Handbook, chapter 5. The bands, and what is odd about them
A newcomer switches the radio on at two in the afternoon, tunes across 80 metres — silence, nothing but a neighbour ten kilometres away. In the evening the same rig, the same antenna, the same frequency give Italy, Germany, Scandinavia. Nothing has changed except the time of day.
This is the commonest question on the air and the worst explained. The answer is forty kilometres of thin air overhead that behaves quite differently by day and by night.
Turn the time of day and watch what happens
MUF ≈ 30,3 MHz (F2 critical frequency — 9,0 MHz, oblique-path factor — 3,37). On 20 m — open, with margin under the MUF.
- 160 mdead by day: the D layer eats the signal
- 80 mdead by day: the D layer eats the signal
- 40 mregional: a few hundred kilometres, not DX
- 20 mopen, with margin under the MUF
- 15 mnear the MUF: the longest reach right now
- 10 mnear the MUF: the longest reach right now
Set the clock to 20:00 and run down the bands: 80 metres has come alive, 20 has died. Go back to 13:00 and it is the other way round. Then drag the solar flux up to 250, and 10 metres goes from empty to the best band on the list.
Three controls, each doing its own thing. Here is what each of them does.
The ionosphere: not a mirror but a lens
The Sun knocks electrons out of air molecules at heights from sixty to four hundred kilometres. Where there are many electrons, a radio wave travels more slowly and gradually bends — exactly as a ray of light does in a glass lens.
The word «reflection» is used out of habit, and it gets in the way of understanding: a mirror would reflect everything alike, whereas the ionosphere bends long waves strongly, short ones weakly, and the shortest not at all — those carry on into space. The whole difference between the bands grows out of exactly this.
There are four layers, and they behave differently:
| Layer | Height | What it does |
|---|---|---|
| D | 60–90 km | returns nothing, only absorbs. Exists only by day |
| E | 100–120 km | returns low frequencies over short distances |
| F1 | 150–220 km | exists only by day, merges with F2 at night |
| F2 | 250–400 km | the main working layer: this is the one that gives you distance |
The D layer is the entire answer to the question at the top. The air there is still dense, an electron collides with molecules millions of times a second, and the wave's energy simply turns into heat. Absorption is roughly inversely proportional to the square of the frequency: at 1.8 MHz it is devastating, at 14 MHz barely noticeable.
And as soon as the Sun sets, the D layer is gone within an hour: dense air puts the electrons back quickly. The F2 layer, in thin air, holds until morning. So night is when the thing that was in the way disappears and the thing that was helping stays.
MUF: a ceiling that moves up and down
The maximum usable frequency (MUF) is the highest frequency the ionosphere will still bring back to the ground on a given path. Above it the wave punches straight through the layer.
The MUF is derived from the critical frequency foF2 — the one that comes back when you look straight up. For an oblique path of a few thousand kilometres it is multiplied by roughly three: what would not return from the zenith returns perfectly well at a shallow angle, because the wave travels a longer way through the layer.
From below the range is limited by absorption in the D layer, and that limit is called the lowest usable frequency. Together they form a window: below it, absorbed; above it, not returned.
By day the window is narrow and sits high. At night the lower limit drops to the very bottom and the upper one follows it down — and the window moves onto the low bands. That is the shift that makes 80 metres a different band in the evening.
The solar cycle: why all this depends on the year as well
The number of electrons depends on ultraviolet light, and the Sun supplies it unevenly, on a cycle of roughly eleven years.
Two numbers measure it. SFI is the Sun's radio flux at 10.7 cm, from about 65 at the minimum to over 250 at the maximum. The sunspot number is an older figure for the same thing. Both are published daily, and both mean one thing: how many electrons there are in the F2 layer today.
The difference between minimum and maximum is not «a bit better». At the minimum, 10 metres is silent for years; at the maximum people work the far side of the planet on it with five watts. Move the SFI slider on the diagram and watch the list of bands rebuild itself completely.
The skip zone: why 500 km is unreachable and 3000 km is not
This is a newcomer's second question, and it is even more confusing than the first.
Two different signals reach the other station. The ground wave creeps along the surface and fades quickly — from two hundred kilometres on 160 metres to a couple of dozen on 10. The sky wave goes up, bends in the F2 layer and comes back down — but only one or two thousand kilometres away.
Between them lies a ring where there is neither. A station 500 km away does not get through, while one 3000 km away sounds as if it were in the next room.
The skip zone is not fixed. It grows with frequency: the shorter the wave, the shallower the ray has to travel in order to come back, and the further away the first hop lands. And it disappears altogether when the frequency falls below the critical one: the wave then returns even from the zenith, and you have contacts at every distance at once. People use this deliberately — a low antenna on 80 or 40 metres radiates upwards and covers the whole region with no gaps.
