One of the first things that confused me when I got into amateur radio was why the bands behave so differently from day to day. Some mornings 20m is wall-to-wall Europe. Other times it’s dead. 10m, supposedly a great band, seemed useless for months. Then one afternoon it opened up and I was hearing stations in South America on a dipole in a loft.

This post is my attempt to explain what’s happening.

The ionosphere

The Earth’s atmosphere has a layer called the ionosphere, sitting roughly 60–1000km up. It’s ionised (free electrons) by solar radiation. These free electrons can bend, reflect, and absorb radio waves depending on their frequency.

Think of it like a mirror that changes its reflectivity depending on what the sun is doing. When conditions are right, HF signals (roughly 3–30 MHz) get bent back down to Earth, landing hundreds or thousands of kilometres away. This is called skywave propagation.

The layers

The ionosphere has several named layers:

  • D layer (lowest) — forms during daylight. Absorbs lower HF frequencies (160m, 80m) heavily — this is why these bands are quiet during the day.
  • E layer — reflects some HF. Responsible for sporadic-E propagation, which causes sudden, unpredictable openings on 6m and 10m.
  • F layer (highest) — the main long-distance layer. Splits into F1 and F2 during daylight. F2 is what makes 20m work globally.

The solar cycle

Solar activity drives ionisation. More UV and X-ray radiation = higher Maximum Usable Frequency (MUF). The sun runs on an approximately 11-year cycle. We’re currently near Solar Cycle 25’s peak, which is good news — higher bands (10m, 12m, 15m) are more usable than they were a few years ago.

The Solar Flux Index (SFI) is the number to watch. Broadly: SFI above 150 means good conditions on higher HF bands. The K-index measures geomagnetic disturbance — high K-index (3+) disrupts propagation, especially at higher latitudes like Scotland.

The greyline

One effect worth knowing about: the greyline (also called the terminator) is the boundary between the sunlit and dark sides of the Earth. For a few minutes either side of sunrise and sunset, the D layer (which normally absorbs lower HF signals) is thin or absent on both sides of the path simultaneously. Lower frequencies that would normally be swallowed get a brief window to propagate much further than usual.

For stations at high latitudes this is particularly interesting. The greyline passes at a shallower angle, which means a longer window and more path options. 40m and 80m contacts that would be impossible at midday can happen easily at dawn. If you’re going to pick one time to monitor the bands, greyline is it.

Why this matters practically

If you’re chasing contacts on HF, understanding propagation means:

  1. Knowing which band to try at a given time of day — 40m works better at night, 20m is reliable during the day, 10m/12m are worth checking when the solar flux is high.
  2. Not giving up on a band because it seemed dead — conditions change in minutes.

Propagation is one of the things that makes HF genuinely interesting to me. It’s a complex, dynamic system with real-time data feeds, patterns to learn, and unpredictable behaviour. That’s not a problem to solve — it’s the whole point.

73 de MM7IUY