E2 · Published 13 October 2026 · By The Ham Desk

Forecasting, Disturbances and the Unusual Paths

The final propagation unit is about the events that ruin a good forecast and the paths that exist because the ionosphere is not a smooth mirror. It covers the indices used to predict a path, the solar events that close it, and the scatter and ducting modes that fill in when the ordinary ones fail.

Band 10 m
Study time 19
Level Stretch
questions 10

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

Pre-flight briefing

The three frequencies of a path

Every HF path has three characteristic frequencies on a given hour. The maximum usable frequency is the highest frequency that the ionosphere will return between those two points, and it is set by the layer's ionisation and the geometry. The lowest usable frequency is set from below by absorption, which grows as frequency falls, and by the noise level at the receiving site. Between them sits the frequency of optimum traffic - usually taken as about 85 per cent of the MUF - which is the frequency that gives the highest probability of a successful contact rather than the highest theoretical ceiling.

Choosing the FOT rather than the MUF is a deliberate hedge. The MUF is a limit that is reached only under ideal conditions, and operating right at it means that any small decline in ionisation pushes the path closed. Twenty per cent below the MUF sits comfortably inside the window while still being high enough to avoid the absorption that dominates at lower frequencies.

The tools that produce these numbers measure rather than guess. An ionosonde sweeps a range of frequencies straight up and records the frequency at which each layer stops returning the signal - the critical frequency - along with the virtual height at which the reflection appears. Networks of ionosondes feed the models that propagation prediction software uses to compute the MUF for a path at a given hour.

When the sun interferes

Solar flares produce a burst of X-rays that reaches earth in minutes and ionises the D region far beyond its normal daytime state. The result is a sudden ionospheric disturbance: absorption rises dramatically, and paths that cross the daylight hemisphere simply vanish at frequencies below about 20 to 30 MHz for minutes to hours. It is detected as a sudden fade on a beacon or a broadcast signal that had been steady moments before.

A more sustained event follows when a coronal mass ejection sends a cloud of plasma toward earth. The resulting geomagnetic storm disturbs the F layer, closes trans-polar HF paths for days, and produces aurora that can carry VHF signals by auroral reflection - the paradox being that a storm which destroys HF at high latitudes creates 6 metre and 2 metre contacts.

A third event is distinct: a solar proton event sends energetic protons down the field lines over the polar caps, where they ionise the D region and absorb HF completely. This is polar cap absorption, and it can last for days, blocking the great-circle routes that pass near the poles. Because these routes are the shortest ones between North America and Asia, the practical effect is that aircraft and maritime HF users must reroute through lower latitudes.

The unusual paths

Not every signal goes up and comes down in a neat hop. In chordal hop propagation the wave is refracted within the ionosphere and travels a long horizontal distance inside the layer before returning to earth, which is how very long single-hop contacts occur without any ground reflection. Forward scatter uses the ionosphere, or the troposphere at VHF, as a diffuse reflector: transmitter and receiver are aimed at a common volume in the sky, and enough energy scatters in the right direction to make the contact.

Two irregularities matter at high frequencies. Spread F is a condition in which the F layer breaks into many reflecting patches, producing echoes that are smeared in time and a signal that fades rapidly - a particular problem on transequatorial paths after dark. Field-aligned irregularities are ducts of enhanced ionisation aligned with the earth's magnetic field, which can carry VHF signals over long distances at low latitudes.

Finally, the sun's 27-day rotation means that active regions come back around. A storm that arrived from a particular active region will often recur 27 days later when that region faces earth again, which is why propagation tables produced a month in advance are worth reading.

Terms you must be able to define
Term	What the examiner wants to hear
MUF	The highest frequency the ionosphere will return between two points at a given time.
Frequency of optimum traffic	About 85 per cent of the MUF; the frequency with the highest probability of a successful contact.
Sudden ionospheric disturbance	X-ray ionisation of the D region during a solar flare, absorbing HF on daylight paths.
Polar cap absorption	Solar protons ionising the polar D region and blocking trans-polar HF for days.
Chordal hop	Propagation that travels horizontally inside the ionosphere for a long distance before returning to earth.

Question deck 10 questions

E3B01 Core

What is the frequency of optimum traffic for an HF path?

E3B02 Core

What causes a sudden ionospheric disturbance?

E3B03 Core

What is polar cap absorption?

E3B04 Core

What does an ionosonde measure?

