General Exam-Radio Wave Propagation

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General Exam-Radio Wave Propagation
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2013-07-15 17:38:32
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Amateur Radio General Exam - G3 Question Set - Radio Wave Propagation
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  1. G3A01 What is the sunspot number?
    A. A measure of solar activity based on counting sunspots and sunspot groups
    B. A 3 digit identifier which is used to track individual sunspots
    C. A measure of the radio flux from the Sun measured at 10.7 cm
    D. A measure of the sunspot count based on radio flux measurements
    • (A)
    • A number of observatories around the world measure solar activity. A weighted average of this data is used to determine the International Sunspot Number (ISN) for each day. These daily sunspot counts are used to produce monthly and yearly average values. The average values are used to see trends and patterns in the measurements.
  2. G3A02 What effect does a Sudden Ionospheric Disturbance have on the daytime ionospheric propagation of HF radio waves?
    A. It enhances propagation on all HF frequencies
    B. It disrupts signals on lower frequencies more than those on higher frequencies
    C. It disrupts communications via satellite more than direct communications
    D. None, because only areas on the night side of the Earth are affected
    • (B)
    • A sudden ionospheric disturbance (SID) is often the result of solar flares that release large amounts of radiation. Ultraviolet and X-ray radiation from the Sun travels at the speed of light, reaching the Earth in about eight minutes. When this radiation reaches the Earth, the level of ionization in the ionosphere increases rapidly. This causes D-layer absorption of radio waves to increase significantly. Absorption of radio signals in the D-layer is always stronger at lower frequencies, affecting lower frequency signals more than higher frequency signals.
  3. G3A03 Approximately how long does it take the increased ultraviolet and X-ray radiation from solar flares to affect radio-wave propagation on the Earth?
    A. 28 days
    B. 1 to 2 hours
    C. 8 minutes
    D. 20 to 40 hours
    • (C)
    • Ultraviolet and X-ray radiation from the Sun travels at the speed of light, reaching the Earth in about eight minutes.
  4. G3A04 Which of the following amateur radio HF frequencies are least reliable for long distance communications during periods of low solar activity?
    A. 3.5 MHz and lower
    B. 7 MHz
    C. 10 MHz
    D. 21 MHz and higher
    • (D)
    • The higher the frequency, the more ionization is needed in the ionosphere in order to refract (bend) the radio signal back to the Earth. When solar activity is low, the higher frequencies will pass through the ionosphere into space instead of being refracted back to Earth. During periods of low solar activity, the 15 meter (21 MHz), 12 meter (24.9 MHz) and 10 meter (28 MHz) bands are the least reliable HF bands for long distance communication.
  5. G3A05 What is the solar-flux index?
    A. A measure of the highest frequency that is useful for ionospheric propagation between two points on the Earth
    B. A count of sunspots which is adjusted for solar emissions
    C. Another name for the American sunspot number
    D. A measure of solar radiation at 10.7 cm
    • (D)
    • Solar flux is the radio energy coming from the Sun. High levels of solar energy produce greater ionization in the ionosphere. The solar flux measurement is taken daily by measuring radio energy from the Sun at 2800 MHz which is a wavelength of 10.7 cm. The measurement is then converted into the solar flux index. Higher values of the solar flux index correspond to higher values of solar flux. The solar-flux measurement may be taken under any weather conditions--the Sun does not have to be visible, as for determining the sunspot number. The radio energy measurement is converted to an open-ended numeric index with a minimum value of 65 (for the minimum amount of energy). Higher values of the solar flux index indicate higher levels of solar activity.
  6. G3A06 What is a geomagnetic storm?
    A. A sudden drop in the solar-flux index
    B. A thunderstorm which affects radio propagation
    C. Ripples in the ionosphere
    D. A temporary disturbance in the Earth's magnetosphere
    • (D)
    • Geomagnetic disturbances result when charged particles from a solar flare reach the Earth. When these charged particles reach the Earth’s magnetic field, they are deflected toward the North and South poles. Radio communications along higher-latitude paths (latitudes greater than about 45 degrees) will be more affected than paths closer to the equator. The charged particles from the Sun may make the F-region seem to disappear or seem to split into many layers, degrading or completely blacking out long-distance radio communications.
