From 46d3eb6f5f07b82de0c1838dcbd575275074f011 Mon Sep 17 00:00:00 2001 From: thomasabishop Date: Fri, 6 Feb 2026 18:36:01 +0000 Subject: [PATCH] radio propagation --- zk/Radio_wave_propagation.md | 83 ++++++++++++++++++++++++++++++++++++ 1 file changed, 83 insertions(+) create mode 100644 zk/Radio_wave_propagation.md diff --git a/zk/Radio_wave_propagation.md b/zk/Radio_wave_propagation.md new file mode 100644 index 0000000..932d296 --- /dev/null +++ b/zk/Radio_wave_propagation.md @@ -0,0 +1,83 @@ +--- +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**.