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Subelement E3
Section E3C
Radio-path horizon; less common propagation modes; propagation prediction techniques and modeling; space weather parameters and amateur radio
What does the term ray tracing describe in regard to radio communications?
  • The process in which an electronic display presents a pattern
  • Modeling a radio wave's path through the ionosphere
  • Determining the radiation pattern from an array of antennas
  • Evaluating high voltage sources for X-Rays

The ionosphere has a non-uniform index of refraction to radio waves propagating through it, since its electron density is a complex function of space and time. At each change of refraction index, a propagating radio wave's direction changes according to Snell's Law (think of the bending of light at an air-water interface).

To model a radio wave path through the ionosphere, a ray tracing code models the ionosphere as a medium with a varying refractive index, and then applies Snell's law at each small change to compute the ray's final path.

VOACAP is one example of a ray tracing code developed over a number of years by a shortwave broadcaster (Voice of America = US government external broadcasts). It is freely available - for more information, see


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What is indicated by a rising A or K index?
  • Increasing disruption of the geomagnetic field
  • Decreasing disruption of the geomagnetic field
  • Higher levels of solar UV radiation
  • An increase in the critical frequency

A and K indices are about the level of activity in the geomagnetic field. If the indices are increasing so are the disturbances in the geomagnetic field.

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Which of the following signal paths is most likely to experience high levels of absorption when the A index or K index is elevated?
  • Transequatorial propagation
  • Polar paths
  • Sporadic-E
  • NVIS

Think of the magnetic fields of Earth being concentrated at the North and South poles. So if there's a magnetic disruption, you'd expect it to effect the magnetic poles the most... therefore "polar paths."

An additional memory tool is to look at the "A" and "K" indexes from the question. AK = Alaska, Alaska has Polar Bears...Polar Paths.

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What does the value of Bz (B sub Z) represent?
  • Geomagnetic field stability
  • Critical frequency for vertical transmissions
  • Direction and strength of the interplanetary magnetic field
  • Duration of long-delayed echoes

Hint: There are two parts of the symbol Bz (B sub Z) The answer contains both parts Direction and Strength

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What orientation of Bz (B sub z) increases the likelihood that incoming particles from the Sun will cause disturbed conditions?
  • Southward
  • Northward
  • Eastward
  • Westward

The orientation of the magnetic field in a coronal mass ejection from the sun is a huge factor in determining the strength of the event's effects here at Earth.

When coronal mass ejections occur on the Sun, a huge amount of energy is released from violent reconfigurations of the complex magnetic field surrounding the star. These events cause high energy charged particles to stream outwards from the sun's outer atmosphere, or corona, into the solar system. Since the particles are charged, they carry with them magnetic field perturbations as they move outwards that are partially (but not completely) reflective of the conditions at the sun when they were released.

If the CME's particles encounter Earth, we have a geomagnetic storm. One of the biggest regulators of the strength of effects of the particles on our magnetosphere and ionosphere is in the disturbance magnetic field's Z orientation (\(+\) = toward our north pole, \(-\) = toward our south pole), labeled as Bz.

If Bz is southward in the incoming particle cloud, this is opposite to our Earth's normal magnetic field - think of the bar magnet repelling a magnetic with opposite polarity. In this case, the CME associated Bz then pushes on the Earth's magnetic field and compresses it, dumping energy into the magnetosphere. The energy input can lead to oscillations in the magnetic field as the Earth 'fights back'.

Since the ionosphere is tightly bound to the background magnetic field lines, it too begins to move and oscillate, quickly developing lots of irregularities, scintillation, and other structure that degrades long distance HF propagation. This is why Bz is a key factor in determining geomagnetic storm strength at Earth.

[FYI, NASA maintains spacecraft monitors just upstream of earth - the Advanced Composition Explorer (ACE) and the new DSCOVR satellite - that have comprehensive space environment monitors, and one of the key variables measured there is the orientation of B.]


Think when things "go south" something bad is happening.


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By how much does the VHF/UHF radio horizon distance exceed the geometric horizon?
  • By approximately 15 percent of the distance
  • By approximately twice the distance
  • By approximately 50 percent of the distance
  • By approximately four times the distance

VHF and UHF have a relatively short range due to the fact that the high frequencies can't get bent by the atmosphere, which makes the range about seeing distance.

You can then guess that the radio waves will extend only slightly farther than the horizon, which is 15% that distance.

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Which of the following descriptors indicates the greatest solar flare intensity?
  • Class A
  • Class B
  • Class M
  • Class X

Solar flares are ranked A, B, C, M, and X with "A" being the lowest and "X" the highest. Intensity increases in alphabetic order.


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What does the space weather term G5 mean?
  • An extreme geomagnetic storm
  • Very low solar activity
  • Moderate solar wind
  • Waning sunspot numbers


G5 is an extreme geomagnetic storm with the following effects:

  • Power systems: Widespread voltage control problems and protective system problems can occur; some grid systems may experience complete collapse or blackouts. Transformers may experience damage.

  • Spacecraft operations: May experience extensive surface charging and problems with orientation, uplink/downlink, and tracking satellites.

