G3 Radio Wave Propagation

A. Higher sunspot numbers generally indicate a greater probability of good propagation at higher frequencies

Sunspot numbers track the Sun's output of extreme ultraviolet radiation, which ionizes the upper atmosphere. More sunspots mean a denser ionosphere, especially the F2 region, so the maximum usable frequency rises and the upper HF bands such as 15, 12 and 10 meters open more often for long-distance contacts.

• B. Sporadic E depends mainly on season and time of day, peaking in summer at mid-latitudes, rather than on the sunspot number, so low sunspot numbers do not make it more likely.
• C. Even with no sunspots the Sun still ionizes the atmosphere every day, so the lower and middle HF bands keep working; mainly the upper HF bands suffer.
• D. The sunspot number does not describe geomagnetic disturbance; that is measured by the K and A indices, and storms from coronal holes can occur even when the sunspot count is zero.

B. It disrupts signals on lower frequencies more than those on higher frequencies

A sudden ionospheric disturbance follows a solar flare: X-rays arriving at the speed of light sharply increase ionization in the D region on the sunlit side of the Earth. D-region absorption is strongest at lower frequencies, so the lower HF bands fade out first and most severely while higher frequencies may still get through.

• A. A sudden ionospheric disturbance increases absorption, so it degrades HF signals rather than enhancing them, sometimes blacking out HF on the sunlit side for minutes to hours.
• C. Satellite links use VHF and higher frequencies, where D-region absorption is small, so they are affected less than HF skywave signals, not more.
• D. The flare's X-rays reach only the sunlit side of the Earth, so a sudden ionospheric disturbance affects daytime propagation, not the night side.

C. 8 minutes

Ultraviolet and X-rays are electromagnetic radiation, so they travel at the speed of light. Crossing the roughly 150 million km from the Sun to Earth takes about 499 seconds, or about 8.3 minutes, so a flare's effects on propagation begin about the moment the flare is seen.

• A. 28 days is close to the Sun's rotation period as seen from Earth (about 27 days), which governs when an active region faces Earth again, not how long radiation takes to arrive.
• B. Radiation traveling at the speed of light covers the Sun to Earth distance in about 8 minutes, not 1 to 2 hours.
• D. Delays of a day or more apply to coronal mass ejections, clouds of charged particles that move far slower than light, not to ultraviolet and X-ray radiation.

D. 15 meters, 12 meters, and 10 meters

The upper HF bands need a dense F2 region to refract them back to Earth. When solar activity is low, the ionosphere is weaker and the maximum usable frequency often stays too low for 15, 12 and 10 meters (21 to 29.7 MHz), so these bands are often closed for long-distance contacts.

• A. 80 and 160 meters need only modest ionization to return to Earth, so they remain usable for long distances at night even when solar activity is low.
• B. 60 and 40 meters are low enough in frequency that the ionosphere still refracts them during low solar activity, so they stay reliable for regional and nighttime long-distance work.
• C. 30 and 20 meters still open for long-distance contacts through most of the solar cycle; 20 meters supports worldwide daytime propagation at any point in the cycle.

D. A measure of solar radiation with a wavelength of 10.7 centimeters

The solar flux index is the Sun's radio output at a wavelength of 10.7 cm (about 2800 MHz), measured in Canada and reported in solar flux units. It tracks the sunspot number and ranges from below 50 to above 300 over a solar cycle; higher values mean a more ionized ionosphere and better conditions on the upper HF bands.

• A. The highest frequency usable for ionospheric propagation between two points is the maximum usable frequency (MUF), not the solar flux index.
• B. A count of sunspots is the sunspot number; the solar flux index is a radio measurement, not a count of spots.
• C. The American sunspot number is a count of sunspots compiled from visual observations, while the solar flux index is a separate measurement of radio emission.

D. A temporary disturbance in Earth’s geomagnetic field

A geomagnetic storm is a temporary, worldwide disturbance of Earth's magnetic field, usually triggered when a coronal mass ejection or a high-speed solar wind stream from a coronal hole reaches Earth. NOAA counts a planetary K index of 5 or more as a storm; storms degrade HF propagation, especially on paths near the poles.

• A. A drop in the solar flux index is a change in the Sun's radio output, not a disturbance of Earth's magnetic field.
• B. A thunderstorm is weather in the lower atmosphere that produces static crashes; a geomagnetic storm involves Earth's magnetic field and is caused by the Sun.
• C. "Ripples in the geomagnetic force" is not how a geomagnetic storm is defined; the term means a temporary disturbance of Earth's magnetic field lasting hours to days.

