G1 · Published 1 October 2026 · By The Ham Desk

Skywave Propagation: Layers, Skips and the Sunspot Cycle

General class operators get the HF bands, and the HF bands change by the hour. This unit explains why 20 metres is open at noon and closed at midnight, what the skip zone is, and which numbers on a propagation page are worth reading.

Band 20 m
Study time 17
Level Core
questions 10

Waveform and antenna diagram for HF Propagation and Space Weather — HF Propagation and Space Weather — waveform overlay and radiation pattern

Pre-flight briefing

The layers and what each one does to your signal

The D layer is the lowest, forming at about 60 to 90 kilometres during daylight and disappearing at night. It does not refract HF usefully; it absorbs it, and the absorption is worst at low frequencies. That is the single fact behind most HF operating strategy: 160, 80 and 40 metres are noisy and short-range by day because the D layer is eating the signal, and long-range and quiet at night when the layer is gone.

The E layer forms around 100 to 120 kilometres and supports short-distance HF contacts by day, with occasional sporadic E patches that open 6 metres unpredictably. The F layer is the one that matters most. By day it splits into F1 and F2, and F2 at 200 to 400 kilometres is high enough that a signal refracted there travels a long distance before returning to earth. At night F1 and F2 merge into a single F layer that still supports long paths on the lower bands.

Skip, the skip zone, and why the band goes long at night

A skywave signal leaves the antenna, bends in the ionosphere and comes back down some distance away. The ground distance covered by that single hop depends on the layer height and the takeoff angle; low angles and high layers give the longest single hop. The area between the limit of ground-wave coverage and the first returning skywave is the skip zone, where nobody can hear you - which is why a 40 metre signal can be perfect at 300 kilometres and inaudible at 80.

The ionosphere changes character as the day ends. The D layer's absorption disappears, so lower frequencies suddenly work; at the same time the F layer's ionisation thins, so the maximum usable frequency falls. The practical consequence is that 20 metres often dies within an hour or two of sunset while 40 and 80 metres open up, and the reverse happens at sunrise - the high bands open first, then the low bands fold.

The numbers that predict all this

The solar flux index measures radio emission from the sun at 10.7 centimetres and correlates well with the ionisation available to HF. High flux - above roughly 150 - means the higher bands open; low flux means 10 and 15 metres are unreliable and the action is on 20 and below. The flux follows an eleven-year sunspot cycle, so the high bands are good for several years and poor for several years after that.

Geomagnetic activity is the other half of the picture, expressed as the K index in three-hour steps from 0 to 9 and the A index as a daily average. When K reaches 5 or more, an ionospheric storm is under way: the F layer becomes disturbed and patchy, high-latitude paths close, and signals that do exist fade and distort. A quiet day with a rising flux is what HF operators wait for.

Two smaller effects are worth knowing. The grey line is the twilight zone along the terminator between day and night, where the D layer is already dark while the F layer is still illuminated - it is the classic time to work long paths on 40 and 80 metres. And near-vertical incidence skywave, NVIS, uses a low horizontal antenna on 40 or 80 metres to fire energy almost straight up and cover the few hundred kilometres that ordinary skip misses.

Terms you must be able to define
Term	What the examiner wants to hear
D layer	The lowest ionised layer; absorbs HF by day, worst at low frequencies, gone at night.
F2 layer	The highest and most important layer for long-distance HF, present by day and merged at night.
Skip zone	The band of silence between the end of ground-wave range and the first returning skywave.
Solar flux index	A 10.7 cm solar radio emission measurement that tracks HF ionisation.
Grey line	The day/night terminator, where low-band DX paths are enhanced.

Question deck 10 questions

G3A01 Recall

Which ionospheric layer absorbs HF signals most strongly during the day?

G3A02 Core

Which layer is chiefly responsible for long-distance HF contacts?

G3A03 Core

What is the skip zone?

G3A04 Core

Why do the lower HF bands work better at night?

G3A05 Core

Why does the 20 metre band often go long and then close shortly after sunset?

G3A06 Core

What does a rising solar flux index usually indicate for HF operators?

G3A07 Core

What is indicated by a geomagnetic K index of 5 or higher?

