E3 Radio Wave Propagation

D. 12,000 miles, if the moon is “visible” by both stations

Earth-Moon-Earth (EME) signals go up to the Moon and back, so any two stations that can both see the Moon at the same time can make contact. The Moon is above the horizon for about half the Earth at once, so the stations can be nearly on opposite sides of the planet: about half of Earth's 24,900-mile circumference, or roughly 12,000 miles apart.

• A. EME is not limited to 2,000 miles; the Moon's distance at perigee changes the path loss slightly but has nothing to do with how far apart the stations can be.
• B. Apogee adds a little path loss because the Moon is farther away, but the separation limit is set by both stations seeing the Moon, which allows far more than 2,000 miles.
• C. Perigee lowers path loss by about 2 dB, but it does not create a 5,000-mile limit; mutual visibility of the Moon allows separations of about 12,000 miles.

B. A fluttery, irregular fading

Libration is the slight apparent rocking of the Moon as seen from Earth. The Moon's rough surface reflects the signal from many points at once, and the rocking keeps changing how those reflections add and cancel, so EME echoes have a rapid, fluttery, irregular fading.

• A. A slow change in pitch comes from the gradually changing Doppler shift as the Earth turns and the Moon moves; libration fading is a rapid change in signal strength.
• C. A gradual loss of signal at sunrise describes daytime absorption of HF signals; EME signals at VHF and higher pass through the ionosphere, and libration fading is a rapid flutter.
• D. An echo shifted in frequency is Doppler shift from the relative motion of the Earth and Moon; it is a frequency offset, while libration fading is a change in amplitude.

A. When the Moon is at perigee

EME path loss grows with the distance to the Moon, so it is lowest at perigee, the Moon's closest point to Earth. The Moon is roughly 363,000 km away at an average perigee and 405,500 km at an average apogee, a difference that changes the round-trip path loss by about 2 dB.

• B. The Moon's phase only describes how much of its sunlit side we see; it does not change the Moon's distance or how it reflects radio signals, so it does not set path loss.
• C. Apogee is the Moon's farthest point from Earth, so path loss is highest then, about 2 dB worse than at perigee.
• D. The MUF describes how well the ionosphere returns HF signals; EME uses VHF and higher frequencies that pass through the ionosphere, so the MUF does not affect EME path loss.

D. It travels at a right angle to the electric and magnetic fields

An electromagnetic wave consists of an electric field and a magnetic field at right angles to each other, and the wave travels in the direction perpendicular to both. If the electric field is vertical and the magnetic field is horizontal, the wave moves forward at a right angle to each of them.

• A. The direction of travel is fixed by the orientation of the two fields, not by the phase angle of the magnetic field.
• B. The wave travels perpendicular to its fields, not parallel to them; both fields lie across the direction of travel.
• C. The phase angle of the electric field does not set the direction of travel, which is at right angles to both fields whatever their phase.

C. They are at right angles

In an electromagnetic wave the electric and magnetic fields are perpendicular to each other, and both are perpendicular to the direction of travel. A vertically polarized wave, for example, has a vertical electric field and a horizontal magnetic field.

• A. The fields are not parallel; they cross at 90 degrees, with both lying across the direction of travel.
• B. Tangential does not describe how the two fields relate; they are perpendicular to each other.
• D. Being 90 degrees out of phase describes timing, not orientation; in a radiated wave far from the antenna, the electric and magnetic fields rise and fall in phase.

B. Switch to a lower frequency HF band

The maximum usable frequency (MUF) is the highest frequency the ionosphere returns to Earth over a given path. After dark the ionization fades and the MUF falls, so signals on a band now above the MUF pass through into space; moving to a lower frequency band keeps the signal below the MUF and the contact going.

• A. A higher frequency band is even further above the falling MUF, so the signal passes through the ionosphere instead of returning to Earth.
• C. A higher takeoff angle shortens each hop and lowers the frequency the ionosphere can return over the path, which makes a long-distance contact harder, not easier.
• D. Beam width does not change the MUF; a broader antenna pattern cannot make the ionosphere return a frequency above the MUF.

