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Amateur Radio Info & Exams - Propagation 2 - Radio horizon, propagation prediction & space weather

Indexes relating to the Ionosphere

Countries for which HF communications are important operate observatories which observe the sun for sunspots and flares, which have various impacts on the ionosphere, and thus HF propagation.

A and K indexes

The K-index quantifies the variation in the horizontal component of the earth's magnetic field. It is calculated 8 times per day, each figure covering 3 hours. Linear magnetometer readings are converted to a pseudo-logarithmic value, ranging from 0 to 9. 1 is calm, while 5 and greater are geomagnetic storms.

NOAA converts USGS K-index numbers to a G-scale. A Kp of 5 is a minor storm at G1. A Kp of 9 is a extreme storm at G5.

The A-Index is derived by converting the K-index to a linear a-index. These 8 values are averaged to produce the daily A-Index.

A and K values are determined for a range of stations, with the Fredericksburg (Virginia) and Australian A-indexes (Afr and Aaus) apparently being of most use to Australian stations, as they are published weekly in the ARNSW News. There are also planetary Kp and Ap indexes.

Increases in these indexes indicate increasing disruption of the geomagnetic field, casusing disruption to HF radio communications, including a depression in the maximum usable frequency (MUF).

Storm level variations also cause things like disruption to power transmission systems, satellites including navigation ones, large currents induced into oil and gas pipelines (hundreds of amps at G5), and aurora at low latitudes.

See also: Wikipedia: K-index

Solar Flares

Flares cause disruption to HF communications. They are classed A, B, C, M, and X, by the X-ray energy they emit, with a numerical suffix further defining the strength. Normally ranging from 1 to 9, the suffix for X is open ended.

Descriptions of the numbers are a little confusing, but using C as an example, it ranges from 10-6 to 10-5 watts per square metre, or 1 to 10 microwatts. Thus C1 is 1 μW, C2 is 2 μW, C3 is 3 μW, etc. From C9, or 9 μW, the next step is M1, at 10 μW. X1 is 100 μW, and X10 1000 μW or 1 mW per square metre. A decimal can be used, such as X8.8.

Class M1 causes minor reductions in HF propagation, becoming worse as the class increases, especially from X1. Class X with large suffixes can cause outages and damage to cable-based telecommunications and power systems. They also cause strong aurora, and aurora at low latitudes. The late 2003 event was likely X45, or 4.5 mW / m².

You can read more at: Wikipedia: Solar Flare

Various fictional accounts discuss the breakdown of society following widespread and prolonged power outages, and example being the initial series of the British TV series "COBRA".


This is the "Voice Of America Coverage Analysis Program", and as the name indicates, this was written for the United State's government, specifically its shortwave radio based propaganda stations. The US Gov aspect makes it ineligible for copyright, like the NEC antenna modelling software, and the NASA images I have used on my Posters. You can thus use it online, or via download, here: Map based online version or Main site with download links

Relevant Questions

These are the actual questions from the Extra licence exam pool, as published by the NCVEC.

What does the radio communication term "ray tracing" describe?
A. The process in which an electronic display presents a pattern
B. Modeling a radio wave's path through the ionosphere
C. Determining the radiation pattern from an array of antennas
D. Evaluating high voltage sources for X-Rays

This is modelling a radio wave's path through the ionosphere, answer B.

What is indicated by a rising A or K index?
A. Increasing disruption of the geomagnetic field
B. Decreasing disruption of the geomagnetic field
C. Higher levels of solar UV radiation
D. An increase in the critical frequency

This result in an increasing disruption of the geomagnetic field, answer A.

Which of the following signal paths is most likely to experience high levels of absorption when the A index or K index is elevated?
A. Transequatorial propagation
B. Polar paths
C. Sporadic-E

Polar paths are impacted, answer B.

