Login or Register for FREE!
Subelement G3
Radio Wave Propagation
Section G3A
Sunspots and solar radiation; ionospheric disturbances; propagation forecasting and indices
What is the significance of the sunspot number with regard to HF propagation?
  • Correct Answer
    Higher sunspot numbers generally indicate a greater probability of good propagation at higher frequencies
  • Lower sunspot numbers generally indicate greater probability of sporadic E propagation
  • A zero sunspot number indicates that radio propagation is not possible on any band
  • A zero sunspot number indicates undisturbed conditions

Higher sunspot numbers generally indicate a greater probability of good propagation at higher frequencies

HF propagation is done through bouncing Electro-Magnetic waves off of charged particles in the earth's atmosphere. High sunspot numbers indicate higher activity in the sun, which shoots off energy into the earth's atmosphere, charging more particles that increases the amount of reflected power on the atmosphere, increasing received signal.

Higher sunspots = Higher probability at Higher frequencies

Last edited by po_supe. Register to edit

Tags: arrl chapter 8 arrl module 35

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

It disrupts signals on lower frequencies more than those on higher frequencies.

During a solar flare, a large amount of energy, including ultraviolet and x-ray radiation, travels out from the sun at the speed of light toward the daylight side of the Earth.

These large bursts of radiated energy cause a sudden increase of ionization in the ionospheric layers of Earth's atmosphere. This is known as a Sudden Ionospheric Disturbance.

SIDs tend to "enhance low frequency propagation" and "absorb high frequency radiation" which appears to be the opposite of the correct answer. The important detail is that the question is about HF radiation specifically and it is the lower end of this range which is typically more affected.

For more info see Wikipedia: Sudden Ionospheric Disturbance (SID)

Last edited by meetar. Register to edit

Tags: arrl chapter 8 arrl module 35

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

RF energy waves, such as ultraviolet and X-ray radiation, travel at the speed of light (approx. 300 million meters per second, or approx. 186,000 miles per second). The earth is about 93 million miles from the sun, and so it takes just over 8 minutes, on average, for a burst of radiation from solar flares to affect radio-wave propagation on earth.

Ultraviolet and X-ray radiation = 8 minutes Particles from coronal mass ejection = 20 - 40 hours
Sun Cycle = 11 years

Last edited by marvsherman419. Register to edit

Tags: arrl chapter 8 arrl module 35

Which of the following are least reliable for long-distance communications during periods of low solar activity?
  • 80 meters and 160 meters
  • 60 meters and 40 meters
  • 30 meters and 20 meters
  • Correct Answer
    15 meters, 12 meters, and 10 meters

High frequency (short wavelength) radio waves are transmitted the farthest when the upper layers of the ionosphere are energized during periods of high solar activity, so they are most affected and least reliable for long distance communications during periods of low solar activity. Therefore, 15 meters, 12 meters and 10 meters, is correct as these are the highest frequencies offered in the answer choices.

Silly hint: Long distance/Long answer.

Remember, low solar activity is bad for low wavelength. The correct answer has lowest [shortest] wavelengths. [KQ4AEY]

Last edited by atrottier. Register to edit

Tags: arrl chapter 8 arrl module 35

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

(D). Measuring solar flux is another way of expressing the amount of solar activity. The solar flux is the intensity of the sun's RF energy emissions.

The Solar flux index is a standardized representative of this radiation energy which is measured at a fixed value of 2800 MHz frequency (10.7 cm wavelength).

The advantage of this measurement over the sunspot index, is that it can be measured during any weather conditions - the sun doesn't have to be visible. The higher the solar flux index number, the greater the amount of solar activity indicated.

For more info see Wikipedia: Solar flux

Last edited by kd7bbc. Register to edit

Tags: arrl chapter 8 arrl module 35

What is a geomagnetic storm?
  • A sudden drop in the solar flux index
  • A thunderstorm that affects radio propagation
  • Ripples in the ionosphere
  • Correct Answer
    A temporary disturbance in Earth's magnetosphere

Easy hint: the correct answer is the only one to have the word "magnet" in it, which is also in the question.

(D). Our earth is protected by outer lines of magnetic force referred to as the magnetosphere. These lines of force flow from pole to pole and help protect the earth from destructive particles. A solar flare can release massive amounts of charged particles toward the earth. When these particles reach the earth they can cause temporary disturbances of the Earth's magnetosphere, called geomagnetic storms.

For more info see Wikipedia: Geomagnetic storms

Last edited by grfd711. Register to edit

Tags: arrl chapter 8 arrl module 35

At what point in the solar cycle does the 20-meter band usually support worldwide propagation during daylight hours?
  • At the summer solstice
  • Only at the maximum point of the solar cycle
  • Only at the minimum point of the solar cycle
  • Correct Answer
    At any point in the solar cycle

The 20 meter (14 MHz) band is less affected by variations in the solar cycle than higher frequency bands. During periods of high solar activity, the band will be deflected for longer distances and with stronger signals, but it is a reliable band for worldwide daylight propagation during any point of the solar cycle.