C. Large bodies of water

A microwave duct forms where a temperature inversion or a sharp change in humidity bends signals back toward Earth and traps them in a layer of air. These conditions are common over large bodies of water, such as seas, coastlines and large lakes, where cool, moist air near the surface lies under warmer, drier air.

• A. Mountain ranges block line-of-sight microwave paths; ducts need stable, layered air of the kind that forms over smooth surfaces such as water.
• B. Stratocumulus clouds are a type of cloud, not a duct; microwave ducts are layers of air with sharp temperature or humidity changes, most often over water.
• D. Nimbus clouds are rain clouds, and rain absorbs and scatters microwave signals rather than guiding them.

A. The E region

Meteors burn up roughly 85 to 120 km (about 50 to 75 miles) above the Earth, the height of the E region. Each meteor leaves a short-lived trail of ionized gas there that can reflect VHF signals, which is the basis of meteor-scatter communication.

• B. The F1 region lies above the altitude where meteors burn up and forms mainly in daytime, so it is not where meteor trails form.
• C. The F2 region is the highest ionospheric region, well above where meteors burn up; it supports long-distance HF skip, not meteor trails.
• D. The D region is the lowest region, below the altitude where most meteor trails form; it mainly absorbs low-frequency HF signals during the day.

C. 28 MHz - 148 MHz

Meteor trails reflect lower frequencies more strongly and for longer, but HF signals are already returned by the regular ionosphere and absorbed by the D region. The 28 to 148 MHz range, which includes the 10, 6 and 2 meter bands, is low enough for useful trail reflections yet high enough that the trails, rather than the normal ionosphere, carry the signal.

• A. 1.8 to 1.9 MHz is in the 160 meter band, where the normal ionosphere and daytime D-region absorption govern propagation, so meteor trails add little.
• B. 10 to 14 MHz signals are routinely returned by the regular ionosphere, so meteor scatter is not a practical mode there.
• D. Reflections from meteor trails become much weaker and briefer as frequency rises, so 220 to 450 MHz is harder to use than the lower VHF range.

D. The index of refraction

The index of refraction (n) of a medium is the ratio of the speed of light in a vacuum to the speed in that medium, so the wave speed is v = c / n. In a medium with n = 1.5, for example, a wave travels at about 200,000 km per second, two thirds of its speed in free space.

• A. Resistance and reactance describe opposition to current in a circuit, not the speed of a wave through a medium.
• B. Evanescence describes a field that decays with distance instead of traveling, such as a signal in a waveguide below its cutoff frequency; it does not set wave speed.
• C. Birefringence is a property of some materials that have two different indices of refraction depending on polarization; the index of refraction itself is what sets the speed.

B. 100 miles to 300 miles

A tropospheric duct traps microwave signals in a layer of air and guides them well beyond line of sight. Typical duct paths on the microwave bands span about 100 to 300 miles, although rare, exceptional ducts support much longer paths.

• A. 10 to 50 miles is within the range of ordinary line-of-sight paths, which need no duct; ducts carry signals much farther.
• C. 1,200 miles is far beyond the typical duct range; paths that long happen only on rare occasions.
• D. 2,500 miles is about the length of rare long duct paths such as California to Hawaii, not a typical range.

C. Severe geomagnetic storms

Auroral propagation uses the ionization of the aurora, which forms when a geomagnetic storm drives charged particles into the upper atmosphere near the poles. The stronger the storm, the farther the aurora extends toward lower latitudes and the more likely VHF signals can be reflected from it.

• A. Meteor showers create brief ionized trails used for meteor scatter, a different mode that does not depend on the aurora.
• B. Quiet geomagnetic conditions keep the aurora weak and close to the poles, so auroral propagation is unlikely.
• D. Low-pressure weather systems are in the lower atmosphere and do not produce the ionization of an aurora.