What does the value of Bz (B sub Z) represent?
A. Geomagnetic field stability
B. Critical frequency for vertical transmissions
C. Direction and strength of the interplanetary magnetic field
D. Duration of long-delayed echoes

BZ indicates the direction and strength of the interplanetary magnetic field, answer C.

What orientation of Bz (B sub z) increases the likelihood that incoming particles from the Sun will cause disturbed conditions?
A. Southward
B. Northward
C. Eastward
D. Westward

Southward, answer A.

By how much does the VHF/UHF radio horizon distance exceed the geometric horizon?
A. By approximately 15 percent of the distance
B. By approximately twice the distance
C. By approximately 50 percent of the distance
D. By approximately four times the distance

It is around 15% greater, answer A.

Which of the following descriptors indicates the greatest solar flare intensity?
A. Class A
B. Class B
C. Class M
D. Class X

This is Class X, answer D.

What does the space weather term G5 mean?
A. An extreme geomagnetic storm
B. Very low solar activity
C. Moderate solar wind
D. Waning sunspot numbers

This indicates an extreme geomagnetic storm, answer A.

These can be disruptive to HF communications.

How does the intensity of an X3 flare compare to that of an X2 flare?
A. 10 percent greater
B. 50 percent greater
C. Twice as great
D. Four times as great

This is 50% greater, answer B.

This is going from 200 μW/m² to 300 μW/m², an increase of 50%.

What does the 304A solar parameter measure?
A. The ratio of X-Ray flux to radio flux, correlated to sunspot number
B. UV emissions at 304 angstroms, correlated to solar flux index
C. The solar wind velocity at 304 degrees from the solar equator, correlated to solar activity
D. The solar emission at 304 GHz, correlated to X-Ray flare levels

This is a measure of solar radiation at Ultraviolet, answer B.

304 Å equates to 30.4 nm, a wavelength significantly shorter, and thus more energetic, than a disco or theatrical UV lamp.

What does VOACAP software model?
A. AC voltage and impedance
B. VHF radio propagation
C. HF propagation
D. AC current and impedance

It models HF propagation, answer C.

How does the maximum distance of ground-wave propagation change when the signal frequency is increased?
A. It stays the same
B. It increases
C. It decreases
D. It peaks at roughly 14 MHz

Coverage decreases as frequency increases, answer C.

In Australia the ABC (the larger publicly funded national broadcaster) tends to be allocated frequencies just above 500 kHz, as these provide coverage over hundreds of kilometres, especially in the evening, while the commercial stations tend to be from a little below 1000 kHz, and up. 2 CR on 549 kHz at 50 kW in Cumnock, between Orange and Peak Hill in rural NSW can cover from the Victorian border to the Queensland one, and in reality, probably further, covering an area similar to that of the UK. BBC Radio Scotland on 810 kHz also provides great coverage to much of Norway, especially in the darkness of winter.

You can watch a 3 minute video on the Cumnock station on the ABC Central West page, or the 40 second version on Facebook.

What type of polarization is best for ground-wave propagation?
A. Vertical
B. Horizontal
C. Circular
D. Elliptical

MW (AM) broadcasting generally aims to generate ground-waves, so as you drive near your local swampy or estuarine area, you may see a large guyed vertical tower, to generate the required vertically polarised signals, answer A.

Why does the radio-path horizon distance exceed the geometric horizon?
A. E-region skip
B. D-region skip
C. Due to the Doppler effect
D. Downward bending due to density variations in the atmosphere

The varying density of the atmosphere bends signals downward, answer D.

What might be indicated by a sudden rise in radio background noise across a large portion of the HF spectrum?
A. A temperature inversion has occurred
B. A solar flare has occurred
C. Increased transequatorial propagation likely
D. Long-path propagation is likely

This may indicate a solar flare has occurred, answer B.

On to: Practices 1 - Test & Measurement Gear

You can find links to lots more on the Learning Material page.

Written by Julian Sortland, VK2YJS & AG6LE, May 2022.

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