E3C07 (D)
Which of the following descriptors indicates the greatest solar flare intensity?
A. Class A
B. Class B
C. Class M
D. Class X


E3C - Radio-path horizon; less common propagation modes; propagation prediction techniques and modeling; space weather parameters and amateur radio

While we say that VHF/UHF communication is “line of sight,” the distance that a VHF/UHF radio wave will travel is slightly longer than the line-of-sight distance. We call this distance the “radio horizon” or “radio-path horizon.” The VHF/UHF radio-path horizon distance exceeds the geometric horizon by approximately 15% of the distance. (E3C06) The radio-path horizon distance exceeds the geometric horizon because of downward bending due to density variations in the atmosphere. (E3C14)

Amateur radio operators may sometimes use ground-wave propagation to communicate. One important thing to know about this type of propagation is that the maximum distance of ground-wave propagation decreases when the signal frequency is increased. (E3C12) Vertical polarization is the best type of polarization for ground-wave propagation. (E3C13) So, if you really want to make a contact via ground wave, use a vertical antenna on the 160m band.

Prediction techniques and modeling

Since it's very advantageous to know the propagation conditions for a particular signal path, many hams use propagation prediction software to choose what frequency bands to use. One of these software packages is called VOACAP, because it was developed by engineers at the Voice of America (VOA).  VOACAP software models HF propagation. (E3C11)

Often, these types of software packages use a technique called ray tracing. In regard to radio communications, the term ray tracing describes the process of modeling a radio wave's path through the ionosphere. (E3C01)

Propagation prediction software uses data such as the A index and K index to model propagation. Both of these provide a measure of geomagnetic activity, which affects HF propagation. A rising A or K index indicates increasing disruption of the geomagnetic field. (E3C02) A high A-index or K-index usually means that HF propagation will be poor. Polar paths are most likely to experience high levels of absorption when the A index or K index is elevated. (E3C03)

The interplanetary magnetic field has a very strong affect on HF propagation. The value of Bz (B subZ) represents the direction and strength of the interplanetary magnetic field. (E3C04) A southward orientation of Bz (B sub z) increases the likelihood that incoming particles from the Sun will cause disturbed conditions. (E3C05) HF propagation is generally poor when Bz is oriented southward and geomagnetic conditions are disturbed.

Space weather parameters and amateur radio

Because solar radiation creates the ionosphere, solar activity has a great impact on radio wave propagation. These conditions are often referred to as space weather.

Solar flares, for example, emit an enormous amount of radiation. Depending on how much radiation they emit, the effect on HF propagation can be good or bad. If the Earth receives only enough radiation to increase ionization in the upper layers of the ionosphere, HF propagation is improved.

If the level of radiation is higher, the lower levels of the ionosphere could become more energized.

When this happens, they absorb more RF energy, and HF propagation is poor. A sudden rise in radio background noise indicates that a solar flare has occurred. (E3C15)

Solar flares are categorized by intensity. Class A solar flares are the least intense. Class X is the descriptor that indicates the greatest solar flare intensity. (E3C07) In between Class A and Class X are Class B, Class C, and Class M.

Within each class, solar flares are assigned a value from 1 – 9. The intensity of an X3 flare is twice as great as that of an X2 flare. (E3C09)

The National Oceanic and Atmospheric Administration (NOAA) has developed what they call Space Weather Scales to communicate to the general public the current and future space weather conditions

and their possible effects on people and systems. The G scale indicates geomagnetic conditions. The space weather term G5 means an extreme geomagnetic storm. (E3C08)

Sunspots are often used to predict HF propagation conditions. The more sunspots, the better HF propagation is. Parameter 304a may, however, be an even better indicator of radio conditions. The 304A

solar parameter measures UV emissions at 304 angstroms, correlated to solar flux index. (E3C10) These UV emissions are one of the principle causes of F2 layer ionization.