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Ahmed > Chapter 2 | Auto Pilot > Flashcards

Chapter 2 | Auto Pilot Flashcards

1
Q

What is necessary to understand the reasons for certain circuits in radio equipment?

A

Knowledge of the propagation of radio waves

Understanding radio wave propagation helps explain frequency preferences and performance limitations.

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2
Q

What surrounds a conductor when a current flows through it?

A

Lines of magnetic force producing a magnetic field

The strength of the magnetic field is proportional to the amount of current.

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3
Q

What is established between the plates of a capacitor when a voltage source is connected?

A

An electrostatic, or electric, field

This electric field is perpendicular to the plates of the capacitor.

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4
Q

What type of wave is generated when both electric and magnetic fields are present?

A

Transverse Electro Magnetic wave (TEM wave)

The TEM wave has electric and magnetic fields that are perpendicular to each other.

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5
Q

What is the impedance of free space?

A

Z = 377 ohms

This value is derived from the parallel plate concept.

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6
Q

What is the velocity of a TEM wave in free space?

A

Approximately 300 x 10^6 meters/second

This is also the velocity of light in free space.

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7
Q

What happens to the radiation pattern of an antenna when it is used near the earth’s surface?

A

It assumes a toroidal or doughnut shape

This is due to the interaction with the earth’s surface.

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8
Q

What are the three main groups of waves radiated by an antenna?

A
  • Space Wave
  • Surface Wave
  • Tropospheric or Sky Wave
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9
Q

What is the effect of diffraction on electromagnetic waves?

A

Waves follow the edge of the material causing the diffraction

This is most pronounced at low frequencies.

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10
Q

What are the two main causes of wave attenuation?

A
  • Spatial attenuation
  • Absorption attenuation
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11
Q

What is the relationship between the conductivity of the surface and wave attenuation?

A

Attenuation is inversely proportional to the conductivity of the surface

Higher conductivity (like water) results in lower attenuation.

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12
Q

What is the plane of polarisation defined as?

A

The plane of the E field

A vertically polarised wave is radiated from a vertical dipole antenna.

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13
Q

What is the ionosphere and where does it extend?

A

A region of ionised gas extending from approximately 30 miles to at least 250 miles above the earth’s surface

Ionisation occurs due to solar radiation.

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14
Q

What happens to a radio wave when it enters an ionised layer?

A

It is refracted towards the earth

The angle of refraction depends on the angle of incidence and the frequency of the wave.

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15
Q

What is the critical angle in relation to radio wave propagation?

A

The angle of incidence below which a ray will not be reflected but escape through the ionised layer

Rays that exceed this angle will escape instead of reflecting back.

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16
Q

Fill in the blank: The power radiated by an isotropic radiator is enclosed within a _______.

A

sphere

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17
Q

True or False: The surface wave is most effective at high frequencies.

A

False

Diffraction is most pronounced at low frequencies.

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18
Q

What is the CRITICAL ANGLE in ionospheric propagation?

A

The angle below which the ray will not be reflected but will escape through the layer.

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19
Q

What is the CRITICAL FREQUENCY?

A

The maximum frequency at which reflection can take place at vertical incidence.

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20
Q

Define MAXIMUM USABLE FREQUENCY (MUF).

A

The maximum frequency which will be reflected at a given angle of incidence.

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21
Q

What is the OPTIMUM WORKING FREQUENCY (OWF)?

A

About 85% of the MUF, chosen to ensure reliable communication.

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22
Q

What is SKIP DISTANCE?

A

The shortest distance at which a frequency will return to earth for a given layer height.

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23
Q

True or False: The skip distance increases as frequency increases.

A

True.

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24
Q

What happens if the frequency is too low?

A

Losses in the ionosphere increase, leading to high attenuation.

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25
Q

How is fading defined in radio wave propagation?

A

Variation in amplitude of the voltage induced in the receiver antenna.

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26
Q

What is INTERFERENCE FADING?

A

Fading caused by variations in phase of rays arriving at the receiver antenna.

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27
Q

What is SELECTIVE FADING?

A

Distortion of the signal due to varying path lengths and attenuation of component frequencies.

28
Q

What causes Sunspots?

A

Cyclic variations in UV radiation affecting MUF.

29
Q

What is the effect of solar flares on ionisation?