G3A08 Stretch

What is the grey line, and why is it useful on 40 and 80 metres?

G3A09 Stretch

What is NVIS propagation used for?

G3A10 Stretch

Why can the same 40 metre signal be strong at 400 kilometres and completely unreadable at 100 kilometres?

Reference sheet

The explanations for all ten questions sit on this sheet. Pick an answer on the left and the matching card lights up.

G3A01 Explanation 1

Key A — The D layer

The D layer is dense enough that electrons collide with neutral atoms before they can refract, so energy is lost as heat instead. Absorption is strongest at low frequencies, which is why the 160 and 80 metre bands are short-range by day and long-range at night.

Rule Fundamentals - D layer absorption

G3A02 Explanation 2

Key D — The F2 layer

Being the highest layer, F2 lets a signal travel furthest before it returns to earth, and it survives into the night after the lower layers have faded. Long-haul HF contacts are almost always F2 contacts.

Rule Fundamentals - F2 propagation

G3A03 Explanation 3

Key C — The area between the limit of ground-wave coverage and the point where the first skywave returns

Inside the skip zone neither ground wave nor skywave reaches, so a station there hears nothing at all. The width of the skip zone depends on the takeoff angle and the layer height, which is why a low dipole on 40 metres covers the range a high beam cannot.

Rule Fundamentals - skip zone

G3A04 Explanation 4

Key B — The D layer that absorbs them disappears after sunset

Absorption, not refraction, is what limits the low bands by day. Once the sun sets, the D layer recombines and the path clears, which is why 80 metres can be a local band at noon and a worldwide band at midnight.

Rule Fundamentals - day/night propagation

G3A05 Explanation 5

Key A — The F layer's ionisation thins as the sun sets, lowering the maximum usable frequency

With declining ionisation the MUF drops below 14 MHz, so 20 metres can no longer be refracted and signals pass through the layer into space. That is the moment when operators move down to 40 and 80 metres for the night.

Rule Fundamentals - MUF and the diurnal cycle

G3A06 Explanation 6

Key D — More ionisation, so better conditions on the higher HF bands

The 10.7 cm flux is the best single measure of the ionising radiation reaching the upper atmosphere, and it drives the F2 layer's density. High flux opens 15 and 10 metres; low flux confines good conditions to 20 metres and below.

Rule Fundamentals - solar flux

G3A07 Explanation 7

Key C — An ionospheric storm that degrades HF propagation, especially at high latitudes

K runs from 0 to 9 over three-hour intervals, with 5 as the usual storm threshold. During a storm the F layer becomes irregular: signals fade, paths through the auroral zone close, and at high latitudes propagation may vanish for days.

Rule Fundamentals - geomagnetic disturbance

G3A08 Explanation 8

Key B — The day/night terminator, where the D layer is dark while the F layer is still illuminated, enhancing low-band DX

Along the terminator one station is in darkness, so the absorbing D layer is gone, while the path still passes through a sunlit F layer that can refract the signal. The window is short - tens of minutes - but it produces contacts that are impossible at any other time of day.

Rule Fundamentals - grey line propagation

G3A09 Explanation 9

Key A — Regional coverage out to a few hundred kilometres, filling the skip zone of a low-angle antenna

NVIS deliberately radiates nearly straight up from a low horizontal antenna on 40 or 80 metres, so the returning signal lands within a few hundred kilometres. Emergencies and regional nets rely on it because it works regardless of the skip zone.

Rule 47 CFR §97.1 - emergency communications

G3A10 Explanation 10

Key A — The nearer station is inside the skip zone, beyond ground-wave range but before the first skywave returns

Ground wave on 40 metres reaches only a few tens of kilometres, and the first skywave hop may land several hundred kilometres away, leaving a gap in between. Stations inside that gap can sometimes be reached by raising the takeoff angle - which is exactly what NVIS does.

Rule Fundamentals - skip zone and takeoff angle

How to use this unit

Work the ten questions in one sitting with the reference sheet covered, then uncover it and read every explanation - including the ones you got right. The wording of the real exam is different, so learn the reasoning rather than the answer letters.