  7. G3A07 At what point in the solar cycle does the 20 meter band usually support worldwide propagation during daylight hours?
    A. At the summer solstice
    B. Only at the maximum point of the solar cycle
    C. Only at the minimum point of the solar cycle
    D. At any point in the solar cycle
    • (D)
    • Even at the minimum point of the solar cycle, world-wide propagation is usually possible on the 20 meter band. As solar activity increases, the band will remain open for longer periods and with stronger signal strengths. For this reason, 20 meters is a favorite band for “DXers".
  8. G3A08 Which of the following effects can a geomagnetic storm have on radio-wave propagation?
    A. Improved high-latitude HF propagation
    B. Degraded high-latitude HF propagation
    C. Improved ground-wave propagation
    D. Improved chances of UHF ducting
    • (B)
    • Geomagnetic disturbances result when charged particles from a solar flare reach the Earth. When these charged particles reach the Earth’s magnetic field, they are deflected toward the North and South poles. Radio communications along higher-latitude paths (latitudes greater than about 45 degrees) will be more affected than paths closer to the equator. The charged particles from the Sun may make the F-region seem to disappear or seem to split into many layers, degrading or completely blacking out long-distance radio communications.
  9. G3A09 What effect do high sunspot numbers have on radio communications?
    A. High-frequency radio signals become weak and distorted
    B. Frequencies above 300 MHz become usable for long-distance communication
    C. Long-distance communication in the upper HF and lower VHF range is enhanced
    D. Microwave communications become unstable
    • (C)
    • When sunspot numbers are high, there is a significant amount of solar activity and there will be more ionization of the ionosphere. The more the ionosphere is ionized, the higher the frequency of radio signals that may be used for long-distance communication. During the peak of a sunspot cycle, the 20 meter (14 MHz) band will be open around the world even through the night. During an unusually good sunspot cycle, even the 6 meter (50 MHz) band can become usable for long-distance communication.
  10. G3A10 What causes HF propagation conditions to vary periodically in a 28-day cycle?
    A. Long term oscillations in the upper atmosphere
    B. Cyclic variation in the Earth’s radiation belts
    C. The Sun’s rotation on its axis
    D. The position of the Moon in its orbit
    • (C)
    • It takes approximately 28 days for the Sun to rotate on its axis. Since active areas on the Sun may persist for more than one rotation, you can expect good propagation conditions to recur approximately every 28 days.
  11. G3A11 Approximately how long is the typical sunspot cycle?
    A. 8 minutes
    B. 40 hours
    C. 28 days
    D. 11 years
    • (D)
    • The internal dynamics of the Sun cause its activity to vary in a cycle lasting approximately 11 years. Sunspots are one indication of solar activity and since they were the earliest phenomenon observed on the Sun, the cycle is called the sunspot cycle.
  12. G3A12 What does the K-index indicate?
    A. The relative position of sunspots on the surface of the Sun
    B. The short term stability of the Earth’s magnetic field
    C. The stability of the Sun's magnetic field
    D. The solar radio flux at Boulder, Colorado
    • (B)
    • The K-index represents readings of the Earth’s geomagnetic field, updated every three hours at Boulder, Colorado. K-index values indicate the stability of the Earth’s geomagnetic field. Steady values indicate a stable geomagnetic field, while rising values indicate an active geomagnetic field. The K-index trends are important indicators of changing propagation conditions. Rising K-index values are generally bad news for HF propagation, especially for propagation paths involving latitudes above 30° north. Values of 4 and rising warn of conditions associated with auroras and degraded HF propagation.
