Radio Wave Propagation Flashcards
Sunspots and solar radiation; geomagnetic field and stability indices
How does a higher sunspot number affect HF propagation?
A. Higher sunspot numbers generally indicate a greater probability of good propagation at higher frequencies
B. Lower sunspot numbers generally indicate greater probability of sporadic E propagation
C. A zero sunspot number indicates that radio propagation is not possible on any band
D. A zero sunspot number indicates undisturbed conditions
A. Higher sunspot numbers generally indicate a greater probability of good propagation at higher frequencies
HF propagation is done through bouncing Electro-Magnetic waves off of charged particles in the earth’s atmosphere. High sunspot numbers indicate higher activity in the sun, which shoots off energy into the earth’s atmosphere, charging more particles that increases the amount of reflected power on the atmosphere, increasing received signal.
Higher sunspots = Higher probability at Higher frequencies
Silly Hint: Both question and answer has “Higher sunspot number”.
What effect does a sudden ionospheric disturbance have on the daytime ionospheric propagation?
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. It disrupts signals on lower frequencies more than those on higher frequencies
During a solar flare, a large amount of energy, including ultraviolet and x-ray radiation, travels out from the sun at the speed of light toward the daylight side of the Earth.
These large bursts of radiated energy cause a sudden increase of ionization in the ionospheric layers of Earth’s atmosphere. This is known as a Sudden Ionospheric Disturbance.
SIDs tend to “enhance low frequency propagation” and “absorb high frequency radiation” which appears to be the opposite of the correct answer. The important detail is that the question is about HF radiation specifically and it is the lower end of this range which is typically more affected.
Approximately how long does it take the increased ultraviolet and X-ray radiation from a solar flare to affect radio propagation on Earth?
A. 28 days
B. 1 to 2 hours
C. 8 minutes
D. 20 to 40 hours
C. 8 minutes
RF energy waves, such as ultraviolet and X-ray radiation, travel at the speed of light (approx. 300 million meters per second, or approx. 186,000 miles per second). The earth is about 93 million miles from the sun, and so it takes just over 8 minutes, on average, for a burst of radiation from solar flares to affect radio-wave propagation on earth.
Ultraviolet and X-ray radiation = 8 minutes
Particles from coronal mass ejection = 20 - 40 hours
Sun Cycle = 11 years
Which of the following are the least reliable bands for long-distance communications during periods of low solar activity?
A. 80 meters and 160 meters
B. 60 meters and 40 meters
C. 30 meters and 20 meters
D. 15 meters, 12 meters, and 10 meters
D. 15 meters, 12 meters, and 10 meters
High frequency (short wavelength) radio waves are transmitted the farthest when the upper layers of the ionosphere are energized during periods of high solar activity, so they are most affected and least reliable for long distance communications during periods of low solar activity. Therefore, 15 meters, 12 meters and 10 meters, is correct as these are the highest frequencies offered in the answer choices.
Silly hint: Long distance/Long answer.
Remember, low solar activity is bad for low wavelength. The correct answer has lowest [shortest] wavelengths.
What is the solar flux index?
A. A measure of the highest frequency that is useful for ionospheric propagation between two points on Earth
B. A count of sunspots that is adjusted for solar emissions
C. Another name for the American sunspot number
D. A measure of solar radiation with a wavelength of 10.7 centimeters
D. A measure of solar radiation with a wavelength of 10.7 centimeters
Measuring solar flux is another way of expressing the amount of solar activity. The solar flux is the intensity of the sun’s RF energy emissions.
The Solar flux index is a standardized representative of this radiation energy which is measured at a fixed value of 2800 MHz frequency (10.7 cm wavelength).
The advantage of this measurement over the sunspot index, is that it can be measured during any weather conditions - the sun doesn’t have to be visible. The higher the solar flux index number, the greater the amount of solar activity indicated.
What is a geomagnetic storm?
A. A sudden drop in the solar flux index
B. A thunderstorm that affects radio propagation
C. Ripples in the geomagnetic force
D. A temporary disturbance in Earth’s geomagnetic field
D. A temporary disturbance in Earth’s geomagnetic field
Easy hint: Storms are “Temporary”
Our earth is protected by outer lines of magnetic force referred to as the magnetosphere. These lines of force flow from pole to pole and help protect the earth from destructive particles. A solar flare can release massive amounts of charged particles toward the earth. When these particles reach the earth they can cause temporary disturbances of the Earth’s magnetosphere, called geomagnetic storms.