D. At any point

20 meters (14 MHz) is low enough that the daytime F2 region refracts it over long distances even when solar activity is low, yet high enough to avoid most daytime D-region absorption. That makes it a dependable daytime long-distance band at any point in the solar cycle.

• A. The summer solstice is a date in Earth's seasons, not a point in the solar cycle, and daytime 20 meter openings do not depend on it.
• B. 20 meters does not need solar maximum; that dependence applies to the upper bands such as 12 and 10 meters.
• C. 20 meters works at solar minimum, but it is not limited to solar minimum; it supports daytime worldwide propagation throughout the cycle.

D. Degrade high-latitude HF propagation

During a geomagnetic storm, charged particles from the Sun are guided by Earth's magnetic field into the polar regions, increasing absorption and disturbing the ionosphere near the auroral zones. HF paths at high latitudes, including those that cross near the poles, suffer fading or complete blackouts.

• A. Geomagnetic storms make high-latitude HF propagation worse, not better, because the auroral zones are where absorption and disturbance are greatest.
• B. Ground wave travels along the Earth's surface and does not depend on the ionosphere, so a geomagnetic storm does not degrade it.
• C. Ground wave does not depend on the ionosphere, so a geomagnetic storm neither improves nor degrades it.

A. Creates auroras that can reflect VHF signals

High geomagnetic activity produces auroras, regions of intense ionization in the high-latitude upper atmosphere. Operators aim VHF signals, commonly on 6 and 2 meters, toward the aurora and reflect them to distant stations; the reflected signals sound raspy and fluttery, so CW often gets through more clearly than voice.

• B. Geomagnetic activity increases absorption in the polar regions, weakening HF signals that pass through them rather than strengthening them.
• C. Geomagnetic activity disturbs the ionosphere and degrades HF propagation in general, including long path routes; it does not improve them.
• D. High geomagnetic activity is not a known way to reduce long delayed echoes; its useful effect for amateurs is auroral reflection of VHF signals.

C. Rotation of the Sun’s surface layers around its axis

The Sun rotates about once every 27 days as seen from Earth, with its surface layers turning faster at the equator than near the poles. Active regions and coronal holes that affect the ionosphere therefore return to face Earth on about that schedule, so propagation conditions tend to repeat roughly 27 days later.

• A. The 26 to 28 day period does not originate in Earth's upper atmosphere; it comes from the Sun's rotation bringing the same active regions back into view.
• B. Earth's radiation belts do not drive HF propagation, and any 27-day variation in them is itself caused by the Sun's rotation.
• D. The Moon's orbit has a similar length, but the Moon's position does not drive ionospheric propagation; the recurring changes come from the Sun.

D. 15 hours to several days

A coronal mass ejection is a cloud of plasma, so it travels far slower than light: it typically takes several days to cross the 150 million km to Earth, and the fastest have arrived in about 18 hours. When it arrives it can trigger a geomagnetic storm that disturbs HF propagation.

• A. 28 days is about the Sun's rotation period, not the travel time of a coronal mass ejection.
• B. 14 days is far longer than a coronal mass ejection takes to reach Earth; even slow ones arrive within several days.
• C. 4 to 8 minutes is about the travel time of light, such as a flare's X-rays (about 8.3 minutes), not of a cloud of plasma.

B. The short-term stability of Earth’s geomagnetic field

The K index measures disturbance of Earth's magnetic field over a 3-hour period, on a scale from 0 (quiet) to 9 (severely disturbed). A planetary K index (Kp) of 5 or more indicates a geomagnetic storm; higher values mean poorer HF conditions, especially on high-latitude paths.

• A. The positions of sunspots are observations of the Sun; the K index measures Earth's magnetic field.
• C. The K index describes Earth's magnetic field, measured by magnetometers on the ground, not the Sun's magnetic field.
• D. Solar radio flux is reported as the 10.7 cm solar flux index; the K index is a geomagnetic measurement, not a measure of solar radio emission.

C. The long-term stability of Earth’s geomagnetic field

The A index is a daily average of eight 3-hourly values converted from the K index, so it summarizes geomagnetic activity over a whole day rather than a 3-hour window. NOAA calls an A index of 0 to 7 quiet and 30 or more a storm; a low A index favors steady HF conditions.

• A. The positions of sunspots describe the Sun, not the stability of Earth's magnetic field.
• B. The A index has nothing to do with polarization of a solar electric field; it describes Earth's magnetic field.
• D. Solar radio flux is the 10.7 cm solar flux index, a separate measurement of the Sun's radio output.