  13. G3A13 What does the A-index indicate?
    A. The relative position of sunspots on the surface of the Sun
    B. The amount of polarization of the Sun's electric field
    C. The long term stability of the Earth’s geomagnetic field
    D. The solar radio flux at Boulder, Colorado
    • (C)
    • The A-index is a daily figure for the state of activity of the Earth’s magnetic field. The A-index tells you mainly about yesterday’s conditions, but it is very revealing when charted regularly, because geomagnetic disturbances nearly always recur at four-week intervals. (It takes the Sun 28 days to rotate once on its axis.)
  14. G3A14 How are radio communications usually affected by the charged particles that reach the Earth from solar coronal holes?
    A. HF communications are improved
    B. HF communications are disturbed
    C. VHF/UHF ducting is improved
    D. VHF/UHF ducting is disturbed
    • (B)
    • The corona is the Sun’s outer layer. Temperatures in the corona are typically about two million degrees Celsius, but can be more than four million degrees Celsius above an active sunspot region. A coronal hole is an area of somewhat lower temperature. Matter ejected through such a “hole” is in the form of plasma, a highly ionized gas made up of electrons, protons and neutral particles. The plasma travels at speeds up to two million miles per hour, and if the “jet” of material is directed toward the Earth it can result in a geomagnetic storm on Earth, disrupting HF communications.
  15. G3A15 How long does it take charged particles from coronal mass ejections to affect radio-wave propagation on the Earth?
    A. 28 days
    B. 14 days
    C. 4 to 8 minutes
    D. 20 to 40 hours
    • (D)
    • The corona is the Sun’s outer layer. Temperatures in the corona are typically about two million degrees Celsius, but can be more than four million degrees Celsius above an active sunspot region. Matter ejected from the corona is in the form of plasma, a highly ionized gas made up of electrons, protons and neutral particles. The plasma travels at speeds of two million miles per hour or more, so it can take about 20 to 40 hours for the plasma to travel the 93 million miles to Earth.
  16. G3A16 What is a possible benefit to radio communications resulting from periods of high geomagnetic activity?
    A. Aurora that can reflect VHF signals
    B. Higher signal strength for HF signals passing through the polar regions
    C. Improved HF long path propagation
    D. Reduced long delayed echoes
    • (A)
    • When the plasma, or charged particles, from a coronal mass ejection reaches the Earth it interacts with the Earth’s magnetic field. The charged particles follow magnetic field lines into the Earth’s atmosphere near the North and South magnetic poles, producing visible aurora borealis at northern latitudes and aurora australis in the south. VHF operators look forward to such conditions because radio signals can be reflected from auroral “patches,” making long-distance contacts possible.
  17. G3B01 How might a sky-wave signal sound if it arrives at your receiver by both short path and long path propagation?
    A. Periodic fading approximately every 10 seconds
    B. Signal strength increased by 3 dB
    C. The signal might be cancelled causing severe attenuation
    D. A well-defined echo might be heard
    • (D)
    • Normally, you will expect radio signals to arrive at your station by following the shortest possible path between you and the transmitting station. This is called short-path propagation. Signals that might have arrived from the opposite direction, 180 degrees different from the short-path signals are normally so weak that you would probably not hear them. Signals that arrive 180 degrees from the short path are called long-path signals. When propagation conditions are suitable, the long-path signals may be strong enough to support communication. In fact, there are times when the long-path propagation may be even better than the short-path propagation. Stations with directional antennas can point their antennas directly away from each other to communicate. (This is not simply communicating using signal radiated “off the back” of the antennas.) If you are listening to signals on your receiver and you hear a well-defined echo, even if it is a weak echo, the chances are you are hearing signals arrive at your station over the long path. The slightly longer time it takes the signals to travel the longer distance around the Earth results in a slight delay when compared to the direct, short-path signals. This is a good indication that you may be able to point your antenna directly away from the received station to communicate.
