E1 · Published 10 September 2026 · By The Ham Desk

Ionospheric Layers, MUF and the Indices That Predict Them

Extra class propagation questions are about mechanism: which layer bends which frequency, why a band dies at night, and which number to look at before deciding whether the path is worth calling.

Band 15 m
Study time 18
Level Core
questions 10

Waveform and antenna diagram for Advanced Propagation — Advanced Propagation — waveform overlay and radiation pattern

Pre-flight briefing

How the layers form and what each one does

Solar ultraviolet and X-ray radiation strips electrons from atoms in the upper atmosphere, and the resulting free electrons refract radio waves back toward the earth. Ionisation density falls off with height above the F region and stops altogether in the D region, roughly 60 to 90 kilometres up, where the atmosphere is still dense enough for collisions to absorb energy rather than refract it. That absorption is why 160 and 80 metres are noisy daytime bands and excellent after dark.

The F region splits into F1 and F2 by day. F2, from about 200 to 400 kilometres, is the layer that makes worldwide HF possible: it is high, thin and long-lived, and at night it merges with F1 into a single F layer that still supports long paths at lower frequencies. The E region, around 100 to 120 kilometres, handles the higher HF bands by day and short skip by night.

Critical frequency, MUF and LUF

The critical frequency is the highest frequency that a vertical signal - straight up, zero degrees - will be returned by a layer. Signals above it pass through. At any angle other than vertical the wave travels further into the layer before turning, so a higher frequency can be returned: the maximum usable frequency is roughly the critical frequency divided by the cosine of the incidence angle. That is why 10 metres opens to a distant continent at a shallow angle while the same band is dead for a nearby station.

The lowest usable frequency is set from below, not above, by absorption: go too low and the D region simply swallows the signal. Between the two lies the window a path actually uses, and it shifts through the day as the sun moves.

Indices, disturbances and unusual modes

The solar flux index measures radiation at 10.7 centimetres and correlates with ionisation, so a flux above about 150 means good HF conditions on the high bands. The A and K indices measure geomagnetic disturbance: K runs from 0 to 9 in three-hour steps and A is a daily average, and a K of 5 or more means a storm that degrades - and at high latitudes can shut down - HF paths.

Beyond the ordinary layers there are the modes that make 6 metres and 2 metres interesting. Sporadic E is dense, patchy ionisation in the E region that appears without warning and refracts 50 MHz signals over a thousand kilometres. Tropospheric ducting traps VHF and UHF signals between layers of different temperature and can carry them far beyond the horizon. Auroral propagation reflects signals off the ionised curtain around the polar regions with a characteristic rasping flutter. Meteor scatter uses the brief ionised trail of a meteor to make contacts of a second or two on 6 metres and 2 metres.

Terms you must be able to define
Term	What the examiner wants to hear
Critical frequency	The highest frequency returned by a layer when transmitted straight up.
MUF	Maximum usable frequency: roughly the critical frequency divided by the cosine of the incidence angle.
LUF	Lowest usable frequency, limited from below by D-region absorption.
Sporadic E	Patchy, unpredictable dense ionisation in the E region that supports 50 MHz contacts.
Tropospheric ducting	Temperature inversion layers that trap and carry VHF/UHF signals beyond the horizon.

Question deck 10 questions

E3A01 Recall

What ionises the layers of the ionosphere?

E3A02 Core

Which ionospheric region is most responsible for long-distance HF communications?

E3A03 Core

What is the critical frequency of an ionospheric layer?

E3A04 Core

How is the maximum usable frequency for a given path related to the critical frequency?

E3A05 Core

What determines the lowest usable frequency for an HF path?

E3A06 Core

What is sporadic E, and why does it matter on 6 metres?

E3A07 Core

What causes tropospheric ducting of VHF and UHF signals?

E3A08 Stretch

A geomagnetic K index of 6 is reported. What should an HF operator expect?

E3A09 Stretch

What is a solar flux index above 150 usually taken to indicate?

E3A10 Stretch

Why are contacts made by meteor scatter usually very short?

Reference sheet

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

E3A01 Explanation 1

Key C — Solar ultraviolet and X-ray radiation

Solar UV and X-rays strip electrons from atmospheric atoms, and the freed electrons are what refract HF signals. Ionisation therefore follows the sun: it peaks near local noon and decays overnight, which is why the bands change character between day and night.

Rule Fundamentals - ionospheric propagation

E3A02 Explanation 2

Key B — The F2 region

The F2 region sits highest of the ionised layers, so a signal refracted there travels the greatest distance per hop, and it persists after dark when the lower layers have faded. The D region absorbs rather than refracts, and the troposphere is not ionised at all.

Rule Fundamentals - F region propagation

E3A03 Explanation 3

Key A — The highest frequency returned when the signal is transmitted straight up

Critical frequency is measured with a vertical-incidence ionosonde and applies to a straight-up signal. Any lower angle of incidence allows a higher frequency to be returned, which is the relationship that produces the MUF concept.

Rule Fundamentals - ionospheric sounding

E3A04 Explanation 4

Key D — It is higher, roughly the critical frequency divided by the cosine of the incidence angle

A signal arriving at a shallow angle penetrates further into the layer before it turns, so the frequency it can carry is the critical frequency divided by the cosine of the incidence angle - always higher, and rising steeply as the angle approaches grazing.

Rule Fundamentals - MUF and incidence angle

E3A05 Explanation 5

Key C — Absorption in the D region, which increases as frequency falls

Absorption is inversely related to frequency squared, so going lower costs signal. The LUF is the frequency at which absorption has eaten enough of the signal that the path no longer supports the required signal-to-noise ratio.

Rule Fundamentals - ionospheric absorption

E3A06 Explanation 6

Key B — Dense patchy ionisation in the E region that unexpectedly refracts 50 MHz signals over long distances

Sporadic E patches appear with little warning, and being dense enough to return 50 MHz they can open the 6 metre band across a continent for minutes or hours. Openings often peak around late spring and early summer and in the local evening.

Rule Fundamentals - sporadic E propagation

E3A07 Explanation 7

Key A — Layers of different temperature that refract signals back toward the earth repeatedly

A temperature inversion traps a signal between two refracting boundaries, and it zigzags along the duct for hundreds of kilometres past the radio horizon. Weather fronts and high pressure over water produce the best ducts.

Rule Fundamentals - tropospheric propagation

E3A08 Explanation 8

Key D — Degraded HF conditions, especially on high-latitude paths

K runs from 0 to 9 in three-hour intervals, and 5 is the usual threshold for a storm. At 6 the F region is disturbed and auroral absorption is heavy, so paths through high latitudes weaken or disappear while low-latitude paths suffer more rapid fading.

Rule Fundamentals - geomagnetic indices

E3A09 Explanation 9

Key C — Enough ionisation for good conditions on the higher HF bands

The 10.7 cm flux is the single best long-term indicator of ionising radiation, and values above about 150 correspond to an F2 region dense enough to open 15 and 10 metres. It says nothing about absorption on the low bands, which depends on time of day rather than on the flux value.

Rule Fundamentals - solar indices

E3A10 Explanation 10

Key C — The ionised trail exists for only a fraction of a second to a couple of seconds

A meteor trail is a thin, short-lived column of ionisation roughly 100 kilometres up, so the two stations must exchange their information in bursts timed to trail events. Operators use high-speed modes and prearranged schedules to fit an exchange into a second or two.

Rule Fundamentals - meteor scatter

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.