1.4 EM Wave Propagation Flashcards
In Refraction, which way do waves bend?
Towards the slower medium.
Which diffracts more, lower or higher frequency signals?
Lower Frequency
What is a “Shaddow Zone” in wave propagation?
What are the three main types of propagation paths through the atmosphere?
Sky wave
Space wave
Ground wave
IOT reduce attenuation of a Ground Wave, would you use vertically or horizontally polarized waves?
Vertically polarized waves because they would have less interaction with the Earth’s surface.
How are Ground Waves attuated? Is saltwater good or bad as a surface?
See picture.
Saltwater is a good surface because it is a good conductor.
What is a Spave Wave?
Travels in a straight line to receiver or bounces off the ground.
What is Fading?
Fading is a decrease in signal strength due to destructive interfece at the receiver. This is caused by a phase difference in the straight line and reflected paths.
Describe Sky Waves. Can you use any frequency you want?
Sky waves are transmitted upwards to teh ionosphere where the are refracted (not reflected) back towards Earth.
Sky waves are limited in frequency by the properties of the ionosphere.
What is the mathematical difference between the visual and radio horizons?
Radio horizon assumes the radius of the earth is 1.33x the actual radius. This must be combined with the height of the transmitter to determine the radio horizon.
What are Temperature Inversions? How are they useful for communications? Are they reliable?
Typically temperature decreases with altitude, however there can be a layer of warm air trapped above a layer of cold air.
Radio waves are refracted towards the slower medium (denser cold air) back towards earth, increasing transmission range.
Temperature Inversions are unreliable because they are unpredictable.
What is Tropospheric Scattering and how does it aid communications?
Turblence in the troposphere can scatter signals in the VHF and UHF bands.
The scattering can be predictable and allow the signal to be scattered back down to Earth, increasing transmission range.
Suffers from high attenuation.
How does Ionization occur?
Ultraviolet light waves strike gaseious atoms and knock and electron free.
What Ionospheric layers are present during the day and at night?
How does Ionization Density in the Ionosphere affect refraction of radio waves?
As the ionization density of a layer increases, it becomes more conductive and the velocity of the EM wave increases. Therefore the EM wave will refract towards the slower medium, towards Earth.
What is the Critical Frequency (ionization).
What a radio wave is transmitted normal to an ionospheric layer, 100% of the wave will refract back to Earth up to the Critical Frequency.
At frequencies greater than this, some or all of the wave will pass through. Either to be lost to space or refract higher up.
Lower frequencies refract more
How does the Angle of Incidence affect the refraction of radio waves off the Ionosphere?
When radio waves hit the inospheric layers at an angle, less refraction is necessary. This allows use of frequencies above the critical frequency.
What is a Skip Zone?
What is the MUF?
Maximum Usable Frequency
The maximum frequency that can be used before none of the radio wave is refracted.
What is the LUF?
Lowert Usable Frequency
Frequencies below the LUF are refracted so sharply by the ionosphere that they return to Earth before reaccing the receiving site.
