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Amateur Radio Info & Exams - Propagation 1 - Sunspot Cycles and Max & Min Usable Frequencies

This covers HF propagation, a topic in some ways more complex than VHF propagation, as it involves sunspot cycles to a greater extent.

At times, especially when there is sunspot activity this works up to and beyond the "11 metre" band, where CBers talk of "working skip". It also extends into the VHF-Low area, such as 30 to 40 MHz, previously used heavily for forestry, electrical transmission repair workers, bush-fire control and police, so when this "skip" was running, the dopey LA police would be trying to tell south-coast NSW bush fire captains to get off "their" frequency.


Over 11 years, the activity of the sun becomes more and less, although in one of every two cycles the maximum is less pronounced.

Sunspots are areas of darkness on the surface on the sun, and these can be view by projecting the image of the sun onto a card using a pinhole in a second card held in front of it, or projected using a small telescope, such as a rifle scope, also with a card around it. These are in areas of high magnetic flux. High sunspots correlate to a more strongly ionised ionosphere, meaning radio waves are more strongly refracted. Low sunspot numbers correlate to poor propagation, especially for upper HF and 6 metre bands. Low sunspot numbers also correlate to a reduction in temperatures on earth, although, in recent years it led only to a pause in the upward trend, getting the science deniers all excited. An extended period of few sunspots from 1645 to about 1715 was termed the "Maunder Minimum" for Annie Russell Maunder and E. Walter Maunder, in combination with volcanic activity, resulted in a cooler period in North America and Europe. This, coupled with the restriction to the flow of the Thames by the medieval London Bridge (not Tower Bridge), allowed the river to freeze, and thus "Frost Fairs" on the frozen surface.

Apparently some hams get all discouraged, and sell their gear during the minimum, rather than pursue different aspects of the hobby.

The sun rotates on its axis in 28 days, or more correctly, rotates such that the same point is facing the earth 28 days later. This means that if there is a high number of sunspots facing earth for several days, this may be repeated 4 weeks later.

Zero sunspots does NOT mean no propagation, but this likely would take place at lower HF frequencies.

If two HF stations are within a few kilometres of each other, and there is not a significant ridge between them, which would also impede a VHF signal, there should be a ground-wave path.

At most times over moderate range, bands such as 80 metres provides reliable night-time propagation, and 40 metres local daytime coverage, and DX at night, with 20 metres giving daytime DX. Above these frequencies, from 15 metres to 6 metres, and 4 metres in Europe and Africa where it is available, high solar activity means that these bands go from being short range to being DX bands. (This also applies on 27 MHz CB.)

The sun emits electromagnetic radiation from radio signals, including microwaves, to light, including ultraviolet, and X-rays. These all take 8 minutes to reach us. The strength (flux) of this emission at 10.7 centimetres (around 2800 MHz or 2.8 GHz) is a predictor of ionospheric behaviour.

Off the exam, if you watch TV, or listen to radio delivered via satellite, either to your home, or to the broadcasting site on the local hill, then near the solstices, the sun may "process behind the satellite", meaning at some point during the day for maybe 20 minutes, the sun's RF emission will swamp the satellite's signal. Where I live this occurs for maybe a week each spring and autumn, for about a week, at around 11 am.


Once we move further apart, at any one time, between two any two points there may or may not be an HF communication path. When there is, there is a LUF - lowest usable frequency, and a MUF - the maximum usable frequency.

When the different between the LUF and MUF close to zero, there is no propagation.

Given that there are bands between the traditional bands, 160, 80, 40, 20, 15, and 10 metres, being the WARC bands, 30, 17, and 12 metres, so there is a reasonable chance that one will fall between the LUF and MUF. At night this may be the 60 metre band.

Perhaps not mentioned on the exam, noise on the lower HF bands, such as 80 metres can be a problem. At the same time, the MUF can be below 7 MHz. In this case, the solution can be to use the 60 metre (5 MHz band).

If you are a ham in a country which is yet to enact the outcome of the latest World Radio Conference to permit Amateur operations on 60 metres, contact the national or federal communications or posts & telecommunications minister, to give the national regulator a prod. This currently includes Australia (but hopefully a new minister in May '22 will fix this).

Around the world, especially in countries where HF communications are important, government agencies provide predictions of the conditions over the upcoming week, and beyond.