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
C. Only at the minimum point
D. At any point
D. At any point
The 20 meter (14 MHz) band is less affected by variations in the solar cycle than higher frequency bands. During periods of high solar activity, the band will be deflected for longer distances and with stronger signals, but it is a reliable band for worldwide daylight propagation during any point of the solar cycle.
Hint: Question “At what point?” - Answer: At any point
How can a geomagnetic storm affect HF propagation?
A. Improve high-latitude HF propagation
B. Degrade ground wave propagation
C. Improve ground wave propagation
D. Degrade high-latitude HF propagation
D. Degrade high-latitude HF propagation
During a Geomagnetic storm, charged particles from increased solar emissions, such as solar flares, are sent toward earth. The particles are deflected by the earth’s magnetosphere along lines of magnetic force from the North and South poles. The regions around the equator are protected. This increased activity is often seen as greater Auroras at the poles. Because the magnetic disturbances are concentrated around the higher latitudes (from about 45 degrees to the poles), HF propagation is distorted and degraded in these high-latitude regions.
How can high geomagnetic activity benefit radio communications?
A. Creates auroras that can reflect VHF signals
B. Increases signal strength for HF signals passing through the polar regions
C. Improve HF long path propagation
D. Reduce long delayed echoes
A. Creates auroras that can reflect VHF signals
Geomagnetic storms are bad news for HF transmissions, especially at higher latitudes. But a period of high geomagnetic activity can be good news for VHF propagation. The magnetic disturbance, which is centered around the poles, can produce aurora that can reflect VHF signals, thereby improving their chances of long-distance propagation.
What causes HF propagation conditions to vary periodically in a 26- to 28-day cycle?
A. Long term oscillations in the upper atmosphere
B. Cyclic variation in Earth’s radiation belts
C. Rotation of the Sun’s surface layers around its axis
D. The position of the Moon in its orbit
C. Rotation of the Sun’s surface layers around its axis
It takes the sun about 28 days to rotate once on its axis. Because solar activity in one region often lasts more than one rotation (such as a group of sunspots), we will typically see the same activity pattern return when the sun returns to that same point in its revolution (and those same sunspots come into range again).
Therefore it is the sun’s rotation on its axis that causes HF propagation conditions to vary periodically in a 28-day cycle.
How long does it take a coronal mass ejection to affect radio propagation on Earth?
A. 28 days
B. 14 days
C. 4 to 8 minutes
D. 15 hours to several days
D. 15 hours to several days
It takes charged particles from coronal mass ejections about 20 to 40 hours to travel to Earth and then affect radio propagation.
The sun is approximately 93 million miles from the earth. Unlike RF energy which travels at the speed of light, the charged particles from a coronal mass ejection take longer to reach Earth.
What does the K-index measure?
A. The relative position of sunspots on the surface of the Sun
B. The short-term stability of Earth’s geomagnetic field
C. The short-term stability of the Sun’s magnetic field
D. The solar radio flux at Boulder, Colorado
B. The short-term stability of Earth’s geomagnetic field
The K-index is a measurement of the short-term stability of Earth’s magnetic field. A high K-index means higher amounts of magnetic disturbance, and so more disruption of HF signals, especially in latitudes from 45 degrees to the poles.
Mnemonic: “K” in German stands for Kurt which means short (same as “curt” in English). Germany is on Earth.
What does the A-index measure?
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 Earth’s geomagnetic field
D. The solar radio flux at Boulder, Colorado
C. The long-term stability of Earth’s geomagnetic field
The A-index, like the K-index, are measures of the earth’s geomagnetic field stability. Whereas the K-index is a short-term measure, the A-index is an averaged daily figure and is charted over the usual rotational period of the sun, so is a better cyclical indicator of the long term stability of the Earth’s geomagnetic field.
Note: Remember that A-index is a longer AVERAGE as opposed to the short term magnetic KICK measured by the K-index.
How is long distance radio communication usually affected by the charged particles that reach Earth from solar coronal holes?
A. HF communication is improved
B. HF communication is disturbed
C. VHF/UHF ducting is improved
D. VHF/UHF ducting is disturbed
B. HF communication is disturbed
The emission of charged particles interacts with the earth’s magnetosphere causing geomagnetic storms or disturbances.
This causes disturbances in the ionosphere, which will adversely affect HF communication.
VHF/UHF is neither improved nor disturbed, since VHF/UHF typically does not rely on the ionosphere for propagation.
Silly hint: Something coming out of an ancient crown (corona) is disturbing.
