G3 - Radio Wave Propagation

A Sudden Ionospheric Disturbance (SID) disrupts signals on lower frequencies more than those on higher frequencies effecting the daytime ionospheric propagation of HF radio waves. An amateur station wishing to continue communications during a sudden ionospheric disturbance could try a higher frequency. It takes approximately 8 minutes for the increased ultraviolet and X-ray radiation from solar flares to affect radio-wave propagation on the Earth.

The solar flux index measures the radio energy emitted by the sun. It is a measure of solar activity at 10.7 cm.

A geomagnetic disturbance is a significant change in the Earth's magnetic field over a short period of time. A geomagnetic storm causes degraded high-latitude HF propagation. The latitudes greater than 45 degrees North or South have propagation paths that are more sensitive to geomagnetic disturbances.

The sunspot number is a measure of solar activity based on counting sunspots and sunspot groups. When sunspot numbers are high long-distance communication in the upper HF and lower VHF range is enhanced. The typical sunspot cycle is approximately 11 years. At any point in the solar cycle the 20 meter band usually supports worldwide propagation during daylight hours. Frequencies above 20 MHz are least reliable for long distance communications during periods of low solar activity.

The K-index is a measure of the short term stability of the Earth’s magnetic field. The A-index is an indicator of the long term stability of the Earth’s geomagnetic field.

When charged particles reach the Earth from solar coronal holes HF radio communications are disturbed. It takes charged particles 20 to 40 hours from Coronal Mass Ejections to affect radiowave propagation on the Earth. A possible benefit to radio communications resulting from periods of high geomagnetic activity is that Aurora that can reflect VHF signals.

If the HF radio-wave propagation (skip) is generally good on the 24-MHz and 28-MHz bands for several days, you might expect a similar condition to occur again 28 days later.

MUF stands for Maximum Usable Frequency for communications between two points. Ionospheric Absorption will be minimum near the maximum usable frequency (MUF). When transmitting on HF, select a frequency just below the MUF for lowest attenuation. Radio waves with frequencies below the maximum usable frequency (MUF) sent into the ionosphere are bent back to the Earth. The 15 meter band should offer the best chance for a successful contact if the maximum usable frequency (MUF) between the two stations is 22 MHz. The 20 meter band should offer the best chance for a successful contact if the maximum usable frequency (MUF) between the two stations is 16 MHz. A reliable way to determine if the maximum usable frequency (MUF) is high enough to support 28-MHz propagation between your station and Western Europe is to listen for signals on a 28 MHz international beacon.

LUF stands for "Lowest Usable Frequency" for communications between two points. Radio waves with frequencies below the lowest usable frequency (LUF) are completely absorbed by the ionosphere. Long distance communication on the 40, 60, 80 and 160 meter bands is more difficult during the day because the D layer absorbs these frequencies during daylight hours. The D layer in the ionosphere is the most absorbent of long skip signals during daylight hours on frequencies below 10 MHz. It is closest to the surface of the Earth.

When the lowest usable frequency (LUF) exceeds the maximum usable frequency (MUF) no HF radio frequency will support communications over the path.

The maximum distance along the Earth's surface that is normally covered in one hop using the E region is 1,200 miles. The maximum distance along the Earth's surface that is normally covered in one hop using the F2 region is 2,500 miles. It can be expected to reach its maximum height at your location at noon during the summer. The F2 region is mainly responsible for the longest distance radio wave propagation because it is the highest ionospheric region.

Factors that affect the maximum usable frequency (MUF) include ALL of the following -- Path distance and location, Time of day and season and Solar radiation and ionospheric disturbance.

The type of radio wave propagation that allows a signal to be detected at a distance too far for ground wave propagation but too near for normal sky wave propagation is called "scatter". If a signal is heard on a frequency above the maximum usable frequency it might be an indication that signals heard on the HF bands are being received via scatter propagation. A characteristic of HF scatter signals is that they have a wavering sound. HF scatter signals often sound distorted because energy is scattered into the skip zone through several radio wave paths. HF scatter signals in the skip zone are usually weak because only a small part of the signal energy is scattered into the skip zone.

A sky-wave signal that arrives at your receiver by both short path and long path propagation will have a well-defined echo.

Short hop sky-wave propagation on the 10 meter band is a good indicator of the possibility of sky-wave propagation on the 6 meter band.

The term “critical angle” used in radio wave propagation means the highest takeoff angle that will return a radio wave to the Earth under specific ionospheric conditions. Near Vertical Incidence Sky-wave (NVIS) propagation is short distance HF propagation using high elevation angles.

A horizontal dipole placed between 1/8 and 1/4 wavelength above the ground will be most effective for skip communications on 40 meters during the day.