One of the markers of 6 metre propagation was long distance propagation of the lower VHF analogue TV channels, but these are rapidly being replaced by digital signals, and while in some cases they are using digital (ATSC) on low VHF, in most cases DVB-T is in the upper VHF and lower UHF bands only.

Even at lower HF, such as 40 metres, propagation from Sydney to Central Western NSW can develop between 9 and 10 am, and even when this path is closed, it is possible to hear stations from Victoria or Tasmania. One odd phenomena was than club members in northern Sydney could talk locally via NVIS or ground-wave, while two members who had moved away, one in Newcastle, the other in the Snowy Mountains, could have a separate conversation on the same frequency, then sometime later re-appear on the Sydney net.

Selection of bands

Note that, for soem hams, they are not seeking the optimal frequency to communicate with a place right now, but may be looking for a time when ther is propagation on a particular band. This is because they are trying to get an award, such as Worked All States, or DXCC on a particular band.


DXCC is the DX Century Club, a certificate for Amateurs who have contacted or "worked" 100 countries, or country-like entities, such as autonomous regions (eg, Åland), the constituent nations and crown dependencies of the UK, and various islands. The United Nations HQ and the ITU HQ are also entities.

Kaliningrad, currently an isolated portion of Russia, and Asiatic Russia are separate to European Russia. Ditto Franz Josef Land in the Arctic. Hopefully Putin will be unequivocally removed from office soon, and Russia's relations normalised.

Entities are added or deleted due to decolonisation, the formation of new countries (South Sudan) and reunification (Germany), or even the whim of a committee re interpretation of rules re the separation distance of islands. I understand that working the same Scottish Amateur the day before, and on the day of Independence will count as two countries.

The island territories around Australia are further examples, each having the VK9 prefix. A few have resident Amateurs, but some are only "activated" by periodic DXpeditions. Likewise, Alaska, and various US related islands count as entities. Perhaps the smart ham club would ensure school, public, or community libraries on populated islands have ham radio licence manuals relevant for licensing by the administering country's government.

A list is here: WIA: DXCC List The & arrows allow sorting by various criteria.

Relevant Questions

These are actual questions from the General exam pool.

What is the significance of the sunspot number with regard to HF propagation?
A. Higher sunspot numbers generally indicate a greater probability of good propagation at higher frequencies
B. Lower sunspot numbers generally indicate greater probability of sporadic E propagation
C. A zero sunspot number indicate radio propagation is not possible on any band
D. A zero sunspot number indicates undisturbed conditions.

A higher sunspot number correlates with better propagation for the upper HF (and low VHF) bands, answer A.

While Low VHF for Americans in 6 metres, much of Europe and parts of Africa have 4 metres at 70-70.6 MHz, technically VHF Mid-band. Ireland has 8 metres (40-45 MHz) with smaller segments elsewhere, and 5 metres, 54 to 69.9 MHz.

WL2XUP, an Experimental station, runs a beacon on 40.6635 MHz, with a USB dial Frequency of 40.662.

What effect does a Sudden Ionospheric Disturbance have on the daytime ionospheric propagation of HF radio waves?
A. It enhances propagation on all HF frequencies
B. It disrupts signals on lower frequencies more than those on higher frequencies
C. It disrupts communications via satellite more than direct communications
D. None, because only areas on the night side of the Earth are affected

Sudden Ionospheric Disturbance impact the lower HF bands more than higher frequencies, answer B.

Approximately how long does it take the increased ultraviolet and X-ray radiation from solar flares to affect radio propagation on the Earth?
A. 28 days
B. 1 to 2 hours
C. 8 minutes
D. 20 to 40 hours

As these are forms of electromagnetic radiation, like visible light, the standard 8 minutes to cover this distance applies, answer C.

Which of the following are least reliable for long distance communications during periods of low solar activity?
A. 80 meters and 160 meters
B. 60 meters and 40 meters
C. 30 meters and 20 meters
D. 15 meters, 12 meters and 10 meters

Upper HF bands and least useful for long range communications during low solar activity, so 15, 12, and 10 metres, answer D. It could be said they behave more like VHF at these times.

