---
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

![](../img/ionosphere-scan-cropped.png)

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**.