The ionosphere in plain language

Above about 60 km, solar radiation strips electrons from atoms in the upper atmosphere. That ionised region is what turns HF from a line-of-sight service into a worldwide one.

The layers

From lowest to highest:

• D layer — around 60–90 km. Forms in daylight, disappears at night. It absorbs lower HF rather than refracting it. This is the layer standing between you and a daytime contact on 80 metres.
• E layer — around 90–150 km. Refracts, supports medium-distance paths. Also produces sporadic-E, the intense patches that open up 10 metres and even 6 metres unexpectedly.
• F1 and F2 layers — 150–500 km. F1 and F2 merge at night into a single F layer. F2 is the workhorse of long-distance HF, because it is highest and therefore gives the longest single hop.

Why the bands behave the way they do

Once you understand the D layer, most of HF behaviour makes sense.

Daytime: the D layer is present and absorbs the lower frequencies. 80 and 40 metres are short-range. Higher bands — 20, 15, 10 — pass through the D layer and refract off F2, so they carry long distances.

Night: the D layer vanishes. 80 and 40 metres open up dramatically for long-distance work. Meanwhile the higher bands lose their ionisation and often close entirely.

That single mechanism explains the daily rhythm of the HF bands.

MUF and LUF

• MUF — Maximum Usable Frequency. The highest frequency that will still bend back to Earth on a given path at a given time. Go above it and your signal goes straight through into space.
• LUF — Lowest Usable Frequency. Below this, absorption is so heavy that nothing gets through.

The workable window sits between them, and it moves through the day. Experienced operators pick a band by asking "where is the MUF right now?" rather than by habit.

The skip zone

A sky wave leaves your antenna at a shallow angle, travels up, refracts, and comes back down hundreds or thousands of kilometres away. Your ground wave covers only the first few tens of kilometres.

In between is the skip zone — an area where neither reaches. This is why you can have a clear contact with a station 1,500 km away while being completely inaudible to someone 300 km away.

Fading

Signals that rise and fall over seconds are fading, reported with the Q code QSB. It happens when conditions in the ionosphere shift, or when two signal paths of slightly different lengths arrive out of phase and partially cancel.

The sun controls everything

Solar activity rises and falls on a roughly 11-year cycle. More solar activity means more ionisation, a higher MUF, and better high-band conditions. Solar flares, on the other hand, can cause sudden ionospheric disturbances that black out HF entirely for hours.

For the exam, know the layers, know the D-layer absorption story, and know what MUF means. For actual operating, that same knowledge tells you which band to try.