What is a characteristic of skywave signals arriving at your location 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 slightly delayed echo might be heard
D. A slightly delayed echo might be heard
The short path of a signal is the most direct straight line path from one location or station to another. The long-path refers to the exact opposite direction, at 180 degrees. Sometimes if there is local interference, the long- path will give you a better signal. However if conditions are good, you may actually get signal reception from both the short and long paths. As is takes slightly longer for the RF signal to travel the 180 degree path (around the world) you might hear that long path signal as a well-defined echo!
What factors affect the MUF?
A. Path distance and location
B. Time of day and season
C. Solar radiation and ionospheric disturbances
D. All these choices are correct
D. All these choices are correct
The Maximum Usable Frequency (MUF) is the highest frequency you can use between two specific points that will allow your signal to reach the ionosphere, and be bent back to earth for reception rather than passing out into space. The MUF is dependent on the location of the two stations. So the path, distance and location are factors. The MUF is also very dependent on the amount of ionization of the atmospheric layers. The ionization of the ionospheric layers used for sky-wave propagation is affected by solar radiation, ionospheric disturbances, along with the time of day and season. Therefore choose answer All of these choices are correct.
Which frequency will have the least attenuation for long-distance skip propagation?
A. Just below the MUF
B. Just above the LUF
C. Just below the critical frequency
D. Just above the critical frequency
A. Just below the MUF
Attenuation or absorption of radio signal is lowest at the frequency just below the Maximum Usable Frequency (MUF). The maximum usable frequency is the highest frequency you can use at a specific time and location which will still allow your signal to be reflected back to earth, rather than continuing into outer space! In this frequency region, the signal strength is higher, and the reflection distances can be longer. Then, just below (about 85% of) the MUF is the Optimum Working Frequency, where the attenuation of the propagated signal is lowest.
Which of the following is a way to determine current propagation on a desired band from your station?
A. Use a network of automated receiving stations on the internet to see where your transmissions are being received
B. Check the A-index
C. Send a series of dots and listen for echoes
D. All these choices are correct
A. Use a network of automated receiving stations on the internet to see where your transmissions are being received
Internet-accessible websites like http://websdr.org/ allow you to listen to receivers world-wide, so you can hear where your transmissions are being received.
Many types of digital transmissions are monitored and reported to https://pskreporter.info/
How does the ionosphere affect radio waves with frequencies below the MUF and above the LUF?
A. They are refracted back to Earth
B. They pass through the ionosphere
C. They are amplified by interaction with the ionosphere
D. They are refracted and trapped in the ionosphere to circle Earth
A. They are refracted back to Earth
This “Goldilock’s” range below the Maximum Usable Frequency (MUF) and above the Lowest Usable Frequency (LUF) is just where you want to be with your transmissions. Above the MUF, is TOO HIGH and your signals will just keep going into outer space, under the LUF and your signals will be TOO LOW and get absorbed and won’t even make it to the ionospheric regions where you want to be. The region in between is JUST RIGHT, and your signals will be bent back to earth.
What usually happens to radio waves with frequencies below the LUF?
A. They are refracted back to Earth
B. They pass through the ionosphere
C. They are attenuated before reaching the destination
D. They are refracted and trapped in the ionosphere to circle Earth
C. They are attenuated before reaching the destination
When your signal is below the Lowest Usable Frequency (LUF), the ionosphere completely absorbs or attenuates the signal rather than reflecting the wave back to earth. The signal just gets lost in the atmospheric noise.
What does LUF stand for?
A. The Lowest Usable Frequency for communications between two specific points
B. Lowest Usable Frequency for communications to any point outside a 100-mile radius
C. The Lowest Usable Frequency during a 24-hour period
D. Lowest Usable Frequency during the past 60 minutes
A. The Lowest Usable Frequency for communications between two specific points
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 for communications between two points
What is the approximate maximum distance along the Earth’s surface 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. 2,500 miles
The F2 region is the highest region of the ionosphere, which is our “sweet spot” for long distance sky-wave transmissions. The F2 is highest during the middle of the day, when the sun’s energy is the greatest. It gets even better during periods of high solar activity and high ionization of the upper atmosphere. The approximate maximum distance along the Earth’s surface that is normally covered in one hop using the F2 region is 2,500 miles!
Note: Just remember that F2 can take you more than 2000 miles!
Silly hint: F2 … only answer with a Five and a 2
Remember 1,200 miles being right? That was E. F2 = 2,500
E = 1,200
What is the approximate maximum distance along the Earth’s surface 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. 1,200 miles
The E region is region of the ionosphere that is the second lowest, just above the D region. The E region, like the F region is more ionized and so more usable for sky-wave signals during the day (especially around noon), but doesn’t hold on to that energy for as much time during the day as the F region.