A. CW

Signals reflected from the moving, irregular ionization of an aurora are spread in frequency, giving them a raspy, buzzing or hissing tone. CW stays readable as on-off keying even when distorted this way, while voice and many data modes become hard to copy.

• B. SSB voice is badly garbled by the buzzing distortion of auroral reflections, so it is harder to copy than CW.
• C. FM uses a wider bandwidth and needs a stronger, steadier signal than CW, so weak, distorted auroral reflections make it a poor choice.
• D. RTTY depends on clean shifts between two tones, which the frequency spreading of auroral reflections smears, so it suffers more than CW.

B. Waves with rotating electric and magnetic fields

In a circularly polarized wave the electric and magnetic fields rotate as the wave travels, making one full turn every wavelength, either clockwise or counterclockwise. Helical antennas and crossed dipoles fed 90 degrees out of phase produce this polarization, which is common in satellite and EME work.

• A. The fields are not bent into a circle in space; the direction of the electric field rotates as the wave advances.
• C. Waves that circle the Earth describe a propagation path, such as round-the-world echoes, not a type of polarization.
• D. A loop antenna radiates a linearly polarized wave whose direction depends on how the loop is oriented; circular polarization comes from antennas such as helicals or phased crossed dipoles.

A. Between points separated by 2,000 miles to 3,000 miles over a path perpendicular to the geomagnetic equator

Transequatorial propagation (TEP) uses dense ionization that forms on both sides of the geomagnetic equator. It links stations roughly equal distances north and south of the geomagnetic equator, typically 2,000 to 3,000 miles apart, on a path that crosses the geomagnetic equator at about a right angle.

• B. TEP crosses the geomagnetic equator; stations located along it would not use the two ionized regions on either side.
• C. Antipodal stations are on opposite sides of the Earth, about 12,000 miles apart, far beyond the reach of TEP.
• D. The terminator is the boundary between day and night, used in gray-line propagation, not TEP, which depends on crossing the geomagnetic equator.

C. 5,000 miles

TEP can carry signals up to about 5,000 miles. The dense ionization on each side of the geomagnetic equator lets a signal travel from one region to the other without returning to Earth in between, so a single TEP path can be about twice as long as a normal F-region hop.

• A. 1,000 miles is shorter than even the 2,000 to 3,000 miles typical of TEP paths.
• B. 2,500 miles is a typical TEP distance, not the maximum.
• D. 7,500 miles is beyond the reach of TEP, which tops out at about 5,000 miles.

C. Afternoon or early evening

TEP depends on dense ionization that builds up during the day on both sides of the geomagnetic equator. These regions are strongest in the afternoon and early evening, so TEP is most likely then.

• A. In the morning the ionization near the geomagnetic equator is still building, so TEP is not yet likely.
• B. At noon the equatorial ionization has not yet reached the afternoon peak that supports TEP.
• D. By late at night the ionization that supports TEP has usually decayed, so it is less likely than in the afternoon or early evening.

B. Independently propagating, elliptically polarized waves created in the ionosphere

The Earth's magnetic field makes the ionosphere affect radio waves differently depending on their sense of rotation. A wave entering the ionosphere splits into two components, the ordinary and extraordinary waves, which are elliptically polarized in opposite senses and are refracted and absorbed differently, so each propagates independently.

• A. The names come from magneto-ionic theory, not from how rare or long the propagation is; both components can cover similar distances.
• C. Long path and short path are the two great-circle directions between stations, not the two components a wave splits into in the ionosphere.
• D. Refraction and reflection are ways a wave changes direction; ordinary and extraordinary waves are both refracted by the ionosphere and differ in polarization.

D. A path entirely in darkness

In daylight the D region heavily absorbs low frequencies such as the 160 meter band (1.8 to 2.0 MHz). After dark the D region largely disappears, so long-distance 160 meter contacts need a path that lies entirely in darkness.

• A. A sunlit path passes through daytime D-region absorption, which stops long-distance signals on 160 meters.
• B. High-latitude paths pass near the auroral zone, where absorption and geomagnetic disturbance often weaken low-frequency signals.
• C. A north-south direction alone does not decide 160 meter propagation; what matters is whether the whole path is dark.