Last edited by po_supe. Register to edit

Tags: arrl chapter 8 arrl module 35

Which of the following effects can a geomagnetic storm have on radio propagation?
  • Improved high-latitude HF propagation
  • Correct Answer
    Degraded high-latitude HF propagation
  • Improved ground wave propagation
  • Degraded ground wave propagation

(B). During a Geomagnetic storm, charged particles from increased solar emissions, such as solar flares, are sent toward earth. The particles are deflected by the earth's magnetosphere along lines of magnetic force from the North and South poles. The regions around the equator are protected. This increased activity is often seen as greater Auroras at the poles. Because the magnetic disturbances are concentrated around the higher latitudes (from about 45 degrees to the poles), HF propagation is distorted and degraded in these high-latitude regions.

For more info see Wikipedia: Geomagnetic storms

Last edited by ki5bzz. Register to edit

Tags: arrl chapter 8 arrl module 35

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

Geomagnetic storms are bad news for HF transmissions, especially at higher latitudes. But a period of high geomagnetic activity can be good news for VHF propagation. The magnetic disturbance, which is centered around the poles, can produce aurora that can reflect VHF signals, thereby improving their chances of long-distance propagation.

Last edited by k6bjy. Register to edit

Tags: arrl chapter 8 arrl module 35

What causes HF propagation conditions to vary periodically in a roughly 28-day cycle?
  • Long term oscillations in the upper atmosphere
  • Cyclic variation in Earth's radiation belts
  • Correct Answer
    The sun's rotation on its axis
  • The position of the moon in its orbit

It takes the sun about 28 days to rotate once on its axis. Because solar activity in one region often lasts more than one rotation (such as a group of sunspots), we will typically see the same activity pattern return when the sun returns to that same point in its revolution (and those same sunspots come into range again).

Therefore it is the sun's rotation on its axis that causes HF propagation conditions to vary periodically in a 28-day cycle.

Last edited by kd7bbc. Register to edit

Tags: arrl chapter 8 arrl module 35

How long does it take charged particles from coronal mass ejections to affect radio propagation on Earth?
  • 28 days
  • 14 days
  • 4 to 8 minutes
  • Correct Answer
    20 to 40 hours

It takes charged particles from coronal mass ejections about 20 to 40 hours to travel to Earth and then affect radio propagation.


The sun is approximately \(93\text{ million miles}\) from the earth. Unlike RF energy which travels at the speed of light, the charged particles from a coronal mass ejection take longer to reach Earth.

Per Wikipedia:

Coronal mass ejections reach velocities from...\(12\) to \(1,988\text{ miles/s}\)...with an average speed of \(304\text{ miles/s}\).

These speeds correspond to transit times from the Sun out to the mean radius of Earth's orbit of about \(13\text{ hours}\) to \(86\text{ days}\) (extremes), with about \(3.5\text{ days}\) as the average.

The average of 3.5 days is closest to 20 to 40 hours. Only light can reach Earth in 4 to 8 minutes -- magnitudes smaller than the 13 hour low-end travel time for CME particles. While some particles might take 14 days, 28 days, or even longer to reach our planet, those particles are much less common, and as a result are less likely to affect radio-wave propagation here on Earth.


For more info see Wikipedia: coronal mass ejection

Hint: It doesn't take days or minutes, it takes "hours".

Last edited by mike881. Register to edit

Tags: arrl chapter 8 arrl module 35

What does the K-index indicate?
  • The relative position of sunspots on the surface of the sun
  • Correct Answer
    The short-term stability of Earth's magnetic field
  • The stability of the sun's magnetic field
  • The solar radio flux at Boulder, Colorado

(B). The K-index is a measurement of the short-term stability of Earth's magnetic field. A high K-index means higher amounts of magnetic disturbance, and so more disruption of HF signals, especially in latitudes from 45 degrees to the poles.

Note: Think of the K index as the earth's magnetic field getting a KICK. Fast, Quick, and a little painful (more disruption of signals with a higher K number).

For more info see Wikipedia: K-index

Last edited by speedmaster. Register to edit

Tags: arrl chapter 8 arrl module 35

What does the A-index indicate?
  • The relative position of sunspots on the surface of the sun
  • The amount of polarization of the sun's electric field
  • Correct Answer
    The long-term stability of Earth's geomagnetic field
  • The solar radio flux at Boulder, Colorado

The A-index, like the K-index, are measures of the earth's geomagnetic field stability. Whereas the K-index is a short-term measure, the A-index is an averaged daily figure and is charted over the usual rotational period of the sun, so is a better cyclical indicator of the long term stability of the Earth's geomagnetic field.

Note: Remember that A-index is a longer AVERAGE as opposed to the short term magnetic KICK measured by the K-index.

For more info see Wikipedia: A-index

Last edited by gconklin. Register to edit

Tags: arrl chapter 8 arrl module 35

How are radio communications usually affected by the charged particles that reach Earth from solar coronal holes?
  • HF communications are improved
  • Correct Answer
    HF communications are disturbed
  • VHF/UHF ducting is improved
  • VHF/UHF ducting is disturbed

The emission of charged particles interacts with the earth's magnetosphere causing geomagnetic storms or disturbances.

This causes disturbances in the ionosphere, which will adversely affect HF communication.

VHF/UHF is neither improved nor disturbed, since VHF/UHF typically does not rely on the ionosphere for propagation.

For more info see Wikipedia: coronal mass ejection

Last edited by glitteribbon. Register to edit

Tags: arrl chapter 8 arrl module 35

Go to G2E Go to G4A