Comm 09-10 - HF Propagation Flashcards
Review Questions
What are the three propagation paths?
Ground wave
Tropospheric Wave Ducting
Sky Wave
Review Questions
What is tropospheric wave ducting?
The part of the radiation that undergoes refraction and reflection in the lower levels of the atmosphere. It is caused by abrupt changes in temperature or moisture.
The ionosphere is a region of what?
Of ionized particles in the upper atmosphere.
Where does the ionosphere extend from and to?
approximately 30 - 600 miles above earth’s surface.
What causes ionization in the ionosphere?
Caused by high energy ultra-violet radiation from the sun being absorbed by atmospheric gases.
Describe the relationship between radio waves and the ionosphere.
The ionosphere refracts radio waves back towards the surface
Ranges are further enhanced when the wave is reflected off the earth’s surface back to ionosphere for a second or third hop, known as multi-hop transmission
What distances and frequency bands accompany a Sky Wave?
A Sky Wave gives reliable comms over very long distances and is useful in the MF and HF bands
True or false
The ionosphere comprises of several distinct layers that vary in ionization.
True
What are the three causes that cause the layers of the ionosphere to fluctuate in height and intensity?
Time of day, the season and sunspot activity
When the sun is at its peak there are 4 designated layers, what are they?
D
E
F1
F2
At night, what happens to the ionospheric layers?
D layer disappears and F1 and F2 combine
Which layer is the most important layer for long range communication?
The F layer
Describe the height of each ionospheric layer.
D - 30-55 miles
E - 55 - 60 miles
F - 80 - 250+ miles
Describe the communication ranges of each ionospheric layer.
D - Considered a hindrance to long range communication, ionization is not sufficient for to contribute to refraction, but atmosphere is still dense enough to cause absorption.
C - Can refract signals up to 20 MHz up to 620 miles
F - Can refract frequencies up to 30 MHz over very long ranges due to a relatively high and constant ionized layer present
In all cases, the more intense the sun’s rays the greater the?
Ionization and consequently, the more refractive the ionosphere.
The ionosphere acts as a?
Conductor or radio waves.
When a radio wave encounters a particle with an electric charge in the ionosphere what happens?
The particle vibrates, absorbs the EM energy from the radio wave re-radiates it. The portion of the re-radiated wave then refracts back to earth.
What is the rule of thumb for HF frequencies?
High sun = higher frequency waves refracted back to earth
Low sun = low frequency waves refracted back to earth
When is ionization at its lowest during the day?
When the sun is at its lowest.
What are the four factors affecting the return of a radio signal back to earth (refraction)?
Angle of transmission
Frequency
Signal strength
Ion density
What is the relationship between transmission angle and the return wave to earth?
As the angle of transmission is increases, greater bending of the wave is required to return it to earth.
Define critical angle.
For a given frequency, the highest angle at which the radio wave can be transmitted and still be refracted back to earth.
Describe the relationship between frequency, refraction and attenuation.
Lower frequencies undergo more refraction but suffer from high attenuation
Higher frequencies refract less but are less affected by attenuation.
Define Critical Frequency
For a given angle of transmission, the highest frequency which will be returned to earth is called the critical frequency.
True or false
All radio transmissions lose energy to attenuation.
True
____ energy signals are required to cover ____ distances and still be of useable level.
High / Long
How can signal strength be increased?
Increased transmitter power or higher frequencies
Higher ion density increases the _________ properties of the atmosphere.
refractive
An atmosphere with higher ion density will do what two things to EM waves?
Refract higher frequencies and increases the amount of bending of any frequency
HF definitions - Propagation Path
The path of radio waves when they are transmitted or propagated form one point of earth to another, or into the atmosphere.
HF definitions - Skip Distance
The distance between the transmitter and the point where the refracted sky wave returns to earth for the first time
Provided the tx angle, signal strength and ionospheric density remain the same, the skip distance will be dependent on what?
The frequency
describe the relationship between the critical angle and the skip distance.
The lower (smaller) the critical angle, the longer the skip distance
HF definitions - Skip Zone
Distance between the end of the useable ground wave and the point where the first sky wave returns to earth.
HF definitions - Fading
Fluctuations in the strength of the radio signal at the receiver due to interference.
What causes Fading?
When numerous signals arrive at a receiver after having followed separate propagation paths. Due to this, they will arrive at the receiver with different phase relationships.
True or false
As ionospheric conditions continually fluctuate, signals may undergo constructive and destructive interference, causing different signal strengths.
True
HF definitions - Maximum Useable Frequency (MUF)
The highest frequency that will give a skip distance equal to the distance from the transmitter to the intended receiving point.
HF definitions - Optimum Working Frequency
The frequency which provides the most consistent comms, generally 85% of the MUF
Why is it important to limit your transmitted power to the minimum required to maintain effective comms?
For tactical reasons and to reduce the electrical draw from the radios.
Should you choose a ground station directly in front/ behind or abeam the aircraft? Why?
Abeam the aircraft, the radiation patterns produced by the HF antennas provide more effective comms with abeam ground stations
Why is it important to be aware of A/C movement relative to the ground station and the time of day?
As you move towards the ground station, you may need to switch to a lower frequency, and vice versa
Additionally, as the sun rises or sets you may need to raise or lower your frequency to keep pace with changing ionospheric conditions
Should you always request a secondary frequency? When should you request another?
Yes, you always should.
Request an additional secondary frequency when you have switch to a new secondary frequency
True or false
Taking note of frequencies that have worked in the past is not necessary as they will most likely not work again.
False, prior frequencies will likely work; however.
When considering a secondary frequency, what rules of thumb should be applied?
High sun/ high freq and distance from the ground station.
Ex. If you T/O in the AM and are flying away from the ground station, ensure your secondary frequency is higher than your frequency
Review Questions
What are the three main propagation paths for radio waves and which one is used for long range comms?
Ground wave
Sky wave
Tropospheric wave ducting
Sky wave used for long range comms
Review Questions
What four factors affect the return of a radio signal back to earth?
Ion density
Signal strength
Frequency
Angle of transmission
Review Questions
What effect does the ionosphere have on radio waves?
Radio waves change velocity as they travel through the ionosphere causing them to bend (refract) back to earth.
Review Questions
What is the Optimum Working Frequency in relation to the MUF?
OWF = 85% of MUF
