ionosphere

Sunspots and solar radiation, ionospheric disturbances, propagation forecasting, and indices

Amateur radio communications are subject to the whims of nature. Many different phenomena affect the propagation of signals, and it behooves you to know a little something about the phenomena. Doing so will make you a more effective amateur radio communicator.

The phenomenon that most affects amateur radio communications on the HF bands is the sunspot cycle. During a cycle, which lasts approximately 11 years, the number of sunspots varies from none to a high of between 100 and 200. In general, the more sunspots, the better is HF propagation.

QUESTION: What is the significance of the sunspot number about HF propagation? (G3A01)
ANSWER: Higher sunspot numbers generally indicate a greater probability of good propagation at higher frequencies

Because counting sunspots is a relatively subjective measure of solar activity, scientists have come up with a more objective measurement, called solar flux. As the sunspot number, or SSN, varies from 0 to around 200, the solar flux varies from around 60 to 300.

QUESTION: What is the solar flux index? (G3A05)
ANSWER: A measure of solar radiation at 10.7 centimeters wavelength

Some bands, such as 20 meters, can usually be counted on to provide good propagation at any point in the solar cycle, at least during daylight hours. The higher frequency bands are less reliable, though.

QUESTION: At what point in the solar cycle does the 20-meter band usually support worldwide propagation during daylight hours? (G3A07)
ANSWER: At any point in the solar cycle

QUESTION: Which of the following are least reliable for long-distance communications during periods of low solar activity? (G3A04)
ANSWER: 15 meters, 12 meters, and 10 meters

The sunspot cycle is a long-term phenomenon. There are other phenomena that affect radio wave propagation in the short term. For example, the sun rotates on its axis every 28 days, and sunspots rotate away from the Earth’s view, and therefore, have less effect on radio propagation

QUESTION: What causes HF propagation conditions to vary periodically in a roughly 28-day cycle? (G3A10)
ANSWER: The Sun’s rotation on its axis

One phenomenon that can have a drastic effect on propagation is a Sudden Ionic Disturbance (SID). During an SID, the sun emits a great deal of ultraviolet and X-ray radiation.

QUESTION: Approximately how long does it take the increased ultraviolet and X-ray radiation from solar flares to affect radio propagation on Earth? (G3A03)
ANSWER: 8 minutes

QUESTION: What effect does a Sudden Ionospheric Disturbance have on the daytime ionospheric propagation of HF radio waves? (G3A02)
ANSWER: It disrupts signals on lower frequencies more than those on higher frequencies

Coronal holes and coronal mass ejections also affect HF communications. They emit charged particles that disrupt propagation. Unlike ultraviolet and X-ray radiation, which travel at the speed of light, it takes 20 to 40 hours for charged particles from coronal mass ejections to affect radio-wave propagation on the Earth.

QUESTION: How are radio communications usually affected by the charged particles that reach Earth from solar coronal holes? (G3A14)
ANSWER: HF communications are disturbed

QUESTION: How long does it take charged particles from coronal mass ejections to affect radio propagation on Earth? (G3A11)
ANSWER: 20 to 40 hours

 

Geomagnetic activity, such as a geomagnetic storm, normally degrades radio propagation. Under some circumstances, though, high geomagnetic activity can aid in the formation of auroras, which can reflect VHF signals.

QUESTION: What is a geomagnetic storm? (G3A06)
ANSWER: A temporary disturbance in the Earth’s magnetosphere

QUESTION: Which of the following effects can a geomagnetic storm have on radio propagation? (G3A08)
ANSWER: Degraded high-latitude HF propagation

QUESTION: What benefit can high geomagnetic activity have on radio communications? (G3A09)
ANSWER: Auroras that can reflect VHF signals

There are two indices that give an indication of the stability of the Earth’s magnetic field: the K-index, which is a short-term indicator, and the A-index, which is a long-term indicator. In general, the lower these numbers are, the better it is for HF propagation.

QUESTION: What does the K-index indicate? (G3A12)
ANSWER: The short term stability of the Earth’s magnetic field

QUESTION: What does the A-index indicate? (G3A13)
ANSWER: The long term stability of the Earth’s geomagnetic field

 

Maximum Usable Frequency, Lowest Usable Frequency, propagation

The two most important parameters for predicting the propagation between two locations are the maximum usable frequency (MUF) and the lowest usable frequency (LUF). The MUF is the highest frequency that you can use to communicate between two points, while the LUF is the lowest frequency that you can use to communicate between two points.

QUESTION: What does MUF stand for? (G3B08)
ANSWER: The Maximum Usable Frequency for communications between two points

QUESTION: What does LUF stand for? (G3B07)
ANSWER: The Lowest Usable Frequency for communications between two points

Many different factors affect the Maximum Usable Frequency, including the path between two stations, time of day, season, solar radiation, and ionospheric disturbances.

QUESTION: What factors affect the MUF? (G3B02)
ANSWER: All these choices are correct
– Path distance and location
– Time of day and season
– Solar radiation and ionospheric disturbances

When they are sent into the ionosphere, radio waves with frequencies below the Maximum Usable Frequency (MUF) and above the Lowest Usable Frequency (LUF) are refracted off the ionosphere back to Earth. Radio waves below the LUF are absorbed by the ionosphere. When the LUF is higher than the MUF, there is no HF frequency available that will allow you to communicate between two points. Note that the MUF and LUF apply for a particular path between two points, not for a particular location.