B. HF communication is disturbed

Coronal holes release high-speed streams of solar wind. When these charged particles reach Earth they can cause geomagnetic storms, which increase absorption and disturb the ionosphere, so HF communication is disturbed, especially on high-latitude paths.

• A. Particles from coronal holes disturb Earth's magnetic field and ionosphere, making HF communication worse, not better.
• C. VHF and UHF ducting is caused by temperature inversions in the lower atmosphere (the troposphere), which solar particles do not improve.
• D. Tropospheric ducting depends on weather in the lower atmosphere, not on solar particles, so coronal holes do not disturb it.

D. A slightly delayed echo might be heard

A signal can reach you along the short great-circle path and also the long way around the Earth. The long path is thousands of kilometers longer, so its copy arrives a fraction of a second later (light takes about 0.13 seconds to circle the Earth), which sounds like a slight echo.

• A. The two paths differ in arrival time by a fraction of a second, producing an echo, not regular fading at 10-second intervals.
• B. The two copies arrive at different times and strengths, so they do not combine into a steady 3 dB increase; the result is an echo.
• C. The long path copy arrives noticeably later than the short path copy, so it is heard as an echo rather than cancelling the signal.

D. All these choices are correct

The MUF depends on how strongly the ionosphere is ionized and on the geometry of the path. Solar radiation and ionospheric disturbances set the level of ionization, time of day and season change how much sunlight the ionosphere receives, and the path distance and location set the angle at which the signal meets the ionosphere and which part of it is used.

• A. Path distance and location do affect the MUF, since longer hops meet the ionosphere at shallower angles, but time of day, season and solar conditions matter too.
• B. Time of day and season affect the MUF by changing how much sunlight ionizes the ionosphere, but path geometry and solar activity matter too.
• C. Solar radiation and ionospheric disturbances affect the MUF by changing ionization levels, but path geometry, time of day and season matter too.

A. Just below the MUF

Ionospheric absorption, mostly in the D region, decreases as frequency rises. The MUF is the highest frequency that still returns to Earth over the path, so a frequency slightly below the MUF suffers the least absorption and gives the strongest long-distance signals.

• B. Slightly above the LUF is the low end of the usable range, where absorption is greatest and signals are weakest.
• C. The critical frequency is the highest frequency returned at vertical incidence; long-distance paths use oblique angles and can use frequencies well above it, with less absorption.
• D. Over long paths the MUF is usually much higher than the critical frequency, so a frequency slightly above the critical frequency still suffers more absorption than one near the MUF.

A. Use a network of automated receiving stations on the internet to see where your transmissions are being received

Networks such as the Reverse Beacon Network, PSK Reporter and WSPRnet use automated receivers around the world that report which stations they hear and how strongly. Transmitting on the band and checking where you are spotted shows where your signal is reaching at that moment.

• B. The A index describes geomagnetic activity over a whole day; it does not show where your signals are reaching on a particular band.
• C. Sending dots and listening for echoes does not reveal which areas your signal reaches.
• D. Checking the A index and listening for echoes do not show propagation from your station on a band, so not all the choices are correct.

A. They are refracted back to Earth

Between the LUF and the MUF lies the usable range of frequencies for a path: low enough for the ionosphere to refract them back to Earth and high enough to avoid being absorbed. Signals in that range return to Earth, which is what makes skywave communication possible.

• B. Frequencies above the MUF pass through the ionosphere into space; those below it are refracted back.
• C. The ionosphere does not amplify radio waves; it refracts them and absorbs some of their energy.
• D. Signals between the LUF and MUF are not trapped circling the Earth; they return to the surface, possibly after several hops.

C. They are attenuated before reaching the destination

The LUF is set mainly by absorption, which is greatest at low frequencies and occurs mostly in the D region by day. Below the LUF, signals lose so much energy passing through the absorbing layers that they are too weak to be heard at the destination.

• A. Refraction back to Earth with usable strength describes frequencies between the LUF and the MUF.
• B. Passing through the ionosphere into space happens to frequencies above the MUF, not below the LUF.
• D. Signals below the LUF are absorbed rather than trapped in the ionosphere.

A. The Lowest Usable Frequency for communications between two specific points

LUF stands for lowest usable frequency: the lowest frequency that gives reliable skywave communication between two specific points. Below it, absorption makes signals too weak, and like the MUF it changes with the path, the time of day and solar conditions.

• B. The LUF is not defined by a 100-mile radius; it applies to the path between two specific points.
• C. The LUF is not a 24-hour figure; it changes through the day as D-region absorption rises and falls with the Sun.
• D. The LUF is not tied to the past 60 minutes; it is a value for a given path under current conditions.