E3B05 Stretch

What is chordal hop propagation?

E3B06 Core

What is forward scatter propagation?

E3B07 Stretch

What is spread F, and why does it matter?

E3B08 Stretch

Why can a geomagnetic storm both destroy HF and create VHF contacts?

E3B09 Stretch

Why do some geomagnetic disturbances recur at roughly 27-day intervals?

E3B10 Stretch

What limits the lowest usable frequency on an HF path?

Reference sheet

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

E3B01 Explanation 1

Key B — About 85 per cent of the maximum usable frequency, giving the highest probability of a successful contact

Operating at the MUF leaves no margin: any decline in ionisation closes the path. Stepping down about 15 per cent keeps the frequency inside the usable window while staying above the absorption that dominates at lower frequencies, which is why prediction programs report the FOT alongside the MUF.

Rule Fundamentals - HF circuit prediction

E3B02 Explanation 2

Key A — X-ray emission from a solar flare ionising the D region and raising absorption

X-rays travel at the speed of light, so a sudden ionospheric disturbance begins within minutes of the flare and affects only the daylight side of the earth. Lower HF frequencies disappear first, and the effect can last from minutes to a few hours.

Rule Fundamentals - solar flares

E3B03 Explanation 3

Key D — Energetic solar protons ionising the polar D region and blocking trans-polar HF paths for days

Protons guided by the geomagnetic field funnel into the polar regions and ionise the D layer there, where HF is most easily absorbed. Because trans-polar routes are the shortest great-circle paths between North America and Asia, a PCA event forces long reroutes rather than merely degrading signals.

Rule Fundamentals - polar cap absorption

E3B04 Explanation 4

Key C — The critical frequency and virtual height of each ionospheric layer

An ionosonde transmits a swept range of frequencies vertically and listens for the returns, recording where each layer stops reflecting. Those measurements are the input to the models that propagation prediction software uses, which is why a forecast is only as good as the last sounding.

Rule Fundamentals - ionospheric sounding

E3B05 Explanation 5

Key B — The signal is refracted within the ionosphere and travels a long horizontal distance inside the layer before returning to earth

Instead of coming down to earth and bouncing again, a chordal hop stays inside the ionosphere where refraction continues to bend it. The result is a single very long hop with no ground reflection - which is how contacts of 8,000 kilometres or more can occur with only one ionospheric encounter.

Rule Fundamentals - chordal hop

E3B06 Explanation 6

Key A — Transmitter and receiver are aimed at a common volume in the atmosphere, and enough energy scatters toward the receiver to make a contact

Forward scatter uses ionospheric or tropospheric irregularities as a diffuse reflector, which is how contacts are made on paths where no direct refraction geometry exists. The signals are weak and subject to deep, rapid fading, so high power and narrow beams are characteristic of scatter work.

Rule Fundamentals - scatter propagation

E3B07 Explanation 7

Key D — The F layer breaks into many reflecting patches, producing smeared echoes and rapid fading, especially on transequatorial paths

Spread F replaces a single clean reflection with many, each at a slightly different height, so the echo is smeared in time and the received signal fluctuates. It is a particular nuisance for transequatorial paths after dark, where it can make an otherwise open band unusable.

Rule Fundamentals - ionospheric irregularities

E3B08 Explanation 8

Key C — It disturbs the F layer and closes high-latitude HF while the aurora it produces reflects VHF signals

The same particle precipitation that makes the F layer irregular and absorptive also creates the auroral ionisation that reflects 6 metre and 2 metre signals. So a storm that silences 20 metres across the polar route may open 6 metres to stations a thousand kilometres north.

Rule Fundamentals - auroral propagation

E3B09 Explanation 9

Key B — That is the sun's rotation period, so the same active region faces earth again

The sun rotates once in about 27 days as seen from earth, so a long-lived active region or a coronal hole comes back into a position where its solar wind stream can reach us. This is why storm activity often repeats on a roughly monthly cadence during quiet parts of the solar cycle.

Rule Fundamentals - solar rotation

E3B10 Explanation 10

Key B — Absorption, which increases as frequency falls, combined with the noise level at the receiver

Lower frequencies suffer more absorption in the D region, and as the received signal falls the ambient noise at the receiving site eventually dominates. The bottom of the usable window is where the signal-to-noise ratio stops supporting the required grade of service.

Rule Fundamentals - LUF

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.