What is the solar flux index?
A. A measure of the highest frequency that is useful for ionospheric propagation between two points on the Earth
B. A count of sunspots which is adjusted for solar emissions
C. Another name for the American sunspot number
D. A measure of solar radiation at 10.7 centimeters wavelength

The sun emits electromagnetic radiation on a wide range of frequencies or wavelengths, including the high UHF (low microwave) band, in this case those with a wavelength of 10.7 cm, the strength (flux) of which is a measure of the activity of the sun, answer D.

What is a geomagnetic storm?
A. A sudden drop in the solar flux index
B. A thunderstorm which affects radio propagation
C. Ripples in the ionosphere
D. A temporary disturbance in the Earth's magnetosphere

Geomagnetic contains words meaning earth and magnetic, so it is a temporary disturbance in the Earth's magnetosphere, answer D.

At what point in the solar cycle does the 20-meter band usually support worldwide propagation during daylight hours?
A. At the summer solstice
B. Only at the maximum point of the solar cycle
C. Only at the minimum point of the solar cycle
D. At any point in the solar cycle

20 metres is a world-wide DX band, and provides daytime propagation at any time during the solar cycle, answer D.

Which of the following effects can a geomagnetic storm have on radio propagation?
A. Improved high-latitude HF propagation
B. Degraded high-latitude HF propagation
C. Improved ground-wave propagation
D. Degraded ground wave propagation

HF performance at high latitudes (polar and near-polar regions) is degraded, answer B.

What benefit can high geomagnetic activity have on radio communications?
A. Auroras that can reflect VHF signals
B. Higher signal strength for HF signals passing through the polar regions
C. Improved HF long path propagation
D. Reduced long delayed echoes

A high sunspot number is usually related to enhanced DX on the upper HF and low VHF bands (15 to 4 metres), answer C.

What causes HF propagation conditions to vary periodically in a 28 day cycle?
A. Long term oscillations in the upper atmosphere
B. Cyclic variation in the Earth’s radiation belts
C. The sun’s rotation on its axis
D. The position of the moon in its orbit

The rotation of the sun causes a variation over a 28 day period, answer C.

How long does it take charged particles from coronal mass ejections to affect radio propagation on Earth?
A. 28 days
B. 14 days
C. 4 to 8 minutes
D. 20 to 40 hours

This question is about particles, which being matter must travel more slowly than light. They are never-the-less fast, arriving in 20 to 40 hours, answer D.

This means that there is a period to prepare for the disruption to HF signals and related issues.

What does the K-index indicate?
A. The relative position of sunspots on the surface of the Sun
B. The short term stability of the Earth’s magnetic field
C. The stability of the Sun's magnetic field
D. The solar radio flux at Boulder, Colorado

It is the short term stability the magnetic field around the Earth, answer B.

What does the A-index indicate?
A. The relative position of sunspots on the surface of the Sun
B. The amount of polarization of the Sun's electric field
C. The long term stability of the Earth’s geomagnetic field
D. The solar radio flux at Boulder, Colorado

This is the long term stability of the geomagnetic field ("geo" meaning relating to Earth), answer C.

How are radio communications usually affected by the charged particles that reach the Earth from solar coronal holes?
A. HF communications are improved
B. HF communications are disturbed C. VHF/UHF ducting is improved
D. VHF/UHF ducting is disturbed

HF communication are disturbed during this time, answer B.

Even in the late 1990s large aircraft such as 747-Classic (-100, -200, -300) and 767 models relied on inertial navigation and manual position reporting, meaning when "the ionosphere wasn't working" the faster 747s had to relay the positions of the slow 767s which they had overtaken flying out of Asia forward to Australia ATC on VHF, remembering that VHF has hundreds of kilometres of range from high altitude.

That said, I have heard an aircraft on something like 8867 kHz USB reporting an air-conditioner failure, while somewhere near the Philippines, at home in Sydney.

What is a characteristic of skywave signals arriving at your location by both short-path and long-path propagation?
A. Periodic fading approximately every 10 seconds
B. Signal strength increased by 3 dB
C. The signal might be cancelled causing severe attenuation
D. A slightly delayed echo might be heard

An echo may be heard, as the signal arrives via the two different paths, answer D.

Using SSB, the difference in the arriving signals will be great enough that they will not cancel each other.