B. 40 meters and 20 meters

Long-path propagation goes the long way around the Earth, more than 12,000 miles, so the band must be open along the entire path. The 40 and 20 meter bands are high enough to avoid heavy D-region absorption yet low enough for the ionosphere to return them over most of such a path, so long-path openings are most frequent there.

• A. 160 and 80 meters suffer daytime D-region absorption, and a long path rarely lies entirely in darkness, so long path is less frequent there.
• C. 10 and 6 meters need a high MUF over the entire long path, which happens far less often than on 40 and 20 meters.
• D. 6 and 2 meters are usually above the MUF for F-region propagation, so long-path contacts there are rare.

C. The distance covered by each hop increases

A signal leaving the antenna at a lower angle travels farther before reaching the ionosphere and farther again before returning to Earth, so each hop covers more distance. Fewer hops are then needed for a given path.

• A. Faraday rotation is a change in a wave's polarization caused by the ionosphere and the Earth's magnetic field; it is not the effect that sets hop distance.
• B. A lower angle meets the ionosphere at a more glancing angle, which raises rather than lowers the highest frequency it can return over the path.
• D. The critical frequency is a property of the ionosphere, measured with signals sent straight up; the antenna's elevation angle does not change it.

C. It decreases

Ground waves travel along the Earth's surface and lose energy to the ground, and these losses increase with frequency. As frequency rises the ground-wave range shrinks, which is why ground waves cover long distances mainly at low and medium frequencies, such as AM broadcasting.

• A. Ground-wave range is not constant; it depends strongly on frequency and on ground conductivity.
• B. Ground losses grow with frequency, so range decreases rather than increases.
• D. Ground-wave range falls steadily as frequency rises; there is no peak near 8 MHz.

A. Around the solstices, especially the summer solstice

Sporadic-E propagation comes from patches of dense ionization in the E region that are most common around the solstices. At mid-latitudes in the Northern Hemisphere the main season is around the summer solstice, roughly May through August, with a smaller peak around the winter solstice.

• B. The winter solstice brings only a smaller, secondary sporadic-E season; the main peak is around the summer solstice.
• C. The equinoxes favor F-region HF propagation, but sporadic E peaks around the solstices, not the spring equinox.
• D. The fall equinox is not a sporadic-E peak; the main season comes around the summer solstice.

A. The signal experiences less loss compared to multi-hop propagation, which uses Earth as a reflector

In chordal-hop propagation the ionosphere refracts a signal several times without sending it down to the ground in between. Each ground reflection and each extra pass through the absorbing lower ionosphere costs signal, so skipping them gives less loss than ordinary multi-hop propagation.

• B. Chordal hop is not defined by a lower MUF; its advantage is avoiding the losses of ground reflections.
• C. Chordal hop changes how the signal travels, not how much atmospheric noise is heard from that direction.
• D. Chordal hop does not make signals travel faster; its benefit is lower loss.

D. Between sunrise and sunset

Sporadic-E ionization forms mostly during daylight, between sunrise and sunset, often peaking in the late morning and again in the late afternoon or early evening. Openings at night are possible but much less common.

• A. Between midnight and sunrise is when sporadic E is least common.
• B. Some openings last past sunset, but sporadic E is most likely during daylight hours.
• C. Nighttime sporadic E is possible but much less likely than daytime openings.

B. Successive ionospheric refractions without an intermediate reflection from the ground

Chordal-hop propagation happens when a signal refracted by the ionosphere stays in the ionosphere and is refracted again, skipping the ground reflection between hops. Between refractions the signal follows a chord across the curved ionosphere, which gives the mode its name.

• A. Propagation away from the great-circle bearing is skewed-path propagation, not chordal hop.
• C. Crossing the geomagnetic equator describes transequatorial propagation; chordal hop is defined by the missing ground reflection, not by where the path goes.
• D. Signals returning to the transmitting station are echoes, such as backscatter or round-the-world echoes, not chordal hop.

