How it Works | Propagation Flashcards
The ionosphere begins at about __ miles up
60
The nitrogen ionises at ___ levels
higher
The oxygen ionises at ___ levels
lower
Ionisation occurs mostly during the ___, and least just before ___
day, sunrise
Recombination is the recombining of the ___ with the ___
atoms electrons
A warmer ionosphere with ___ ionisation reflects signals ___
increased, better
The reflection is best towards the ___ in the mornings, to the South around ___, and the west in the ___ and ___.
East, Midday, evenings, evenings
_ layers of electrically charged gas atoms are produced between __ and ___km above the Earth
4 50 300
The D layer is between __ and __ km
50 100
The D layer exists during __ hours
daylight
The D layer experiences ___ ionisation as ___ UV waves penetrate so far into the atmosphere.
low, few
The D layer can refract _LF when large antenna and ___ power transmitters are used.
VLF high
The D layer is mainly responsible for absorbing and blocking _F and _F waves, whilst being transparent to _F.
LF MF HF
As it fades, _F signals can reach higher layers where they can be reflected back.
M
The E layer is between __ and ___ km
100 150
The E layer returns _F and _F bands, and waves longer than ___m
LF HF 100
The E layer is normally transparent to _HF
VHF
Sporadic E are strong patches of ionisation which can return _HF
VHF
The F layer exists in ___, and splits into __ and __
daylight, F1, F2
The F layer is responsible for most _F communications, wavelengths of less than ___m
HF, 10m
F layer atoms can stay ionised all ___ during maximum sunspot activity
night
The single hop capability in the F layer is up to ____ miles for horizontal waves.
3000
As the circumference of a wave front increases, the energy ___ per unit of length
reduces
The signal strength at any point is called the ___ strength, and is measured in ___
field, volts
Attenuation concerns the loss of ___ and ___ in various parts of the radio wave
energy velocity
Attenuation is the result of the radio wave’s energy being absorbed by the ___/___/___, on top of the normal ___ power with range.
Earth/atmosphere/ionosphere, decreasing
The only way to control attenuation is by alteration of the __
frequency
Radar attenuation ___ with frequency, and can be affected by water droplets.
increases
Radio waves can change direction as a result of ___, ___, and ___
refraction, reflection, diffraction
Refraction is the change of ___/___ as the wave travels across different media.
speed/bending
Refraction happens at the ionosphere with _F, according to the ___, ___, and ___
HF, temp, pressure, humidity.
The ___ does not change when refraction occurs
frequency
The initial and reflected waves have the ____ angle
same
A phase shift occurs, depending on the angle of ___ and the wave ___
incidence, polarisation
Diffraction of the wave is the ___ of the wave as it passes through a gap/around an edge
spreading
Radio waves bend around ___/___, and over mountains
dawn dusk
Multipath propagation is the receiving of signals from ___ sources, out of ___ with each other at the antenna
many, phase
AM will generally reflect off of different layers of the ___, arriving out of ___ at the antenna
ionosphere, phase
AM VHF will reflect off the ____, but not with SSB.
ionosphere
Reflection off of the ionosphere can severely affect ___ modulation used by GPS sats, but not in the ___ latitudes.
PSK, mid
Ground waves are associated with _LF and _F, sometimes _F
VLF, LF, MF
Ground waves don’t leave the ___ atmosphere
lower
The two factors which influence how closely ground waves follow the curve of the Earth are ___ and ___
scattering attenuation
Radio waves tend to be reflected by objects larger than about ___ their wavelength
half
Lower frequency waves will curve around ___ obstacles, such as hills
small
As the _ field cuts the Earth’s surface, currents are created, with the energy coming from the wave itself
H
Contact with the surface will ___ the circumference of the wave
widen
Widening the circumference of the wave will cause ___, curving it downward, and causing it to be ___
attenuation, absorbed
The approximate length of LF/MF ground waves are ___ to ___ nm
1000/500
MF ground waves must be ___ polarised to induce ground currents.
vertically
Range depends on ___, type of ___, ___
wavelength, ground, polarisation
A lower frequency/longer wavelength will mean a ___ reception over long distances.
better
Higher frequency will mean a ___ attenuation
greater
Attenuation is _ times greater over land than sea
3
___ polarised have the least attenuation
vertically
Ground reflected waves bounce off the ground on the way to the receiver. They travel ___ than the ground wave due to ___ attenuation, but can have their phase ___ at the point of reflection.
further, lower, reversed
Space waves are contained within the ___
troposphere
Space waves are ___ and do not bounce like HF waves
direct
VHF/SHF/UHF are line of sight and (won’t) curve to follow the Earth’s surface.
won’t
A/G transmissions are limited to __nm in the UK
25
Tower transmissions are limited to ___ft
4,000
Approach frequencies are limited to ___ft
10,000
Theoretical direct wave range is given by the formula:
( 1.23 x Sqrt( H ) ) + ( 1.23 x Sqrt( h ) )
With H and h being the height of the aircraft/height of the ground antenna.