  • Other systems: Pipeline currents can reach hundreds of amps, HF (high frequency) radio propagation may be impossible in many areas for one to two days, satellite navigation may be degraded for days, low-frequency radio navigation can be out for hours, and auroras have been seen as low as Florida and southern Texas (typically 40° geomagnetic lat.).

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How does the intensity of an X3 flare compare to that of an X2 flare?
  • 10 percent greater
  • 50 percent greater
  • Twice as great
  • Four times as great

The expected answer is twice as great, so that is what you should choose on the exam. This is, however, incorrect.

The A, B, C, M, X scale is logarithmic, where each letter in the index represents a 10-fold increase. So an X-class flare is 10 times more powerful than an M-class flare, and 100 times more powerful than a C-class flare.

Within each class there is a linear scale that goes from 1–9.

(NOTE: as there is no letter beyond X, scientists continue the linear scale beyond X9. A very powerful flare in 2003 measured X28 before the sensors cut out!)

Imagine that this is how big an X1 is:

  • [----]

That would make an X2 this big:

  • [----][----]

And an X3 would be this big:

  • [----][----][----]

In other words, X3 is not twice as big as X2, but only 50% bigger.



Wikipedia's article on Solar flares is actually a little confusing. I'd suggest the NASA and UNC links above.

Note: The last great X2 occurred in October 2013.

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What does the 304A solar parameter measure?
  • The ratio of X-Ray flux to radio flux, correlated to sunspot number
  • UV emissions at 304 angstroms, correlated to solar flux index
  • The solar wind velocity at 304 degrees from the solar equator, correlated to solar activity
  • The solar emission at 304 GHz, correlated to X-Ray flare levels

304A can be thought of as 304 Å (Angstroms). The word angstroms only occurs in the correct answer: UV emissions at 304 angstroms, correlated to solar flux index.

But what is the Solar Flux Index, and what does it have to do with Ultraviolet Light at 304 Å?

Solar Flux Index (SFI) is measured in solar flux units (SFUs). SFI is defined as the amount of flux (radio noise) emitted at 2800 MHz. Because 2800 MHz correlates with a wavelength of 10.7 cm, it is also referred to as the 10.7 cm Flux Index.

An ångström or angstrom unit (Symbol: Å or A) is a unit of length equal to 10–10 meters, used principally to express the wavelengths of electromagnetic radiation. It is named after Swedish physicist Anders Jonas Ångström.

This problem refers to λ = 304 Å = 30.4 nm.

The 304A Index, often shortened to just 304A, is a

"NOAA reported value from 0 to unknown. Indicates relative strength of total solar radiation at a wavelength of 304 angstroms (or 30.4 nm), emitted primarily by ionized helium in the sun's photosphere. Two measurements are available for this parameter, one measured by the Solar Dynamics Observatory, using the EVE instrument, and the other, using data from the SOHO satellite, using its SEM instrument. Responsible for about half of all the ionization of the F layer in the ionosphere. 304A looseley correlates to SFI. The background level - at solar minimums - will typically be around 134, and at solar maxima can exceed 200 or more. Updated hourly."


Additional references:







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What does VOACAP software model?
  • AC voltage and impedance
  • VHF radio propagation
  • HF propagation
  • AC current and impedance

VOACAP stands for Voice of America Coverage Analysis Program. It is used to predict HF propagation.

VOA is the clue contained in the question. As a shortwave broadcast operation, HF propagation would be the one thing they might be most interested in.

See HF predictions map.

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How does the maximum distance of ground-wave propagation change when the signal frequency is increased?
  • It stays the same
  • It increases
  • It decreases
  • It peaks at roughly 14 MHz

Ground waves are a result of interaction of the radio signal with the ground. The interaction has the effect of causing the signal to follow the curvature of the earth. Ground wave propagation is most useful on the 1.8MHz and 3.5MHz bands during daytime.

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What type of polarization is best for ground-wave propagation?
  • Vertical
  • Horizontal
  • Circular
  • Elliptical

Ground waves are a result of interaction of the radio signal with the ground. The interaction has the effect of causing the signal to follow the curvature of the earth. Ground wave propagation is most useful on the 1.8 MHz and 3.5 MHz bands during daytime.

One way to remember this answer is to consider commercial AM broadcast: ground-wave propagation via vertical towers that polarize vertically.

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Why does the radio-path horizon distance exceed the geometric horizon?
  • E-region skip
  • D-region skip
  • Downward bending due to aurora refraction
  • Downward bending due to density variations in the atmosphere

The effect is caused by inversion layers (density changes) in the atmosphere. This effect is more commonly referred to as tropospheric ducting. An excellent write up can be found here: https://en.wikipedia.org/wiki/Tropospheric_propagation

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What might a sudden rise in radio background noise indicate?
  • A meteor ping
  • A solar flare has occurred
  • Increased transequatorial propagation likely
  • Long-path propagation is occurring

Solar flares cause a sudden rise in background noise. So, if you detect a sudden rise in radio background noise, it might have been caused by a solar flare.

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