EM Wave Propagation Flashcards
What are the properties of EM waves?
Frequency
Wavelength
Amplitude
Polarization (direction of electric field wrt the horizon)
Define Wave Impedance
Measure of the relationship between electric and magnetic fields in the EM wave as it propagaates.
What does the impedance change with?
Permittivity and permeability of the material.
Define Poynting Vector
Depicts the direction and rate of transfer of energy (power) due to electromagnetic fields.
S = E X H
What are the factors which affect signal reception?
Attenuation
Spreading
Refraction
Diffraction
Define Attenuation
Gradual reduction in the EM energy due to reflection, scattering, and absorption.
Longer wavelengths -> less attenuation
Define Spreading
As energy leaves an antenna, it spreads out.
Inverse Square Law.
Define Reflection
EM waves reflected by conductors.
Areas may be shielded from radio waves.
Define Refraction
Waves are bent when changing mediums.
Causes Radio Line of Sight to be further than visual LOS (ducting).
What is Refraction Ducting?
Refracting EM waves off different temperature layers in the atmosphere and the ground IOT increase range.
Define Dispersion
Different frequencies will be refracted by different amounts, causing rays to take different paths and allowing EM waves to see around objects.
At what frequencies is Diffraction most prominent?
Low frequencies
Define Interference
Waves may either constructively or destrucvely interfere with each other.
Describe the Knife-edge effect
An object of significant sharpness may act as a secondary source at which waves are diffracted, extending the range.
What are the 3 main propagation paths of radio waves?
Space Wave (LOS)
Sky Wave (refractst off ionosphere)
Ground Wave (follows earth)
What type of polarization is required for Ground Waves? What frequency is it good up to?
Vertical polarization
2 MHz
What is the Ionosphere?
A layer of charged particles located 50-600km above the surface of the earth.
Charged particles are generated by solar radiation.
What does the Ionosphere do to radio waves?
The charged particles in the ionosphere refracts and absorbs radio waves.
Lower freq. absorbed more, higher less.
When are there the greatest changes in the Ionosphere layers?
Dawn and dusk
What is the Critical Frequency?
The upper frequency that will be reflected by the ionosphere when the ray is launched vertically.
How many layers to the Ionosphere during day and during night?
4 during the day. D, E, F1, F2
1 during the night. F
What is the MUF?
Maximum Usable Frequency
Same as critical frequency
How is the amount of sun related to the MUF?
More sunspots on the surface of the sun, more ionization, higher MUF.
What is the SSN?
Smoothed Sunspot Number
used when predicting EM propagation do to Ionospheric Variation