The E region still can support an approximate maximum distance along the Earth’s surface in one hop of 1,200 miles.
Note; Remember that the longest distance achievable in one hop is using the F2 region for over 2,000 miles, whereas the E region gives us about half that distance or an “Everyday” level of 1,200 miles.
What happens to HF propagation when the LUF exceeds the MUF?
A. Propagation via ordinary skywave communications is not possible over that 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. Propagation via ordinary skywave communications is not possible over that path
In the case where the Lowest Usable Frequency (LUF) exceeds the Maximum Usable Frequency (MUF), there is no HF radio frequency that will support ordinary skywave communications over the path between the two points.
The combination of atmospheric conditions and frequency could cause any HF signal between two points to be totally absorbed or attenuated and no signal will get through.
Just remember that both MUF and LUF are dependent on station locations as well as atmospheric conditions.
Its possible that you may be able to make that same path transmission at a different time with better ionization levels, or you might be able to communicate with another station at that time.
Which of the following is typical of the lower HF frequencies during the summer?
A. Poor propagation at any time of day
B. World-wide propagation during daylight hours
C. Heavy distortion on signals due to photon absorption
D. High levels of atmospheric noise or static
D. High levels of atmospheric noise or static
Thunderstorms in tropical areas cause high levels of static in summer. The noise from lightning strikes causes “static crashes” that can be heard on HF frequencies.
A simple memory device: Summer Storms result in Static
Which ionospheric region is closest to the surface of Earth?
A. The D region
B. The E region
C. The F1 region
D. The F2 region
A. The D region
The layers of the atmosphere are in alphabetical order; the D, E, and F layers are considered the ionosphere and the D layer (at a height of 30 to 60 miles) is the region closest to the surface of the earth.
The ionosphere is the portion of the atmosphere responsible for Skywave Propagation (bouncing signals “off of the sky” to come back down long distances away), which makes it of particular interest to Amateur Radio operators.
For more info see Wikipedia: D region
To remember this you may want to remember D for Down.
What is meant by the term “critical frequency” at a given incidence angle?
A. The highest frequency which is refracted back to Earth
B. The lowest frequency which is refracted back to Earth
C. The frequency at which the signal-to-noise ratio approaches unity
D. The frequency at which the signal-to-noise ratio is 6 dB
A. The highest frequency which is refracted back to Earth
Each HF frequency is affected differently by the ionosphere at different times of day, season and angle it strikes the ionosphere. The maximum frequency that will be refracted back to earth, and not just continue into space, at a given time is called the “critical frequency”.
Why is skip propagation via the F2 region longer than that via the other ionospheric regions?
A. Because it is the densest
B. Because of the Doppler effect
C. Because it is the highest
D. Because of temperature inversions
C. Because it is the highest
The F2 region appears when the highest layer of the ionosphere (the F region) becomes strongly ionized. As the ionization increases, the F layer splits into the lower F1 region and the higher F2 region. Because it is the highest ionospheric region, long distances may be reached in one bend or “hop” of the signal (up to about 2,500 miles!).
This is why the F2 region is mainly responsible for the longest distance radio wave propagation.
What does the term “critical angle” mean, as applied to 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 Earth under specific ionospheric conditions
D. The highest takeoff angle that will return a radio wave to Earth under specific ionospheric conditions
D. The highest takeoff angle that will return a radio wave to Earth under specific ionospheric conditions
Silly hint: Your health is critical with high blood pressure.
In radio wave propagation, the term critical angle refers to the highest takeoff angle that will return a radio wave to the Earth under specific ionospheric conditions. If the angle of the signal leaving your antenna is too low, it may not reach the ionosphere, or if parallel to the earth, not even make it around the curvature of the earth and be lost. If the angle is too great the signal may go straight through the atmosphere and be lost into space. The critical angle, like the critical frequency levels of MUF and LUF are dependent on the conditions of the ionosphere, as the height of layers change with ionization levels, and the angle needed to reach those levels will change with those factors.
Why is long-distance communication on the 40-, 60-, 80-, and 160-meter bands more difficult during the day?
