Three ways a signal travels

• Ground wave: low frequencies hug the earth’s surface. It’s how AM broadcast stations cover a region by day. On the ham bands it matters mostly on 160 m and 80 m, over short distances.
• Line of sight: VHF and UHF mostly travel in straight lines, a little past the visual horizon. Height is everything, which is why repeaters sit on hilltops.
• Sky wave: on HF, signals bend back down from the ionosphere and land hundreds or thousands of kilometres away. That’s what makes worldwide contacts possible with a wire in a tree.

The ionosphere in plain English

Sunlight strips electrons off gas high in the atmosphere, creating layers that can bend radio waves:

• D layer (60–90 km): only exists in daylight. It doesn’t bend signals; it absorbs them, especially on the low bands. It’s why 80 m and 160 m are quiet at noon.
• E layer (about 100 km): a daytime layer that occasionally forms dense patches, called sporadic-E, that open 10 m and 6 m in summer.
• F layer (200–400 km): the main reflector for long-distance HF. It weakens at night but doesn’t disappear.

Why conditions change

• Day and night: the higher bands need daylight to stay open; the low bands need darkness to escape D-layer absorption.
• Seasons: winter days often favour the higher bands; summer brings more static.
• The 11-year solar cycle: more sunspots mean a denser ionosphere and better high bands.
• Space weather: flares and geomagnetic storms can disrupt everything for hours or days.

Putting it to use

Every page in this section builds on these ideas. PropMap turns them into a live forecast for your location, and the other propagation pages explain each piece in more depth.

Where to go next

• MUF & Usable Frequency
• NVIS vs. Skip Zone
• Solar & Space Weather for Radio
• PropMap

Last updated October 2026.