B. The Maximum Usable Frequency for communications between two points

MUF stands for maximum usable frequency: the highest frequency that the ionosphere reliably refracts back to Earth over the path between two points. Above it signals pass through into space, and the MUF changes with the path, time of day, season and solar activity.

• A. MUF stands for maximum, not minimum; the lowest usable frequency on a path is the LUF.
• C. MUF means maximum, not minimum, and it applies to a path between two points rather than a 24-hour period.
• D. The MUF applies to the path between two points under current conditions; it changes through the day, so it is not a 24-hour figure.

C. 2,500 miles

The F2 region is the highest ionospheric region, peaking a few hundred kilometers up. A low-angle signal refracted from that height comes back to Earth up to about 2,500 miles (about 4,000 km) away, so longer paths need several hops.

• A. 180 miles is far short of the reach of a single F2 hop, which normally extends to about 2,500 miles.
• B. About 1,200 miles is the maximum one-hop distance for the lower E region, not the F2 region.
• D. 12,000 miles is about half the Earth's circumference (about 24,900 miles) and requires several F2 hops.

B. 1,200 miles

The E region lies about 90 to 130 km up, lower than the F2 region, so the geometry of a single hop is shorter. A low-angle signal refracted from the E region comes back to Earth up to about 1,200 miles (about 2,000 km) away.

• A. 180 miles is much less than an E-region hop can cover; the maximum one-hop E distance is about 1,200 miles.
• C. About 2,500 miles is the one-hop limit for the higher F2 region.
• D. 12,000 miles is about half the Earth's circumference and requires many hops.

A. Propagation via ordinary skywave communications is not possible over that path

Usable frequencies for a path lie between the LUF, below which absorption weakens the signal too much, and the MUF, above which it passes into space. If the LUF rises above the MUF, for example during heavy absorption after a solar flare, no frequency is both refracted and strong enough, so ordinary skywave over that path is not possible.

• B. When the LUF exceeds the MUF there is no usable frequency on the path, so communication is lost, not enhanced.
• C. Double-hop propagation also needs frequencies that are refracted and not absorbed, so it fails too rather than becoming more common.
• D. No HF frequency works over the path in this condition, so propagation is not enhanced on any of them.

D. High levels of atmospheric noise or static

Lightning produces strong radio noise that is most intense at lower frequencies and travels long distances. Thunderstorms are more frequent in summer, so bands such as 160, 80 and 40 meters suffer high levels of static (static crashes, also called QRN).

• A. The lower HF bands still work in summer, especially at night; the typical summer problem is noise, not a complete lack of propagation.
• B. Worldwide daytime propagation is typical of higher bands such as 20 meters; daytime D-region absorption limits the lower HF bands to shorter distances.
• C. Signals on the lower bands are not distorted by photon absorption; the typical summer problem is static from thunderstorms.

A. The D region

The ionospheric regions in order of height are D (about 50 to 90 km), E (about 90 to 130 km), and F (about 130 to 500 km), which splits by day into F1 and F2. The D region is the lowest and exists mainly in daylight, when it absorbs lower-frequency HF signals.

• B. The E region lies above the D region, at about 90 to 130 km.
• C. The F1 region forms by day within the F region, above both the D and E regions.
• D. The F2 region is the highest ionospheric region, not the lowest.

A. The highest frequency which is refracted back to Earth

For a given angle of incidence, the critical frequency is the highest frequency the ionosphere still refracts back to Earth; higher frequencies pass through into space. It is usually quoted for vertical incidence (a signal sent straight up) and shows how strongly a region is ionized.

• B. The critical frequency is an upper limit; the low end of the usable range on a path is set by absorption and is described by the LUF.
• C. A signal-to-noise ratio near unity describes a signal at the noise level of a receiver, which is unrelated to ionospheric refraction.
• D. A 6 dB signal-to-noise ratio is a receiver or link performance figure, not a property of the ionosphere.

C. Because it is the highest

The higher a signal travels before it is bent back, the farther away it returns to Earth. The F2 region is the highest ionospheric region, so its hops are the longest: up to about 2,500 miles, compared with about 1,200 miles for the E region.

• A. The F2 region's density sets which frequencies it can return, but the length of a skip comes from the region's height.
• B. The Doppler effect is a frequency shift caused by motion; it does not set the length of a skip.
• D. Temperature inversions occur in the lower atmosphere and cause VHF and UHF ducting; they do not affect ionospheric skip.

