Radio Wave Propagation Flashcards
What is diffraction and what causes it?
More or less diffraction at lower frequencies?
Spreading of the wave in to a circular waveform caused by the wave travelling through a hole or over a sharp object
More
What is attenuation?
Can it happen in a vacuum?
loss of signal strength of a wave
no because there is nothing to give up energy to
Ionosphere divided into … layers labelled as …
three
D, E & F
What is ionospheric attenuation?
increases with?
Loss of energy to electrically charged atoms at the edge of space
reducing frequency
Formula for max range of VHF/UHF space wave in NM?
(1.23 x square of H1) + (1.23 x square of H2)
H1 = height of transmitter in feet AMSL
H2 = height of receiver in feet AMSL
Radar reflection most likely when the wavelength is what size?
Same size or smaller than target
What radio waves refract the most?
Lower frequencies/longer wavelengths
The gap between the surface wave and first returning sky wave where no signal will be received is called …
dead space
What layer of ionosphere is only present by day and is … to earth?
D
Closest
VHF and UHF have what kind of propagation?
Space wave (line of sight)
What is atmospheric attenuation?
Increases with increase in … and becomes significant above …
electrons in particles of dust and water absorb some of the energy in the wave
frequency
1GHz
What is refraction and what causes it?
A change in direction or bending of the radio wave as the medium the wave is travelling through changes density
The amount of refraction of a radio wave depends on?
The change of speed of the wave, it’s wavelength and the angle at which it hits the new medium
Combination of space waves and surface waves is called?
ground wave
What is surface attenuation?
Increases with …
Fastest speeds over …
Loss of energy and slowing of the wave as it loses energy to the earth’s surface it is flowing over
frequency
water
In what layer is the density of the ionosphere the greatest?
Is this closer or further from earth?
F - split into F1 and F2 by day and combine to become one at night
Further
What is sporadic E?
Unusually long reception range of VHF waves caused by more heavily ionised E layer due to increased solar activity
What are sky waves?
Waves refracted in the ionosphere causing them to return to earth over large distances
Distance from transmitter to first returning sky wave is called … and will increase/decrease with increase in frequency?
minimum skip distance
decrease (because higher frequency waves do not refract as much, giving shorter skip distance)
Reflection sometimes occurs when it otherwise wouldnt when?
Waves hit a surface at a very shallow angle
List the three things that influence reflection
wavelength
density of materials
angle of incidence
Why is HF better than lower frequencies for sky waves?
Although lower frequency waves refract the most, they are also the ones that suffer most ionospheric attenuation
What are surface waves?
What limits the distance of them?
What frequencies travel furthest via surface wave and why?
Waves following the curvature of the earth caused by diffraction of the wave as it passes over hills and buildings etc
limited by surface attenuation
low frequencies travel furthest because less affected by surface attenuation
What is atmospheric ducting?
what frequencies?
also known as?
unusually long range propagation of VHF, UHF, SHF and EHF caused by atmospheric conditions creating an atmospheric duct, such as an inversion and rapid decrease in humidity with height. Extends line of sight range by approx 15%.
Also referred to as super refraction
What is ionospheric ducting?
what frequency?
distance?
In the VLF band the wavelength is so low that sky waves not normally expected but the point is reached where the waves are no longer refracted but are reflected from ionosphere. Barely any attenuation because the waves barely enter the ionosphere and gap between earth and ionosphere acts as a waveguide
4000nm distances
What causes static?
what is noise?
natural phenomenon caused by electrical discharge in convective clouds
insignificant above VHF
man made interference caused by unshielded electrical equipment when ac current and sparking produce electromagnetic fields
affects VHF and above