A. The F region absorbs signals at these frequencies during daylight hours
B. The F region is unstable during daylight hours
C. The D region absorbs signals at these frequencies during daylight hours
D. The E region is unstable during daylight hours
C. The D region absorbs signals at these frequencies during daylight hours
HF frequencies such as the 40, 60, 80 and 160 meter bands are harder to use for long distance communication during the day. The same factors that make the upper E and F layers great for the higher VHF frequencies make the lower D layer more “absorbant” of HF wave signals. The lower HF signals are attenuated or absorbed and so do not pass through for bending at higher ionospheric levels. Ionization is higher during daylight hours, and so the D layer absorbs these frequencies much more during daylight hours. This makes the band much more useful for nighttime use than during the day.
Remember “D” for Daylight, Dull, Diminished.
What is a characteristic of HF scatter?
A. Phone signals have high intelligibility
B. Signals have a fluttering sound
C. There are very large, sudden swings in signal strength
D. Scatter propagation occurs only at night
B. Signals have a fluttering sound
HF scatter signals often sound distorted because the energy is scattered into the skip zone through several different radio wave paths. When this happens, the original signal is Split or Scattered and will be de-fragmented and may sound distorted at the receiver.
What makes HF scatter signals often sound distorted?
A. The ionospheric region 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 paths
D. Energy is scattered into the skip zone through several different paths
HF scatter signals often sound distorted because the energy is scattered into the skip zone through several different radio wave paths.
Note:
A signal which is properly adjusted for angle and frequency is like tossing a clump of clay into the air: it makes a nice arc and comes back to earth at one spot, landing with a nice “thunk”.
A signal which is scattered into the skip zone is like tossing up that same lump of clay into the air, but this time it hits the leaves of an overhanging tree which breaks up the clump. Some parts gets stuck in the tree, and the parts that do come down land in a wider area, making lots of smaller “scattered” sounds.
Silly, but easy, tip: the word “scatter” is used in the answer.
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 via ground waves, which absorb most of the signal energy
D. Propagation is via ducts in the F region, which absorb most of the energy
A. Only a small part of the signal energy is scattered into the skip zone
An HF signal in the skip zone gets broken up by the ionospheric conditions, where only a small part of the signal energy is scattered into the skip zone. The signal that does get back to the receiving station arrives in “little bits” at slightly different angles, and so produces a weak and wavering sound.
Hint: ‘skip zone’ in both question and answer.
What type of propagation allows signals to be heard in the transmitting station’s skip zone?
A. Faraday rotation
B. Scatter
C. Chordal hop
D. Short-path
B. Scatter
With radio signals you have ground wave, meaning your signal goes between point a-b directly. Then you have sky waves, where your signal hits the ionosphere and bounces back down to earth possibly hundreds of miles away. What happens if you have a station you want to talk to in between the reach of your ground wave and sky wave? Those are the stations in your “skip zone.” The best way to reach them is by scatter, meaning your signal will scatter off the ionosphere and possibly hit your station in the skip zone. NVIS or Near Vertical Incidence Skywave is scatter propogation.
Silly Hint: Imagine yourself “scattering” papers about trying to figure out why the signal is heard in the skip zone.
The shortest answer is the correct answer.
What is near vertical incidence skywave (NVIS) propagation?
A. Propagation near the MUF
B. Short distance MF or HF propagation at high elevation angles
C. Long path HF propagation at sunrise and sunset
D. Double hop propagation near the LUF
B. Short distance MF or HF propagation at high elevation angles
Near Vertical Incidence Sky-wave (NVIS) propagation as the name indicates uses signals which are sent out at very high angles, yet below the critical angle, or these signals would go right out into space! Because these signals take a shorter path through the ionospheric layers, they often have less absorption, attenuation or distortion of the signal. This method can be great for high quality short distance HF propagation using these high elevation angles, especially where there are interferences in the way of direct ground-wave propagation.
Hint: Near vertical implies high elevation angle.
For more info see Wikipedia: Near Vertical Incidence Sky-wave (NVIS)
Oversimplification: basically, instead of an antenna that radiates parallel to the ground, like a mag mount on your car or a Yagi pointed across town, you tip it over 90 degrees so you’re sending RF at the sky (Yagi pointed at zenith in “Christmas Tree Mode”). These are also known as “cloud warmers.”
Which ionospheric region is the most absorbent of signals below 10 MHz during daylight hours?
A. The F2 region
B. The F1 region
C. The E region
D. The D region
D. The D region
The daylight ionization that makes the upper ionospheric regions great for VHF sky-wave propagation works against HF frequencies below 10 MHz. During the daytime the D layer absorbs these signals. Think of the D layer as a “Daylight Dud”. This layer becomes much more usable after dark, when ionization of the band decreases its “absorbent” character.