  18. G3B02 Which of the following is a good indicator of the possibility of sky-wave propagation on the 6 meter band?
    A. Short skip sky-wave propagation on the 10 meter band
    B. Long skip sky-wave propagation on the 10 meter band
    C. Severe attenuation of signals on the 10 meter band
    D. Long delayed echoes on the 10 meter band
    • (A)
    • As the maximum usable frequency (MUF) for a given path increases, the ionosphere also supports shorter single-hop distance at lower frequencies. Suppose you are operating on the 10 meter band, and are contacting stations that are 800 to 1000 miles away. After making a few more contacts you start to notice that you are contacting stations only about 500 miles away, and then you notice that you are contacting stations even closer, perhaps only out to a few hundred miles. This can be an excellent indication that the MUF for the longer-path stations has moved up to a higher frequency, perhaps even above 50 MHz. It is a good time to check for a band opening on 6 meters!
  19. G3B03 Which of the following applies when selecting a frequency for lowest attenuation when transmitting on HF?
    A. Select a frequency just below the MUF
    B. Select a frequency just above the LUF
    C. Select a frequency just below the critical frequency
    D. Select a frequency just above the critical frequency
    • (A)
    • Ionospheric absorption (attenuation) is lowest just below the Maximum Usable Frequency (MUF), the highest frequency that will allow the radio wave to reach its desired destination using E or F-region propagation. Use a frequency just below the MUF for the highest received signal strength.
  20. G3B04 What is a reliable way to determine if the Maximum Usable Frequency (MUF) is high enough to support skip propagation between your station and a distant location on frequencies between 14 and 30 MHz?
    A. Listen for signals from an international beacon
    B. Send a series of dots on the band and listen for echoes from your signal
    C. Check the strength of TV signals from Western Europe
    D. Check the strength of signals in the MF AM broadcast band
    • (A)
    • Beacon stations transmit signals so that amateur operators can evaluate propagation conditions. By listening for beacon stations from Western Europe, you will be able to determine if the MUF is high enough for 10 meter communications to that area.
  21. G3B05 What usually happens to radio waves with frequencies below the Maximum Usable Frequency (MUF) and above the Lowest Usable Frequency (LUF) when they are sent into the ionosphere?
    A. They are bent back to the Earth
    B. They pass through the ionosphere
    C. They are amplified by interaction with the ionosphere
    D. They are bent and trapped in the ionosphere to circle the Earth
    • (A)
    • The Maximum Usable Frequency (MUF) and Lowest Useable Freuqency (LUF) relate to a particular desired destination. The MUF is the highest frequency that will allow the radio wave to reach its desired destination using E or F-region propagation. The LUF is the lowest frequency at which ionospheric absorption is low enough to allow a signal to reach the desired destination. There is no single MUF or LUF for a given transmitter location; it will vary depending on the direction and distance to the station you are attempting to contact. Signals with frequencies above the LUF and lower than the MUF are generally bent back to Earth and at frequencies higher than the MUF will pass through the ionosphere instead of being bent back to the Earth.
  22. G3B06 What usually happens to radio waves with frequencies below the Lowest Usable Frequency (LUF)?
    A. They are bent back to the Earth
    B. They pass through the ionosphere
    C. They are completely absorbed by the ionosphere
    D. They are bent and trapped in the ionosphere to circle the Earth
    • (C)
    • The Lowest Usable Frequency (LUF) is the frequency below which ionospheric absorption attenuates the radio signals to below the atmospheric noise levels. Since absorption increases with decreasing frequency, signals at frequencies below the LUF can not be received via sky-wave communication.
  23. G3B07 What does LUF stand for?
    A. The Lowest Usable Frequency for communications between two points
    B. The Longest Universal Function for communications between two points
    C. The Lowest Usable Frequency during a 24 hour period
    D. The Longest Universal Function during a 24 hour period
    • (A)
    • The Lowest Usable Frequency (LUF) is the frequency below which ionospheric absorption attenuates the radio signals to below the atmospheric noise levels. Since absorption increases with decreasing frequency, signals at frequencies below the LUF can not be received via sky-wave communication.