A

Increased UV radiation leads to sudden fadeout of ionospheric wave reception.

30
Q

Define Ionospheric Storms.

A

Disturbances caused by ionised particles from the sun affecting radio wave communication.

31
Q

What are Magnetic Storms?

A

Fluctuations in the earth’s magnetic field caused by ionised particles from the sun.

32
Q

What is Meteor Ionisation?

A

Local volume of increased ionisation caused by meteors entering the atmosphere.

33
Q

What does SID stand for?

A

Sudden Ionospheric Disturbances.

34
Q

What is the maximum range for ionospheric scatter propagation?

A

About 1200 miles.

35
Q

What is Super Refraction?

A

A phenomenon where waves are trapped in a duct formed by temperature and humidity changes.

36
Q

How does the surface wave and sky wave respond to obstructions?

A

Slightly affected, but the space wave is seriously attenuated by obstructions.

37
Q

What is the effective range for VLF communication?

A

Up to 1000 miles.

38
Q

What is the service radius of a medium power MF station?

A

Around 100 miles.

39
Q

What frequency range is considered VLF?

A

30 kHz.

40
Q

What frequency range is considered MF?

A

300 kHz to 3 MHz.

41
Q

Fill in the blank: The phase change during reflection for horizontally polarised waves is _______.

A

180°.

42
Q

What is the main wave path for frequencies below 300 KHz?

A

Ground Wave.

43
Q

What happens to the received signal strength if the direct and reflected rays arrive in phase?

A

Reception will be maximum.

44
Q

What is the primary cause of atmospheric interference in VLF and LF bands?

A

Electrical discharges from clouds.

45
Q

What is the nature of VLF and LF antennas?

A

They must be comparable in length to the operating wavelength.

46
Q

What is the service radius of a medium power broadcasting station?

A

Approximately 100 miles

47
Q

What factors affect the attenuation of the sky wave?

A

State of ionosphere, time of day, and season

48
Q

When does the greatest variation in sky wave attenuation occur?

A

At sunrise and sunset

49
Q

How far can the sky wave be received at night with low ionisation?

A

Up to 1000 miles, subject to severe fading

50
Q

What is space diversity reception?

A

A system connecting two or three widely spaced antennas to the receiver to overcome fading

51
Q

What type of antennas are commonly used in MF broadcasting?

A

Fairly large, usually omni-directional antennas

52
Q

What is the frequency range for HF communication?

A

3 - 30 MHz

53
Q

Why is the ground wave used only for short-range communication in HF?

A

Due to high attenuation of the ground wave

54
Q

What is required for long-range point-to-point communication in HF?

A

Reflection of the sky wave from the ionosphere

55
Q

What happens to radio waves above 30 MHz?

A

They penetrate the ionosphere and are not reflected

56
Q

How does ionization affect the reflection of radio waves?

A

Reflection depends on the degree of ionization, varying with time of day and season

57
Q

What is the relationship between frequency and ionospheric attenuation?

A

Attenuation varies with frequency and decreases as frequency increases

58
Q

What frequency change might a station operating on 6 MHz during the day make at night?

A

It might use 2 MHz at night

59
Q

What types of interference are less severe on HF frequencies?

A

Atmospheric and man-made interference

60
Q

What type of interference noise is produced above 20 MHz?

A

Hissing interference noise from solar and galactic radiation

61
Q

What are the frequency ranges for VHF, UHF, and SHF communication?

A

VHF: 30 - 300 MHz, UHF: 300 - 3000 MHz, SHF: 3000 - 30000 MHz

62
Q

What is the primary mode of communication at VHF, UHF, and SHF frequencies?

A

Using space waves for short distances

63
Q

What types of systems use VHF, UHF, and SHF carrier frequencies?

A
  • Television
  • FM broadcasting
  • Pulse transmission systems
  • Frequency division multiplex systems
64
Q

What is the air-to-ground communication frequency range?

A

100 - 156 MHz (VHF) and 220 - 400 MHz (mainly UHF)

65
Q

What propagation method is used for frequencies up to 60 MHz?

A

Ionospheric scatter propagation

66
Q

What is the range for ionospheric scatter propagation?

A

Approximately 1000 miles

67
Q

What limits reception above 500 MHz?

A

Tropospheric scatter provides limited reception up to 400 miles

Ahmed (2 decks)

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