The model in the diagram is deliberately crude: a single reflection height, a flat Earth, equinox conditions, one Sun over the whole path. It gets the direction right — raise the frequency and the hop lengthens; the Sun sets and the absorption goes — and that is what it is here for.
A real path prediction is computed differently (VOACAP and the like), with real geometry, an ionospheric model and statistics. And the real state of the air is not computed at all — it is observed, which is covered below.
QSB: why the volume drifts on its own
The signal comes in steadily, then within ten seconds sinks into the noise, then comes back. Nobody touched anything.
Not one ray but several reach the receiver: one that bent once, another twice, a third that left at a slightly different angle. The path lengths differ by hundreds of metres, and on 20 metres the wavelength is twenty metres. The rays add in phase, then out of phase, and the level swings by tens of decibels.
On top of that the plane of polarisation rotates in the Earth's magnetic field: the wave arrives vertically polarised at one moment and horizontally at the next, and the antenna only takes what matches it.
There is one practical conclusion and it is not obvious: change nothing during a fade. A newcomer starts turning the antenna and adding power at exactly the moment when the thing to do is wait twenty seconds.
The grey line: two hours a day when you hear furthest
This is something no propagation model has and everyone who has worked the low bands knows.
Along the line that separates day from night, for a few tens of minutes a situation arises that is impossible either by day or by night. The D layer has already dissolved — the absorption is gone. The E and F layers are still ionised — the bending remains. A kind of corridor forms along the terminator, in which a signal travels thousands of kilometres on a power that at any other time would not reach the next region.
Three things worth knowing in order to use it:
- there are only two directions. Your corridor runs to wherever it is also twilight: west, following the Sun, in the evening, and east, towards it, in the morning. Anything outside the corridor is inaudible at that time;
- the window is short. From half an hour to an hour, and sometimes ten minutes. Miss it and you wait until the next day;
- it works best on 160, 80 and 40 metres. That is where D-layer absorption was the main obstacle, and that is where its disappearance pays off most.
In practice this means your longest contacts on the low bands will happen not at midnight but an hour before sunrise and right after sunset. It is an inconvenient time, which is exactly why there is less competition there.
The character of the bands, briefly
| Band | By day | At night | Skip zone |
|---|---|---|---|
| 160 m · 1.81–2.0 | local only, up to 50 km | thousands of kilometres in winter at the cycle minimum | — |
| 80 m · 3.5–3.8 | 150–300 km | Europe, and America towards morning | small |
| 40 m · 7.0–7.2 | 500–800 km in summer, up to 1500 in winter | any distance | tens of km by day to hundreds at night |
| 20 m · 14.0–14.35 | the whole world | closes in winter at the cycle minimum | 500–1000 km by day, 1500–2000 at night |
| 15 m · 21.0–21.45 | half the day when the Sun is active | sometimes, at the maximum | large |
| 10 m · 28.0–29.7 | only at high SFI | sometimes, at the maximum | 2000–2500 km |
Three things are visible in this table and in no other.
The skip zone grows with frequency. On 40 metres by day there is practically none, which makes it the band for contacts within the country. On 10 metres it is two and a half thousand kilometres: you will never hear the next region there, but Brazil comes easily.
Forty metres is the all-rounder. The only band that works around the clock: your region by day, the world at night. If you are going to have one antenna, build it for this band.
One hundred and sixty metres is not for a beginner. Not because it is difficult, but because of the size: a quarter-wave vertical there is forty metres tall. It is a band for people who already have land and experience.
⚠️ The band edges are given for IARU Region 1, which Ukraine belongs to. Exactly how much of that spectrum your licence class is allowed, and at what power, is set out in the band plan and in the Regulations; these are different things and are best not confused.
A case of its own is sporadic E: dense patches of ionisation in the E layer that appear mostly between May and August and pay no attention to the solar cycle. They suddenly open 10 and 6 metres for one or two thousand kilometres for a few hours. They are why 10 metres sometimes comes alive in the deadest year.
How to find out what is open without guessing
The most useful advice in this chapter, and it is simple: do not guess. The state of the air is visible free of charge and in real time.
- Listen before you call. Five minutes on a band tell you more than any forecast.
- Beacons. The international beacon network transmits in turn on five bands from one and the same site. Hear a beacon and you know the path is open.
- Networks of automatic receivers. CW skimmers and digital-mode reporters show on a map who is being heard from where, right now. That is a measurement, not a model.
- WSPR. If you want to know whether your antenna reaches your target, two watts are enough to find out within an hour.
The wavelength calculator converts frequency into metres and back — you will need it every time a band has to become an antenna dimension.
Next — chapter 6, decibels: why a hundred watts against five is not «twenty times better».