G-3

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SUBELEMENT G3 – RADIO WAVE PROPAGATION

[3 Exam Questions – 3 Groups]

 

G3A – Sunspots and solar radiation; ionospheric disturbances; propagation forecasting and indices

Higher sunspot numbers generally indicate a greater probability of good propagation at higher frequencies. The Sun’s rotation on its axis causes HF propagation conditions to vary periodically in a 28 day cycle. Long-distance communication in the upper HF and lower VHF range are enhanced by high sunspot numbers. At any point in the solar cycle the 20-meter band usually support worldwide propagation during daylight hours. 15 meters, 12 meters and 10 meters are least reliable for long distance communications during periods of low solar activity.

Measure of solar radiation at 10.7 centimeters wavelength is the solar flux index. K-index is the short term stability of the Earth’s magnetic field. A-index is the long term stability of the Earth’s geomagnetic field. A temporary disturbance in the Earth’s magnetosphere is called a geomagnetic storm. A geomagnetic storm degrades high-latitude HF propagation

It takes approximately 8 minutes for ultraviolet and X-ray radiation from solar flares to affect radio propagation on the Earth. It takes 20 to 40 hours for charged particles from coronal mass ejections to affect radio propagation on the Earth. Charged particles that reach the Earth from solar coronal holes cause HF communications to be disturbed

Sudden Ionospheric Disturbance disrupts signals on lower frequencies more than those on higher frequencies. Radio communications resulting from periods of high geomagnetic activity (Auroras) that can reflect VHF signals.

G3B – Maximum Usable Frequency; Lowest Usable Frequency; propagation

 MUF stand for the Maximum Usable Frequency for communications between two points. LUF stands for the Lowest Usable Frequency for communications between two points. Radio waves with frequencies below the LUF are completely absorbed by the ionosphere. When radio waves with frequencies below the MUF and above the LUF are sent into the ionosphere they are bent back to the Earth. When selecting a frequency for lowest attenuation when transmitting on HF select a frequency just below the MUF. When the LUF exceeds the MUF no HF radio frequency will be support by ordinary sky-wave communications over the path. Path distance, location, time of day, season, solar radiation and ionospheric disturbances affect the MUF. Listen for signals from an international beacon in the frequency range you plan to use 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.

2,500 miles is the approximate maximum distance along the Earth’s surface that is normally covered in one hop using the F2 region. 1,200 miles is the approximate maximum distance along the Earth’s surface that is normally covered in one hop using the E region. Short skip sky-wave propagation on the 10-meter band is a good indicator of the possibility of sky-wave propagation on the 6-meter band. When a sky-wave signal arrives at your receiver by both short path and long path propagation, a well-defined echo might be heard

G3C – Ionospheric layers; critical angle and frequency; HF scatter; Near Vertical Incidence Sky-wave

Ionospheric layer called the D layer is closest to the surface of the Earth. Long distance communication on the 40-meter, 60-meter, 80-meter and 160-meter bands more difficult during the day because the D layer absorbs signals at these frequencies during daylight hours. D layer is the most absorbent of long skip signals during daylight hours on frequencies below 10 MHz. F2 region mainly is responsible for the longest distance radio wave propagation because it is the highest ionospheric region. When the Sun is overhead ionospheric layers reach their maximum height. The term “critical angle” means the highest takeoff angle that will return a radio wave to the Earth under specific ionospheric conditions

Short distance MF or HF propagation using high elevation angles is Near Vertical Incidence Sky-wave (NVIS) propagation. 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

Scatter propagation allows a signal to be detected at a distance too far for ground wave propagation but too near for normal sky-wave propagation. 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 wavering sound is a characteristic of HF scatter signals. Energy is scattered into the skip zone through several different radio wave paths makes HF scatter signals often sound distorted. The signal is heard on a frequency above the Maximum Usable Frequency is an indication that signals heard on the HF bands are being received via scatter propagation.

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