Why 40 Metres at Night and 20 at Noon

Ask why 40 metres is dead at lunchtime and alive at midnight and you will get answers that amount to "that is just how it is". It is not folklore. There is a specific layer of the atmosphere doing a specific thing, appearing and disappearing on a schedule set by the sun, and once you know which layer is doing what, choosing a band stops being superstition and becomes a decision you can defend.

Four layers, two of which matter most

Ultraviolet and X-ray from the sun strip electrons off the thin upper atmosphere, producing regions of free electrons at different altitudes. Those regions bend radio waves — and whether they bend or simply absorb depends on the electron density, the air density around it, and the frequency.

The D layer sits roughly 60 to 90 km up and exists only in daylight: the air there is dense enough that free electrons recombine within minutes of sunset, so the layer collapses at dusk and rebuilds at dawn. Crucially, at high frequencies it does not refract. It absorbs — an electron set moving by the passing wave collides with a neutral molecule and the energy becomes heat. That absorption falls off roughly as the square of frequency, which is the single most useful fact here: halve the frequency and you roughly quadruple the daytime absorption.

The E layer is around 90 to 150 km, also daylight-driven, and it does refract, supporting short and medium daytime paths. It also produces sporadic E: intense, patchy, short-lived clouds of ionisation, most common in late spring and summer, throwing single hops of several hundred to a couple of thousand kilometres on the higher bands whatever the sun is doing overall.

F1 sits around 150 to 220 km and exists only by day. F2, the one that matters, runs from roughly 250 to 400 km and higher. Up there the air is so thin that electrons take hours to find anything to recombine with, so F2 survives the night at reduced density. That single fact — one layer that persists in darkness — is why night-time long-distance work exists at all.

Why 40 metres is two different bands

Put those together. In daylight the D layer absorbs, and it absorbs low frequencies hardest. A 7 MHz signal launched at a low angle passes through it twice on every hop and comes out the far side with most of its energy gone. What survives is the high-angle stuff that goes almost straight up, hits the F layer and lands within a few hundred kilometres. So daytime 40 metres is a regional band.

At sunset the D layer collapses within about half an hour. The absorption disappears, the F layer is still there, and the same transmitter into the same antenna is suddenly working across an ocean. Nothing about your station changed. A layer 70 km up stopped existing.

Run the logic upward and the higher bands invert it. At 14 MHz the D layer absorption is roughly a quarter of what it is at 7 MHz, so daylight is no obstacle — but 14 MHz needs enough F-layer ionisation to be bent back at all, and after midnight the F region has thinned and the band closes. Hence the shorthand: higher frequencies by day, lower by night.

MUF and the ceiling

A signal sent straight up at an ionised layer either comes back or punches through, and the dividing line is the critical frequency — for the F2 layer, written foF2 and measured continuously by ionosondes around the world. Send anything above foF2 vertically and it is gone into space.

Send it at a shallow angle and the ceiling rises: the flatter the angle, the higher the frequency the layer can still turn around, roughly as the secant of the angle of incidence. For a long path the maximum usable frequency can be around three times the critical frequency — which is why a long-haul contact works on a band where a station two hundred kilometres away is inaudible. The dead ground between them is the skip zone, and on 20 metres at midday it can be a thousand kilometres wide.

There is a floor as well, the lowest usable frequency, set by absorption and noise. Going lower past that point just feeds the D layer. The best band for a given path at a given hour is usually the highest one open, because that is the one paying the least absorption tax. Working just under the MUF is the whole game.

Grey line

The terminator — the moving line of sunrise and sunset — is the best thing that happens to the low bands. Along it the D layer has either collapsed or not yet formed, while the F layer is still ionised from the day or already ionising for it. For twenty or forty minutes either side, a path running along the terminator gets F-layer refraction with no D-layer absorption, and 160, 80 and 40 metres do things they cannot do at any other hour. It is short and it moves, so you plan for it rather than stumbling into it.

The sun over eleven years and over three hours

The solar cycle runs about eleven years. More sunspots means more ultraviolet, more F2 ionisation, higher critical frequencies and a higher MUF. The number to watch is the 10.7 cm solar flux index, reported daily. When it is high, 15, 12 and 10 metres come alive for hours and a small station can work the world. Near solar minimum those bands are quiet outside sporadic E season and 20 metres becomes the long-haul band by default.

The geomagnetic indices work over hours instead, and people routinely confuse the two timescales. The K index runs 0 to 9, reported every three hours; the A index is its daily cousin. High values mean a disturbed magnetic field, usually after a coronal mass ejection, and the consequence is extra absorption at high latitudes, auroral flutter on polar paths, and on a bad day a blackout on the sunlit side. A K of 0 to 2 is quiet and good; 5 or more means the polar paths are in trouble.

What to expect, band by band

160 m

From 1.8 MHz. Winter nights only — summer atmospheric noise buries it. Big antennas, high local noise, and some of the most satisfying long-distance work in the hobby when the grey line cooperates.

80 m

From 3.5 MHz. Regional by day, continental and beyond after dark. Noisy in thunderstorm season. A dipole for it is 40 m of wire, which is the honest reason most people skip it.

40 m

From 7 MHz. The all-rounder and the one to learn on. Regional in daylight, long-haul after sunset, something open at almost any hour of any year. Crowded, and in some regions it shares space with broadcasters after dark.

30 m

From 10.1 MHz. Narrow, CW and data only, no contests permitted, and consequently the quietest band on the list. Sits between the day and night behaviours and often works when its neighbours do not.

20 m

From 14 MHz. The long-distance workhorse: open somewhere most days, often into the evening, and at solar minimum the most dependable band you have. If you only ever put up one antenna, put it up for this.

17 m

From 18.068 MHz. A quieter 20 with a shorter opening. Needs moderate solar flux, carries no contest traffic.

15 m

From 21 MHz. Daytime long-haul, excellent when the flux is up, closes not long after dark. At solar maximum it is often better than 20.

12 m and 10 m

From 24.89 and 28 MHz. Need real solar activity for F2, and then they are spectacular — a few watts and a wire crossing oceans. Outside that, sporadic E in late spring and summer gives short, sharp openings.

Band edges vary between the three ITU regions, so check what your own licence allows before transmitting near the bottom of one.

Three rules that cover most of it

Higher frequency in daylight, lower after dark. The best band open is usually the highest band open. And if a band sounds empty, listen for another thirty seconds before concluding it is closed — an open band with nobody on it and a closed band are indistinguishable until somebody transmits, which is why people underestimate 30 and 17 metres for years at a time.

None of it helps if the antenna is putting its energy at the wrong angle, which is the other half of the problem and the subject of the wire antenna dispatch. Height sets your radiation angle, and radiation angle decides whether you are talking to the next province or the next continent. On low power band choice matters more still — see five watts and a wire. And how any of this was discovered is a good story, told on the history page.