A. Vertical

A ground wave travels along the Earth's surface, and the conducting ground short-circuits a horizontal electric field, so a horizontally polarized ground wave dies out quickly. A vertically polarized wave, with its electric field perpendicular to the ground, is supported, which is why AM broadcast stations use vertical towers.

• B. A horizontal electric field next to conducting ground is largely canceled, so horizontal polarization is not supported by ground-wave propagation.
• C. A circularly polarized wave includes a horizontal component that the ground quickly attenuates, so ground waves do not keep circular polarization.
• D. An elliptically polarized wave also includes a horizontal component that the ground attenuates; only the vertical component travels well as a ground wave.

D. Solar flares

A solar flare releases a burst of X-rays that reaches Earth at the speed of light, about 8 minutes later. The X-rays sharply increase ionization in the D region on the sunlit side of Earth, which absorbs HF signals and causes radio blackouts lasting minutes to hours. NOAA rates these blackouts from R1 to R5.

• A. Coronal mass ejections take many hours to days to reach Earth and cause geomagnetic storms, not the short-term blackouts that begin as soon as a flare is seen.
• B. Sunspots mark active regions where flares can start, but sunspots themselves do not cause radio blackouts, wherever they lie on the Sun.
• C. A north-oriented interplanetary magnetic field tends to keep geomagnetic conditions quiet; it does not cause radio blackouts.

A. Increasing disturbance of the geomagnetic field

The K-index (a 3-hour value from 0 to 9) and the A-index (a daily value) measure how disturbed the Earth's magnetic field is. A rising value means increasing geomagnetic disturbance, which usually degrades HF propagation, especially on high-latitude paths.

• B. Falling, not rising, A and K values indicate a calming geomagnetic field.
• C. Solar UV output is tracked by the solar flux index and sunspot number; the A and K indices measure the geomagnetic field.
• D. Rising A and K values signal geomagnetic disturbance, which tends to degrade the ionosphere rather than raise the critical frequency.

B. Through the auroral oval

When the A or K index is elevated, charged particles stream into the upper atmosphere around the magnetic poles and increase ionization in the auroral oval. Signals on paths through the auroral oval suffer heavy absorption and fading, so polar paths are hit hardest during geomagnetic disturbances.

• A. Transequatorial paths cross the geomagnetic equator, far from the auroral zone, so they are less affected by geomagnetic disturbance.
• C. Sporadic E comes from patches of dense E-region ionization, mostly in summer at mid-latitudes, and is not driven by geomagnetic disturbance.
• D. NVIS uses high-angle signals over short distances, usually away from the auroral zone, so it is not the path most affected.

C. North-south strength of the interplanetary magnetic field

Bz is the north-south component of the interplanetary magnetic field (IMF), the magnetic field carried outward from the Sun by the solar wind. Spacecraft upstream of Earth measure it in nanotesla, and its direction helps forecast geomagnetic disturbances.

• A. Geomagnetic field stability is described by the A and K indices, not by Bz.
• B. Critical frequency is a property of the ionosphere, found by sounding it with vertical signals, not a component of the solar wind's magnetic field.
• D. Long-delayed echoes are a rare propagation curiosity; Bz is a magnetic field component, not a duration.

A. Southward

Earth's magnetic field points northward at the sunward boundary of the magnetosphere. When Bz points south, opposite to Earth's field, the two fields connect and energy and charged particles from the solar wind enter the magnetosphere, so geomagnetic disturbances become much more likely and more severe.

• B. A northward Bz is parallel to Earth's field at the magnetosphere boundary and transfers little solar wind energy, so conditions usually stay quieter.
• C. Bz is by definition the north-south component of the field; an eastward direction belongs to a different component and is not what Bz measures.
• D. Westward also describes a different component of the field; Bz measures only the north-south direction, and the southward case drives disturbances.