D. The highest takeoff angle that will return a radio wave to Earth under specific ionospheric conditions

Signals that leave the antenna at steeper angles meet the ionosphere more nearly head-on and are harder to bend back. The critical angle is the highest takeoff angle that still returns to Earth at a given frequency and set of ionospheric conditions; steeper signals pass through into space, which creates the skip zone.

• A. The long path azimuth is the compass direction to a station the long way around the Earth, not a limit on takeoff angle.
• B. The short path azimuth is the compass direction along the shorter great circle, not a limit on takeoff angle.
• C. Lower takeoff angles return to Earth more easily; the critical angle is the highest angle that returns, not the lowest.

C. The D region absorbs signals at these frequencies during daylight hours

Sunlight creates the D region, the lowest ionospheric layer, during the day. It absorbs energy from signals passing through it, and the absorption is much stronger at lower frequencies, so long-distance signals on 160, 80, 60 and 40 meters fade out by day and return after dark when the D region disappears.

• A. The F region refracts these signals rather than absorbing them; the absorption happens lower, in the D region.
• B. F-region instability is not the cause; these signals are absorbed in the D region before they can reach the F region.
• D. E-region instability is not the cause; daytime absorption in the D region is what weakens these lower bands.

B. Signals have a fluttering sound

HF scatter signals arrive by many paths at once, scattered from irregularities in the ionosphere or from the ground, and each path changes constantly. Their combination makes the signal weak with a rapid fluttering or hollow sound.

• A. Scatter signals are weak and distorted, so phone signals are usually hard to understand rather than highly intelligible.
• C. Scatter signals are characterized by a weak, rapid flutter, not by very large, sudden swings in strength.
• D. Scatter is not limited to night; it can occur whenever the band supports skip propagation, day or night.

D. Energy is scattered into the skip zone through several different paths

Scatter signals reach the skip zone over several different paths, each with a different length and delay. The copies arrive slightly out of step and interfere with each other, which makes the signal sound distorted and fluttery.

• A. The distortion comes from energy arriving over several paths of different lengths, not from instability of one ionospheric region.
• B. Ground waves do not cause the distortion; in the skip zone the ground wave has already faded out.
• C. Whether the E region is present does not explain scatter distortion; the cause is energy arriving over several different paths.

A. Only a small part of the signal energy is scattered into the skip zone

Most of a signal's energy follows the normal skip path and returns to Earth beyond the skip zone. Only a small fraction is scattered back into the skip zone, so scatter signals heard there are weak.

• B. HF scatter comes from irregularities in the ionosphere and from the ground where skip signals land, not from the magnetosphere.
• C. The ground wave has already faded out by the skip zone; scatter signals there arrive from the ionosphere or by ground scatter.
• D. Scatter signals are not carried in absorbing ducts in the F region; they are weak because only a small part of the energy is scattered into the skip zone.

B. Scatter

The skip zone lies between the end of the ground wave and the point where the first skip returns to Earth. Scatter from irregularities in the ionosphere, and from the ground where the skip lands, sends a small part of the signal back into that zone, so stations there may hear a weak, fluttery copy.

• A. Faraday rotation is a twisting of a wave's polarization as it passes through the ionosphere; it does not carry signals into the skip zone.
• C. Chordal hop is long-distance propagation in which the signal is refracted repeatedly in the ionosphere without touching the Earth between hops; it does not fill the skip zone.
• D. Short path is the shorter great-circle route to a distant station, not a mechanism for reaching the skip zone.

B. Short distance MF or HF propagation at high elevation angles

Near vertical incidence skywave sends MF or HF signals almost straight up, typically from a low horizontal antenna. On a frequency below the critical frequency the ionosphere returns them almost straight down, covering the area within a couple of hundred miles of the station with no skip zone; it is used mainly on bands such as 80 and 40 meters.

• A. Operating near the MUF suits long-distance paths at low angles; NVIS uses frequencies below the critical frequency for short distances.
• C. Long path propagation at sunrise and sunset is a long-distance effect along the twilight band, not short-distance NVIS.
• D. Double hop propagation near the LUF describes long-distance multihop paths, while NVIS is a single, nearly vertical hop for short distances.

D. The D region

The D region, about 50 to 90 km up, exists mainly in daylight. Its air is dense enough that free electrons set in motion by a radio wave collide often with gas molecules, turning signal energy into heat; this absorption grows as frequency falls, so signals below 10 MHz are hit hardest.

• A. The F2 region refracts HF signals for long-distance propagation; it is not where most daytime absorption occurs.
• B. The F1 region forms by day high in the ionosphere, where collisions are rare, so it absorbs far less than the D region.
• C. The E region absorbs some energy, but the D region below it causes most daytime absorption of signals below 10 MHz.