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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
  • Correct Answer
    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


Hint: You trace a path.

Last edited by katzelover. Register to edit

Tags: arrl chapter 10 arrl module 10a

What is indicated by a rising A or K index?
  • Correct Answer
    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
  • Correct Answer
    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.

Cheat: “Elevated -> North -> Polar

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

The interplanetary magnetic field (IMF) is that part of the sun's magnetic field which is spread throughout the solar system by solar wind. \(B_Z\) describes the component of the IMF's direction and strength most relevant to space weather, and thus to radio.


In physics, the symbol B is used to denote a magnetic field. Field strength is often described as a three-dimensional vector with \(x\), \(y\), and \(z\) components. In the case of the IMF, the axes are oriented such that \(x\) points from the Earth to the sun, \(y\) is in the plane of the ecliptic tangent to the Earth's orbit, and \(z\) is perpendicular to both*, roughly along the Earth's magnetic axis from north to south.**

Normally, the Earth's magnetosphere repels most of the energy from the solar wind, which prevents our atmosphere from blowing away.*** But when \(B_Z\) tends southward, the polarity of the sun's magnetic field is opposite Earth's, which allows the two fields to connect rather than repulse each other.

When this happens during solar storms (and particularly coronal mass ejections), large amounts of energy from the solar wind may transfer along these connections and even temporarily reconfigure the Earth's magnetosphere, leading to many dramatic and disruptive phenomena including aurora and geomagnetic storms.

Further reading (with diagrams): http://blog.aurorasaurus.org/?p=178

* This is the Geocentric Solar Magnetospheric coordinate system (GSM).
** Technically \(B_Z\) points toward the "ecliptic pole."
*** This is what happened to Mars.

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What orientation of Bz (B sub z) increases the likelihood that incoming particles from the Sun will cause disturbed conditions?
  • Correct Answer
  • 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 Z orientation (\(+\) = toward our north pole, \(-\) = toward our south pole) of the disturbance's magnetic field. That Z orientation is labeled as Bz.

If \(B_Z\) is southward in the incoming particle cloud, this is opposite to our Earth's normal magnetic field -- think of a bar magnet repelling a magnet 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 structures that degrade 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?
  • Correct Answer
    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 very much, which makes the range about seeing distance.

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

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Which of the following descriptors indicates the greatest solar flare intensity?
  • Class A
  • Class B
  • Class M
  • Correct Answer
    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. After X it goes X1, X2, X3, etc. The highest-rated solar flare ever was an X26 in 1859

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

The NOAA Geomagnetic Storm Scale indicates the severity of geomagnetic storms. It is denoted by a G followed by a number from 1 to 5, with 1 being a minor event, and 5 being an extreme event. Power systems: Weak power grid fluctuations can occur.

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.).

See also the NOAA Space Weather Scales

Hint: The G in G5, and geomagnetic.

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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
  • Correct Answer
    Twice as great
  • Four times as great

NOTE: This question appears in both the 2016-2020 and the 2020-2024 pool, but unfortunately in the official question pool for 2016-2020 it incorrectly has the answer flagged as "twice as great" when the correct answer is actually "50 percent greater". The answer is correct in the 2020-2024 version of the extra class question pool.

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
  • Correct Answer
    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
  • Correct Answer
    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
  • Correct Answer
    It decreases
  • It peaks at roughly 14 MHz

Ground waves are waves emitted at less than a wavelength above the ground that travel in contact with the surface of the earth. At higher frequencies like most of HF and above this is especially bad; the waves are severely attenuated and nothing amazingly good happens.

At 160m and below frequencies with vertical polarization though they may be able to travel a few hundred miles since they can get refracted in such a way as to follow the curve of the earth without too much attenuation, whereas at 10m they tend to die out in 10 miles or less. Note that this requires vertical polarization; horizontal waves will short circuit immediately since the whole electrical field tends to end up in the somewhat conductive ground.

How well surface wave propagation works depends on the conductivity of the soil or water that makes up the earth's surface on the path.

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?
  • Correct Answer
  • 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.

This only works with vertical waves; horizontal waves will short circuit immediately since the whole electrical field tends to end up in the somewhat conductive ground.

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
  • Correct Answer
    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
  • Correct Answer
    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. -k6yxh

Silly way to remember:

voices rise as tempers flare

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