In the early days of radio, most communications was conducted at low frequencies of just a few megahertz.   Transmission distance was achieved through Ground Wave propagation that tended to hug the earth’s surface using brute force of high power transmitters.   When radio experimenters began to operate with increasingly high frequencies the effects of reflections between ionospheric layers the sea allowed for very long distance communications with only a few watts of power.  This introduced the concept of Sky Wave propagation.  

Ground waves travel over the Earth’s surface. The distance over which they travel is determined by their radio frequency.

Sky waves are radiated upwards at all angles from the antenna, until they reach the Ionosphere, where they can reflect back to the ground well beyond the curvature of the earth.

This also led to communication dead zones beyond the reach of ground waves, but less distance than where the Sky Waves first return to Earth.  This is called the Skip Zone.

Ionospheric variations

The reflective Ionosphere is not a static layer.  Its density varies with the influence of the sun through the rotation of the earth.  Layers expand and contract at different times of the day providing very chaotic paths between distant locations.  Changes in ionospheric density determine how well a radio signal will be reflected, or in some cases Refracted or ‘bent’ allowing a lot of radio energy to pass through the ionosphere and into space.

Solar Weather

The charged particles from the sun which energises the ionosphere will also vary in intensity from short term solar flares and sunspots and longer term solar sunspot cycles of eleven year intervals.

A more comprehensive description of ionospheric propagation is detailed in the Foundation level Study Guide (FLSG)

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The Foundation Level Study Guide

Information about basic Ionospheric Propagation relevant to Foundation Level Qualifications  can be found in this book from Chapter 6, on Pages 49 to 53

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ACMA Syllabus Extract

According to the ACMA Foundation Syllabus, the required knowledge on this topic is:

6.1   Propagation Basics

Recall that radio waves travel in straight lines, unless diffracted, reflected or refracted.

6.2 Effect of Distance on Radio Waves

Recall that radio waves get weaker with distance as they propagate from the antenna.

6.6   The Ionosphere

Recall, using supplied reference material, that the ionosphere comprises layers of ionised gas at varying heights above ground.

6.7  Factors affecting HF propagation

Recall that ionospheric propagation is dependent on time of day, season, frequency and solar activity.

6.8   Ionospheric refraction Recall that long-distance HF communication relies on propagation by ionospheric refraction

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