A. It is approximately 15 percent farther

The atmosphere bends VHF and UHF signals slightly back toward the Earth, so they travel beyond the geometric horizon. This is modeled with an Earth radius 4/3 of its real size; since horizon distance grows with the square root of the radius, the radio horizon is about 1.15 times, or about 15 percent, farther than the geographic horizon.

• B. Refraction bends signals around the curve of the Earth, so the radio horizon is farther than the geographic horizon, not nearer.
• C. 50 percent overstates the effect; normal refraction extends the horizon by about 15 percent.
• D. Atmospheric refraction makes the radio horizon about 15 percent farther than the geographic horizon, so they are not the same.

D. Class X

Solar flares are classified by their peak X-ray brightness in the letter sequence A, B, C, M and X, where each letter is 10 times stronger than the one before. Class X is the most intense, so an X1 flare is 10 times an M1 and 100 times a C1.

• A. Class A is the weakest solar flare class, about 100 times less intense than class C at the same number.
• B. There is no class Z on the solar flare scale, which ends at class X.
• C. Class M flares are one step below class X, with one tenth the X-ray output of an X flare of the same number.

D. G5

NOAA's space weather scales rate geomagnetic storms from G1 (minor) to G5 (extreme). A G5 storm corresponds to a planetary K-index of 9, the top of the scale.

• A. B9 is a solar flare X-ray class, a weak flare near the top of class B, not a geomagnetic storm level.
• B. X5 is a strong solar flare X-ray class associated with radio blackouts, not a geomagnetic storm level.
• C. M9 is a solar flare X-ray class, a medium-sized flare near the top of class M, not a geomagnetic storm level.

D. Digital-mode and CW signals

Amateur propagation reporting networks such as the Reverse Beacon Network, PSK Reporter and WSPRnet collect automatic reports of which CW and digital-mode signals are being received, and where. These real-time spots show which bands and paths are open.

• A. Solar flux is measured by radio observatories, not by amateur reporting networks, which report received signals.
• B. Electric field intensity is measured with field-strength instruments; amateur networks report which stations' signals are heard.
• C. Magnetic declination is the angle between true north and magnetic north, used in navigation, not a propagation report.

B. UV emissions at 304 angstroms, correlated to the solar flux index

The 304A parameter is the Sun's extreme ultraviolet emission at a wavelength of 304 angstroms (30.4 nm), produced by ionized helium. This radiation helps ionize the upper atmosphere, and its level tracks the solar flux index, so it serves as another measure of solar activity for propagation.

• A. The 304A parameter is a single ultraviolet emission line, not a ratio of X-ray flux to radio flux.
• C. The 304 is a wavelength in angstroms, not an angle in degrees, and the parameter does not measure solar wind speed.
• D. The 304 refers to angstroms, a unit of wavelength, not to 304 GHz, and the parameter is an ultraviolet measurement, not a flare level.

C. HF propagation

VOACAP (Voice of America Coverage Analysis Program) predicts HF propagation between two points, roughly 2 to 30 MHz. It estimates which frequencies and times will work and the expected signal strength and reliability of the path.

• A. VOACAP is a propagation program, not a tool for analyzing AC voltage and impedance in circuits.
• B. VOACAP models ionospheric propagation in the HF range; VHF propagation depends on other mechanisms that it does not model.
• D. AC current and impedance are circuit quantities; VOACAP predicts HF radio propagation.

B. A coronal mass ejection impact or a solar flare has occurred

Solar flares and coronal mass ejection (CME) impacts are sudden solar disturbances that can raise radio noise over a wide range of frequencies at once. A sudden rise in background noise across much of the HF spectrum, rather than on a single frequency, points to one of these solar events.

• A. A temperature inversion affects VHF and higher frequencies through tropospheric ducting; it does not raise HF background noise.
• C. Transequatorial propagation on 6 meters is a VHF opening along particular paths and is not signaled by broadband HF noise.
• D. Long-path openings on the higher HF bands depend on the ionosphere along the path and are not signaled by a sudden rise in noise.