Sky waves reflect off of the ___ when the angle is correct, reaching a further distance.
ionosphere.
The HF frequencies allocated to commercial aviation range from _ to __ _Hz
2 22 MHz
HF waves hitting the ionosphere can be ___ if the angle is correct
bent
The side of the HF wave hitting the ionosphere will speed up, due to the ___ delectric constant from ionisation
reduced
The speed of radio waves in the atmosphere is determined by the ___ ___
dielectric property
The dielectric property depends upon ___, ___, and ___
temp, pressure, and relative humidity
Dielectric constant is ___ proportional to pressure and relative humidity, and ___ proportional to temperature.
directly, inversely.
Deceasing dielectric constant is why radio waves ___ their speed with height
increase
As a radio wave moves away from the surface, the decreasing dielectric constant will cause the top to move ___, bending it back towards the Earth
faster
The dielectric constant is the ratio of the ___ of the condenser in a given medium, to its capacity in a vacuum, or a measure of the ___ of the air to wave propagation.
capacity, resistance
The dielectric constant causes the radio wave to follow a curved path with a radius . times the visual horiozon.
1.3
The angle at which the bending of a wave first happens is the ___ angle
critical
Any rays which are more vertical than the critical angle are called ___ rays, typically used for ___.
escape, satellites
The critical frequency is the frequency at which ___ occurs. It depends upon the ___ of the layer concerned.
bending, density
The first wave to reach the ground after being refracted/reflected is called the ___ ___ ___
first returning sky wave
Between the point at which the ground wave fades, and the first returning sky wave comes back from the ionosphere is the ___ zone
skip
The ___ ___ is the Earth distance taken by a signal after each reflection
skip distance
30 MHz do (not) return as they are too high frequency, __-__ _Hz is the most typical for bouncing.
not, 15-25
With increasing ionization, the skip zone gets ___
narrower
With lower frequency , and longer wavelength, the ___ the skip distance
smaller
The skip zone of HF transmissions will ___ with higher frequencies and higher reflecting ionospheric layers.
increase
At night time, the skip distance will ___ for the same frequency.
increase
A lower frequency will ___ the critical angle and the skip distance will ___
reduce decrease
Skip is best when the sun is ___ between you and the area from which you are receiving.
halfway
Long path skip takes the ___ way around the Earth, usually because ionospheric conditions are better that way
long
Beyond the ground wave zone, we depend upon the ionosphere for all __ contacts
HF
ADF suffers from night effect around ___/___, when the needle swings erratically as the plane of polarisation changes after reflections.
sunrise sunset
During the night is when you will receive ___ ADF stations best
distant
HF comms are always possible when the frequency is ___ enough to be reflected, and ___ enough to not be attenuated.
low, high
HF Datalink uses the ___ sideband of a selected frequency to send ___ modulated digital information
upper., phase
Normal refractivity is existent for about __% of the time
50
Normal refractivity will cause the wave to be bent ___ from its usual straight line.
downward
Normal reactivity exists when moisture, pressure, and temp, ___ with altitude
decrease
When lapse rates differ significantly from normal, _HF, _HF, and _HF can follow different curved paths.
V U S
An inversion and steep lapse rates can bend the wave ___ toward the surface of the Earth, and ___ propagation distances; known as ___ refraction.
more, increase, super
Super refraction can cause the waves to become trapped between the ___ and the ___ ___ causing the refraction.
Earth, negative gradient
Super refraction ducts are associated with regions of ___ pressure, over __ terrain, and the sea.
high, flat
For radar, ducting will ___ the ground clutter
reduce
_LF signals can travel long distances through a process similar to super refraction, involving ionospheric layers and a ___ wave, which is ___ rather than refracted.
VLF, conduit, reflected
HF frequencies must be higher during the ___, or when you’re ___ ___ the station
day, .further from
As the ionosphere is higher at night, you can use ___ frequencies.
lower
The maximum usable frequency is the point at which refraction is ____
no longer possible
The level of ionisation is less by night than by day, meaning that a ___ frequency can be used to get the same type of refraction
lower
Attenuation is ____ by night
reduced
HF transmitter use frequencies between _-__ _Hz
2 20 MHz
The lowest usable frequency is the point below which refraction ____ ____
cannot start