  24. G3B08 What does MUF stand for?
    A. The Minimum Usable Frequency for communications between two points
    B. The Maximum Usable Frequency for communications between two points
    C. The Minimum Usable Frequency during a 24 hour period
    D. The Maximum Usable Frequency during a 24 hour period
    • (B)
    • The Maximum Usable Frequency (MUF) relates to a particular desired destination. The MUF is the highest frequency that will allow the radio wave to reach its desired destination using E or F-region propagation. There is no single MUF for a given transmitter location; it will vary depending on the direction and distance to the station you are attempting to contact. Signals with frequencies lower than the MUF are generally bent back to Earth and at higher than the MUF will pass through the ionosphere instead of being bent back to the Earth.
  25. G3B09 What is the approximate maximum distance along the Earth's surface that is normally covered in one hop using the F2 region?
    A. 180 miles
    B. 1,200 miles
    C. 2,500 miles
    D. 12,000 miles
    • (C)
    • Layers in the F region form and decay in correlation with the daily passage of the sun. The F1 and F2 layers form when the F region splits into two parts due to high radiation from the Sun, recombining into a single F layer at night. The more solar radiation the F region receives, the more it is ionized so it reaches maximum ionization shortly after noon during the summertime. The ionization tapers off very gradually towards sunset and the F2 layer remains usable into the night. The F2 region is the highest of the ionosphere, reaching as high as 300 miles at noon in the summertime. Because it is the highest, it is the region mainly responsible for long-distance communications. A one-hop transmission can travel a maximum distance of about 2,500 miles using F2 propagation.
  26. G3B10 What is the approximate maximum distance along the Earth's surface that is normally covered in one hop using the E region?
    A. 180 miles
    B. 1,200 miles
    C. 2,500 miles
    D. 12,000 miles
    • (B)
    • The E region of the ionosphere is the second lowest, just above the D region. The E layer forms at an altitude of about 70 miles above the Earth. The E region ionizes during the daytime, but does not stay ionized very long after sunset. Ionization in the E region is at a maximum around midday. During the daytime, a radio signal can travel a maximum distance of about 1,200 miles in one hop using E-region propagation.
  27. G3B11 What happens to HF propagation when the Lowest Usable Frequency (LUF) exceeds the Maximum Usable Frequency (MUF)?
    A. No HF radio frequency will support ordinary skywave communications over the path
    B. HF communications over the path are enhanced
    C. Double hop propagation along the path is more common
    D. Propagation over the path on all HF frequencies is enhanced
    • (A)
    • Signals at frequencies below the LUF will be absorbed in the ionosphere rather than returning to Earth. Occasionally the LUF may be higher than the maximum usable frequency (MUF). This means that for the highest possible frequency that will propagate through the ionosphere for that path, the signal absorption is so large that even signals at the MUF are absorbed. Under these conditions it is impossible to establish sky-wave communication between those two points no matter what frequency is used! (Communications between either location and other locations may be possible, since the LUF and MUF depend on the end points of the communication path.)
  28. G3B12 What factors affect the Maximum Usable Frequency (MUF)?
    A. Path distance and location
    B. Time of day and season
    C. Solar radiation and ionospheric disturbances
    D. All of these choices are correct
    • (D)
    • The Maximum Usable Frequency (MUF) is the highest frequency that will provide sky-wave propagation between two specific locations. For example, suppose you live in Illinois and want to communicate with another amateur in Ecuador. You might find that the MUF for this contact is about 18 MHz at 14:00 UTC. You may also find that the MUF to communicate with a station in Spain at that same time is 12 MHz. Different communications path distances and directions will often result in very different MUF values. The MUF depends on conditions in the ionosphere, and those conditions will vary by time of day as well as the season of the year. The amount of solar radiation striking the ionosphere varies significantly depending on the timing of the 11-year sunspot cycle. Any solar flares, coronal-mass ejections and other disturbances on the Sun can also result in ionospheric disturbances that will affect the MUF. Answer choices A, B and C all describe factors that will affect the MUF for a given sky-wave propagation path, so answer choice D is correct.