QUESTION: What usually happens to radio waves with frequencies below the MUF and above the LUF when they are sent into the ionosphere? (G3B05)
ANSWER: They are bent back to the Earth

QUESTION: What usually happens to radio waves with frequencies below the LUF? (G3B06)
ANSWER: They are completely absorbed by the ionosphere

QUESTION: What happens to HF propagation when the LUF exceeds the MUF? (G3B11)
ANSWER: No HF radio frequency will support ordinary skywave communications over the path

Frequencies just below the MUF for a particular path will have the lowest attenuation. One way to find that frequency might be to look at online propagation maps. Another way would be to listen for beacons on the upper frequency bands. If you can hear the beacons, then you’re sure that propagation is favorable between your location and the location of the beacon.

QUESTION: Which of the following applies when selecting a frequency for lowest attenuation when transmitting on HF? (G3B03)
ANSWER: Select a frequency just below the MUF

QUESTION: 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? (G3B04)
ANSWER: Listen for signals from an international beacon in the frequency range you plan to use

While signals most often take the shortest path from point to point, sometimes the best path for radio propagation is in the opposite direction, also called the “long path.” Sometimes signals can be propagated between two points via the long path and the short path, and the receiving station will actually hear both signals. When this happens, the operator will hear what appears to be an echo.

QUESTION: What is a characteristic of skywave signals arriving at your location by both short-path and long-path propagation? (G3B01)
ANSWER: A slightly delayed echo might be heard

 

Ionospheric layers, critical angle and frequency, HF scatter, Near Vertical Incidence Sky-wave

The ionosphere is what makes long-distance radio communications possible on the shortwave bands. The ionosphere is made up of three layers of charged particles, labelled D, E, and F. The charged particles in the ionosphere are created by the sun’s radiation hitting the atmosphere and ionizing the gases.

QUESTION: Where on Earth do ionospheric layers reach their maximum height? (G3C02)
ANSWER: Where the Sun is overhead

During the day, there are two F layers, F1 and F2. F2 is higher than F1, so the radio signals that refract off that layer travel farther than signals that bounce off the F1 or E layers. Radio waves refracted by the F2 layer travel about 2,500 miles in a single hop using the F2 region. At night, F1 and F2 combine into a single F layer.

QUESTION: Why is the F2 region mainly responsible for the longest distance radio wave propagation? (G3C03)
ANSWER: Because it is the highest ionospheric region

QUESTION: What is the approximate maximum distance along the Earth’s surface that is normally covered in one hop using the F2 region? (G3B09)
ANSWER: 2,500 miles

While the F2 layer is the layer that normally reflects HF radio signals, the E layer can also reflect signals under some conditions. Since it’s lower than the F layers, the distance a radio wave travels when refracted by the E layer is shorter than the distance it would travel by being refracted from one of the F layers.

QUESTION: What is the approximate maximum distance along the Earth’s surface that is normally covered in one hop using the E region? (G3B10)
ANSWER: 1,200 miles

The D layer is the ionospheric layer closest to the surface of the Earth. Unlike the other layers, it generally absorbs signals at HF frequencies in the 40m band and below during the day. Fortunately, the D layer mostly disappears at night, allowing us to use the 40, 60, 80 and 160 meter bands for long-distance communications.

QUESTION: Which ionospheric layer is closest to the surface of Earth? (G3C01)
ANSWER: The D layer

QUESTION: Which ionospheric layer is the most absorbent of long skip signals during daylight hours on frequencies below 10 MHz? (G3C11)
ANSWER: The D layer

QUESTION: Why is long-distance communication on the 40-meter, 60-meter, 80-meter, and 160-meter bands more difficult during the day? (G3C05)
ANSWER: The D layer absorbs signals at these frequencies during daylight hours

For a radio wave to be refracted by the ionosphere, it has to hit the ionosphere at an appropriate angle. If the angle is too steep, the radio wave will not be refracted, but instead pass right through the ionosphere. The angle at which this begins to happen is called the critical angle.

QUESTION: What does the term “critical angle” mean, as used in radio wave propagation? (G3C04)
ANSWER: The highest takeoff angle that will return a radio wave to Earth under specific ionospheric conditions

One interesting propagation phenomenon is scatter propagation. When scatter propagation occurs, a small portion of the signal is reflected back in the direction of the transmitting station. This scattered signal can then be heard in the skip zone, which is the area between the transmitting station and where the first hop hits the Earth. This distance is too far for ground wave propagation, but too near for normal sky-wave propagation. Indications that you might be receiving a signal via scatter propagation is they are weaker than you would expect and may sound wavering or distorted.

QUESTION: What type of propagation allows signals to be heard in the transmitting station’s skip zone? (G3C09)
ANSWER: Scatter

QUESTION: Why are HF scatter signals in the skip zone usually weak? (G3C08)
ANSWER: Only a small part of the signal energy is scattered into the skip zone

QUESTION: What is a characteristic of HF scatter? (G3C06)
ANSWER: They have a wavering sound

QUESTION: What makes HF scatter signals often sound distorted? (G3C07)
ANSWER: Energy is scattered into the skip zone through several different radio wave paths

Another interesting phenomenon is Near Vertical Incidence Skywave (NVIS) propagation. Antennas designed to take advantage of this propagation mode send signals at an angle close to 90 degrees, and if conditions are right, the ionosphere reflects that signal back to the Earth at a very short distance from the transmitting station.

QUESTION: What is Near Vertical Incidence Skywave (NVIS) propagation? (G3C10)
ANSWER: Short distance MF or HF propagation using high elevation angles