VHF/UHF propagation is mostly “line of sight,” meaning that without any objects to reflect the radio waves, they will travel directly from the transmitter to the receiver. In practice, 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,” and it’s about 15% longer than the line-of-sight distance. The radio-path horizon distance extends beyond the geometric horizon because of downward bending due to density variations in the atmosphere.
QUESTION: By how much does the VHF/UHF radio horizon distance exceed the geometric horizon? (E3C06)
ANSWER: By approximately 15 percent of the distance
QUESTION: Why does the radio-path horizon distance exceed the geometric horizon? (E3C14)
ANSWER: Downward bending due to density variations in the atmosphere
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. Vertical polarization is the best type of polarization for ground-wave propagation. So, if you really want to make a contact via ground wave, use a vertical antenna on the 160m band.
QUESTION: How does the maximum range of ground-wave propagation change when the signal frequency is increased? (E3C12)
ANSWER: It decreases
QUESTION: What type of polarization is best for ground-wave propagation? (E3C13)
ANSWER: Vertical
Prediction techniques and modeling
If you’re interesting in contacting distant stations on the HF bands, it’s very advantageous to know the propagation conditions for a particular signal path. That’s why 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.
QUESTION: What does VOACAP software model? (E3C11)
ANSWER: HF propagation
Often, software packages that model HF propagation use a technique called ray tracing. Ray tracing models a radio wave’s path through the ionosphere.
QUESTION: What does the radio communication term “ray tracing” describe? (E3C01)
ANSWER: Modeling a radio wave’s path through the ionosphere
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, and generally 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.
QUESTION: What is indicated by a rising A or K index? (E3C02)
ANSWER: Increasing disruption of the geomagnetic field
QUESTION: Which of the following signal paths is most likely to experience high levels of absorption when the A index or K index is elevated? (E3C03)
ANSWER: Polar
The interplanetary magnetic field has a very strong affect on HF propagation. The value of Bz (B sub Z) represents the direction and strength of the interplanetary magnetic field. A southward orientation of Bz (B sub z) increases the likelihood that incoming particles from the Sun will cause disturbed conditions. HF propagation is generally poor when Bz is oriented southward and geomagnetic conditions are disturbed.
QUESTION: What does the value of Bz (B sub Z) represent? (E3C04)
ANSWER: Direction and strength of the interplanetary magnetic field
QUESTION: What orientation of Bz (B sub z) increases the likelihood that incoming particles from the sun will cause disturbed conditions? (E3C05)
ANSWER: Southward
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.
QUESTION: What might be indicated by a sudden rise in radio background noise across a large portion of the HF spectrum? (E3C15)
ANSWER: A solar flare has occurred
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. 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. An X3 flare is 50% more intense than an X2 flare.
QUESTION: Which of the following descriptors indicates the greatest solar flare intensity? (E3C07)
ANSWER: Class X
QUESTION: How does the intensity of an X3 flare compare to that of an X2 flare? (E3C09)
ANSWER: 50 percent greater
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.
QUESTION: What does the space weather term G5 mean? (E3C08)
ANSWER: An extreme geomagnetic storm
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, and these emissions are proportional to the solar flux index. These UV emissions are one of the principle causes of F2 layer ionization.
QUESTION: What does the 304A solar parameter measure? (E3C10)
ANSWER: UV emissions at 304 angstroms, correlated to the solar flux index
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