  29. G3C01 Which of the following ionospheric layers is closest to the surface of the Earth?
    A. The D layer
    B. The E layer
    C. The F1 layer
    D. The F2 layer
    • (A)
    • The D region of the ionosphere is the lowest, forming the D layer at a height of 30 to 60 miles. Because it is the lowest, it is also the densest and its ionization disappears by dark.
  30. G3C02 Where on the Earth do ionospheric layers reach their maximum height?
    A. Where the Sun is overhead
    B. Where the Sun is on the opposite side of the Earth
    C. Where the Sun is rising
    D. Where the Sun has just set
    • (A)
    • The F region forms and decays in correlation with the daily passage of the Sun. The F1 and F2 layers form when the F region splits into two parts due to receiving high radiation from the Sun, recombining into a single F region at night. The more solar radiation the F region receives, the more it is ionized so it reaches maximum ionization shortly after noon during the summertime. The ionization tapers off very gradually towards sunset and the F2 region remains usable into the night. The F2 region is the highest of the ionosphere, reaching as high as 300 miles at noon in the summertime.
  31. G3C03 Why is the F2 region mainly responsible for the longest distance radio wave propagation?
    A. Because it is the densest ionospheric layer
    B. Because it does not absorb radio waves as much as other ionospheric regions
    C. Because it is the highest ionospheric region
    D. All of these choices are correct
    • (C)
    • Because the F2 region is the highest ionospheric region, it is the region mainly responsible for long-distance communications. A one-hop transmission can travel a maximum distance of about 2500 miles using the F2 region.
  32. G3C04 What does the term “critical angle” mean as used in radio wave propagation?
    A. The long path azimuth of a distant station
    B. The short path azimuth of a distant station
    C. The lowest takeoff angle that will return a radio wave to the Earth under specific ionospheric conditions
    D. The highest takeoff angle that will return a radio wave to the Earth under specific ionospheric conditions
    • (D)
    • At each frequency there is a maximum angle at which the radio wave can leave the antenna and still be refracted back to Earth by the ionosphere instead of simply passing through it and proceeding out into space. The critical angle changes depending on the ionization of the ionosphere.
  33. G3C05 Why is long distance communication on the 40, 60, 80 and 160 meter bands more difficult during the day?
    A. The F layer absorbs signals at these frequencies during daylight hours
    B. The F layer is unstable during daylight hours
    C. The D layer absorbs signals at these frequencies during daylight hours
    D. The E layer is unstable during daylight hours
    • (C)
    • Think of the D region as the "Darned Daylight" region. Instead of bending high frequency signals back to Earth, it absorbs energy from them. Signals at lower frequencies (longer wavelengths such as 160, 80, 60 and 40 meters) are absorbed more than at higher frequencies. The ionization created by the sunlight does not last very long in the D region, disappearing by sunset.
  34. G3C06 What is a characteristic of HF scatter signals?
    A. They have high intelligibility
    B. They have a wavering sound
    C. They have very large swings in signal strength
    D. All of these choices are correct
    • (B)
    • The area between the farthest reach of ground-wave propagation and the point where signals are refracted back from the ionosphere (sky-wave propagation) is called the skip zone. Since some of the transmitted signal is scattered in the atmosphere or from ground reflections, communication may be possible in the skip zone by the use of scatter signals. The amount of signal scattered in the atmosphere will be quite small and the signal received in the skip zone will arrive from several radio-wave paths. This tends to produce a weak, distorted signal with a fluttering or wavering sound.
  35. G3C07 What makes HF scatter signals often sound distorted?
    A. The ionospheric layer involved is unstable
    B. Ground waves are absorbing much of the signal
    C. The E-region is not present
    D. Energy is scattered into the skip zone through several different radio wave paths
    (D)

    The amount of signal scattered back towards the transmitting station from the ionosphere or ground will be quite small. The signal received in the skip zone will also arrive from several radio-wave paths. This tends to produce a weak, distorted signal with a fluttering or wavering sound.
