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Comms > Comm 09-10 - HF Propagation > Flashcards

Comm 09-10 - HF Propagation Flashcards

1
Q

Review Questions

What are the three propagation paths?

A

Ground wave
Tropospheric Wave Ducting
Sky Wave

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2
Q

Review Questions

What is tropospheric wave ducting?

A

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.

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3
Q

The ionosphere is a region of what?

A

Of ionized particles in the upper atmosphere.

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4
Q

Where does the ionosphere extend from and to?

A

approximately 30 - 600 miles above earth’s surface.

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5
Q

What causes ionization in the ionosphere?

A

Caused by high energy ultra-violet radiation from the sun being absorbed by atmospheric gases.

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6
Q

Describe the relationship between radio waves and the ionosphere.

A

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

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7
Q

What distances and frequency bands accompany a Sky Wave?

A

A Sky Wave gives reliable comms over very long distances and is useful in the MF and HF bands

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8
Q

True or false

The ionosphere comprises of several distinct layers that vary in ionization.

A

True

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9
Q

What are the three causes that cause the layers of the ionosphere to fluctuate in height and intensity?

A

Time of day, the season and sunspot activity

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10
Q

When the sun is at its peak there are 4 designated layers, what are they?

A

D
E
F1
F2

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11
Q

At night, what happens to the ionospheric layers?

A

D layer disappears and F1 and F2 combine

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12
Q

Which layer is the most important layer for long range communication?

A

The F layer

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13
Q

Describe the height of each ionospheric layer.

A

D - 30-55 miles
E - 55 - 60 miles
F - 80 - 250+ miles

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14
Q

Describe the communication ranges of each ionospheric layer.

A

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

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15
Q

In all cases, the more intense the sun’s rays the greater the?

A

Ionization and consequently, the more refractive the ionosphere.

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16
Q

The ionosphere acts as a?

A

Conductor or radio waves.

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17
Q

When a radio wave encounters a particle with an electric charge in the ionosphere what happens?

A

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.

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18
Q

What is the rule of thumb for HF frequencies?

A

High sun = higher frequency waves refracted back to earth

Low sun = low frequency waves refracted back to earth

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19
Q

When is ionization at its lowest during the day?

A

When the sun is at its lowest.

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20
Q

What are the four factors affecting the return of a radio signal back to earth (refraction)?

A

Angle of transmission
Frequency
Signal strength
Ion density

21
Q

What is the relationship between transmission angle and the return wave to earth?

A

As the angle of transmission is increases, greater bending of the wave is required to return it to earth.

22
Q

Define critical angle.

A

For a given frequency, the highest angle at which the radio wave can be transmitted and still be refracted back to earth.

23
Q

Describe the relationship between frequency, refraction and attenuation.

A

Lower frequencies undergo more refraction but suffer from high attenuation

Higher frequencies refract less but are less affected by attenuation.

24
Q

Define Critical Frequency

A

For a given angle of transmission, the highest frequency which will be returned to earth is called the critical frequency.

25
Q

True or false

All radio transmissions lose energy to attenuation.

A

True

26
Q

____ energy signals are required to cover ____ distances and still be of useable level.

A

High / Long

27
Q

How can signal strength be increased?

A

Increased transmitter power or higher frequencies

28
Q

Higher ion density increases the _________ properties of the atmosphere.

A

refractive

29
Q

An atmosphere with higher ion density will do what two things to EM waves?

A

Refract higher frequencies and increases the amount of bending of any frequency

30
Q

HF definitions - Propagation Path

A

The path of radio waves when they are transmitted or propagated form one point of earth to another, or into the atmosphere.

31
Q

HF definitions - Skip Distance

A

The distance between the transmitter and the point where the refracted sky wave returns to earth for the first time

32
Q

Provided the tx angle, signal strength and ionospheric density remain the same, the skip distance will be dependent on what?

A

The frequency

33
Q

describe the relationship between the critical angle and the skip distance.

A

The lower (smaller) the critical angle, the longer the skip distance

34
Q

HF definitions - Skip Zone

A

Distance between the end of the useable ground wave and the point where the first sky wave returns to earth.

35
Q

HF definitions - Fading

A

Fluctuations in the strength of the radio signal at the receiver due to interference.

36
Q

What causes Fading?

A

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.

37
Q

True or false

As ionospheric conditions continually fluctuate, signals may undergo constructive and destructive interference, causing different signal strengths.

A

True

38
Q

HF definitions - Maximum Useable Frequency (MUF)

A

The highest frequency that will give a skip distance equal to the distance from the transmitter to the intended receiving point.

39
Q

HF definitions - Optimum Working Frequency

A

The frequency which provides the most consistent comms, generally 85% of the MUF

40
Q

Why is it important to limit your transmitted power to the minimum required to maintain effective comms?

A

For tactical reasons and to reduce the electrical draw from the radios.

41
Q

Should you choose a ground station directly in front/ behind or abeam the aircraft? Why?

A

Abeam the aircraft, the radiation patterns produced by the HF antennas provide more effective comms with abeam ground stations

42
Q

Why is it important to be aware of A/C movement relative to the ground station and the time of day?

A

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

43
Q

Should you always request a secondary frequency? When should you request another?

A

Yes, you always should.

Request an additional secondary frequency when you have switch to a new secondary frequency

44
Q

True or false

Taking note of frequencies that have worked in the past is not necessary as they will most likely not work again.

A

False, prior frequencies will likely work; however.

45
Q

When considering a secondary frequency, what rules of thumb should be applied?

A

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

46
Q

Review Questions

What are the three main propagation paths for radio waves and which one is used for long range comms?

A

Ground wave
Sky wave
Tropospheric wave ducting

Sky wave used for long range comms

47
Q

Review Questions

What four factors affect the return of a radio signal back to earth?

A

Ion density
Signal strength
Frequency
Angle of transmission

48
Q

Review Questions

What effect does the ionosphere have on radio waves?

A

Radio waves change velocity as they travel through the ionosphere causing them to bend (refract) back to earth.

49
Q

Review Questions

What is the Optimum Working Frequency in relation to the MUF?

A

OWF = 85% of MUF

Comms (9 decks)

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