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