted** and **reflected**.
+
+![](../img/reflection-diffraction-refraction.png)
+
+- **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.
+
+- **Diffraction** is when the wave bends _around_ obstacles or spreads outwards
+  after passing through a narrow opening.
+
+- **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**.