The Ionosphere — Your Mirror in the Sky

Why HF Radio Can Travel Around the World

HF radio signals (3–30 MHz) can travel thousands of kilometres — not by line of sight, but by bouncing off the ionosphere, a layer of electrically charged gas in the upper atmosphere (60–500 km up).

The Sun's ultraviolet radiation strips electrons from gas molecules up there, creating layers of charged particles that can bend (refract) radio waves back to Earth.

The Layers — What Each One Does

D Layer (60–90 km) — The Signal Absorber

The D layer is the enemy of low-frequency HF signals during the day:

• Only exists during daylight — disappears quickly after sunset
• Absorbs signals, especially below 10 MHz — this is why 80m and 160m are much better at night
• It doesn't usefully reflect signals — it just soaks them up

E Layer (90–150 km) — The Middle Player

• Can reflect signals up to about 10 MHz during the day
• Mostly disappears at night
• Home of the exciting Sporadic E phenomenon (more on that later)

F Layer (150–500 km) — The DX Layer

• The most important layer for long-distance HF communication
• Splits into F1 and F2 during the day; merges into one F layer at night
• The F2 layer is the highest and has the most ionisation — capable of the longest skip distances
• Unlike the D layer, the F layer persists at night (though weaker)