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Radio Navigation > Radio propagation (14-05-18) > Flashcards

Radio propagation (14-05-18) Flashcards

1
Q

What is a sky wave?

A

A wave that returns to the earth via refraction via the ionosphere.

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2
Q

What band of frequencies are sky waves?

A

Mainly HF.

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3
Q

What are ground waves?

A

Anything that isnt a sky wave:

Space waves (VHF+)

Surface waves (MF and below)

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4
Q

How do space waves propagate?

A

In straight lines (LOS)

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5
Q

How do surface waves propagate?

A

Some defraction occurs around the earths surface.

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6
Q

What is attenuation?

A

A gradual loss of signal strength through a medium.

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7
Q

What is the effect of frequency on the level of attenuation in the atmosphere and ionosphere?

A

In the atmosphere: freq up, attenuation up

ionosphere is opposite: freq up attenuation down

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8
Q

At what frequency does attenuation in the atmosphere become significant?

A

1GHz

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9
Q

What are escape waves?

A

Waves that have not refracted enogh to return.

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10
Q

Regarding refraction, what is the critical angle?

A

The angle at which the first returning sky wave occurs.

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11
Q

What is the minimum skip distance?

A

The range that you can’t transmit sky waves due to critical angle.

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12
Q

What is the dead zone?

A

The area where you can’t transmit a signal with either sky waves or ground waves.

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13
Q

What is the effect of frequency on refraction?

A

A lower frequency (therefore longer wavelength) will refract more.

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14
Q

What happens to the height of the ionosphere daily?

A

It is lower in day time due to increased solar radiation causing more ionisation.

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15
Q

What is the effect of time of day on the minimum skip distance?

A

At night the ionosphere is higher and therefore refraction will be higher so minimum skip distance will be longer.

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16
Q

What is the effect of frequency on minimum skip distance?

A

A lower frequency will refect more, reducing cirtical angle and therefore minimum skip distance

17
Q

What is the problem with low frequency signals?

A

Poor clarity, very susceptible to static and noise.

18
Q

In order to keep the minimum skip distance the same, what must happen to signal frequency in the day and at night?

A

Sun up, frequency up.

19
Q

What is the optimum frequency and do we like to use it?

A

Where the first returning sky wave is exactly on the receiver and no because it isn’t safe enough

20
Q

What is maximum usable freq (MUF)?

A

optimum freq - 100nm (therefore has a lower frequency than optimum)

21
Q

What limits the lowest usable frequency for transmissions?

A

the level of static

22
Q

What is the structure of the ionosphere?

A

Day: D, E, F1 & F2

Night: E & F

D is weakest

F is strongest and gives refraction

E & D give attenuation

F is at 225km

E is at 125km

D is at 75km

23
Q

What is the equation for maximum theoretical range of a transmission?

A

MTR = 1.23 x (root(HtTx) + root(HtRx))

HtTx - height of the transmitter

HtRx - height of the receiver

24
Q

What is the ratio of attenuation levels of deserts to ice?

A

3:1

25
Q

What is the optimum frequency for diffraction?

A

VLF

26
Q

What is meant by antiphase?

A

2 signals of equal freq are out of phase by 180deg, and oppose eachother.

27
Q

What is meant by a phase being “behind” or “lagging”?

A

The signal out of phase starts after the main signal.

28
Q

What is super-refraction, what frequency bands does it affect and what causes it?

A

It is a type of refraction that occurs via an inversion that causes VHF+ frequencies to be refracted thus possibly causing destructive interference at the receiver.

29
Q

What is sub-refraction, what frequency bands does it affect and what causes it?

A

It is a reduction in the defraction at the surface in a section of instability which straightens out a signals propagation, reducing it’s range to LOS, affecting VHF+ bands.

30
Q

What is sporadic E, what frequency bands does it affect and what causes it?

A

Refraction of VHF bands in strong E layer conditions in the ionosphere, causing destructive interference at the receiver.

31
Q

What frequency bands are affected by static & noise?

A

VLF - HF

32
Q

What is the effect of altitude on static?

A

Higher altitude means less static

33
Q

What is the signal to noise ratio?

A

The point when a signal becomes loud enough to be heard over noise.

Radio Navigation (17 decks)

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