Radio propagation (14-05-18) Flashcards
What is a sky wave?
A wave that returns to the earth via refraction via the ionosphere.
What band of frequencies are sky waves?
Mainly HF.
What are ground waves?
Anything that isnt a sky wave:
Space waves (VHF+)
Surface waves (MF and below)
How do space waves propagate?
In straight lines (LOS)
How do surface waves propagate?
Some defraction occurs around the earths surface.
What is attenuation?
A gradual loss of signal strength through a medium.
What is the effect of frequency on the level of attenuation in the atmosphere and ionosphere?
In the atmosphere: freq up, attenuation up
ionosphere is opposite: freq up attenuation down
At what frequency does attenuation in the atmosphere become significant?
1GHz
What are escape waves?
Waves that have not refracted enogh to return.
Regarding refraction, what is the critical angle?
The angle at which the first returning sky wave occurs.
What is the minimum skip distance?
The range that you can’t transmit sky waves due to critical angle.
What is the dead zone?
The area where you can’t transmit a signal with either sky waves or ground waves.
What is the effect of frequency on refraction?
A lower frequency (therefore longer wavelength) will refract more.
What happens to the height of the ionosphere daily?
It is lower in day time due to increased solar radiation causing more ionisation.
What is the effect of time of day on the minimum skip distance?
At night the ionosphere is higher and therefore refraction will be higher so minimum skip distance will be longer.
What is the effect of frequency on minimum skip distance?
A lower frequency will refect more, reducing cirtical angle and therefore minimum skip distance
What is the problem with low frequency signals?
Poor clarity, very susceptible to static and noise.
In order to keep the minimum skip distance the same, what must happen to signal frequency in the day and at night?
Sun up, frequency up.
What is the optimum frequency and do we like to use it?
Where the first returning sky wave is exactly on the receiver and no because it isn’t safe enough
What is maximum usable freq (MUF)?
optimum freq - 100nm (therefore has a lower frequency than optimum)
What limits the lowest usable frequency for transmissions?
the level of static
What is the structure of the ionosphere?
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
What is the equation for maximum theoretical range of a transmission?
MTR = 1.23 x (root(HtTx) + root(HtRx))
HtTx - height of the transmitter
HtRx - height of the receiver
What is the ratio of attenuation levels of deserts to ice?
3:1
What is the optimum frequency for diffraction?
VLF
What is meant by antiphase?
2 signals of equal freq are out of phase by 180deg, and oppose eachother.
What is meant by a phase being “behind” or “lagging”?
The signal out of phase starts after the main signal.
What is super-refraction, what frequency bands does it affect and what causes it?
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.
What is sub-refraction, what frequency bands does it affect and what causes it?
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.
What is sporadic E, what frequency bands does it affect and what causes it?
Refraction of VHF bands in strong E layer conditions in the ionosphere, causing destructive interference at the receiver.
What frequency bands are affected by static & noise?
VLF - HF
What is the effect of altitude on static?
Higher altitude means less static
What is the signal to noise ratio?
The point when a signal becomes loud enough to be heard over noise.