What factors affect the MUF?
A. Path distance and location
B. Time of day and season
C. Solar radiation and ionospheric disturbances
D. All these choices are correct

The paths Alaska to Hawaiʻi, Canberra to Afghanistan, or Florida to Spain each will behave differently, including because the distances are different, at a specific time, the time of day and seasons will be different in the areas each path crosses. Solar behaviours and ionospheric disturbances will also vary these factors over time. Thus it is all of these, answer D.

Which of the following applies when selecting a frequency for lowest attenuation when transmitting on HF?
A. Select a frequency just below the MUF
B. Select a frequency just above the LUF
C. Select a frequency just below the critical frequency
D. Select a frequency just above the critical frequency

The higher frequency just below the maximum usable frequency is likely to give the best signal, answer A.

What is a reliable way to determine if the MUF is high enough to support skip propagation between your station and a distant location on frequencies between 14 and 30 MHz?
A. Listen for signals from an international beacon in the frequency range you plan to use
B. Send a series of dots on the band and listen for echoes from your signal
C. Check the strength of TV signals from Western Europe
D. Check the strength of signals in the MF AM broadcast band

The IARU International beacon network is useful for assessing propagation, answer A.

What is wrong with the last two answers? Yep, these services are well outside the bands in the question. Also, Europe has mostly closed analogue TV, replacing it with DVB-T, which is less easy to decode under DX conditions. Incidentally, choosing ATSC over the near global DVB-T was all about having consumers pay for equipment made more expensive by the inclusion of patented techniques, which require the payment of fees by the manufacturers.

Only if the signal travels around the earth and back to your station would you hear the dots, and this is not necessary for you to communicate with stations across the ocean, or the continent. The time between transmission and the signal arriving is about 1/7 second, and this is not necessarily a common thing.

What usually happens to radio waves with frequencies below the MUF and above the LUF when they are sent into the ionosphere?
A. They are bent back to the Earth
B. They pass through the ionosphere
C. They are amplified by interaction with the ionosphere
D. They are bent and trapped in the ionosphere to circle the Earth

Signals between the LUF and MUF are refracted (bent) back to Earth, allowing communications, answer A.

What usually happens to radio waves with frequencies below the LUF?
A. They are bent back to the Earth
B. They pass through the ionosphere
C. They are completely absorbed by the ionosphere
D. They are bent and trapped in the ionosphere to circle the Earth

These signals are absorbed by the ionosphere, answer C.

What does LUF stand for?
A. The Lowest Usable Frequency for communications between two points
B. The Longest Universal Function for communications between two points
C. The Lowest Usable Frequency during a 24 hour period
D. The Longest Universal Function during a 24 hour period

It is the Lowest Usable Frequency usable between two points, answer A.

This can be used to discuss current conditions, or part of a prediction of the best frequency to use in the future.

What does MUF stand for?
A. The Minimum Usable Frequency for communications between two points
B. The Maximum Usable Frequency for communications between two points
C. The Minimum Usable Frequency during a 24 hour period
D. The Maximum Usable Frequency during a 24 hour period

It is the maximum usable frequency between two places, answer B.

Again, this can be now, or part of a prediction of the best frequency to use in the future. If you have brain-fade during the exam, remember that the alternative is LUF / Lowest, so M must be Maximum.

What is the approximate maximum distance along the Earth's surface that is normally covered in one hop using the F2 region?
A. 180 miles
B. 1,200 miles
C. 2,500 miles
D. 12,000 miles

At its greatest, a single hop allows a signal to cross a continent such as Australia or the US, or link VK2 to ZL, around 4000 km, or 2,500 miles, answer C.

What is the approximate maximum distance along the Earth's surface that is normally covered in one hop using the E region?
A. 180 miles
B. 1,200 miles
C. 2,500 miles
D. 12,000 miles

This covers something like Brisbane to Adelaide. The answer is 1900 km, or 1,200 miles, answer B.

What happens to HF propagation when the LUF exceeds the MUF?
A. No HF radio frequency will support ordinary sky-wave communications over the path
B. HF communications over the path are enhanced
C. Double hop propagation along the path is more common
D. Propagation over the path on all HF frequencies is enhanced

It means there are no frequencies which allow ionospheric propagation, answer A.

When I used to listen to shortwave broadcasts in the 1980s, I believe this was most often the case between South Africa and south-eastern Australia. I certainly never heard a SABC station.

On to: HF Propagation 2 - Layers, Critical angle, and NVIS

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

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

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