  36. G3C08 Why are HF scatter signals in the skip zone usually weak?
    A. Only a small part of the signal energy is scattered into the skip zone
    B. Signals are scattered from the magnetosphere which is not a good reflector
    C. Propagation is through ground waves which absorb most of the signal energy
    D. Propagations is through ducts in F region which absorb most of the energy
    • (A)
    • The amount of signal scattered back towards the transmitting station from the ionosphere or ground will be quite small. The signal received in the skip zone will also arrive from several radio-wave paths. This tends to produce a weak, distorted signal with a fluttering or wavering sound.
  37. G3C09 What type of radio wave propagation allows a signal to be detected at a distance too far for ground wave propagation but too near for normal sky-wave propagation?
    A. Faraday rotation
    B. Scatter
    C. Sporadic-E skip
    D. Short-path skip
    • (B)
    • The area between the farthest reach of ground-wave propagation and the point where signals are refracted back from the ionosphere (sky-wave propagation) is called the skip zone. Since some of the transmitted signal is scattered in the atmosphere, communication may be possible in the skip zone by the use of scatter signals.
  38. G3C10 Which of the following might be an indication that signals heard on the HF bands are being received via scatter propagation?
    A. The communication is during a sunspot maximum
    B. The communication is during a sudden ionospheric disturbance
    C. The signal is heard on a frequency below the Maximum Usable Frequency
    D. The signal is heard on a frequency above the Maximum Usable Frequency
    • (D)
    • Frequencies above the Maximum Usable Frequency (MUF) normally pass through the ionosphere out into space rather than being bent back, although atmospheric scatter from the ionosphere will sometimes allow communication on these frequencies. Amateurs trying to communicate on frequencies that seem to be above the MUF may notice that they can communicate using these scattered signals. Had they been using a frequency below the MUF they may not have noticed any scattered signals.
  39. G3C11 Which of the following antenna types will be most effective for skip communications on 40 meters during the day?
    A. Vertical antennas
    B. Horizontal dipoles placed between 1/8 and 1/4 wavelength above the ground
    C. Left-hand circularly polarized antennas
    D. Right-hand circularly polarized antenna
    • (B)
    • Low horizontal antennas, such as dipoles between 1/8 and ¼-wavelength above the ground work best for daytime skip communications on low frequencies. Signals from such antennas are radiated at high vertical angles that can be reflected by the ionosphere, but have a minimum amount of attenuation from the D and E layers.
  40. G3C12 Which ionospheric layer is the most absorbent of long skip signals during daylight hours on frequencies below 10 MHz?
    A. The F2 layer
    B. The F1 layer
    C. The E layer
    D. The D layer
    • (D)
    • Think of the D region as the "Darned Daylight" region. Instead of bending high frequency signals back to Earth, it absorbs energy from them. Signals at lower frequencies (longer wavelengths such as 160, 80, 60 and 40 meters) are absorbed more than at higher frequencies. The ionization created by the sunlight does not last very long in the D region, disappearing by sunset.
  41. G3C13 What is Near Vertical Incidence Sky-wave (NVIS) propagation?
    A. Propagation near the MUF
    B. Short distance HF propagation using high elevation angles
    C. Long path HF propagation at sunrise and sunset
    D. Double hop propagation near the LUF
    • (B)
    • Near Vertical Incidence Sky-wave (NVIS) propagation refers to communication using sky-wave signals transmitted at very high vertical angles. The frequencies used are below the critical frequency, meaning that their critical angle (see question G3C04) is ninety degrees, meaning they can be reflected straight back down to Earth. Because the signals travel at high angles, they have a minimum amount of attenuation from the D and E layers. The result is good communications in a region around the transmitter over distances higher than supported by ground-wave propagation

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